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
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:
A knowledge of probable defect type, position, and orientation.
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.
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.
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.
basic steps in performing an inspection with a surface probe are
Select and setup the instrument and probe.
Select a frequency to produce the desired depth of penetration.
Adjust the instrument to obtain an easily recognizable defect
response using a calibration standard or setup specimen.
Place the inspection probe (coil) on the component surface
and null the instrument.
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.
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.