TABLE OF CONTENTS Eddy current nondestructive testing involves the induction of eddy currents in an object under test and the detection of induced currents by pickup coils or magnetometers. Eddy current inspection may be used to detect defects including pits and inclusions in conductors and may also be used to sort materials by permeability and conductivity.
The impedance of an AC current carrying coil will be affected by the presence of a nearby conductor. Local variations in conductivity will also contribute to the impedance change in the coil. Monitoring the coil impedance enables the detection of flaws in conducting structures such as pipes, aircraft, and metallic structural elements. The impedance change in the coil is given by Ohm's law, where the eddy current induced voltage is
| (6.93) |  |
Integrating the above expression around a single filamentary loop gives
| (6.94) |  |
If the coil is located above a semi-infinite conducting plane, the coil impedance change is
| (6.95) |  |
In general, we can write the impedance change as the sum of a resistive and a reactive term
| (6.96) |  |
? Z is typically plotted in the impedance plane plotting resistance versus reactance.
Taking the real and imaginary parts of expression (6.95) gives the coil resistance
| (6.97) |  |
and reactance
| (6.98) |  |
In the case of infinite conductivity the impedance change is purely reactive since Im(-1) = 0 giving
| (6.99) |  |
so that there is no power loss in a perfect conductor with no change in drive coil resistance. Expressions for ? X and ?
