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BMFB 4283

NDT & FAILURE ANALYSIS

Lectures for Week 6

Prof. Qumrul Ahsan, PhDDepartment of Engineering Materials

Faculty of Manufacturing Engineering

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6.0 Eddy Current6.1 Introduction6.2 Electromagnetic Principles6.3 Equipment and Testing Techniques6.4 Inspection6.5 Application

Issues to address

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Introduction

This module is intended to present information onthe NDT method of eddy current inspection.

Eddy current inspection is one of several methods

that use the principal of electromagnetism as thebasis for conducting examinations. Several othermethods such as Remote Field Testing (RFT), FluxLeakage and Barkhausen Noise also use thisprinciple.

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Outline

Electromagnetic induction

Generation of eddy currents

Inspection applications

Equipment utilized in eddy current inspectionProbes/Coils

Instrumentation

Reference standard

Advantages and Limitations

Glossary of Terms

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Electromagnetic Induction

Eddy currents are created through a process calledelectromagnetic induction.

When alternating current is applied to the conductor, suchas 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.

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Electromagnetic Induction (cont.)

If another electrical conductor is brought into the proximity ofthis changing magnetic field, the reverse effect will occur.Magnetic field cutting through the second conductor will causean induced current to flow in this second conductor. Eddy

currents are a form of induced currents!

Current Flow

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Generation of Eddy Currents

Eddy currents are induced electrical currents that flow in acircular path. They get their name from eddies that areformed when a liquid or gas flows in a circular path aroundobstacles when conditions are right.

Test Probe

Eddy Currents

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Generation of Eddy Currents (cont.)

When an electrically conductive material is placed in the coils

dynamic magnetic field electromagnetic induction will occur

and eddy currents will be induced in the material.

Eddy currents flowing in the material will generate their own

secondary magnetic field which will oppose the coils

primary magnetic field.

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This entire electromagnetic induction process to produceeddy currents may occur from several hundred to severalmillion times each second depending upon inspectionfrequency.

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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 materialcutting through the coils magnetic field,

and the condition of the material (i.e.

whether it contains cracks or other

defects.)

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Depth

Eddy Current Density

Dep

th

Eddy Current Density

Low Frequency

Low ConductivityLow Permeability

High Frequency

High ConductivityHigh Permeability

Standard Depthof

Penetration(Skin Depth)

1/e or 37 %of surface density

Generation of Eddy Currents (cont.)Eddy currents are strongest at the surface of the material and decrease instrength below the surface. The depth that the eddy currents are only 37% asstrong as they are on the surface is known as the standard depth ofpenetration or skin depth. This depth changes with probe frequency, material

conductivity and 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 ().Although eddy currents penetrate deeper than one standard depth ofpenetration, they decrease rapidly with depth. At two standard depths ofpenetration (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 downto only 5% of the surface density.

Where:d = Standard Depth of

Penetration (mm)

pi = 3.14

f = Test Frequency (Hz)

= Magnetic Permeability

(H/mm)

= Electrical Conductivity (%IACS)

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Inspection Data

There are three characteristics of the specimen that affectthe strength of the induced eddy currents.

The electrical conductivity of the material

The magnetic permeability of the material The amount of solid material in the vicinity of the test

coil.

Information about the strength of the eddy currents withinthe specimen is determined by monitoring changes involtage and/or current that occur in the coil.

The strength of the eddy currents changes the electricalimpedance (Z) of the coil.

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Inspection Data (cont.)

R~

XL

TestCoil

Impedance (Z) in an eddy current coil isthe total opposition to current flow. In acoil, Z is made up of resistance (R) andinductive reactance (XL).

Definitions:

Resistance - The opposition of currentflow, resulting in a change of electricalenergy into heat or another form ofenergy.

Inductive Reactance (XL) - Resistance toAC current flow resulting fromelectromagnetic induction in the coil.

Impedance (Z) - The combinedopposition to current flow resulting from

inductive reactance and resistance.

In an AC coil, induction fromthe magnetic field of one loopof the coil causes a secondarycurrent in all other loops. Thesecondary current opposes the

primary current.

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Lift off in Eddy Current

The distance that the coil is from the conductive material is

called liftoff, and this distance affects the mutual-inductance of

the circuits.

Fig. 1.a) Impedance plane diagram representation in conventional eddy

current testing, b) zoomed-in and rotated area on the operating point;the lift-off variation is kept along the horizontal axis.

With increase offrequency will give widerangle (i.e diff in 20 kHzand 2 MHz

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Phase Lag in Eddy Current Phase lag is to obtain information about

the depth of a defect within a material.

Phase lag is the shift in time betweenthe eddy current response from a

disruption on the surface and a

disruption at some distance below the

surface.

Disruptions in the eddy currents awayfrom 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 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 depthsalong its length.

Phase lag can be calculated with

the following equation.

Where:

=Phase Lag (Rad or Degrees)x=Distance Below Surface (in or mm)

=Standard Depth of Penetration (in ormm)

I d Pl ( dd )

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Impedance Plane (eddy scope)the strength of the eddy currents and the magnetic permeability of the testmaterial cause the eddy current signal on the impedance plane to react in avariety of different ways.

If the eddy current circuit is balanced in

air and then placed on a piece ofaluminum,the resistance component will increase(eddy currents are being generated in thealuminum and this takes energy awayfrom the coil, which shows up as

resistance)the inductive reactance of the coildecreases (the magnetic field created bythe eddy currents opposes the coil'smagnetic field and the net effect is aweaker magnetic field to produce

inductance).If a crack is present in the material,fewer eddy currents will be able to formand the resistance will go back down andthe inductive reactance will go back up.Changes in conductivity will cause theeddy current signal to change in a

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Impedance Plane (eddy scope)the strength of the eddy currents and the magnetic permeability of the testmaterial cause the eddy current signal on the impedance plane to react in avariety of different ways.

When a probe is placed on a magnetic

material such as steel, somethingdifferent happens.eddy currents form, taking energyaway from the coil, which shows up asan increase in the coils resistance.the eddy currents generate their own

magnetic field that opposes the coilsmagnetic field.the reactance increases. This isbecause the magnetic permeability ofthe steel concentrates the coil'smagnetic field. This increase in themagnetic field strength completelyovershadows the magnetic field of theeddy currents.The presence of a crack or a changein the conductivity will produce a

change in the eddy current signal

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Equipment

Equipment for eddy current inspection is very diversified.Proper equipment selection is important if accurateinspection data is desired for a particular application.

As a minimum, at least three basic pieces of equipment are

needed for any eddy current examination: Instrumentation

Probes

Reference Standards

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Equipment

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 changeacross the coil. An ammeter could also be used to measure

the current change in the circuit instead of using the

voltmeter.

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Instrumentation - Meters

Meters are typically thesimplest form of eddycurrent instrumentation.

The two generalcategories of meters aredigital and analog.

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Digital Meters

Digital meters are typically designed to examine one specificattribute of a test component such as conductivity ornonconductive coating thickness. These meters tend to haveslightly higher accuracy than analog devices.

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Analog Meters

Analog meters can be used formany different inspectionapplications such as crackdetection, material thickness

measurements, nonconductivecoating measurements orconductive coatingmeasurements.

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Analog Meters (cont.)

The display read-out found on most analog instruments is

typically either a calibrated or uncalibrated display.

Calibrated displays have an inherent scalingfactor which correlates to the property theinstrument is designed to measure such asconductivity.

Uncalibrated displays are typically moreflexible in the variety of different tests

they can perform. These types ofinstruments, however, require the use ofdata extrapolation techniques ifquantitative data is desired.

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Portable Eddy Scopes

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Portable Eddy Scopes (cont.)

Portable eddy scopes are another category ofinstrumentation and they present the inspection data in theform of an impedance plane diagram.

On the impedance diagram,

the total impedance isdisplayed by plotting itsresistance component andinductive reactancecomponent at 90 degrees toeach other.

This is beneficial for bothseparation and identificationof test variables that caneffect inspection results.

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Modern eddy scopes are usually digital based instrumentswhich can often be purchased as either a single or dualfrequency tester. Dual frequency instruments are capable ofsequentially driving a probe at two different inspectionfrequencies.

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Digital scopes often have an RS232 (serial) connection forinterfacing with a serial printer or computer as well asprovisions for output of signals to recording devices such as astrip-chart recorder. In addition, these instruments contain asmall amount of RAM so that equipment settings as well as

screen presentations can be stored for later reference.

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Multi-Frequency Eddy

Current Instruments

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Current Instruments (cont.)

Multi-Frequency instruments usually refer to equipmentthat can drive inspection coils at more than twofrequencies either sequentially (multiplexing) orsimultaneously.

This type of instrumentation is used extensively for tubinginspection in the power generation, chemical andpetrochemical industries.

These instruments are often capable of being computernetworked and may have as many as four probes attachedto them at one time.

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Advantages of Multi-frequency inspections:

Allows increased inspection information to be collected from oneprobe pulling.

Provides for comparison of same discontinuity signal at differentfrequencies.

Allows mixing of frequencies which helps to reduce or eliminatesources of noise.

Often improves detection, interpretation and sizing capabilities ofdiscontinuities.

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Screen of multi-frequency instrument during inspection.

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Eddy Current Probes

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Eddy Current Probes (cont.)

Probes selection is critical to acquiring adequate inspectiondata.

Several factors to consider include:

Material penetration requirements (surface vs.

subsurface)

Sensitivity requirements

Type of probe connections on eddy current instrument(many variations)

Probe and instrument impedance matching (will probework with instrument)

Probe size (smaller probes penetrate less)

Probe type (absolute, differential, reflection or hybrid)

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Surface probes are coils that are typically mounted close to oneend of a plastic housing. As the name implies, the technicianmoves the coil end of the probe over the surface of the testcomponent.

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Some surface probes are specifically designed for crackdetection of fastener holes. These include sliding probes, ringprobes and hole probes.

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Surface probes can bevery small in size to allowaccessibility to confinedareas.

Finger Probe

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Inside Diameter (I.D.) probes, also known as bobbin probes,are coils that are usually wound circumferentially around aplastic housing. These probes are primarily designed forinspection inside of tubular materials.

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Outside Diameter (O.D.) probes are coils that are wound thecircumference of a hollow fixture. The coil is designed such thatthe test part is ran through the middle of the coil. These probescan be used to inspect bars, rods as well as tubes.

Eddy Current Probes (cont )

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Eddy Current Probes (cont.) Absolute probes generally have a single test coil that is used

to generate the eddy currents and sense changes in the eddy

current field 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 withinthe 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, information can be gained about the test material.


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Absolute coils can be used for

flaw detection,

conductivity measurements,

liftoff measurements

thickness measurements.

Since absolute probes are sensitive to things such asconductivity, 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 tohave a fixed "air loaded" reference coil that compensates for

ambient temperature variations.


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Eddy Current Probes (cont.) Differential Probes

Differential probes have two active coils usually wound in opposition

When the two coils are over a flaw-free area of test sample, there is nodifferential 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 relativelyinsensitive 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 aflaw 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.

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Reflection Probes

Reflection probes have two coils similar to a differential

probe

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.

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Reference Standards

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Reference Standards (cont.)

In order to give the eddy current inspector useful data whileconducting an inspection, signals generated from the testspecimen must be compared with known values.

Reference standards are typically manufactured from thesame or very similar material as the test specimen.

Many different types of standards exist for due to the varietyof eddy current inspections performed.

The following slides provide examples of specific types ofstandards.

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Material thickness standards used to help determine suchthings as material thinning caused by corrosion or erosion.

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Crack Standards:

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ASME Tubing Pit Standard:

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Nonconductive coating (paint) standard with various thicknessof paint on aluminum substrate.

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Inspection Applications

One of the major advantages of eddy current as an NDT tool isthe variety of inspections that can be performed.

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Application of EC

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.

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Material Thickness Measurement

Thickness measurements are possible with eddy currentinspection within certain limitations.

Only a certain amount of eddy currents can form in agiven volume of material.

Therefore, thicker materials will support more eddycurrents than thinner materials.

The strength (amount) of eddy currents can be measuredand related to the material thickness.

Eddy Currents

Magnetic Field

From Probe

TestMaterial

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Material Thickness Measurement

(cont.)

Eddy current inspection is often used in the aviation

industries to detect material loss due to corrosion and

erosion.

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Material Thickness Measurement (cont.)

Eddy current inspection is used extensively to inspecttubing at power generation and petrochemical facilities forcorrosion and erosion.

Crack Detection

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Crack DetectionCrack detection is one of the primary uses of eddy currentinspection. Cracks cause a disruption in the circular flowpatterns of the eddy currents and weaken their strength. This

change in strength at the crack location can be detected.especially high sensitivity to detection of surface breakingcracks.

Magnetic FieldFrom Test Coil

MagneticField FromEddy

Currents

Eddy Currents

Crack

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Crack Detection (cont.)

Eddy current inspection of bead seat area on aircraft wheelfor cracks using special probe that conforms to the shape of therim.

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Crack Detection (cont.)

Loading points, such as fastener holes, are high stress areasand often the site of service induced fatigue cracking. Rotatingprobe guns can be used to inspect a large number of holes in ashort period of time. The photo on the right is a waterfall plotof the cross section of a fastener hole. Each horizontal line

represents one rotation of the probe gun. A vertical signalindicates a crack.

Thickness Measurements of Nonconducting Coatings

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Thickness Measurements of Nonconducting Coatings

on Conductive MaterialsThe thickness of nonmetallic coatings on metal substrates canbe 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 spacerbetween the probe and the conductive surface. As the distancebetween 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 m can be measured to anaccuracy between 10% for lower values and 4% for higher

values. Contributions to impedance changes due toconductivity variations should be phased out, unless it is knownthat conductivity variations are negligible, as normally found at

higher frequencies.

Fairly precise measurements can be made with a standard eddycurrent flaw detector and a calibration specimen. The probe is nulled inair and the direction of the lift-off signal is established. The location ofthe signal is marked on the screen as the probe is placed on thecalibration specimen in areas of decreasing coating thickness. Whenthe probe is placed on the test surface, the position of the signal willmove from the air null position to a point that can be correlated to thecalibration markings.

Nonconductive Coating Measurement

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gNonconductive coatings on electrically conductive substratescan be measured very accurately with eddy currentinspection. (Accuracy of less that one mil is not uncommon.)

ConductiveBase Metal

NonconductiveCoating

Eddy Currents

This reduction in strength can be measured and related to coatingthickness.

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 magneticfield can interact with the base metal.


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(cont.)

The photo to the left shows an aircraft panel paint thicknessinspection. On the right, the display of a digital eddy currentinspection instrument shows the different signals obtainedby measuring eight different thicknesses of paint onaluminum.

Increasing paintthickness

Monitoring Conductivity and

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Monitoring Conductivity and

Permeability Variations

Eddy current inspection is sensitive to changes in a materialselectrical conductivity and magnetic permeability. Thissensitivity allows the inspection method to be used for suchinspection procedures as:

Material Identification Material Sorting

Determination of heat damage

Cladding and plating thickness measurement

Case depth determination

Heat treatment monitoring

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Conductivity Measurements

Boeing employees in Philadelphia were given the privilegeof evaluating the Liberty Bell for damage using NDTtechniques. Eddy current methods were used to measurethe electrical conductivity of the Bell's bronze casing at avarious points to evaluate its uniformity.

Advantages of Eddy Current

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Advantages of Eddy Current

Inspection 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

Limitations of Eddy Current

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Limitations of Eddy Current

Inspection Only conductive materials can be inspected

Surface must be accessible to the probe

Skill and training required is more extensive than othertechniques

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

Documents

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