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MME 131: Lecture 32

Non-destructive Testing

Prof. A.K.M.B. Rashid Department of MME BUET, Dhaka

Nondestructive inspections fundamentals

Classification of nondestructive inspections

Radiographic inspection

Magnetic particle inspection

Penetrant inspection

Ultrasonic inspection

Eddy current inspection

Reference:

SH Avner. Introduction to Physical Metallurgy, 2nd Ed., pp.45-60.

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Nondestructive Inspection

It is an examination of an object in a manner which will not

impair the future usefulness of the object.

Does not provide a direct measurement of mechanical

properties of the object.

Very valuable in locating material defects that could impair the

performance of the object when placed in service.

Common reasons for performing nondestructive inspections (NDI):

To detect faulty material before it is formed or machined into component parts

To detect faulty component before assembly

To discover defects that may have developed during service

For routine examination in service, permitting their removal before failure occurs

To improve and control manufacturing process to make products more reliable,

safe and economical.

Five basic elements in any nondestructive inspection:

SOURCE

provides a probing medium that can be used to inspect the item under test

MODIFICATION

the probing medium must change or be modified as a result of the variations

or discontinuities within the object being tested

DETECTION

a detector capable of determining the changes in the probing medium

INDICATION

a means of indicating or recording the signals from the detector

INTERPRETATION

a method of interpreting these indications

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Classification of Nondestructive Inspection

Radiographic inspection

Magnetic particle inspection

Die penetrant inspection

Ultrasonic inspection

Eddy current inspection

Five most common nondestructive inspection methods:

Radiographic Inspection

Radiography uses penetrating radiation

that is directed towards a component.

The component stops some of the radiation.

The amount that is stopped or absorbed is affected by

material density and thickness differences.

These differences in “absorption” are recorded

on film, or electronically.

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General principle

The part is placed between

the radiation source and the

radiographic film. The part

will stop some of the radiation.

Thicker and more dense area

will stop more of the radiation.

The film darkness (density)

will vary with the amount of

radiation reaching the film

through the test object.

more exposure

less exposure

Top view of developed film

X-ray film

Some radiographic images

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The radiation used in radiography testing is either X-ray or gamma-ray, a higher energy (shorter wavelength) version of the electromagnetic waves capable of penetrating relatively large thickness of metal.

Industrial radiography is often subdivided into “X-ray Radiography” or “Gamma-ray Radiography”, depending on the source of radiation used.

Technique is not limited by material type or density.

Can inspect assembled components.

Minimum surface preparation required.

Sensitive to changes in thickness, corrosion, voids, cracks, and material density changes.

Detects both surface and subsurface defects.

Provides a permanent record of the inspection.

Advantages

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Disadvantages

Many safety precautions for the use of high intensity radiation.

Many hours of technician training prior to use.

Orientation of equipment and flaw can be critical.

Determining flaw depth is impossible without additional angled exposures.

Expensive initial equipment cost.

Magnetic particle inspection can detect both production

discontinuities (inclusions, seams, laps, tears, grinding

cracks and quenching cracks) and in-service damage

(fatigue and overload cracks) in ferromagnetic materials

such as iron and steel.

Magnetic Particle Inspection

Can detect surface discontinuities too fine to be detected

by the naked eye, and will also detect discontinuities

which lie slightly below the surface.

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How does it work?

A ferromagnetic test specimen is magnetized with a strong

magnetic field created by a magnet or special equipment. If the specimen has a discontinuity, the discontinuity will

interrupt the magnetic field flowing through the specimen and a leakage field will occur.

Finely milled iron particles coated with a dye pigment are applied

to the test specimen. These particles are attracted to leakage fields and will cluster to form an approximate shape of the surface projection of the discontinuity. This indication can be visually detected under proper lighting conditions (e.g., ultraviolet light).

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Some examples

Advantages

Can detect both surface and near sub-surface defects.

Can inspect parts with irregular shapes easily.

Pre-cleaning of components is not as critical as it is for some

other inspection methods. Most contaminants within a flaw

will not hinder flaw detectability.

Method of inspection is fast and indications are visible

directly on the specimen surface.

Considered low cost compared to many other NDI methods.

A very portable inspection method especially when used with

battery powered equipment.

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Disadvantages

Cannot inspect non-ferrous materials such as aluminum,

magnesium or most stainless steels.

Inspection of large parts may require use of equipment

with special power requirements.

Some parts may require removal of coating or plating to

achieve desired inspection sensitivity.

Limited subsurface discontinuity detection capabilities.

Maximum depth sensitivity is approximately 0.6” (under

ideal conditions).

Post cleaning, and post demagnetization is often necessary.

Alignment between magnetic flux and defect is important.

Die Penetrant Inspection

Penetrant Testing, or PT, is a nondestructive testing method that builds on the principle of Visual Inspection.

PT increases the “seeability” of small discontinuities that the human eye might not be able to detect alone.

It is a very sensitive inspection method of detecting minute discontinuities such as cracks, shrinkage, and porosity that are open to the surface.

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How does it work?

In penetrant testing, a liquid with high

surface wetting characteristics is applied

to the surface of a component under test.

The penetrant “penetrates” into surface

breaking discontinuities via capillary

action and other mechanisms.

Excess penetrant is removed from the

surface.

A developer (powder) is applied to pull

the trapped penetrant out of the defect

and spread it on the surface where it can

be seen.

With good inspection technique (under

UV light), visual indications of any

discontinuities present become apparent.

Applying penetrant

Washing of excess penetrant

Applying developer

Inspection

What CAN be tested using PT?

All defects that are open to the surface.

Rolled products – cracks, seams, laminations.

Castings – cold shuts, hot tears, porosity, blow holes, shrinkage.

Forgings – cracks, laps, external bursts.

Welds – cracks, porosity, overlap, lack of fusion, lack of penetration

What CANNOT be tested using PT?

Components with rough surfaces, such as sand castings, that trap and hold

penetrant.

Porous ceramics

Wood and other fibrous materials.

Plastic parts that absorb or react with the penetrant materials.

Components with coatings that prevent penetrants from entering defects.

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Advantages

Relative ease of use.

Can be used on a wide range of material types.

Large areas or large volumes of parts/materials can be

inspected rapidly and at low cost.

Parts with complex geometries are routinely inspected.

Indications are produced directly on surface of the part

providing a visual image of the discontinuity.

Initial equipment investment is low.

Aerosol spray cans can make equipment very portable.

Disadvantages

Only detects surface breaking defects.

Requires relatively smooth nonporous material.

Precleaning is critical. Contaminants can mask defects.

Requires multiple operations under controlled conditions.

Chemical handling precautions necessary (toxicity, fire, waste).

Metal smearing from machining, grinding and other operations

inhibits detection. Materials may need to be etched prior to

inspection.

Post cleaning is necessary to remove chemicals.

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Ultrasonic Inspection

Ultrasonic testing uses high frequency sound energy to

conduct examinations and make measurements.

Sound is produced by a vibrating body and travels in the form

of a wave. Sound waves travel through materials by vibrating

the particles that make up the material.

The pitch of the sound is determined by the frequency of the

wave. Ultrasound is sound with a pitch too high (1-5 million

Hz) to be detected by the human ear.

Ultrasonic waves are introduced into a material by a transducer

where they travel in a straight line and at a constant speed until

they encounter a surface.

At surface interfaces some of the wave energy is reflected and

some is transmitted.

The amount of reflected or transmitted energy can be detected

and provides information about the size of the reflector.

The travel time of the sound can be measured and this provides

information on the distance that the sound has traveled.

How does it work?

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Ultrasonic testing is a very versatile inspection method,

and inspections can be accomplished in either of the

following ways:

Pulse-echo One transducer is used in one side of the sample, both as

transmitter and receiver

Through Transmission Two transducers are used in both sides of the sample, one as

transmitter and the other as receiver

Testing techniques

0 2 4 6 8 10

Pulse-echo system

sample plate

crack

initial pulse

crack echo

back surface echo

oscilloscope, or flaw detector screen

A transducer sends out a pulse of energy and the same transducer listens for reflected

energy (an echo) from the discontinuities (if any) and the surfaces of the test article.

The amount of reflected sound energy is displayed versus time, which provides the

inspector information about the size and the location of features that reflect the sound.

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Through-transmission system

Two transducers located on opposing sides of the

test specimen are used. One transducer acts as

a transmitter, the other as a receiver.

Discontinuities in the sound path will result in a

partial or total loss of sound being transmitted and

be indicated by a decrease in the received signal

amplitude.

Through transmission is useful in detecting

discontinuities that are not good reflectors, and

when signal strength is weak. It does not provide

depth information.

0 2 4 6 8 10

2

1 1

T R

T R

1 1

2

Flaw detection (cracks, inclusions, porosity, etc.)

Erosion and corrosion thickness gauging

Assessment of bond integrity in adhesively joined and brazed components

Estimation of void content in composites and plastics

Measurement of case hardening depth in steels; part or coating thickness

Estimation of grain size in metals

What can be tested?

A considerable amount of information about the part being

examined can be collected some of which are mentioned below:

Ultrasonic examinations can be conducted on a wide variety

of material forms including castings, forgings, welds, and

composites.

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Advantages

Sensitive to both surface and subsurface discontinuities.

Depth of penetration for flaw detection or measurement is superior to

other methods.

Only single-sided access is needed when pulse-echo technique is used.

High accuracy in determining reflector position and estimating size and

shape.

Minimal part preparation required.

Electronic equipment provides instantaneous results.

Detailed images can be produced with automated systems.

Has other uses such as thickness measurements, in addition to flaw

detection.

Disadvantages

Surface must be accessible to transmit ultrasound.

Skill and training is more extensive than with some other methods.

Normally requires a coupling medium to promote transfer of sound

energy into test specimen.

Materials that are rough, irregular in shape, very small,

exceptionally thin or not homogeneous are difficult to inspect.

Cast iron and other coarse grained materials are difficult to inspect

due to low sound transmission and high signal noise.

Linear defects oriented parallel to the sound beam may go

undetected.

Reference standards are required for both equipment calibration,

and characterization of flaws.

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Eddy-Current Inspection

How does it work?

Eddy current testing uses electromagnetic

induction to detect flaws.

conductive material

Coil Coil's magnetic field

Eddy currents

Eddy current's magnetic field

A varying magnetic field

is produced if a source of

alternating current is

connected to a coil.

When this field is

placed near a test

specimen capable of

conducting an electric

current, eddy currents

will be induced in the

specimen.

A small surface probe is scanned over the part surface in an attempt to detect a crack

The detection unit will measure this new magnetic field and convert the signal

into a voltage that can read on a meter or cathode-ray tube.

Variations in the electrical conductivity or magnetic permeability of the test object

or the presence of any flaws will cause a change in eddy current and a

corresponding change in the phase and amplitude of the measured current.

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Advantages

Detection of very small surface and sub-surface cracks

and other irregularities.

Minimal preparation of surface.

Samples with complex geometry can be investigated

Variations in composition and heat treatment conditions.

Measurement of electrical conductivity and coating thickness.

Disadvantages

Only conductive materials can be tested.

Surface of the material must be accessible.

Depth of penetration is limited by materials’ conductivity

Flaws lie parallel to the probe cannot be detected.

Bad surface finish can cause bad reading.

Nondestructive Inspection: A Summary

Table 1.8: Major nondestructive methods

Indicating when to use, where to use, advantages, and

limitations of each nondestructive inspection method

described earlier.

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