T.O. 33B-1-1
4-3
of interest. As will be discussed later, there are some situations where the permeability in the area of interest is not an
interfering parameter and eddy current inspection can be successfully applied.
4.1.7
Geometry.
Geometric features such as edges, curved surfaces, changes in thickness, and non-conductive coatings (such as paint)
on surfaces affect the distribution and strength of eddy currents. The eddy current response as a probe approaches an
edge is known as edge effect and appears similar to a response from a crack. Similarly, curved surfaces and non-
conductive coatings can vary the distance between the probe coil and the part. These changes are known as lift-off, and
the consequent effects on the eddy current signal are called lift-off effects. Lift-off usually cannot be completely
prevented; therefore compensating for some lift-off is part of the calibration procedure for most eddy current
inspections. Part thickness variations can also produce an interfering response in some eddy current inspections when
the thickness is in the range of the depth of penetration of the eddy current field (see paragraph 4.2.3.2).
4.1.8
Lift off And Fill Factor.
The effects of lift-off are mentioned in a later section and can be exploited if the measurement of coating thickness is
desired. Changes in lift-off can be calibrated to allow measurements of nonconductive coating thicknesses. Fill-factor
applies to parts passed through an encircling coil and, in a manner similar to lift-off, can be used to gauge some
dimensions.
4.1.9
Flaw Detection.
When eddy currents are induced in a metal in the region of a crack or other flaw, the eddy current flow is distorted.
The distortion results in a localized decrease in electrical conductivity. In this manner an eddy current inspection is
able to detect cracks or other flaws.
4.1.10
Components of an Eddy Current Inspection System.
In its simplest form, an eddy current inspection system consists of the following components: (1) an oscillator, (2) a coil
assembly, (3) a bridge circuit, (4) signal processing circuits, and (5) a readout. A block diagram of an inspection
system is shown in Figure 4-2 with the coil applied to a test part. Systems may be constructed for multiple purposes or
for very specialized functions. In general, instruments designed for specific tasks, such as measuring coating thickness
or electrical conductivity, are easier to calibrate and operate than general purpose instruments but also are limited to
their designed application.
4.1.10.1
Oscillator.
The oscillator provides an alternating current of one or more frequencies to the test coil. The frequency used is
determined by the intent of the inspection and the material being inspected. Frequencies used for eddy current
inspection range from less than 50 Hz to greater than 6 MHz.
4.1.10.2
Coil Assembly (Probe).
The coil assembly induces eddy currents into the part being inspected and detects changes in eddy current flow. For
some applications, a single coil is used for both functions. More commonly, multiple coils are employed in an
assembly. A common configuration has one coil inducing the eddy current flow and separate coils used as detectors.
Another configuration uses one coil as both an inducer and a detector on the test part.
