Introduction to Eddy Current Testing

1.- Introduction

Basic Principles History of ET Present State of ET

2.- The Physics

Properties of Electricity Current Flow & Ohm's Law Induction & Inductance Self Inductance Mutual Inductance Circuits & Phase Impedance Depth & Current Density Phase Lag

3.- Instrumentation

Eddy Current Instruments Resonant Circuits Bridges Impedance Plane Display - Analog Meter

4.- Probes/Coil Design

Probes - Mode of Operation Probes - Configuration Probes - Shielding & Loading Coil Design

5.- Procedure Issues Reference Standards Signal Filtering

6.- Applications Surface Breaking Cracks SBC using Sliding Probes Metal Thinning (Corrosion) Tube Inspection Conductivity Heat Treat Verification Thickness of Thin Mat'ls Thickness of Coatings

7.- Advanced Techniques Scanning Multi-Frequency Tech. Swept Frequency Tech. Pulsed ET Tech. Background Pulsed ET Remote Field Tech. Impedance Matching

8.- Quizzes

9.- Formulae & Tables EC Standards & Methods EC Material Properties

1 INTRODUCTION

Basic Principles of Eddy Current Inspection

Eddy current inspection is one of several NDT methods that use the principal of

“electromagnetism” as the basis for conducting examinations. Several other

methods such as Remote Field Testing (RFT), Flux Leakage and Barkhausen

Noise also use this principle.

Eddy currents are created through a process called electromagnetic induction.

When alternating current is applied to the conductor, such as copper wire, a

magnetic field develops in and around the conductor. This magnetic field

expands as the alternating current rises to maximum and collapses as the current

is reduced to zero. If another electrical conductor is brought into the close

proximity to this changing magnetic field, current will be induced in this second

conductor. Eddy currents are induced electrical currents that flow in a circular

path. They get their name from “eddies” that are formed when a liquid or gas

flows in a circular path around obstacles when conditions are right.

View movie clip of an eddy current inspection. (430Kb)

One of the major advantages of eddy current as an NDT tool is the variety of

inspections and measurements that can be performed. In the proper

circumstances, eddy currents can be used for:

Crack detection

Material thickness measurements

Coating thickness measurements

Conductivity measurements for:

o Material identification

o Heat damage detection

o Case depth determination

o Heat treatment monitoring

Some of the advantages of eddy current inspection include:

Sensitive to small cracks and other defects

Detects surface and near surface defects

Inspection gives immediate results

Equipment is very portable

Method can be used for much more than flaw detection

Minimum part preparation is required

Test probe does not need to contact the part

Inspects complex shapes and sizes of conductive materials

Some of the limitations of eddy current inspection include:

Only conductive materials can be inspected

Surface must be accessible to the probe

Skill and training required is more extensive than other techniques

Surface finish and and roughness may interfere

Reference standards needed for setup

Depth of penetration is limited

Flaws such as delaminations that lie parallel to the probe coil winding and

probe scan direction are undetectable

History of Eddy Current Testing

Eddy current testing has its origins with

Michael Faraday's discovery

ofelectromagnetic induction in 1831.

Faraday was a chemist in England during

the early 1800's and is credited with the

discovery of electromagnetic induction,

electromagnetic rotations, the magneto-

optical effect, diamagnetism, and other

phenomena. In 1879, another scientist

named Hughes recorded changes in the

properties of a coil when placed in

contact with metals of

different conductivityand permeability.

However, it was not until the Second

World War that these effects were put to

practical use for testing materials. Much

work was done in the 1950's and 60's,

particularly in the aircraft and nuclear

industries. Eddy current testing is now a

widely used and well-understood

inspection technique.

Present State of Eddy Current Inspection

Eddy current inspection is used in a

variety of industries to find defects and

make measurements. One of the

primary uses of eddy current testing is

for defect detection when the nature of

the defect is well understood. In

general, the technique is used to inspect

a relatively small area and the probe

design and test parameters must be

established with a good understanding

of the flaw that is to be detected. Since

eddy currents tend to concentrate at the

surface of a material, they can only be

used to detect surface and near surface defects.

In thin materials such as tubing and sheet stock, eddy currents can be used to

measure the thickness of the material. This makes eddy current a useful tool for

detecting corrosion damage and other damage that causes a thinning of the

material. The technique is used to make corrosion thinning measurements on

aircraft skins and in the walls of tubing used in assemblies such as heat

exchangers. Eddy current testing is also used to measure the thickness of paints

and other coatings.

Eddy currents are also affected by the electrical conductivity and magnetic

permeability of materials. Therefore, eddy current measurements can be used to

sort materials and to tell if a material has seen high temperatures or been heat

treated, which changes the conductivity of some materials.

Eddy current equipment and probes can be purchased in a wide variety of

configurations. Eddyscopes and a conductivity tester come packaged in very

small and battery operated units for easy portability. Computer based systems are

also available that provide easy data manipulation features for the laboratory.

Signal processing software has also been developed for trend removal,

background subtraction, and noise reduction. Impedance analyzers are also

sometimes used to allow improved quantitative eddy-current measurements.

Some laboratories have multidimensional scanning capabilities that are used to

produce images of the scan regions. A few portable scanning systems also exist

for special applications, such as scanning regions of aircraft fuselages.

Research to Improve Eddy current measurements

A great deal of research continues to be done to improve eddy current

measurement techniques. A few of the these activities, which are being

conducted at Iowa State University, are described below.

Photoinductive Imaging (PI)

A technique known as photoinductive imaging (PI) was pioneered at CNDE and

provides a powerful, high-resolution scanning and imaging tool. Microscopic

resolution is available using standard-sized eddy-current sensors. Development

of probes and instrumentation for photoinductive (PI) imaging is based on the use

of a medium-power (5 W nominal power) argon ion laser. This probe provides

high resolution images and has been used to study cracks, welds, and diffusion

bonds in metallic specimens. The PI technique is being studied as a way to image

local stress variations in steel.

Pulsed Eddy Current Research is currently being conducted on the use of a technique called pulsed

eddy current (PEC) testing. This technique can be used for the detection and

quantification of corrosion and cracking in multi-layer aluminum aircraft

structures. Pulsed eddy-current signals consist of a spectrum of frequencies

meaning that, because of the skin effect, each pulse signal contains information

from a range of depths within a given test specimen. In addition, the pulse signals

are very low-frequency rich which provides excellent depth penetration. Unlike

multi-frequency approaches, the pulse-signals lend themselves to convenient

analysis. .

Measurements have been carried out both in the laboratory and in the field.

Corrosion trials have demonstrated how material loss can be detected and

quantified in multi-layer aluminum structures. More recently, studies carried out

on three and four layer structures show the ability to locate cracks emerging from

fasteners. Pulsed eddy-current measurements have also been applied to

ferromagnetic materials. Recent work has been involved with measuring the case

depth in hardened steel samples.

2 THE PHYSICS

Properties of Electricity

Since eddy current inspection makes use of electromagnetic induction, it is important to

know about the scientific principles of electricity and magnetism. For a review of these

principles, the Science of NDT materials on this Internet site may be helpful. A review of

the key parameters will be provided here.

Electricity

It is well known that one of the subatomic particles

of an atom is the electron. Atoms can and usually

do have a number of electrons circling its nucleus.

The electrons carry a negative electrostatic charge

and under certain conditions can move from atom

to atom. The direction of movement between atoms

is random unless a force causes the electrons to

move in one direction. This directional movement

of electrons due to some imbalance of force is what

is known as electricity.

Amperage

The flow of electrons is measured in units called amperes or amps for short. An

amp is the amount of electrical current that exists when a number of electrons,

having one coulomb of charge, move past a given point in one second.

A coulomb is the charge carried by 6.25 x 1018electrons or

6,250,000,000,000,000,000 electrons.

Electromotive Force

The force that causes the electrons to move in an electrical circuit

is called the electromotive force, or EMF. Sometimes it is

convenient to think of EMF as electrical pressure. In other words,

it is the force that makes electrons move in a certain direction

within a conductor. There are many sources of EMF, the most

common being batteries and electrical generators.

The Volt

The unit of measure for EMF is the volt. One volt is defined as the electrostatic

difference between two points when one joule of energy is used to move one

coulomb of charge from one point to the other. A joule is the amount of energy

that is being consumed when one watt of power works for one second. This is

also known as a watt-second. For our purposes, just accept the fact that one joule

of energy is a very, very small amount of energy. For example, a typical 60-watt

light bulb consumes about 60 joules of energy each second it is on.

Resistance

Resistance is the opposition of a body or substance to the flow of electrical

current through it, resulting in a change of electrical energy into heat, light, or

other forms of energy. The amount of resistance depends on the type of material.

Materials with low resistance are good conductorsof electricity. Materials with

high resistance are good insulators.

Current Flow and Ohm's Law

Ohm's law is the most important, basic law of electricity. It defines the

relationship between the three fundamental electrical quantities: current, voltage,

and resistance. When a voltage is applied to a circuit containing only resistive

elements (i.e. no coils), current flows according to Ohm's Law, which is shown

below.

I = V / R

Where:

I = Electrical Current

(Amperes)

V = Voltage (Voltage)

R = Resistance (Ohms)

Ohm's law states that the electrical current (I) flowing in an circuit is

proportional to the voltage(V) and inversely proportional to the resistance (R).

Therefore, if the voltage is increased, the current will increase provided the

resistance of the circuit does not change. Similarly, increasing the resistance of

the circuit will lower the current flow if the voltage is not changed. The formula

can be reorganized so that the relationship can easily be seen for all of the three

variables.

The Java applet below allows the user to vary each of these three parameters in

Ohm's Law and see the effect on the other two parameters. Values may be input

into the dialog boxes, or the resistance and voltage may also be varied by moving

the arrows in the applet. Current and voltage are shown as they would be

displayed on an oscilloscope with the X-axis being time and the Y-axis being the

amplitude of the current or voltage. Ohm's Law is valid for both direct current

(DC) and alternating current (AC). Note that in AC circuits consisting of purely

resistive elements, the current and voltage are always in phase with each other.

Exercise: Use the interactive applet below to investigate the relationship of the

variables in Ohm's law. Vary the voltage in the circuit by clicking and dragging

the head of the arrow, which is marked with the V. The resistance in the circuit

can be increased by dragging the arrow head under the variable resister, which is

marked R. Please note that the vertical scale of the oscilloscope screen

automatically adjusts to reflect the value of the current.

See what happens to the voltage and current as the resistance in the circuit is

increased. What happens if there is not enough resistance in a circuit? If the

resistance is increased, what must happen in order to maintain the same level of

current flow?

Induction and Inductance

Induction

In 1824, Oersted discovered that current passing

though a coil created a magnetic field capable of

shifting a compass needle. Seven years later, Faraday

and Henry discovered just the opposite. They noticed

that a moving magnetic field would induce current in

an electrical conductor. This process of generating

electrical current in a conductor by placing the

conductor in a changing magnetic field is

calledelectromagnetic induction or just induction.

It is called induction because the current is said to be induced in the conductor by

the magnetic field.

Faraday also noticed that the rate at which the magnetic field changed also had an

effect on the amount of current or voltage that was induced. Faraday's Law for

an uncoiled conductor states that the amount of induced voltage is proportional to

the rate of change of flux lines cutting the conductor. Faraday's Law for a straight

wire is shown below.

Where:

VL = the induced voltage in volts

dø/dt = the rate of change of magnetic flux in webers/second

Induction is measured in unit of Henries (H) which reflects this dependence on

the rate of change of the magnetic field. One henry is the amount of inductance

that is required to generate one volt of induced voltage when the current is

changing at the rate of one ampere per second. Note that current is used in the

definition rather than magnetic field. This is because current can be used to

generate the magnetic field and is easier to measure and control than magnetic

flux.

Inductance

When induction occurs in an electrical

circuit and affects the flow of electricity it is

called inductance,L. Self-inductance, or

simply inductance, is the property of a

circuit whereby a change in current causes a

change in voltage in the same circuit. When

one circuit induces current flow in a second

nearby circuit, it is known as mutual-

inductance. The image to the right shows an

example of mutual-inductance. When an AC

current is flowing through a piece of wire in a circuit, an electromagnetic field is

produced that is constantly growing and shrinking and changing direction due to

the constantly changing current in the wire. This changing magnetic field will

induce electrical current in another wire or circuit that is brought close to the wire

in the primary circuit. The current in the second wire will also be AC and in fact

will look very similar to the current flowing in the first wire. An electrical

transformer uses inductance to change the voltage of electricity into a more

useful level. In nondestructive testing, inductance is used to generate eddy

currents in the test piece.

It should be noted that since it is the changing magnetic field that is responsible

for inductance, it is only present in AC circuits. High frequency AC will result in

greater inductive reactance since the magnetic field is changing more rapidly.

Self-inductance and mutual-inductance will be discussed in more detail in the

following pages.

Self-Inductance and Inductive Reactance

The property of self-inductance is a particular form of electromagnetic induction.

Self inductance is defined as the induction of a voltage in a current-carrying wire

when the current in the wire itself is changing. In the case of self-inductance, the

magnetic field created by a changing current in the circuit itself induces a voltage

in the same circuit. Therefore, the voltage is self-induced.

The term inductor is used to describe a circuit element possessing the property of

inductance and a coil of wire is a very common inductor. In circuit diagrams, a

coil or wire is usually used to indicate an inductive component. Taking a closer

look at a coil will help understand the reason that a voltage is induced in a wire

carrying a changing current. The alternating current running through the coil

creates a magnetic field in and around the coil that is increasing and decreasing

as the current changes. The magnetic field forms concentric loops that surround

the wire and join to form larger loops that surround the coil as shown in the

image below. When the current increases in one loop the expanding magnetic

field will cut across some or all of the neighboring loops of wire, inducing a

voltage in these loops. This causes a voltage to be induced in the coil when the

current is changing.

By studying this image of a coil, it can be seen that the number of turns in the

coil will have an effect on the amount of voltage that is induced into the circuit.

Increasing the number of turns or the rate of change of magnetic flux increases

the amount of induced voltage. Therefore,Faraday's Law must be modified for a

coil of wire and becomes the following.

Where:

VL = induced voltage in volts

N = number of turns in the coil

dø/dt = rate of change of magnetic flux in

webers/second

The equation simply states that the amount of induced voltage (VL) is

proportional to the number of turns in the coil and the rate of change of the

magnetic flux (dø/dt). In other words, when the frequency of the flux is increased

or the number of turns in the coil is increased, the amount of induced voltage will

also increase.

In a circuit, it is much easier to measure current than it is to measure magnetic

flux, so the following equation can be used to determine the induced voltage if

the inductance and frequency of the current are known. This equation can also be

reorganized to allow the inductance to be calculated when the amount of inducted

voltage can be determined and the current frequency is known.

Where:

VL = the induced voltage in volts

L = the value of inductance in henries

di/dt = the rate of change of current in amperes per second

Lenz's Law

Soon after Faraday proposed his law of induction, Heinrich Lenz developed a

rule for determining the direction of the induced current in a loop.

Basically, Lenz's law states that an induced current has a direction such that

its magnetic field opposes the change in magnetic field that induced the current. This means that the current induced in a conductor will oppose the

change in current that is causing the flux to change. Lenz's law is important in

understanding the property of inductive reactance, which is one of the properties

measured in eddy current testing.

Inductive Reactance

The reduction of current flow in a circuit due to induction is called inductive

reactance. By taking a closer look at a coil of wire and applying Lenz's law, it

can be seen how inductance reduces the flow of current in the circuit. In the

image below, the direction of the primary current is shown in red, and the

magnetic field generated by the current is shown in blue. The direction of the

magnetic field can be determined by taking your right hand and pointing your

thumb in the direction of the current. Your fingers will then point in the direction

of the magnetic field. It can be seen that the magnetic field from one loop of the

wire will cut across the other loops in the coil and this will induce current flow

(shown in green) in the circuit. According to Lenz's law, the induced current

must flow in the opposite direction of the primary current. The induced current

working against the primary current results in a reduction of current flow in the

circuit.

It should be noted that the inductive reactance will increase if the number of

winds in the coil is increased since the magnetic field from one coil will have

more coils to interact with.

Similarly to resistance, inductive reactance reduces the flow of current in a

circuit. However, it is possible to distinguish between resistance and inductive

reactance in a circuit by looking at the timing between the sine waves of the

voltage and current of the alternating current. In an AC circuit that contains only

resistive components, the voltage and the current will be in-phase, meaning that

the peaks and valleys of their sine waves will occur at the same time. When there

is inductive reactance present in the circuit, the phase of the current will be

shifted so that its peaks and valleys do not occur at the same time as those of the

voltage. This will be discussed in more detail in the section on circuits.

Mutual Inductance

(The Basis for Eddy Current Inspection)

The magnetic flux through a circuit can be related to the current in that circuit

and the currents in other nearby circuits, assuming that there are no nearby

permanent magnets. Consider the following two circuits.

The magnetic field produced by circuit 1 will intersect the wire in circuit 2 and

create current flow. The induced current flow in circuit 2 will have its own

magnetic field which will interact with the magnetic field of circuit 1. At some

point P, the magnetic field consists of a part due toi1 and a part due to i2. These

fields are proportional to the currents producing them.

The coils in the circuits are labeled L1 and L2 and this term represents the self

inductance of each of the coils. The values of L1 and L2 depend on the

geometrical arrangement of the circuit (i.e. number of turns in the coil) and the

conductivity of the material. The constant M, called themutual inductance of

the two circuits, is dependent on the geometrical arrangement of both circuits. In

particular, if the circuits are far apart, the magnetic flux through circuit 2 due to

the current i1 will be small and the mutual inductance will be small. L2 and M are

constants.

We can write the flux, B through circuit 2 as the sum of two parts.

B2 = L2i2 + i1M

An equation similar to the one above can be written for the flux through circuit 1.

B1 = L1i1 + i2M

Though it is certainly not obvious, it can be shown that the mutual inductance is

the same for both circuits. Therefore, it can be written as follows:

M1,2 = M2,1

How is mutual induction used in eddy current

inspection?

In eddy current inspection, the eddy currents are

generated in the test material due to mutual

induction. The test probe is basically a coil of wire

through which alternating current is passed.

Therefore, when the probe is connected to an

eddyscope instrument, it is basically represented by circuit 1 above. The second

circuit can be any piece of conductive material.

When alternating current is passed through the coil, a

magnetic field is generated in and around the coil.

When the probe is brought in close proximity to a

conductive material, such as aluminum, the probe's

changing magnetic field generates current flow in the

material. The induced current flows in closed loops

in planes perpendicular to the magnetic flux. They are named eddy

currents because they are thought to resemble the eddy currents that can be seen

swirling in streams.

The eddy currents produce their own magnetic fields

that interact with the primary magnetic field of the

coil. By measuring changes in the resistance and

inductive reactance of the coil, information can be

gathered about the test material. This information

includes the electrical conductivity and magnetic

permeability of the material, the amount of material

cutting through the coils magnetic field, and the

condition of the material (i.e. whether it contains cracks or other defects.) The

distance that the coil is from the conductive material is called liftoff, and this

distance affects the mutual-inductance of the circuits. Liftoff can be used to make

measurements of the thickness of nonconductive coatings, such as paint, that

hold the probe a certain distance from the surface of the conductive material.

It should be noted that if a sample is ferromagnetic, the magnetic flux is

concentrated and strengthened despite opposing eddy current effects. The

increase inductive reactance due to the magnetic permeability of ferromagnetic

materials makes it easy to distinguish these materials from nonferromagnetic

materials.

In the applet below, the probe and the sample are shown in cross-section. The

boxes represent the cross-sectional area of a group of turns in the coil. The liftoff

distance and the drive current of the probe can be varied to see the effects of the

shared magnetic field. The liftoff value can be set to 0.1 or less and the current

value can be varied from 0.01 to 1.0. The strength of the magnetic field is shown

by the darkness of the lines.

Circuits and Phase

A circuit can be thought of as a closed path in which current flows through the

components that make up the circuit. The current (i) obeys Ohm's Law, which is

discussed on the page oncurrent flow. The simple circuit below consists of a

voltage source (in this case an alternating current voltage source) and a resistor.

The graph below the circuit diagram shows the value of the voltage and the

current for this circuit over a period of time.

This graph shows one complete cycle of an alternating current source. From the

graph, it can be seen that as the voltage increases, the current does the same. The

voltage and the current are said to be "in-phase" since their zero, peak, and valley

points occur at the same time. They are also directly proportional to each other.

In the circuit below, the resistive component has been replaced with an inductor.

When inductance is introduced into a circuit, the voltage and the current will be

"out-of-phase," meaning that the voltage and current do not cross zero, or reach

their peaks and valleys at the same time. When a circuit has an inductive

component, the current (iL) will lag the voltage by one quarter of a cycle. One

cycle is often referred to as 360o, so it can be said that the current lags the voltage

by 90o.

This phase shift occurs because the inductive reactance changes with changing

current. Recall that it is the changing magnetic field caused by a changing

current that produces inductive reactance. When the change in current is

greatest, inductive reactance will be the greatest, and the voltage across the

inductor will be the highest. When the change in current is zero, the inductive

reactance will be zero and the voltage across the inductor will be zero. Be

careful not to confuse the amount of current with the amount of change in the

current. Consider the points where the current reaches it peak amplitude and

changes direction in the graph below (0o, 180o, and 360o). As the current is

changing directions, there is a split second when the change in current is

zero. Since the change in current is zero, no magnetic field is generated to

produce the inductive reactance. When the inductive reactance is zero, the

voltage across the inductor is zero.

The resistive and inductive components are of primary interest in eddy current

testing since the test probe is basically a coil of wire, which will have both

resistance and inductive reactance. However, there is a small amount of

capacitance in the circuits so a mention is appropriate. This simple circuit below

consists of an alternating current voltage source and a capacitor. Capacitance in a

circuit caused the current (ic) to lead the voltage by one quarter of a cycle

(90o current lead).

When there is both resistance and inductive reactance (and/or capacitance) in a

circuit, the combined opposition to current flow is known as impedance.

Impedance will be discussed more on the next page.

Impedance

Electrical Impedance (Z), is the total opposition that a circuit presents to

alternating current. Impedance is measured in ohms and may include resistance

(R), inductive reactance (XL), and capacitive reactance (XC). However, the

total impedance is not simply the algebraic sum of the resistance, inductive

reactance, and capacitive reactance. Since the inductive reactance and capacitive

reactance are 90o out of phase with the resistance and, therefore, their maximum

values occur at different times, vector addition must be used to calculate

impedance.

In the image below, a circuit diagram is shown that represents an eddy current

inspection system. The eddy current probe is a coil of wire so it contains

resistance and inductive reactance when driven by alternating current. The

capacitive reactance can be dropped as most eddy current probes have little

capacitive reactance. The solid line in the graph below shows the circuit's total

current, which is affected by the total impedance of the circuit. The two dashed

lines represent the portion of the current that is affected by the resistance and the

inductive reactance components individually. It can be seen that the resistance

and the inductive reactance lines are 90o out of phase, so when combined to

produce the impedance line, the phase shift is somewhere between zero and 90o.

The phase shift is always relative to the resistance line since the resistance line is

always in-phase with the voltage. If more resistance than inductive reactance is

present in the circuit, the impedance line will move toward the resistance line and

the phase shift will decrease. If more inductive reactance is present in the circuit,

the impedance line will shift toward the inductive reactance line and the phase

shift will increase.

The relationship between impedance and its individual components (resistance

and inductive reactance) can be represented using a vector as shown below. The

amplitude of the resistance component is shown by a vector along the x-axis and

the amplitude of the inductive reactance is shown by a vector along the y-axis.

The amplitude of the the impedance is shown by a vector that stretches from zero

to a point that represents both the resistance value in the x-direction and the

inductive reactance in the y-direction. Eddy current instruments with impedance

plane displays present information in this format.

The impedance in a circuit with resistance and inductive reactance can be

calculated using the following equation. If capacitive reactance was present in the

circuit, its value would be added to the inductance term before squaring.

The phase angle of the circuit can also be calculated using some trigonometry.

The phase angle is equal to the ratio between the inductance and the resistance in

the circuit. With the probes and circuits used in nondestructive testing,

capacitance can usually be ignored so only inductive reactance needs to be

accounted for in the calculation. The phase angle can be calculated using the

equation below. If capacitive reactance was present in the circuit, its value would

simply be subtracted from the inductive reactance term.

or

The applet below can be used to see how the variables in the above equation are

related on the the vector diagram (or the impedance plane display). Values can be

entered into the dialog boxes or the arrow head on the vector diagram can be

dragged to a point representing the desired values. Note that the capacitive

reactance term has been included in the applet but as mentioned before, in eddy

current testing this value is small and can be ignored.

Impedance and Ohm's Law

In previous pages, Ohm's Law was discussed for a purely resistive circuit. When

there is inductive reactance or capacitive reactance also present in the circuit,

Ohm's Law must be written to include the total impedance in the circuit.

Therefore, Ohm's law becomes:

I = V / Z

Ohm's law now simply states that the current (I), in amperes, is proportional to

the voltage (V), in volts, divided by the impedance (Z), in ohms.

The applet below can be used to see how the current and voltage of a circuit are

affected by impedance. The applet allows the user to vary the inductance (L),

resistance (R), voltage (V)and current (I). Voltage and current are shown as they

would be displayed on an oscilloscope. Note that the resistance and/or the

inductive reactance values must be changed to change the impedance in the

circuit.

Also note that when there is inductance in the circuit, the voltage and current are

out of phase. This is because the voltage across the inductor will be a maximum

when the rate of change of the current is greatest. For a sinusoidal wave form like

AC, this is at the point where the actual current is zero. Thus the voltage applied

to an inductor reaches its maximum value a quarter-cycle before the current does,

and the voltage is said to lead the current by 90o.

Depth of Penetration & Current Density

Eddy currents are closed loops of induced

current circulating in planes perpendicular to

the magnetic flux. They normally travel

parallel to the coil's winding and flow is limited

to the area of the inducing magnetic field. Eddy

currents concentrate near the surface adjacent

to an excitation coil and their strength

decreases with distance from the coil as shown

in the image. Eddy current density decreases

exponentially with depth. This phenomenon is known as the skin effect.

The skin effect arises when the eddy currents flowing in the test object at any

depth produce magnetic fields which oppose the primary field, thus reducing the

net magnetic flux and causing a decrease in current flow as the depth increases.

Alternatively, eddy currents near the surface can be viewed as shielding the coil's

magnetic field, thereby weakening the magnetic field at greater depths and

reducing induced currents.

The depth that eddy currents penetrate into a material is affected by the

frequency of the excitation current and the electrical conductivity and magnetic

permeability of the specimen. The depth of penetration decreases with increasing

frequency and increasing conductivity and magnetic permeability. The depth at

which eddy current density has decreased to 1/e, or about 37% of the surface

density, is called the standard depth of penetration ( ). The word 'standard'

denotes plane wave electromagnetic field excitation within the test sample

(conditions which are rarely achieved in practice). Although eddy currents

penetrate deeper than one standard depth of penetration, they decrease rapidly

with depth. At two standard depths of penetration (2 ), eddy current density has

decreased to 1/e squared or 13.5% of the surface density. At three depths (3 ),

the eddy current density is down to only 5% of the surface density.

Since the sensitivity of an eddy current inspection depends on the eddy current

density at the defect location, it is important to know the strength of the eddy

currents at this location. When attempting to locate flaws, a frequency is often

selected which places the expected flaw depth within one standard depth of

penetration. This helps to assure that the strength of the eddy currents will be

sufficient to produce a flaw indication. Alternately, when using eddy currents to

measure the electrical conductivity of a material, the frequency is often set so that

it produces three standard depths of penetration within the material. This helps to

assure that the eddy currents will be so weak at the back side of the material that

changes in the material thickness will not affect the eddy current measurements.

The applet below illustrates how eddy current density changes in a semi-infinite

conductor. The applet can be used to calculate the standard depth of penetration.

The equation for this calculation is:

Where:

= Standard Depth of Penetration (mm)

= 3.14

f = Test Frequency (Hz)

= Magnetic Permeability (H/mm)

= Electrical Conductivity (% IACS)

(Note: The applet has an input box for relative permeability since this is often the

more readily available value. The applet multiplies the relative permeability of

the material by the permeability of free space to get to H/mm units.)

The applet also indicates graphically the phase lag at one and two standard

depths of penetration. Phase lag will be discussed on the following page.

Phase Lag

Phase lag is a parameter of the eddy current signal that makes it possible to

obtain information about the depth of a defect within a material. Phase lag is the

shift in time between the eddy current response from a disruption on the surface

and a disruption at some distance below the surface. The generation of eddy

currents can be thought of as a time dependent process, meaning that the eddy

currents below the surface take a little longer to form than those at the

surface. Disruptions in the eddy currents away from the surface will produce

more phase lag than disruptions near the surface. Both the signal voltage and

current will have this phase shift or lag with depth, which is different from the

phase angle discussed earlier. (With the phase angle, the current shifted with

respect to the voltage.)

Phase lag is an important parameter in eddy current testing because it makes it

possible to estimate the depth of a defect, and with proper reference specimens,

determine the rough size of a defect. The signal produced by a flaw depends on

both the amplitude and phase of the eddy currents being disrupted. A small

surface defect and large internal defect can have a similar effect on the magnitude

of impedance in a test coil. However, because of the increasing phase lag with

depth, there will be a characteristic difference in the test coil impedance vector.

Phase lag can be calculated with the following equation. The phase lag angle

calculated with this equation is useful for estimating the subsurface depth of a

discontinuity that is concentrated at a specific depth. Discontinuities, such as a

crack that spans many depths, must be divided into sections along its length and a

weighted average determined for phase and amplitude at each position below the

surface.

In

Radians

In

Degrees

Where:

=Phase Lag (Rad or Degrees)

x=Distance Below Surface (in or mm)

=Standard Depth of Penetration (in or mm)

At one standard depth of penetration, the phase lag is one radian or 57o. This

means that the eddy currents flowing at one standard depth of penetration ( )

below the surface, lag the surface currents by 57o. At two standard depths of

penetration (2 ), they lag the surface currents by 114o. Therefore, by measuring

the phase lag of a signal the depth of a defect can be estimated.

On the impedance plane, the liftoff signal serves as the reference phase

direction. The angle between the liftoff and defect signals is about twice the

phase lag calculated with the above equation. As mentioned above,

discontinuities that have a significant dimension normal to the surface, will

produce an angle that is based on the weighted average of the disruption to the

eddy currents at the various depths along its length.

In the applet below, the relationship between the depth and dimensions of a

discontinuity and the rotation produced on the impedance plane is explored. The

red lines represent the relative strength of the magnetic field from the coil and the

dashed lines indicate the phase lag of the eddy currents induced at a particular

depth.

3 EINSTRUMENTATION

Eddy Current Instruments

Eddy current instruments can be

purchased in a large variety of

configurations. Both analog and digital

instruments are available. Instruments

are commonly classified by the type of

display used to present the data. The

common display types are analog

meter, digital readout, impedance plane and time versus signal amplitude. Some

instruments are capable of presenting data in several display formats.

The most basic eddy current testing instrument consists of an alternating current

source, a coil of wire connected to this source, and a voltmeter to measure the

voltage change across the coil. An ammeter could also be used to measure the

current change in the circuit instead of using the voltmeter.

While it might actually be possible to detect some types of defects with this type

of equipment, most eddy current instruments are a bit more sophisticated. In the

following pages, a few of the more important aspects of eddy current

instrumentation will be discussed.

Resonant Circuits

Eddy current probes typically have a frequency or a range of frequencies that

they are designed to operated. When the probe is operated outside of this range,

problems with the data can occur. When a probe is operated at too high of a

frequency, resonance can occurs in the circuit. In a parallel circuit with resistance

(R), inductance (XL) and capacitance (XC), as the frequency increases

XL decreases and XC increase. Resonance occurs when XL and XC are equal but

opposite in strength. At the resonant frequency, the total impedance of the circuit

appears to come only from resistance since XL and XC cancel out. Every circuit

containing capacitance and inductance has a resonant frequency that is inversely

proportional to the square root of the product of the capacitance and inductance.

In eddy current probes and cables, it is commonly stated that capacitance is

negligible. However, even circuits not containing discreet components for

resistance, capacitance, and inductance can still exhibit their effects. When two

conductors are placed side by side, there is always some capacitance between

them. Thus, when many turns of wire are placed close together in a coil, a

certain amount of stray capacitance is produced. Additionally, the cable used to

interconnect pieces of electronic equipment or equipment to probes, often has

some capacitance, as well as, inductance. This stray capacitance is usually very

small and in most cases has no significant effect. However, they are not

negligible in sensitive circuits and at high frequencies they become quite

important.

The applet below represents an eddy current probe with a default resonant

frequency of about 1.0 kHz. An ideal probe might contain just the inductance, but

a realistic probe has some resistance and some capacitance. The applet initially

shows a single cycle of the 1.0 kHz current passing through the inductor.

Exercise 1: Using your mouse, adjust the resistance by sliding the slide bar.

Does the frequency change?

Exercise 2: Note that changing the inductance and/or the capacitance changes

the resonant frequency of this resonant circuit. Can you find several

combinations of capacitance and inductance that resonate at 1.0 kHz?

Bridges

The bridge circuit shown in the applet below is known as the Maxwell-Wien

bridge (often called the Maxwell bridge), and is used to measure unknown

inductances in terms of calibrated resistance and capacitance. Calibration-grade

inductors are more difficult to manufacture than capacitors of similar precision,

and so the use of a simple "symmetrical" inductance bridge is not always

practical. Because the phase shifts of inductors and capacitors are exactly

opposite each other, a capacitive impedance can balance out an inductive

impedance if they are located in opposite legs of a bridge, as they are here.

Unlike this straight Wien bridge, the balance of the Maxwell-Wien bridge is

independent of the source frequency. In some cases, this bridge can be made to

balance in the presence of mixed frequencies from the AC voltage source, the

limiting factor being the inductor's stability over a wide frequency range.

Exercise: Using the equations within the applet, calculate appropriate values for

C and R2 for a set of probe values. Then, using your calculated values, balance

the bridge. The oscilloscope trace representing current (brightest green) across

the top and bottom of the bridge should be minimized (straight line).

In the simplest implementation, the standard capacitor (C) and the resistor in

parallel with it are made variable, and both must be adjusted to achieve balance.

However, the bridge can be made to work if the capacitor is fixed (non-variable)

and more than one resistor is made variable (at least the resistor in parallel with

the capacitor, and one of the other two). However, in the latter configuration it

takes more trial-and-error adjustment to achieve balance as the different variable

resistors interact in balancing magnitude and phase.

Another advantage of using a Maxwell bridge to measure inductance rather than

a symmetrical inductance bridge is the elimination of measurement error due to

the mutual inductance between the two inductors. Magnetic fields can be difficult

to shield, and even a small amount of coupling between coils in a bridge can

introduce substantial errors in certain conditions. With no second inductor to

react within the Maxwell bridge, this problem is eliminated.

Display - Complex Impedance Plane (eddy scope)

Electrical Impedance (Z), is the total opposition that a

circuit presents to an alternating current. Impedance,

measured in ohms, may include resistance (R), inductive

reactance (XL), andcapacitive reactance (XC). Eddy

current circuits usually have only R and (XL)

components. As discussed in the page on impedance, the

resistance component and the reactance component are

not in phase, so vector addition must be used to relate

them with impedance. For an eddy current circuit with

resistance and inductive reactance components, the total

impedance is calculated using the following equation.

You will recall that this can be graphically displayed using the impedance plane

diagram as seen above. Impedance also has an associated angle, called the phase

angle of the circuit, which can be calculated by the following equation.

The impedance plane diagram is a very useful way of displaying eddy current

data. As shown in the figure below, the strength of the eddy currents and the

magnetic permeability of the test material cause the eddy current signal on the

impedance plane to react in a variety of different ways.

If the eddy current circuit is balanced in air and then placed on a piece of

aluminum, the resistance component will increase (eddy currents are being

generated in the aluminum and this takes energy away from the coil, which

shows up as resistance) and the inductive reactance of the coil decreases (the

magnetic field created by the eddy currents opposes the coil's magnetic field and

the net effect is a weaker magnetic field to produce inductance). If a crack is

present in the material, fewer eddy currents will be able to form and the

resistance will go back down and the inductive reactance will go back up.

Changes in conductivity will cause the eddy current signal to change in a

different way.

When a probe is placed on a magnetic material such as steel, something different

happens. Just like with aluminum (conductive but not magnetic), eddy currents

form, taking energy away from the coil, which shows up as an increase in the

coils resistance. And, just like with the aluminum, the eddy currents generate

their own magnetic field that opposes the coils magnetic field. However, you will

note for the diagram that the reactance increases. This is because the magnetic

permeability of the steel concentrates the coil's magnetic field. This increase in

the magnetic field strength completely overshadows the magnetic field of the

eddy currents. The presence of a crack or a change in the conductivity will

produce a change in the eddy current signal similar to that seen with aluminum.

In the applet below, liftoff curves can be generated for several nonconductive

materials with various electrical conductivities. With the probe held away from

the metal surface, zero and clear the graph. Then slowly move the probe to the

surface of the material. Lift the probe back up, select a different material and

touch it back to the sample surface.

Experiment

Generate a family of liftoff curves for the different materials available in the

applet using a frequency of 10kHz. Note the relative position of each of the

curves. Repeat at 500kHz and 2MHz. (Note: it might be helpful to capture an

image of the complete set of curves for each frequency for comparison.)

1) Which frequency would be best if you needed to distinguish between two high

conductivity materials?

2) Which frequency would be best if you needed to distinguish between two low

conductivity materials?

The impedance calculations in the above applet are based on codes by Jack Blitz

from "Electrical and Magnetic Methods of Nondestructive Testing," 2nd ed.,

Chapman and Hill.

Display - Analog Meter

Analog instruments are the

simplest of the instruments

available for eddy current

inspections. They are used for

crack detection, corrosion

inspection, or conductivity testing.

These types of instruments contain

a simple bridge circuit, which

compares a balancing load to that

measured on the test specimen. If

any changes in the test specimen

occur which deviate from normal

you will see a movement on the instruments meter.

Analog meters such as the D'Arsonval design pictured in the applet below, must

"rectify" the AC into DC. This is most easily accomplished through the use of

devices called diodes. Without going into elaborate detail over how and why

diodes work as they do, remember that they each act like a one-way valve for

electrons to flow. They act as a conductor for one polarity and an insulator for

another. Arranged in a bridge, four diodes will serve to steer AC through the

meter movement in a constant direction.

An analog meter can easily measure just a few microamperes of current and is

well suited for use in balancing bridges.

Exercise: Using the equations within the applet, calculate appropriate values for

C and R2 for a set of probe values. Then balance the bridge using your calculated

values. The analog meter should swing close to the left end if its scale indicates

little or no current across the bridge. Across the bridge should be minimized

(straight line).

4 Probes / Coil Desing

Probes - Mode of Operation

Eddy current probes are available in a

large variety of shapes and sizes. In

fact, one of the major advantages of

eddy current inspection is that probes

can be custom designed for a wide

variety of applications. Eddy current

probes are classified by the

configuration and mode of operation of

the test coils. The configuration of the

probe generally refers to the way the

coil or coils are packaged to best "couple" to the test area of interest. An example

of different configurations of probes would be bobbin probes, which are inserted

into a piece of pipe to inspect from the inside out, versus encircling probes, in

which the coil or coils encircle the pipe to inspect from the outside in. The mode

of operation refers to the way the coil or coils are wired and interface with the

test equipment. The mode of operation of a probe generally falls into one of four

categories: absolute, differential, reflection and hybrid. Each of these

classifications will be discussed in more detail below.

Absolute Probes

Absolute probes generally have a single test

coil that is used to generate the eddy currents

and sense changes in the eddy current field. As

discussed in the physics section, AC is passed

through the coil and this sets up an expanding

and collapsing magnetic field in and around

the coil. When the probe is positioned next to

a conductive material, the changing magnetic

field generates eddy currents within the

material.

The generation of the eddy currents take energy from the coil and this appears as

an increase in the electrical resistance of the coil. The eddy currents generate

their own magnetic field that opposes the magnetic field of the coil and this

changes the inductive reactance of the coil. By measuring the absolute change in

impedance of the test coil, much information can be gained about the test

material.

Absolute coils can be used for flaw detection, conductivity measurements, liftoff

measurements and thickness measurements. They are widely used due to their

versatility. Since absolute probes are sensitive to things such as conductivity,

permeability liftoff and temperature, steps must be taken to minimize these

variables when they are not important to the inspection being performed. It is

very common for commercially available absolute probes to have a fixed "air

loaded" reference coil that compensates for ambient temperature variations.

Differential Probes

Differential probes have two active

coils usually wound in opposition,

although they could be wound in

addition with similar results. When

the two coils are over a flaw-free

area of test sample, there is no

differential signal developed

between the coils since they are

both inspecting identical material.

However, when one coil is over a

defect and the other is over good

material, a differential signal is

produced. They have the advantage

of being very sensitive to defects

yet relatively insensitive to slowly

varying properties such as gradual dimensional or temperature variations. Probe

wobble signals are also reduced with this probe type. There are also

disadvantages to using differential probes. Most notably, the signals may be

difficult to interpret. For example, if a flaw is longer than the spacing between

the two coils, only the leading and trailing edges will be detected due to signal

cancellation when both coils sense the flaw equally.

Reflection Probes

Reflection probes have two coils similar to a differential probe, but one coil is

used to excite the eddy currents and the other is used to sense changes in the test

material. Probes of this arrangement are often referred to as driver/pickup probes.

The advantage of reflection probes is that the driver and pickup coils can be

separately optimized for their intended purpose. The driver coil can be made so

as to produce a strong and uniform flux field in the vicinity of the pickup coil,

while the pickup coil can be made very small so that it will be sensitive to very

small defects.

Some absolute and differential "transformer" type eddy current probes.

The through-transmission method is sometimes used when complete penetration

of plates and tube walls is required.

Hybrid Probes

An example of a hybrid probe is the split D,

differential probe shown to the right. This probe has

a driver coil that surrounds two D shaped sensing

coils. It operates in the reflection mode but

additionally, its sensing coils operate in the

differential mode.

This type of probe is very sensitive to surface cracks. Another example of a

hybrid probe is one that uses a conventional coil to generate eddy currents in the

material but then uses a different type of sensor to detect changes on the surface

and within the test material. An example of a hybrid probe is one that uses a Hall

effect sensor to detect changes in the magnetic flux leaking from the test surface.

Hybrid probes are usually specially designed for a specific inspection

application.

Probes - Configurations

As mentioned on the previous page, eddy current probes are classified by the

configuration and mode of operation of the test coils. The configuration of the

probe generally refers to the way the coil or coils are packaged to best "couple"

to the test area of interest. Some of the common classifications of probes based

on their configuration include surface probes, bolt hole probes, inside diameter

(ID) probes, and outside diameter (OD) probes.

Surface Probes

Surface probes are usually designed to be

handheld and are intended to be used in

contact with the test surface. Surface

probes generally consist of a coil of very

fine wire encased in a protective housing.

The size of the coil and shape of the

housing are determined by the intended

use of the probe. Most of the coils are

wound so that the axis of the coil is

perpendicular to the test surface. This coil

configuration is sometimes referred to as

a pancake coil and is good for detecting

surface discontinuities that are oriented

perpendicular to the test surface.

Discontinuities, such as delaminations,

that are in a parallel plane to the test

surface will likely go undetected with this coil configuration.

Wide surface coils are used when scanning large areas for relatively large

defects. They sample a relatively large area and allow for deeper penetration.

Since they do sample a large area, they are often used for conductivity tests to get

more of a bulk material measurement. However, their large sampling area limits

their ability to detect small discontinuities.

Pencil probes have a small surface coil that is encased in a long slender housing

to permit inspection in restricted spaces. They are available with a straight shaft

or with a bent shaft, which facilitates easier handling and use in applications such

as the inspection of small diameter bores. Pencil probes are prone to wobble due

to their small base and sleeves are sometimes used to provide a wider base.

Bolt Hole Probes

Bolt hole probes are a special type of surface probe that is designed to be used

with a bolt hole scanner. They have a surface coil that is mounted inside a

housing that matches the diameter of the hole being inspected. The probe is

inserted in the hole and the scanner rotates the probe within the hole.

ID or Bobbin Probes

ID probes, which are also referred to as

Bobbin probes or feed-through probes,

are inserted into hollow products, such

as pipes, to inspect from the inside out.

The ID probes have a housing that

keep the probe centered in the product

and the coil(s) orientation somewhat

constant relative to the test surface.

The coils are most commonly wound

around the circumference of the probe

so that the probe inspects an area around the entire circumference of the test

object at one time.

OD or Encircling Coils

OD probes are often called encircling coils.

They are similar to ID probes except that the

coil(s) encircle the material to inspect from the

outside in. OD probes are commonly used to

inspect solid products, such as bars.

Probes - Shielding & Loading

One of the challenges of performing an eddy current

inspection is getting sufficient eddy current field strength

in the region of interest within the material. Another

challenge is keeping the field away from nonrelevant

features of the test component. The impedance change

caused by nonrelevant features can complicate the

interpretation of the signal. Probe shielding and loading

are sometimes used to limit the spread and concentrate

the magnetic field of the coil. Of course, if the magnetic

field is concentrated near the coil, the eddy currents will also be concentrated in

this area.

Probe Shielding

Probe shielding is used to prevent or reduce the interaction of

the probe's magnetic field with nonrelevent features in close

proximity of the probe. Shielding could be used to reduce edge

effects when testing near dimensional transitions such as a step

or an edge. Shielding could also be used to reduce the effects

of conductive or magnetic fasteners in the region of testing.

Eddy current probes are most often shielded using magnetic shielding or eddy

current shielding. Magnetically shielded probes have their coil surrounded by a

ring of ferrite or other material with high permeability and low conductivity. The

ferrite creates an area of low magnetic reluctance and the probe's magnetic field

is concentrated in this area rather than spreading beyond the shielding. This

concentrates the magnetic field into a tighter area around the coil.

Eddy current shielding uses a ring of highly conductive but nonmagnetic

material, usually copper, to surround the coil. The portion of the coil's magnetic

field that cuts across the shielding will generate eddy currents in the shielding

material rather than in the nonrelevent features outside of the shielded area. The

higher the frequency of the current used to drive the probe, the more effective the

shielding will be due to the skin effect in the shielding material.

Probe Loading with Ferrite Cores

Sometimes coils are wound around a ferrite core. Since

ferrite is ferromagnetic, the magnetic flux produced by

the coil prefers to travel through the ferrite as opposed to

the air. Therefore, the ferrite core concentrates the

magnetic field near the center of the probe. This, in turn,

concentrates the eddy currents near the center of the

probe. Probes with ferrite cores tend to be more sensitive

than air core probes and less affected by probe wobble

and lift-off.

Coil (Probe) Design

The most important feature in eddy current testing is the way in which the eddy

currents are induced and detected in the material under test. This depends on the

design of the probe. As discussed in the previous pages, probes can contain one

or more coils, a core and shielding. All have an important effect on the probe,

but the coil requires the most design consideration.

A coil consists of a length of wire wound in a helical manner around the length of

a former. The main purpose of the former is to provide a sufficient amount of

rigidity in the coil to prevent distortion. Formers used for coils with diameters

greater than a few millimeters (i.e. encircling and pancake coils), generally take

the form of tubes or rings made from dielectric materials. Small-diameter coils

are usually wound directly onto a solid former.

The region inside the former is called the core, which can consist of either a solid

material or just air. When the core is air or a nonconductive material, the probe

is often referred to as an air-core probe. Some coils are wound around a ferrite

core which concentrates the the coil's magnetic field into a smaller area. These

coils are referred to as "loaded" coils.

The wire used in an eddy current probe is typically made from copper or other

nonferrous metal to avoid magnetic hysteresis effects. The winding usually has

more than one layer so as to increase the value of inductance for a given length of

coil. The higher the inductance (L) of a coil, at a given frequency, the greater the

sensitivity of eddy current testing.

It is essential that the current through the coil is as low as possible. Too high a

current may produce:

a rise in temperature, hence an expansion of the coil, which increases the

value of L.

magnetic hysteresis, which is small but detectable when a ferrite core is

used.

The simplest type of probe is the single-coil probe, which is in widespread use.

The following applet may be used to calculate the effect of the inner and outer

diameters, length, number of turns and wire diameter of a simple probe design on

the probe's self inductance. Dimensional units are in millimeters.

A more precise value of L is given by:

L = Kn2 p [ (ro2 - rc

2) - µrrc2] µo / l

ro is the mean radius of the coil.

rc is the radius of the core.

l is the length of the coil.

n is the number of turns.

µr is the relative magnetic permeability of the core.

µo is the permeability of free space (i.e. 4 pi x 10-7 H/m).

K is a dimensionless constant characteristic of the length and the external

and internal radii.

5 Procedure Issues

Reference Standards

In eddy current testing, the use of reference standards in setting up the equipment

is particularly important since signals are affected by many different variables

and slight changes in equipment setup can drastically alter the appearance of a

signal. As with most other NDT methods, the most useful information is obtained

when comparing the results from an unknown object to results from a similar

object with well characterized features and defects. In almost all cases, eddy

current inspection procedures require the equipment to be configured using

reference standards.

For crack detection, corrosion thinning and other material damage, reference

standards are used to setup the equipment to produce a recognizable signal or set

of signals from a defect or set of defects. In many cases, the appearance of a test

signal can be related to the appearance of a signal from a known defect on the

reference standard to estimate the size of a defect in the test component. Signals

that vary significantly from the responses produced by the reference standard

must be further investigated to the determine the source of the signal.

The reference standard should be of the same material as the test article. If this is

not possible or practical, it should be of material that has the same electrical

conductivity and magnetic permeability. Component features (material thickness,

geometry, etc.) should be the same in the reference standard as those in the test

region of interest. If the reference standard is the type with intentional defects,

these defects should be as representative of actual defects in the test component

as possible. The closer the reference standard is to the actual test component, the

better. However, since cracks and corrosion damage are often difficult and costly

to produce, artificial defects are commonly used. Narrow notches produced with

electron discharge machining (EDM) and saw cuts are commonly used to

represent cracks, and drilled holes are often used to simulate corrosion pitting.

Common eddy current reference standards include:

Conductivity standards.

Flat plate discontinuity standards.

Flat plate metal thinning standards (step or tapered wedges).

Tube discontinuity standards.

Tube metal thinning standards.

Hole (with and without fastener) discontinuity standards

Signal Filtering

Signal filtering is often used in eddy current testing to eliminate unwanted

frequencies from the receiver signal. While the correct filter settings can

significantly improve the visibility of a defect signal, incorrect settings can

distort the signal presentation and even eliminate the defect signal completely.

Therefore, it is important to understand the concept of signal filtering.

Filtering is applied to the received signal and, therefore, is not directly related to

the probe drive frequency. This is most easily understood when picturing a time

versus signal amplitude display. With this display mode, it is easy to see that the

signal shape is dependent on the time or duration that the probe coil is sensing

something. For example, if a surface probe is placed on the surface of conductor

and rocked back and forth, it will produce a wave like signal. When the probe is

rocked fast, the signal will have a higher frequency than when the probe is

rocked slowly back and forth.

The signal does not need a wavelike appearance to have frequency content and

most eddy current signals will be composed of a large number of frequencies.

Consider a probe that senses a notch for 1/60th of a second. In a period of one

second the probe could (in theory) go over the notch 60 times, resulting in the

notch signal having a frequency of 60 Hz. But, imposed on this same signal,

could be the signal resulting from probe wobble, electronic noise, a conductivity

shift and other factors which occur at different frequencies.

Filters Effects

The two standard filters found in most

impedance plane display instruments are the

‘High Pass Filter’ (HPF) and ‘Low Pass

Filter’ (LPF). Some instruments also have

a‘Band Pass Filter’ (BPF), which is a

combination high and low pass filter. Filters

are adjusted in Hertz (Hz).

The HPF allows high frequencies to pass and

filters out the low frequencies. The HPF is

basically filtering out changes in the signal

that occur over a significant period of time.

The LPF allows low frequency to pass and

filters out the high frequency. In other words,

all portions of the signal that change rapidly (have a high slope) are filtered, such

as electronic noise.

In the image above, the gradual (low frequency) changes were first filtered out

with a HPF and then high frequency electronic noise was filtered with a LPF to

leave a clearly visible flaw indication. It should also be noted that since flaw

indication signals are comprised of multiple frequencies, both filters have a

tendency to reduce the indication signal strength. Additionally, scan speed must

be controlled when using filters. Scan over a flaw too slow and the HPF might

filter out the flaw indication. Scan over the flaw too fast and the LPF might

eliminate the flaw indication.

Filter Settings

If the spectrum of the signal frequency

and the signal amplitude or attenuation

are plotted, the filter responses can be

illustrated in graphical form. The

image to the right shows the response

of a LPF of 20Hz and a HPF of 40Hz.

The LPF allows only the frequencies

in yellow to pass and the HPF only

allow those frequencies in the blue

area to pass. Therefore, it can be seen

that with these settings there are no

frequencies that pass (i.e. the frequencies passed by the LPF are filtered out by

the HPF and visa versa).

To create a window of acceptance for

the signals, the filters need to overlap.

In the image to the right, the LPF has

been adjusted to 60Hz and the HPF to

10Hz. The area shown in gray is where

the two frequencies overlap and the

signal is passed. A signal of 30Hz will

get through at full amplitude, while a

signal of 15Hz will be attenuated by

approximately 50%. All frequencies

above or below the gray area (the pass

band) will be rejected by one of the

two filters.

Use of Filters

The main function of the LPF is to remove high frequency interference noise.

This noise can come from a variety of sources including the instrumentation

and/or the probe itself. The noise appears as an unstable dot that produces jagged

lines on the display as seen in the signal from a surface notch shown in the left

image below. Lowering the LPF frequency will remove more of the higher

frequencies from the signal and produce a cleaner signal as shown in the center

image below. When using a LPF, it should be set to the highest frequency that

produces a usable signal. To reduce noise in large surface or ring probes, it may

be necessary to use a very low LPF setting (down to 10Hz). The lower the LPF

setting, the slower the scanning speed must be and the more closely it must be

controlled. The image on the right below shows a signal that has been clipped

due to using a scan speed too fast for the selected HPF setting.

The HPF is used to eliminate low frequencies which are produced by slow

changes, such as conductivity shift within a material, varying distance to an edge

while scanning parallel to it, or out-of-round holes in fastener hole inspection.

The HPF is useful when performing automated or semiautomatic scans to keep

the signal from wandering too far from the null (balance) point. The most

common application for the HPF is the inspection of fastener holes using a

rotating scanner. As the scanner rotates at a constant RPM, the HPF can be

adjusted to achieve the desired effect.

Use of the HPF when scanning manually is not recommended, as keeping a

constant scanning speed is difficult, and the signal deforms and amplitude

decreases. The size of a signal decreases as the scan speed decreases and a flaw

indication can be eliminated completely if the scan is not done with sufficient

speed. In the images below, it can be seen that a typical response from a surface

notch in aluminum without HPF (left image) looks considerably different when

the HPF is activated (right image). With the HPF, looping signals with a positive

and similar negative deflection are produced on the impedance plane.

The use of a minimal HPF setting (1 or 2 Hz) may be used when manually

scanning, provided the operator can largely control the scan speed and becomes

familiar with the indication signal changes as scan speed is varied slightly. An

good example of such an application would be the manual scan of the radius of a

wheel that is rotated by hand, but the speed of rotation can be kept relatively

constant.

6 Applications

Surface Breaking Cracks

Eddy current equipment can be used for a variety of

applications such as the detection of cracks

(discontinuities), measurement of metal thickness,

detection of metal thinning due to corrosion and

erosion, determination of coating thickness, and the

measurement of electrical conductivity and

magnetic permeability. Eddy current inspection is

an excellent method for detecting surface and near

surface defects when the probable defect location

and orientation is well known.

Defects such as cracks are detected when they

disrupt the path of eddy currents and weaken their

strength. The images to the right show an eddy

current surface probe on the surface of a conductive

component. The strength of the eddy currents under

the coil of the probe ins indicated by color. In the lower image, there is a flaw

under the right side of the coil and it can be see that the eddy currents are weaker

in this area.

Of course, factors such as the type of material, surface finish and condition of the

material, the design of the probe, and many other factors can affect the sensitivity

of the inspection. Successful detection of surface breaking and near surface

cracks requires:

1. A knowledge of probable defect type, position, and orientation.

2. Selection of the proper probe. The probe should fit the geometry of the

part and the coil must produce eddy currents that will be disrupted by the

flaw.

3. Selection of a reasonable probe drive frequency. For surface flaws, the

frequency should be as high as possible for maximum resolution and high

sensitivity. For subsurface flaws, lower frequencies are necessary to get

the required depth of penetration and this results in less sensitivity.

Ferromagnetic or highly conductive materials require the use of an even

lower frequency to arrive at some level of penetration.

4. Setup or reference specimens of similar material to the component being

inspected and with features that are representative of the defect or

condition being inspected for.

The basic steps in performing an

inspection with a surface probe

are the following:

1. Select and setup the

instrument and probe.

2. Select a frequency to

produce the desired depth

of penetration.

3. Adjust the instrument to

obtain an easily recognizable defect response using a calibration standard

or setup specimen.

4. Place the inspection probe (coil) on the component surface and null the

instrument.

5. Scan the probe over part of the surface in a pattern that will provide

complete coverage of the area being inspected. Care must be taken to

maintain the same probe-to-surface orientation as probe wobble can affect

interpretation of the signal. In some cases, fixtures to help maintain

orientation or automated scanners may be required.

6. Monitor the signal for a local change in impedance that will occur as the

probe moves over a discontinuity.

The applet below depicts a simple eddy current probe near the surface of a

calibration specimen. Move the probe over the surface of the specimen and

compare the signal responses from a surface breaking crack with the signals from

the calibration notches. The inspection can be made at a couple of different

frequencies to get a feel for the effect that frequency has on sensitivity in this

application.

Surface Crack Detection Using Sliding Probes

Many commercial aircraft applications involve the use of multiple fasteners to

connect the multi-layer skins. Because of the fatigue stress that is caused by the

typical application of any commercial aircraft, fatigue cracks can be induced in

the vicinity of the fastener holes. In order to inspect the fastener holes in an

adequate amount of time, sliding probes are an efficient method of inspection.

Sliding probes have been named so because they move over fasteners in a sliding

motion. There are two types of sliding probes, fixed and adjustable, which are

usually operated in the reflection mode. This means that the eddy currents are

induced by the driver coil and detected by a separate receiving coil.

Sliding probes are one of the fastest methods to inspect large numbers of fastener

holes. They are capable of detecting surface and subsurface discontinuities, but

they can only detect defects in one direction. The probes are marked with a

detection line to indicate the direction of inspection. In order to make a complete

inspection there must be two scans that are orthogonal (90 degrees) to each other.

Probe Types

Fixed Sliding Probes

These probes are generally used for thinner material compared to the adjustable

probes. Maximum penetration is about 1/8 inch. Fixed sliding probes are

particularly well suited for finding longitudinal surface or subsurface cracks such

as those found in lap joints. Typical frequency range is

from 100 Hz to 100 kHz.

Adjustable Sliding Probes These probes are well suited for finding subsurface cracks

in thick multi-layer structures, like wing skins. Maximum

penetration is about 3/4 inch. The frequency range for

adjustable sliding probes is from 100 Hz to 40 kHz.

Adjustable probes, as the name implies, are adjustable with the use of spacers,

which will change the penetration capabilities. The spacer thickness between the

coils is normally adjusted for the best detection. For tangential scans or 90 degree

scanning with an offset from the center, a thinner spacer is often used.

The spacer thickness range can vary from 0 (no

spacer) for inspections close to the surface and

small fastener heads to a maximum of about 0.3

inch for deep penetration with large heads in the

bigger probe types. A wider spacer will give more

tolerance to probe deviation as the sensitive area

becomes wider but the instrument will require more

gain. Sliding probes usually penetrate thicker

materials compared to the donut probes.

Reference Standards

Reference/calibration standards for setup of sliding probes typically consist of

three or four aluminum plates that are fastened together within a lap joint type

configuration. EDM notches or naturally/artificially- induced cracks are located

in the second or third layer of the standard.

Reference standards used should be manufactured from the same material type,

alloy, material thickness, and chemical composition that will be found on the

aircraft component to be inspected. Sizes and tolerances of flaws introduced in

the standards are usually regulated by inspection specifications.

Inspection Variables

Liftoff Signal Adjustment Liftoff is normally adjusted to be relatively horizontal. The term "relatively

horizontal" is used here because the liftoff signal often appears a curved line

rather than a straight line. Sometimes liftoff can be a sharp curve and may need

to be adjusted to run slightly upwards before moving downwards. See Figures 1

and 2.

Scan Patterns A typical scan is centralized over the fastener head and moves along the axis of

the fastener holes. This scan is generally used to detect cracks positioned along

the axis of the fastener holes. For detecting cracks located transverse or 90

degrees from the axis of the fastener holes, a scan that is 90 degrees from the axis

of the fastener holes is recommended.

Signal Interpretation

When the probe moves over a fastener hole with a crack, the indication changes

and typically will create a larger vertical movement. The vertical amplitude of

the loop depends on the crack length, with longer cracks giving higher

indications.

If the crack is in the far side of the fastener, as the probe moves over it, the dot

will follow the fastener line first but will move upwards (clockwise) as it goes

over the crack. If the crack is in the near side, it will be found first and the dot

will move along the crack level before coming down to the fastener level.

If two cracks on opposite sides of the fastener hole are present, the dot will move

upwards to the height by the first crack length and then come back to the fastener

line and balance point. If the second crack is longer than the first one, the dot will

move even higher and complete the loop (clockwise) before going down to the

balance point. See Figures 3 and 4.

Probe Scan Deviation Most probes are designed to give a narrow indication for a good fastener hole so

that the loops from the cracks are more noticeable. Some probes and structures

can give wider indications and a similar result can be obtained if the probe is not

straight when it approaches the fastener. It is important to keep the probe

centralized over the fastener heads. Doing this will give you a maximum

indication for the fastener and a crack.

If the probe deviates from the center line, the crack indication will move along

the loop that we saw in Figure 5 and is now present in Figure 6. The crack

indication is at "a" when the probe is centralized and moves toward "b" as it

deviates in one direction, or "c" as it deviates in the opposite direction. Point "b"

gives an important indication even if it loses a small amount of amplitude it has

gained in phase, giving a better separation angle. This is because we deviated to

the side where the crack is located.

Crack Angle Deviation

A reduction in the crack indication occurs when the crack is at an angle to the

probe scan direction. This happens if the crack is not completely at 90 degrees to

the normal probe scan or changes direction as it grows. Both the fixed and

adjustable sliding probes are capable of detecting cracks up to about 30 degrees

off angle. See Figures 7 and 8.

Electrical Contact When inspecting fasteners that have just been installed or reference standards

that have intimate contact with the aluminum skin plate, it is not unusual to

obtain a smaller than normal indication. In some extreme cases, the fastener

indication may disappear almost completely. This is due to the good electrical

contact between the fastener and the skin. This condition allows the eddy

currents to circulate without encountering a boundary, and therefore, no obstacle

or barrier. Because of this effect, it is recommended to paint the holes before

fastener installation.

Tube Inspection

Eddy current inspection is often used to detect corrosion,

erosion, cracking and other changes in tubing. Heat

exchangers and steam generators, which are used in power

plants, have thousands of tubes that must be prevented

from leaking. This is especially important in nuclear power

plants where reused, contaminated water must be

prevented from mixing with fresh water that will be

returned to the environment. The contaminated water flows

on one side of the tube (inside or outside) and the fresh

water flows on the other side. The heat is transferred from

the contaminated water to the fresh water and the fresh

water is then returned back to is source, which is usually a

lake or river. It is very important to keep the two water sources from mixing, so

power plants are periodically shutdown so the tubes and other equipment can be

inspected and repaired. The eddy current test method and the related remote field

testing method provide high-speed inspection techniques for these applications.

A technique that is often used

involves feeding a differential bobbin

probe into the individual tube of the

heat exchanger. With the differential

probe, no signal will be seen on the

eddy current instrument as long as no

metal thinning is present. When metal

thinning is present, a loop will be seen

on the impedance plane as one coil of

the differential probe passes over the flawed area and a second loop will be

produced when the second coil passes over the damage. When the corrosion is on

the outside surface of the tube, the depth of corrosion is indicated by a shift in the

phase lag. The size of the indication provides an indication of the total extent of

the corrosion damage.

A tube inspection using a bobbin probe is simulated below. Click the "null"

button and then drag either the absolute or the differential probe through the tube.

Note the different signal responses provided by the two probes. Also note that the

absolute probe is much more sensitive to dings and the build up of magnetite on

the outside of the tube than the differential probe is.

Conductivity Measurements

One of the uses of eddy current instruments

is for the measurement of electrical

conductivity. The value of the electrical

conductivity of a metal depends on several

factors, such as its chemical composition

and the stress state of its crystalline

structure. Therefore, electrical conductivity

information can be used for sorting metals,

checking for proper heat treatment, and

inspecting for heat damage.

The technique usually involves nulling an absolute probe in air and placing the

probe in contact with the sample surface. For nonmagnetic materials, the change

in impedance of the coil can be correlated directly to the conductivity of the

material. The technique can be used to easily sort magnetic materials from

nonmagnetic materials but it is difficult to separate the conductivity effects from

the magnetic permeability effects, so conductivity measurements are limited to

nonmagnetic materials. It is important to control factors that can affect the results

such as the inspection temperature and the part geometry. Conductivity changes

with temperature so measurements should be made at a constant temperature and

adjustments made for temperature variations when necessary. The thickness of

the specimen should generally be greater than three standard depths of

penetration. This is so the eddy currents at the back surface of the sample are

sufficiently weaker than the variations in the specimen thickness that are not seen

in the measurements.

Generally large pancake type, surface

probes are used to get a value for a

relatively large sample area. The instrument

is usually setup such that a ferromagnetic

material produces a response that is nearly

vertical. Then, all conductive but

nonmagnetic materials will produce a trace

that moves down and to the right as the

probe is moved toward the surface.

Think back to the discussion on the impedance plane and these type of responses

make sense. Remember that inductive reactance changes are plotted along the y-

axis and resistance changes are plotted in the x-axis. Since ferromagnetic

materials will concentrate the magnetic field produced by a coil, the inductive

reactance of the coil will increase. The effects on the signal from the magnetic

permeability overshadow the effects from conductivity since they are so much

stronger.

When the probe is brought near a conductive but nonmagnetic material, the coil's

inductive reactance goes down since the magnetic field from the eddy currents

opposes the magnetic field of the coil. The resistance in the coil increases since it

takes some of the coil's energy to generate the eddy currents and this appears as

additional resistance in the circuit. As the conductivity of the materials being

tested increases, the resistance losses will be less and the inductive reactance

changes will be greater. Therefore, the signals will be come more vertical as the

conductivity increases, as shown in the image above.

To sort materials using an impedance plane device, the signal from the unknown

sample must be compared to a signal from a variety of reference

standards. However, there are devices available that can be calibrated to produce

a value for electrical conductivity which can then be compared to published

values of electrical conductivity in MS/m or percent IACS (International

Annealed Copper Standard). Please be aware that the conductivity of a particular

material can vary significantly with slight variations in the chemical composition

and, thus, a conductivity range is generally provided for a material. The

conductivity range for one material may overlap with the range of a second

material of interest, so conductivity alone can not always be used to sort

materials. The electrical conductivity values for a variety of materials can be

found in the material properties reference tables.

The following applet is based on codes for nonferrous materials written by Back

Blitz from his book, "Electrical and Magnetic Methods of Nondestructive

Testing", 2nd ed., Chapman & Hill (1997). The applet demonstrates how an

impedance plane eddy current instrument can be used for the sorting of materials.

Conductivity Measurements for the Verification of Heat Treatment

With some materials, such as solution heat

treatable aluminum alloys, conductivity

measurements are often made verifying that

parts and materials have received the proper

heat treatment. High purity aluminum is soft

and ductile, and gains strength and hardness

with the addition of alloying elements. A few

such aluminum alloys are the 2000 series

(2014, 2024, etc.), 6000 series (6061, 6063,

etc.), and 7000 series (7050, 7075, etc.). The 2xxx series aluminum alloys have

copper, the 6xxx series have magnesium, and the 7xxx have zinc as their major

alloying elements.

Heat treatment of aluminum alloys is accomplished in two phases - solution heat

treatment and then aging. In the solution heat treatment step, the alloys are heated

to an elevated temperature to dissolve the alloying elements into solution. The

metal is then rapidly cooled or quenched to “freeze” the atoms of the alloying

elements in the lattice structure of the aluminum. This distorts and stresses the

structure, making electron movement more difficult, thereby decreasing the

electrical conductivity. In this condition, the alloys are still relatively soft but

start to gain strength as the alloying elements begin to precipitate out of solution

to form extremely small particles that impede the movement of dislocations

within the material.

The formation of the precipitates can be controlled for many alloys by heating

and holding the material at an elevated temperature for a period of time (artificial

aging). As the alloying elements precipitate out of solid solution, the conductivity

of the material gradually increases. By controlling the amount of precipitated

particles within the aluminum, the properties can be controlled to produce peak

strength or some combinations of strength and corrosion resistance. Sometimes,

the material must be annealed or put into the softest, most ductile condition

possible in order to perform forming operations. Annealing allows all of the

alloying elements to precipitate out of solution to form a coarse, widely spaced

precipitate. The electrical conductivity is greatest when the material is in the

annealed condition.

Since solution heat-treated and aged materials are stronger, components can be

made using less material. A lighter or more compact design is often of great

importance to the designer and well worth the cost of the heat treating process.

However, think of the consequences that could arise if a component that was

supposed to be solution heat-treated and aged somehow left the manufacturing

facility and was put into service unheat-treated or annealed. This is a real

possibility since heat-treated aluminum parts look exactly like unheat-treated

parts. Consider 2024 aluminum as an example. Select tensile properties and its

electrical conductivity for various heat treatment conditions are given in the

following table.

Properties for Alclad 2024 Aluminum

Heat Treatment Condition Ultimate Strength Yield Strength Electrical

Conductivity

Annealed (O) 26 ksi (180 MPa) 11 ksi (75 MPa) 50 % IACS

Solution Heat Treated and

Naturally Aged (T42) 64 ksi (440 MPa) 42 ksi (290 MPa) 30 % IACS

Solution Heat Treated,

Coldworked and

Artificially Aged (T861)

70 ksi (485 MPa) 66 ksi (455 MPa) 38 % IACS

It can be seen that the yield strength for the material is 42 kilopounds/square inch

(ksi) (290 MPa) in the solution heat-treated and naturally aged condition (T42

condition). The yield strength can be increased to 66 ksi (455 MPa) when

coldworked and artificially aged (T861 condition). But in the annealed condition,

the yield strength is reduced to 11 ksi (75 MPa). If an annealed part were

accidentally used where a part in the T42 or T861 was intended, it would likely

fail prematurely. However, a quick check of the conductivity using an eddy

current instrument of all parts prior to shipping would prevent this from

occurring.

Thickness Measurements of Thin Material

Eddy current techniques can be used to

perform a number of dimensional

measurements. The ability to make rapid

measurements without the need for couplant

or, in some cases even surface contact,

makes eddy current techniques very useful.

The type of measurements that can be made

include:

thickness of thin metal sheet and foil,

and of metallic coatings on metallic and nonmetallic substrate

cross-sectional dimensions of cylindrical tubes and rods

thickness of nonmetallic coatings on metallic substrates

Corrosion Thinning of Aircraft Skins

One application where the eddy current technique

is commonly used to measure material thickness is

in the detection and characterization of corrosion

damage on the skins of aircraft. Eddy current

techniques can be used to do spot checks or

scanners can be used to inspect small areas. Eddy

current inspection has an advantage over

ultrasound in this application because no

mechanical coupling is required to get the energy

into the structure. Therefore, in multi-layered areas

of the structure like lap splices, eddy current can

often determine if corrosion thinning is present in

buried layers.

Eddy current inspection has an advantage over

radiography for this application because only single

sided access is required to perform the inspection.

To get a piece of film on the back side of the aircraft skin might require removing

interior furnishings, panels, and insulation which could be very costly. Advanced

eddy current techniques are being developed that can determine thickness

changes down to about three percent of the skin thickness.

Thickness Measurement of Thin Conductive Sheet, Strip and Foil

Eddy current techniques are used to measure the thickness of hot sheet, strip and

foil in rolling mills, and to measure the amount of metal thinning that has

occurred over time due to corrosion on fuselage skins of aircraft. On the

impedance plane, thickness variations exhibit the same type of eddy current

signal response as a subsurface defect, except that the signal represents a void of

infinite size and depth. The phase rotation pattern is the same, but the signal

amplitude is greater. In the applet, the lift-off curves for different areas of the

taper wedge can be produced by nulling the probe in air and touching it to the

surface at various locations of the tapered wedge.

If a line is drawn between the end points of the lift-off curves, a comma shaped

curve is produced. As illustrated in the second applet, this comma shaped curve

is the path that is traced on the screen when the probe is scanned down the length

of the tapered wedge so that the entire range of thickness values are measured.

When making this measurement, it is important to keep in mind that the depth of

penetration of the eddy currents must cover the entire range of thicknesses being

measured. Typically, a frequency is selected that produces about one standard

depth of penetration at the maximum thickness. Unfortunately, at lower

frequencies, which are often needed to get the necessary penetration, the probe

impedance is more sensitive to changes in electrical conductivity. Thus, the

effects of electrical conductivity cannot be phased out and it is important to

verify that any variations of conductivity over the region of interest are at a

sufficiently low level.

Measurement of Cross-sectional Dimensions of Cylindrical Tubes and Rods

Dimensions of cylindrical tubes and rods can be measured with either OD coils

or internal axial coils, whichever is appropriate. The relationship between change

in impedance and change in diameter is fairly constant, except at very low

frequencies. However, the advantages of operating at a higher normalized

frequency are twofold. First, the contribution of any conductivity change to the

impedance of the coil becomes less important and it can easily be phased out.

Second, there is an increase in measurement sensitivity resulting from the higher

value of the inductive component of the impedance. Because of the large phase

difference between the impedance vectors corresponding to changes in fill-factor

and conductivity (and defect size), simultaneous testing for dimensions,

conductivity, and defects can be carried out.

Typical applications include measuring eccentricities of the diameters of tubes

and rods and the thickness of tube walls. Long tubes are often tested by passing

them at a constant speed through encircling coils (generally differential) and

providing a close fit to achieve as high a fill-factor as possible.

An important application of tube-wall thickness measurement is the detection and

assessment of corrosion, both external and internal. Internal probes must be used

when the external surface is not accessible, such as when testing pipes that are

buried or supported by brackets. Success has been achieved in measuring

thickness variations in ferromagnetic metal pipes with the remote field technique.

Thickness Measurement of Thin Conductive Layers

It is also possible to measure the thickness of a thin layer of metal on a metallic

substrate, provided the two metals have widely differing electrical conductivities

(i.e. silver on lead where = 67 and 10 MS/m, respectively). A frequency must be

selected such that there is complete eddy current penetration of the layer, but not

of the substrate itself. The method has also been used successfully for measuring

thickness of very thin protective coatings of ferromagnetic metals (i.e. chromium

and nickel) on non-ferromagnetic metal bases.

Depending on the required degree of penetration, measurements can be made

using a single-coil probe or a transformer probe, preferably reflection type.

Small-diameter probe coils are usually preferred since they can provide very high

sensitivity and minimize effects related to property or thickness variations in the

underlying base metal when used in combination with suitably high test

frequencies. The goal is to confine the magnetizing field, and the resulting eddy

current distribution, to just beyond the thin coating layer and to minimize the

field within the base metals

Thickness Measurements of Nonconducting Coatings on

Conductive Materials

The thickness of nonmetallic coatings on metal substrates can be determined

simply from the effect of liftoff on impedance. This method has widespread use

for measuring thickness of paint and plastic coatings. The coating serves as a

spacer between the probe and the conductive surface. As the distance between

the probe and the conductive base metal increases, the eddy current field strength

decreases because less of the probe's magnetic field can interact with the base

metal. Thicknesses between 0.5 and 25 µm can be measured to an accuracy

between 10% for lower values and 4% for higher values. Contributions to

impedance changes due to conductivity variations should be phased out, unless it

is known that conductivity variations are negligible, as normally found at higher

frequencies.

Fairly precise measurements can be made with a standard eddy current flaw

detector and a calibration specimen. The probe is nulled in air and the direction

of the lift-off signal is established. The location of the signal is marked on the

screen as the probe is placed on the calibration specimen in areas of decreasing

coating thickness. When the probe is placed on the test surface, the position of

the signal will move from the air null position to a point that can be correlated to

the calibration markings.

Specialized eddy current coating thickness detectors are also available and are

often pocket-sized with the probe resembling a small pencil. They are usually

operated by a small battery and provide a digital read-out in the appropriate units.

Calibration adjustments, some of which are laid down by standards such as BS

EN 2360 (1995) and ASTM B 244 and E 376, may be assisted by the use of an

inbuilt microprocessor.

7 Advanced Techniques

Scanning

Eddy current data can be collected using automated scanning systems to improve

the quality of the measurements and to construct images of scanned areas. The

most common type of scanning is line scanning where an automated system is

used to push the probe at a fixed speed. Line scan systems are often used when

performing tube inspections or aircraft engine blade slot inspections, where

scanning in one dimension is needed. The data is usually presented as a strip

chart recording. The advantage of using a linear scanning system is that the probe

is moved at a constant speed, so indications on the strip chart can be correlated to

a position on the part being scanned. As with all automated scanning systems,

operator variables, such as wobble of the probe, are reduced.

Two-dimensional scanning systems are used to scan a two-dimensional area.

This could be a scanning system that scans over a relatively flat area in a X-Y

raster mode, or it could be a bolt hole inspection system that rotates the probe as

it is moved into the hole. The data is typically displayed as a false-color plot of

signal strength or phase angle shift as a function of position, just like an

ultrasonic C-scan presentation. Shown below is a portable scanning system that is

designed to work on the skins of aircraft

fuselage and wing sections.

Listed below are some automated scanning

advantages:

minimizes changes in liftoff or fill factor

resulting from probe wobble, uneven

surfaces, and eccentricity of tubes caused

by faulty manufacture or damage

accurate indexing

repeatability

high resolution mapping

Multiple Frequency Techniques

Multiple frequency eddy current techniques simply involve collecting data at

several different frequencies and then comparing the data or mixing the data in

some way.

Why the need for multiple frequencies? - Some background information

The impedance of an eddy current probe may be affected by the following

factors:

variations in operating frequency

variations in electrical conductivity and the magnetic permeability of a

object or structure, caused by structural changes such as grain structure,

work hardening, heat treatment, etc.

changes in liftoff or fill factor resulting from probe wobble, uneven

surfaces, and eccentricity of tubes caused by faulty manufacture or

damage

the presence of surface defects such as cracks, and subsurface defects such

as voids and nonmetallic inclusions

dimensional changes, for example, thinning of tube walls due to corrosion,

deposition of metal deposits or sludge, and the effects of denting

the presence of supports, walls, and brackets

the presence of discontinuities such as edges

Several of these factors are often present simultaneously. In the simple case

where interest is confined to detecting defects or other abrupt changes in

geometry, a differential probe can be used to eliminate unwanted factors,

providing they vary in a gradual manner. For example, variations in electrical

conductivity and tube thinning affect both coils of a differential probe

simultaneously. However, if unwanted parameters that occur abruptly are

affecting the measurements, they can sometimes be negated by mixing signals

collected at several frequencies.

An example of where a multi-frequency eddy current inspection is used is in heat

exchanger tube inspections. Heat exchanger assemblies are often a collection of

tubing that have support brackets on the outside. When attempting to inspect the

full wall thickness of the tubing, the signal from the mounting bracket is often

troublesome.

By collecting a signal at the frequency necessary to inspect the full thickness of

the tube and subtracting a second signal collected at a lower frequency (which

will be more sensitive to the bracket but less sensitive to features in the tubing),

the effects of the bracket can be reduced.

There are a number of commercially available multi-frequency eddy current

instruments. Most operate at only two frequencies at a time but some units can

collect data at up to four frequencies simultaneously. Multi-frequency

measurements can also be made using an impedance analyzer but this equipment

is generally not suitable for field measurements. A typical impedance analyzer

system is shown below. The interest in pulsed eddy current instruments is largely

due to their ability to, in essence, perform multi-frequency measurements very

quickly and easily.

Swept Frequency

Swept frequency eddy current techniques involve collecting eddy current data at

a wide range of frequencies. This usually involves the use of a specialized piece

of equipment such as an impedance analyzer, which can be configured to

automatically make measurements over a range of frequencies. The swept-

frequency technique can be implemented with commercial equipment but it is a

difficult and time-consuming measurement. The advantage of a swept frequency

measurement is that depth information can be obtained since eddy current depth

of penetration varies as a function of frequency.

Swept frequency measurements are useful in applications such as measuring the

thickness of conductive coatings on conductive base metal, differentiating

between flaws in surface coatings and flaws in the base metal and differentiating

between flaws in various layers of built-up structure. An example application

would be the lap splice of a commercial aircraft. Swept frequency measurements

would make it possible to tell if cracking was occurring on the outer skin, the

inner skin or a double layer. Below is an example of the type of data that can be

obtained from swept-frequency measurements.

Data from swept-frequency measurements on two heats of material.

It can be seen that in the etched condition, the material labeled "good" exhibits a

much different signal response than the material labeled "bad." It can also be

seen that a frequency of around 2.2 MHz provides the largest separation in the

curves. Therefore, this frequency should be used if a single frequency is used to

sort the parts made from the two metals.

Pulsed Eddy Current Inspection

Conventional eddy current inspection techniques use sinusoidal alternating

electrical current of a particular frequency to excite the probe. The pulsed eddy

current technique uses a step function voltage to excite the probe. The advantage

of using a step function voltage is that it contains a continuum of frequencies. As

a result, the electromagnetic response to several different frequencies can be

measured with just a single step. Since the depth of penetration is dependent on

the frequency of excitation, information from a range of depths can be obtained

all at once. If measurements are made in the time domain (that is by looking at

signal strength as a function of time), indications produced by flaws or other

features near the inspection coil will be seen first and more distant features will

be seen later in time.

To improve the strength and ease interpretation of the signal, a reference signal is

usually collected, to which all other signals are compared (just like nulling the

probe in conventional eddy current inspection). Flaws, conductivity, and

dimensional changes produce a change in the signal and a difference between the

reference signal and the measurement signal that is displayed. The distance of the

flaw and other features relative to the probe will cause the signal to shift in time.

Therefore, time gating techniques (like in ultrasonic inspection) can be used to

gain information about the depth of a feature of interest.

Additional Background Information on Pulsed Eddy Current Techniques

Background on Pulsed Eddy Current (adapted from Blitz, 1997)

The use of pulsed eddy currents has long been considered for testing metals

(Libby, 1971) and it has been applied to operations in specialized areas, such as

in the nuclear energy industry, where testing equipment is often constructed to

order. However, significant progress in this direction has taken place only

recently after appropriate advances in technology (Krzwosz et al. 1985; Sather,

1981; Waidelich, 1981; Wittig and Thomas 1981), but at the time of writing,

commercial equipment was not yet available. The method has the potential

advantages of greater penetration, the ability to locate discontinuities from time-

of-flight determinations, and a ready means of multi-frequency measurement. At

present, it does not generally have the precision of the conventional methods. The

apparatus is somewhat complicated in design and not readily usable by the

average operator who is experienced with the conventional eddy current

equipment. Its main successes are in the testing of thin metal tubes and sheets, as

well as metal cladding for measuring thickness and for the location and sizing of

internal defects.

When comparing the pulsed method with the conventional eddy current

technique, the conventional technique must be regarded as a continuous wave

method for which propagation takes place at a single frequency or, more

correctly, over a very narrow frequency bandwidth. With pulse methods, the

frequencies are excited over a wide band, the extent of which varies inversely

with the pulse length; this allows multi-frequency operation. As found with

ultrasonic testing, the total amount of energy dissipated within a given period of

time is considerably less for pulsed waves than for continuous waves having the

same intensity. For example, with pulses containing only one or two wavelengths

and generated 1000 times per second, the energy produced is only about 0.002 of

that for continuous waves having the same amplitude. Thus, considerably higher

input voltages can be applied to the exciting coil for pulsed operation than for

continuous wave operation.

Pulsed waves can reasonably be expected to allow penetration of measurable

currents through a metal sample to a depth of about 10 times the standard

penetration depth, provided a suitable probe is used (i.e. a shielded ferrite-cored

coil, see section 5.3).

Therefore, penetration is possible through a 2 mm thick plate at frequencies of 1-

3 kHz for non-ferromagnetic metals having corresponding electrical

conductivities ranging from 60 down to 20MS/m. However, with an

unmagnetized steel plate 2 mm thick, where sigma = 5 MS/m and µr = 100, the

maximum frequency for through-penetration is only 100 Hz.

Pulsed eddy currents may be generated by a thyratron connected in series with

the exciting coil through a capacitor (e.g. Waidelich, 1981). A direct voltage, on

the order of 1200 V, slowly charges the capacitance and when the thyratron

conducts there is an abrupt discharge through the coil in which free-damped

harmonic oscillations occur. This is repeated periodically (i.e. at 1 kHz), so as to

propagate the eddy current pulses through the metal.

The currents are detected by a receiving probe located either adjacent to or on the

opposite side of the metal sample from the exciting probe when access is

possible. The range of propagated frequencies depends on the logarithmic

decrement of the exciting circuit, and because the speed of the waves is a

function of frequency, dispersion takes place and the pulse changes in shape as it

progresses through the metal. As one would expect, the height of the peak and its

time delay can be related to the thickness of the metal. Waidelich reports a

maximum penetration of 90 mm for aluminum sheet and 10 mm for steel. For 6

mm thick sheets, the peak value of the received pulse voltage was 13 V for

aluminum but only 20 mV for steel. Krzwosz et al. (1985) has shown how pulses

that result from the presence of internal simulated defects produce broadening

with an increase in depth.

The frequency content of the pulses depends on their lengths, and in the extreme,

contains continuous spectra ranging from less than 100 Hz to 1 or 2 kHz. By

performing a Fourier transformation, the pulse obtained by the receiving probe

can be displayed in the form of the variation of amplitude (or phase) with

frequency. By sampling different delay times within a pulse, different parts of the

spectrum can be evaluated (Sather, 1981). If both amplitude and phase are

measured, two parameters (i.e. presence of defects, variations in tube thickness,

and changes in fill-factor or liftoff) can be evaluated for each frequency selected

in the same way as with the multi-frequency method, although, at present, with a

lower degree of precision.

Dodd et al.(1988) has designed and developed a pulsed magnetic saturation

method for the eddy current testing of ferromagnetic metals. The DC field pulses

are generated by passing a high-current pulse through an electromagnet so as to

produce saturation in the metal object; the pulse length is made equal to the

thickness of the object, thus ensuring complete eddy current penetration where

feasible. The DC pulse, on the order of 1 ms duration, simultaneously produces

an eddy current pulse, which is detected by a probe; the output of the probe is

characteristic of the material being tested.

This technique has the advantage of producing high magnetic peak powers with

low average powers, thus keeping any heating of the test sample down to an

acceptable level. It has been applied successfully to the internal testing of the

walls of steel steam generator tubes, and tubes of diameter 10.9 mm and wall

thickness 5 mm have been examined with peak powers of 500 kW. Small defects

close to the external surfaces can be detected, and by taking advantage of the

multi-frequency properties of pulsed eddy currents, their indications can be

resolved from those that originate from other characteristics of the tubes.

More recent work on the use of pulsed eddy currents has been reported by Gibbs

and Campbell (1991), who inspected cracks under fasteners in aluminum aircraft

structures. Here, a Hall element was used as a receiver. Radial position,

approximate depth, and relative size of defects hidden under fastener heads could

be determined in countersunk areas for defect depths of up to 7 mm for

nonferrous fasteners and 14 mm for ferrous fasteners.

Lebrun et al. (1975) reported the detection of deep cracks in ferromagnetic

samples using an emission coil excited by square pulses of high intensity and

employing highly sensitive magneto-resistive sensors to measure the resultant

magnetic fields. Defects of 1 mm x 1 mm could be detected at a depth of 5 mm

and 3 mm x 4 mm at a depth of 20 mm.

Remote Field Sensing

Eddy current testing for external defects in tubes where external access is not

possible (e.g. buried pipelines), is conducted using internal probes. When testing

thick-walled ferromagnetic metal pipes with conventional internal probes, very

low frequencies (e.g. 30 Hz for a steel pipe 10 mm thick) are necessary to

achieve the through-penetration of the eddy currents.

This situation produces a very low sensitivity of flaw detection. The degree of

penetration can, in principle, be increased by the application of a saturation

magnetic field. However, because of the large volume of metal present, a large

saturation unit carrying a heavy direct current may be required to produce an

adequate saturating field.

The difficulties encountered in the internal testing of ferromagnetic tubes can be

greatly alleviated with the use of the remote field eddy current method. This

method provides measurable through penetration of the walls at three times the

maximum frequency possible with the conventional direct field method. This

technique was introduced by Schmidt in 1958. Although it has been used by the

petroleum industry for detecting corrosion in their installations since the early

1960s, it has only recently evoked general interest. This interest is largely

because the method is highly sensitive to variations in wall thickness, but

relatively insensitive to fill-factor changes. The method has the added advantage

of allowing equal sensitivities of detection at both the inner and outer surfaces of

a ferromagnetic tube. It cannot, however, differentiate between signals from

these respective surfaces.

For more information on Remote Field Testing can be found in the Specialized

NDT Methodssection of this site.

Impedance Matching

Eddy current testing requires us to determine the components of the impedance

of the detecting coil or the potential difference across it. Most applications

require the determination only of changes in impedance, which can be measured

with a high degree of sensitivity using an AC bridge.

The principles of operation of the most commonly used eddy current instruments

are based on Maxwell's inductance bridge, in which the components of the

impedance of the detecting coil, commonly called a probe, are compared with

known variable impedances connected in series and forming the balancing arm of

the bridge. Refer back to Bridges.

The input to the bridge is an AC

oscillator, often variable in both

frequency and amplitude. The

detector arm takes the form of either

a meter or a storage cathode-ray

oscilloscope, a phase-sensitive

detector, a rectifier to provide a

steady indication, and usually an

attenuator to confine the output

indication within a convenient

range. Storage facilities are

necessary in the oscilloscope in

order to retain the signal from the

detector for reference during

scanning with the probe.

The highest sensitivity of detection

is achieved by properly matching the

impedance of the probe to the impedance of the measuring instrument. Thus,

with a bridge circuit that is initially balanced, a subsequent but usually small

variation in the impedance of the probe upsets the balance, and a potential

difference appears across the detector arm of the bridge.

Although the Maxwell inductance bridge forms the basis of most eddy current

instruments, there are several reasons why it cannot be used in its simplest form

(i.e. Hague, 1934), including the creation of stray capacitances, such as those

formed by the leads and leakages to earth. These unwanted impedances can be

eliminated by earthing devices and the addition of suitable impedances to

produce one or more wide-band frequency (i.e. low Q) resonance circuits.

Instruments having a wide frequency range (i.e. from 1 kHz to 2 MHz) may

possess around five of these bands to cover the range. The value of the

impedance of the probe is therefore an important consideration in achieving

proper matching and, as a result, it may be necessary to change the probe when

switching from one frequency band to another.

8 QUIZZES

Eddy Current Inspection Quizzes

These quizzes draw from the same database of questions and differ only in the

number of questions presented. Each time a quiz is opened, a new set of random

questions will be produced from the database. The Collaboration for NDE

Education does not record the names of individuals taking a quiz or the results of

a quiz.

20 Question ET Quiz

1

Which of the following are a common eddy current

reference standard?

Conductivity standards

Tube discontinuity standards

Hole discontinuity standards

All of the above

2

Sliding probes usually operate in the:

Reflection mode

Through-transmission mode

Pulsed mode

Differential mode

3

When using the liftoff trace of an impedance plane

instrument to distinguish between several materials with

high electrical conductivity, it is best to test using a:

High frequency

Variable frequency

Low frequency

None of the above

4

The depth of penetration is affected by:

Magnetic permeability

Electrical resistivity

Probe drive frequency

All of the above

5

The HPF allows:

Low frequencies to pass and filters out the high frequencies

High frequencies to pass and filters out the low frequencies

For the combined frequencies to be adjusted to a harmonic

balance

The shape of the waveform to be clipped beyond it

frequency balance

6

Probe shielding is used to:

Shape the eddy currents to the curvature of the part

Reduce the inductive coupling of the probe and part

Increase the probe impedance

Reduce the effects of nonrelevant features in close proximity

to the probe

7

The main purpose of an iron core in a probe is to:

Shift the transmission of eddy currents in order to decrease

penetration

Allow the probe to operate at a higher frequency

Concentrate the magnetic field near the center of the probe

Allow testing of very dense materials

8

The principles of operation of the most commonly used eddy

current instruments are based on:

Roentgen's formulas

Maxwell's inductance bridge

Reciprocity

The Inverse Square Law

9

When maintaining constant liftoff is a problem, what type of

probe should be used?

A absolute probe

A differential probe

A reflection probe

Both B and C

10

Eddy current testing can be used to:

Detect surface and near surface cracks

Measure electrical conductivity

Measure the thickness of nonconductive coatings on plastics

Both A and B

11

Sliding probes are used to test:

Large flat surfaces

Inside bolt holes

In radiuses

Around the edges of fasteners

12

When testing for surface flaws, the probe drive frequency

used:

Should be as high as possible

Should be as low as possible

Depends on the conductivity and permeability of the

material

Depends only on the material conductivity

13

Which component features should be similar to the reference

standard?

Material thickness

Material geometry

Material conductivity

All of the above

14

Relative permeability is:

The permeability in a vacuum

A unitless value

The ratio of the measured permeability and the permeability

in a vacuum

Both B and C

15

When an absolute probe is brought near a ferromagnetic

material, the inductive reactance of the coil will:

Remain unchanged

Increase because the material will concentrate the magnetic

field in and around the coil

Decrease because the material will concetrate the magnetic

field in and around the coil

Decrease because the magnetic field of the coil will be

weakened by the material

16

Which of the following are a common eddy current reference

standard?

Conductivity standards

Tube discontinuity standards

Hole discontinuity standards

All of the above

17

The higher the frequency of the current used to drive the

probe, the:

More effective shielding will be due to skin effect

Deeper the eddy currents will penetrate

Stronger the probe's magnetic field will be

Both A and C

18

Drilled holes are commonly used to represent:

Pitting

Cracks

Delaminations

None of the above

19

An eddy current test circuit will have:

Resistance

Inductive reactance

A small amount of capacitance

All of the above

20

Inductance is caused by:

The interaction of a changing magnetic field with a

conductor

Direct current

Resistance in the coil

None of the above

35 Question ET Quiz

1

Probe shielding is used to:

Shape the eddy currents to the curvature of the part

Reduce the inductive coupling of the probe and part

Increase the probe impedance

Reduce the effects of nonrelevant features in close proximity

to the probe

2

Filters are adjusted in:

Hz

KHz

MHz

THz

3

Eddy current testing can be used to:

Detect surface and near surface cracks

Measure electrical conductivity

Measure the thickness of nonconductive coatings on plastics

Both A and B

4

The coil in an eddy current probe is most often made from:

Iron

Copper

Silver

Platinum

5

LPF stands for:

Low Pulse Frequency

Low Pass Filter

Last Pass Filter

Low Pass Frequency

6

An eddy current test circuit will have:

Resistance

Inductive reactance

A small amount of capacitance

All of the above

7

Which of the following are a common eddy current reference

standard?

Conductivity standards

Tube discontinuity standards

Hole discontinuity standards

All of the above

8

The main function of the LPF is to:

Control probe wobble

Adjust the machine to the proper conductivity standard

Shift the waveform to the left of the screen

Remove high frequency interference noise

9

When testing for subsurface flaws, the frequency should be:

As high as possible

As low as possible

Calculated to produce a 90o difference between the liftoff

and flaw signals

None of the above

Sliding probes usually operate in the:

10

Reflection mode

Through-transmission mode

Pulsed mode

Differential mode

11

Which type of probe has a long slender housing to permit

inspection in restricted spaces?

Pancake probes

Pencil probes

Encircling probes

Sliding probes

12

Pencil probes are prone to:

Energy spikes

Low frequency noise

Wobble

Both A and C

13

Relative permeability is:

The permeability in a vacuum

A unitless value

The ratio of the measured permeability and the permeability

in a vacuum

Both B and C

14

Sliding probes are used to test:

Large flat surfaces

Inside bolt holes

In radiuses

Around the edges of fasteners

15

When a probe is brought near a conductive but nonmagnetic

material, the coil's inductive reactance will:

Increase

Decrease

Remain the same

Remain the same until the probe touches the material

16

What is the relationship between electrical conductivity and

electrical resistivity?

They are directly proportional

They are not related

One is the inverse of the other

It depends on the test frequency

17

A bolt hole probe and scanner is used to inspect:

Flat surfaces

Radiuses

Holes

Both B and C

18

Some common classifications of probes include:

Surface probes

Bolt hole probes

ID probes

All of the above

19

Most surface probe coils are wound so that:

They transmit a frequency that will slightly resonate the part

surface

They create a static magnetic field

The axis of the coil is perpendicular to the test surface

Both B and C

20

Discontinuities, such as delaminations, that are in a plane

that is parallel with the test surface will likely:

Be easily detected with a surface probe

Be easily detected with an internal probe

Be easily detected with an external probe

None of the above

21

When making a conductivity measurement, the thickness of

the material should be at least ___ times the standard depth

of penetration.

1

2

3

4

22

Phase lag:

Increase with discontinuity depth

Decrease with discontinuity depth

Is the same as phase angle

Is the same as the angle separating the liftoff and flaw

signals on an impedance plane

23

The depth of penetration is affected by:

Magnetic permeability

Electrical resistivity

Probe drive frequency

All of the above

24

Which type of probe is most often used to inspect the inside

diameter of a machined hole?

Pencil probes

Surface probes

Bolt hole probes

Bobbin probes

25

A probes that can be used to inspect the entire circumference

of test objects are:

Encircling or bobbin probes

Circumference probes

Pencil probes

None of the above

26

Which type of probe is most commonly sued to inspect solid

products such as bar stock?

Bobbin probes

Surface coils

Encircling coils

Pencil probes

27

Probe shielding and loading are sometimes used to:

Prevent external electrical interference

Limit the spread and concentrate the magnetic field of the

coil

Magnetically saturate the part

None of the above

28

Since eddy current signals are affected by many different

variables, it is particularly important to use what when

setting up the equipment?

Couplant

Fluorescent particles

Reference standards

Non abrasive cleaners

29

When an absolute probe is brought near a ferromagnetic

material, the inductive reactance of the coil will:

Remain unchanged

Increase because the material will concentrate the magnetic

field in and around the coil

Decrease because the material will concetrate the magnetic

field in and around the coil

Decrease because the magnetic field of the coil will be

weakened by the material

30

Use of the HPF is not recommended:

On thin parts

On thick parts

When scanning manually

On ferrous parts

31

Probes for inspection of pipe and tubing are typically of the:

Surface probe variety

Bolt hole variety

Bobbin (ID) variety

All of the above are correct

32

Probes with iron cores tend to:

Be more sensitive than air core probes and less affected by

probe wobble

Be more difficult to use

Increase the background noise of the signal

Both B and C

33

Which component features should be similar to the reference

standard?

Material thickness

Material geometry

Material conductivity

All of the above

34

The principles of operation of the most commonly used eddy

current instruments are based on:

Roentgen's formulas

Maxwell's inductance bridge

Reciprocity

The Inverse Square Law

35

The higher the inductance of a coil at a given frequency:

The more penetrating the eddy currents will be

The less sensitive the coil will be

The more sensitive the coil will be

None of the above

50 Question ET Quiz

Probe shielding is used to:

1

Shape the eddy currents to the curvature of the part

Reduce the inductive coupling of the probe and part

Increase the probe impedance

Reduce the effects of nonrelevant features in close proximity

to the probe

2

Drilled holes are commonly used to represent:

Pitting

Cracks

Delaminations

None of the above

3

A bolt hole probe and scanner is used to inspect:

Flat surfaces

Radiuses

Holes

Both B and C

4

Discontinuities, such as delaminations, that are in a plane

that is parallel with the test surface will likely:

Be easily detected with a surface probe

Be easily detected with an internal probe

Be easily detected with an external probe

None of the above

5

When testing for subsurface flaws, the frequency should be:

As high as possible

As low as possible

Calculated to produce a 90o difference between the liftoff

and flaw signals

None of the above

6

Eddy current testing can be used to:

Detect surface and near surface cracks

Measure electrical conductivity

Measure the thickness of nonconductive coatings on plastics

Both A and B

7

The depth of penetration is affected by:

Magnetic permeability

Electrical resistivity

Probe drive frequency

All of the above

8

Phase lag:

Increase with discontinuity depth

Decrease with discontinuity depth

Is the same as phase angle

Is the same as the angle separating the liftoff and flaw

signals on an impedance plane

9

The principles of operation of the most commonly used eddy

current instruments are based on:

Roentgen's formulas

Maxwell's inductance bridge

Reciprocity

The Inverse Square Law

10

Narrow EDM notches and saw cuts:

Are never used because they are too wide

Are never used due to their heat affected zones

Are commonly used to represent cracks

Both A and B

11

When making a conductivity measurement, the thickness of

the material should be at least ___ times the standard depth

of penetration.

1

2

3

4

12

Wide surface probes are used when scanning:

Large areas for very small cracks

Small areas for delaminations

Large areas for relatively large defects

None of the above

13

The HPF is used to:

Eliminate low frequencies which are produced by slow

changes, such as a conductivity shift

Adjust the bandwidth to a neutral frequency in order to

maximize depth of penetration

Remove any standing waves in the output signal

Shift the waveform from positive to negative when a

rejectable defect is identified

14

Which type of probe has a long slender housing to permit

inspection in restricted spaces?

Pancake probes

Pencil probes

Encircling probes

Sliding probes

15

Most surface probe coils are wound so that:

They transmit a frequency that will slightly resonate the part

surface

They create a static magnetic field

The axis of the coil is perpendicular to the test surface

Both B and C

16

When maintaining constant liftoff is a problem, what type of

probe should be used?

A absolute probe

A differential probe

A reflection probe

Both B and C

17

Some common classifications of probes include:

Surface probes

Bolt hole probes

ID probes

All of the above

18

Since eddy current signals are affected by many different

variables, it is particularly important to use what when

setting up the equipment?

Couplant

Fluorescent particles

Reference standards

Non abrasive cleaners

HPF stands for:

19

High Pulse Filter

Harmonic Pulse Filter

High Pass Filter

High Pulse Factor

20

LPF stands for:

Low Pulse Frequency

Low Pass Filter

Last Pass Filter

Low Pass Frequency

21

When using eddy currents to measure the thickness of a

nonconductive coating applied to a conductive base, the

measurement is based on:

A frequency change due to liftoff

An impedance change due to a change in conductivity

An impedance change due to liftoff

Both A and C

22

The HPF allows:

Low frequencies to pass and filters out the high frequencies

High frequencies to pass and filters out the low frequencies

For the combined frequencies to be adjusted to a harmonic

balance

The shape of the waveform to be clipped beyond it

frequency balance

23

Scanning speed must be controlled:

When using a small transducer

When using a large transducer

When using a high pass filter

When using a large low frequency probe

24

Which type of probe is most often used to inspect the inside

diameter of a machined hole?

Pencil probes

Surface probes

Bolt hole probes

Bobbin probes

25

What is the relationship between electrical conductivity and

electrical resistivity?

They are directly proportional

They are not related

One is the inverse of the other

It depends on the test frequency

26

Which type of probe is most commonly sued to inspect solid

products such as bar stock?

Bobbin probes

Surface coils

Encircling coils

Pencil probes

27

Probe shielding and loading are sometimes used to:

Prevent external electrical interference

Limit the spread and concentrate the magnetic field of the

coil

Magnetically saturate the part

None of the above

When testing for surface flaws, the probe drive frequency

28

used:

Should be as high as possible

Should be as low as possible

Depends on the conductivity and permeability of the

material

Depends only on the material conductivity

29

What material(s) is/are commonly used to shield an eddy

current probe?

Ferrite

Aluminum

Lead

Both A and B

30

Which of the following are a common eddy current reference

standard?

Conductivity standards

Tube discontinuity standards

Hole discontinuity standards

All of the above

31

The main purpose of an iron core in a probe is to:

Shift the transmission of eddy currents in order to decrease

penetration

Allow the probe to operate at a higher frequency

Concentrate the magnetic field near the center of the probe

Allow testing of very dense materials

32

Use of the HPF is not recommended:

On thin parts

On thick parts

When scanning manually

On ferrous parts

33

An eddy current test circuit will have:

Resistance

Inductive reactance

A small amount of capacitance

All of the above

34

Inductance is identified by the letter:

L

M

Z

X

35

Inductance is caused by:

The interaction of a changing magnetic field with a

conductor

Direct current

Resistance in the coil

None of the above

36

The higher the inductance of a coil at a given frequency:

The more penetrating the eddy currents will be

The less sensitive the coil will be

The more sensitive the coil will be

None of the above

37

Probes with iron cores tend to:

Be more sensitive than air core probes and less affected by

probe wobble

Be more difficult to use

Increase the background noise of the signal

Both B and C

38

Relative permeability is:

The permeability in a vacuum

A unitless value

The ratio of the measured permeability and the permeability

in a vacuum

Both B and C

39

The higher the frequency of the current used to drive the

probe, the:

More effective shielding will be due to skin effect

Deeper the eddy currents will penetrate

Stronger the probe's magnetic field will be

Both A and C

40

The main function of the LPF is to:

Control probe wobble

Adjust the machine to the proper conductivity standard

Shift the waveform to the left of the screen

Remove high frequency interference noise

41

In almost all cases, eddy current inspection procedures

require the equipment to be calibrated to:

A reference standard

An identified defect

A crack which is � the rejection criteria

A crack twice the rejection criteria

42

The coil in an eddy current probe is most often made from:

Iron

Copper

Silver

Platinum

43

Eddy currents are generated when:

A conductive material is placed in a changing magnetic field

When a conductive material is moved through a static

magnetic field

When a static magnetic field is moved across the surface of a

conductive material

All of the above

44

Pencil probes are prone to:

Energy spikes

Low frequency noise

Wobble

Both A and C

45

A probes that can be used to inspect the entire circumference

of test objects are:

Encircling or bobbin probes

Circumference probes

Pencil probes

None of the above

46

A probe that is often intended to be used in contact with the

test surface is called a:

Reference probe

Surface probe

Transmission probe

Reflection probe

47

Probes for inspection of pipe and tubing are typically of the:

Surface probe variety

Bolt hole variety

Bobbin (ID) variety

All of the above are correct

48

When using the liftoff trace of an impedance plane

instrument to distinguish between several materials with

high electrical conductivity, it is best to test using a:

High frequency

Variable frequency

Low frequency

None of the above

49

Sliding probes are used to test:

Large flat surfaces

Inside bolt holes

In radiuses

Around the edges of fasteners

50

Filtering is applied to the received signal and, therefore:

It should be added to the base signal

It is not directly related to the probe drive frequency

Should be added to the pick-up coil

Should be subtracted from the amplitude of the dB

9 Formulae & Tables

EC Standards and Methods

STANDARDS

British Standards (BS) and American Standards (ASTM) relating to magnetic

flux leakage and eddy current methods of testing are given below. National

standards are currently being harmonized across the whole of Europe, and British

Standards are no exception. Harmonized standards will eventually be identified

by the initials BS EN; for example, BS 5411 has been revised and is now known

as BS EN 2360. Harmonization is unlikely to be completed before 2001. The

year of updating a British Standard is given in brackets. ASTM standards are

published annually and updated when necessary.

FLUX LEAKAGE METHODS (INCLUDING MAGNETIC PARTICLE

INSPECTION)

British Standards (BS)

BS 6072:1981 (1986) Magnetic particle flaw detection

BS 4489:1984 Black light measurement

BS 5044:1973 (1987) Contrast aid paints

BS 5138:1974 (1988) Forged and stamped crankshafts

BS 3683 (part 2):1985 Glossary

BS 4069:1982 Inks and powders

American Society for Testing and Materials (ASTM)

ASTM E 709 Magnetic particle inspection practice

ASTM E 125 Indications in ferrous castings

ASTM E 1316 Definition of terms

ASTM E 570 Flux leakage examination of ferromagnetic steel tubular products

EDDY CURRENT METHODS

British Standards (BS)

BS 3683 (part 5):1965 (1989) Eddy current flaw detection glossary

BS 3889 (part 2A): 1986 (1991) Automatic eddy current testing of wrought steel

tubes

BS 3889 (part 213): 1966 (1987) Eddy current testing of nonferrous tubes

BS 5411 (part 3):1984 Eddy current methods for measurement of coating

thickness of nonconductive coatings on nonmagnetic base material. Withdrawn:

now known as BS EN 2360 (1995).

American Society for Testing and Materials (ASTM)

ASTM A 450/A450M General requirements for carbon, ferritic alloys and

austenitic alloy steel tubes

ASTM B 244 Method for measurement of thickness of anodic coatings of

aluminum and other nonconductive coatings on nonmagnetic base materials with

eddy current instruments

ASTM B 659 Recommended practice for measurement of thickness of metallic

coatings on nonmetallic substrates

ASTM E 215 Standardizing equipment for electromagnetic testing of seamless

aluminum alloy tube

ASTM E 243 Electromagnetic (eddy current) testing of seamless copper and

copper alloy tubes

ASTM E 309 Eddy current examination of steel tubular products using magnetic

saturation

ASTM E 376 Measuring coating thickness by magnetic field or eddy current

(electromagnetic) test methods

ASTM E 426 Electromagnetic (eddy current) testing of seamless and welded

tubular products austenitic stainless steel and similar alloys

ASTM E 566 Electromagnetic (eddy current) sorting of ferrous metals

ASTM E 571 Electromagnetic (eddy current) examination of nickel and nickel

alloy tubular products

ASTM E 690 In-situ electromagnetic (eddy current) examination of nonmagnetic

heat-exchanger tubes

ASTM E 703 Electromagnetic (eddy current) sorting of nonferrous metals

ASTM E 1004 Electromagnetic (eddy current) measurements of electrical

conductivity

ASTM E 1033 Electromagnetic (eddy current) examination of type F

continuously welded (CW) ferromagnetic pipe and tubing above the Curie

temperature

ASTM E 1316 Definition of terms relating to electromagnetic testing

ASTM G 46 Recommended practice for examination and evaluation of pitting

corrosion

Material Properties Tables

Electrical Conductivity and Resistivity

-

Aluminum & Aluminum Alloys (htm) (pdf)

Copper & Copper Alloys (htm) (pdf)

Iron & Iron Alloys (htm) (pdf)

Magnesium & Magnesium Alloys (htm) (pdf)

Nickel & Nickel Alloys (htm) (pdf)

Titanium & Titanium Alloys (htm) (pdf)

Misc. Materials & Alloys (htm) (pdf)

TOMADO DE LA WEB:

http://www.ndt-ed.org/EducationResources/CommunityCollege/EddyCurrents/cc_ec_index.htm