Eddy Current Testing - Basic Principle

Eddy Current Testing-Basic TrainingEddy Current NDT is a technique that can test metals for flaws either during the manufacturing process or as a consequence of age or environment.It is a highly sensitive technique and completely Non-Destructive. (No damage to test object)It is quick, safe and cost-effective to use.

Eddy Current Testing-Basic TrainingEddy Current Theory

When a voltage is applied to a circuit containing resistive elements, current flows according to ohms lawI=V/R or V=I.R

Eddy Current Testing-Basic TrainingEddy Current Theory

If a circuit consists of more than one element, the overall voltages, resistance and capacitance can be calculated as followsV1=I.R1V2=I.R2Vtot = V1+V2=I.R1+I.R2=I(R1+R2)=I.Rtot

Eddy Current Testing-Basic TrainingTheory Electromagnetic Induction

Current passing through a coil creates a magnetic field

A moving magnetic field would induce a voltage in an electrical conductor

Eddy Current Testing-Basic TrainingThe meter needle will deflect one way when current is applied and then back the other way when it is removedA voltage is only induced when the magnetic field is changing.

Eddy Current Testing-Basic TrainingA voltage is also induced in the first winding, apposing the change in the applied voltageInductance is defined as

Induced Voltage = L di/dt

Eddy Current Testing-Basic Training

If an AC current flows through an inductor,the voltage across the inductor will be at maximum when the rate of change of current is greatest. For a sinusoidal wave, this is at the point where the actual current is zero

Eddy Current Testing-Basic TrainingThe voltage applied to an inductor reaches its maximum value a quarter cycle before the current does

The voltage leads the current by 90 degrees

Eddy Current Testing-Basic TrainingThe value of the voltage and current can be calculated as follows

V = I.XL

XL = inductive reactance (XL=2 pi f L)

F = frequency in Hz

Eddy Current Testing-Basic TrainingCurrent flow in AC Circuits Impedance

Same basic principle as in DC circuits, but the maximum voltage across the resistance coincides with the zero voltage across the inductor and vice versa

Eddy Current Testing-Basic TrainingCurrent flow in AC circuits - Impedance

Eddy Current Testing-Basic TrainingWe can represent this graphically using a vector diagram

Eddy Current Testing-Basic TrainingEddy Current Theory Testing

When an AC current flows in a coil in close proximity to a conducting surface, the magnetic field of the coil will induce circulating(eddy) currents in that surface.The magnitude and phase of the eddy currents will affect the loading on the coil and thus its impedance

Eddy Current Testing-Basic TrainingEddy Current Theory - Testing

Eddy Current Testing-Basic TrainingEddy Current Theory - testing

Eddy Current Testing-Basic TrainingA deep crack in the surface below the coil will interrupt or reduce the eddy current flow, thus decreasing the loading of the coil and increasing its effective impedenceBy monitoring the voltage across the coil we can detect changes in the test material

Eddy Current Testing-Basic TrainingEddy Current Theory Testing

Cracks MUST interrupt the surface eddy current flow to be detected

Eddy Current Testing-Basic TrainingFactors affecting Eddy Current response

Material conductivity Greater the conductivity greater the eddy current flow- conductivity depends on material composition, heat treatment, work hardening etc etc


PermeabilityDescribed as the ease with which a material can be magnetisedFor nFe metals and austinitic S.Steel, the permeability (mr)is 1 (as for free space)For Fe metals the value of mr may be several hundred, thus influencing the eddy current responsePermeability may vary within a metal part due to localised stresses, heating effects etc


Frequency

Eddy current response is greatly affected by the test frequency, but this property can be controlled


GeometryCurvature,edges,grooves etc will affect the eddy current responseWhen the material thickness is less than the effective depth of penetration, this will also affect the eddy current response


Proximity / Lift-offThe closer the probe coil to the surface, greater the effect on the coil. This has two main effects1. Lift off signal as the probe is moved on and off the surface2. A reduction in sensitivity as the coil to product spacing increases


Depth of PenetrationThe eddy current density is greatest on the surface of the metal and declines with depthDepth of penetration-decreases with an increase in frequency-decreases with an increase in conductivity-decreases with an increase in permeability

Eddy Current Testing-Basic TrainingEffective Depth of Penetration

It is defined as three times the standard depth, where eddy current density has fallen to 3% of the surface value

Eddy Current Testing-Basic TrainingThe Impedance Plane

This is a graphical representation of the complex probe impedance where the X-value represents the resistance and the Y-value represents the inductive reactance


The display of CRT instrument represents a window in to the impedance plane

Above example shows probe on Aluminium

Eddy Current Testing-Basic TrainingCoil Configurations Absolute (Single Coil)

Surface probes probe axis normal to the surfaceEncircling coils the product to be tested is inserted through a circular coilID probes for heat exchanger inspection(normally wound with the coil axis along the center of the tube)Absolute probes are good for metal sorting and detection of cracks

Eddy Current Testing-Basic TrainingCoil Configurations Differential (two coils)

Differential probe has two sensing elements looking at different areas of the material being tested. The instrument shows the difference between the eddy current conditions at the two pointsGood for detection of small defects and are relatively unaffected by lift-off, temperature changes and external interference

Eddy Current Testing-Basic TrainingDifferential probe response

The figure of 8 is formed based on response from first element and then the second element move over the defectThe lift off is cancelled out, but there is still wobble response as the probe is moved and tilted slightly

Eddy Current Testing-Basic TrainingCoil Configurations Reflection/driver pick-up probes

They have a primary winding driven from the oscillator and one of more sensor windings connected to the measurement circuit

Eddy Current Testing-Basic TrainingCoil Configurations Reflection/driver pickup probes

AdvantagesDriver and pickup coils can be separately optimised for their intended purposeWider frequency rangeThe larger driver coil gives more even field, resulting in better penetration and lift-off characteristics

Eddy Current Testing-Basic TrainingTypical Coil Connections Bridge

The two coils (differential or absolute + balancing coil) form the legs of the bridge. When the bridge is balanced, the measured voltage will be zero. Any change in the condition of either coil will result in an unbalanced bridge, the degree of imbalance corresponds to the change in coil impedance

Eddy Current Testing-Basic TrainingTypical Coil Connections Driver pickup

The essential elements are same as bridge type. The necessary changes can be achieved by simple switching or probe connection changes

Eddy Current Testing-Basic TrainingPractical testing requirements

-A suitable probe-An instrument with the necessary capabilities-A good idea of size, location and type of flaws-A suitable test standard to set up the equipment-A procedure or accept/reject criteria-Operator expertise to understand and interpret the results

Eddy Current Testing-Basic TrainingEddy Current Instruments

-Special purpose equipment-Crack Detectors-Portable Impedance plane equipment-Systems Eddy Current units

Eddy Current Testing-Basic TrainingSpecial Purpose Equipment

Coating Thickness Meters, Conductivity meters etc

Generally designed to give a digital readout without demanding interpretation of an indication

Eddy Current Testing-Basic TrainingCrack Detectors

Generally operates at a restricted number of frequencies typically several hundred KHz, meter or bargraph displaySuitable for surface crack detection and simple sorting applications

Eddy Current Testing-Basic TrainingPortable Impedance Plane Equipment

Impedance plane display on CRT, wide frequency ranges, multifrequency operation to reduce specific interfering effects

Eddy Current Testing-Basic TrainingSystems Eddy Current Units

Intended for factory operation, often in automatic or semi-automatic inspection mode.

Eddy Current Testing-Basic TrainingEddy Current testing Applications

-Surface crack detection-Non-ferrous metal sorting-Sub-surface crack/corrosion detection-Heat exchanger tube testing-In-line inspection of steel tubing-Ferrous Weld Inspection

Eddy Current Testing-Basic Training

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Eddy Current Testing - Basic Principle