Inspection with Eddy Currents.
The eddy currents induced in an electrical conductor vary in magnitude and distribution in response to specimen
properties such as electrical conductivity, magnetic permeability, geometry and discontinuities as well as inspection
parameters such as the coil-to-specimen separation (also called lift-off or fill-factor, depending on the type of coil used)
and coil assembly design. A consequence of this is that often the eddy current inspection for one condition, i.e.
presence of discontinuities, can be hampered by variations in properties not of concern, i.e. specimen geometry. In
most cases the effects of variations in properties not of interest can be minimized or suppressed. Minimization or
suppression of unwanted eddy current responses is essential to the conduct of an effective inspection and will be
discussed in a later section of this chapter.
Eddy Current Inspection Techniques.
A wide variety of eddy current inspection techniques have been developed. Test frequencies, coil arrangements, data
analyses, and data displays are some of the parameters which define a technique. When the ferromagnetic properties of
the specimen are of interest, magnetoinductive testing is the more correct term; for the purposes of this chapter eddy
current inspection will be the term of choice.
In most cases, the properties that are detectable by eddy current testing are of little interest by themselves. However, in
many cases they correlate to characteristics that are of interest, such as the presence of defects, heat treat condition or
coating thickness. Some of the characteristics that eddy current techniques can detect and/or measure are presented in
Table 4-1, categorized according to the actual material property or inspection parameter measured.
Table 4-1. Common Applications of Eddy Current Inspection.
Diameter (e.g., of bar stock
with encircling coil)
* Ferromagnetic Materials Only
Electrical conductivity is a measure of the ability of electric currents to flow in a material. The conductivity of a
material can depend on a materials heat treat condition, chemical composition, and the presence of cracks or other
discontinuities. Therefore, eddy current inspection can be used to detect variations in alloy composition, hardness (heat
treat condition), thermal exposure and a wide variety of flaw conditions. For example, the electrical conductivity of
aluminum alloys is significantly influenced by heat treat condition. Consequently, it is often possible to use eddy
current testing to determine if a part of a known aluminum alloy has been properly heat treated or has been exposed to
temperatures that can degrade the heat treat condition. The dependence of conductivity on chemical composition can
also allow eddy current inspection to sort between different alloys. Eddy current inspection is also able to detect the
localized variation in conductivity associated with the presence of a discontinuity.
For ferromagnetic materials (see Chapter 3 for a discussion of ferromagnetic materials), the relative magnetic
permeability is the principal property that affects eddy current response. The relative permeability depends on a wide
variety of parameters; alloy composition, degree of magnetization, heat treat and residual stress, just to name a few. In
many cases, variations in permeability due to non-flaw conditions mask effects from discontinuities or other conditions