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S Breaking

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 coatingthickness, and the measurement of electrical conductivity and

magnetic permeability. Eddy current inspection is an excellentmethod 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 aconductive 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 seethat the eddy currents are weaker in this area.

Of course, factors such as the type of material, surface finish and condition of the material, thedesign 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 evenlower 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 orcondition being inspected for.

The basic steps in performing aninspection with a surface probe are the

following:

1.  Select and setup the instrument

and probe.

2.  Select a frequency to produce thedesired depth of penetration.

3.  Adjust the instrument to obtain an

easily recognizable defect response using a calibration standard or setup specimen.

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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-surfaceorientation 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 movesover 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.