δ = skin depth of penetration (mm).
ƒ = excitation frequency (Hz).
μ = magnetic permeability (H/mm).
σ = electrical conductivity (%IACS).
IACS = International Annealed Copper Standard.
When attempting to locate flaws, a frequency is selected that places the expected flaw depth within one standard depth of penetration into the material; this ensures the density of current in the material is sufficient to affect the impedance of the coil and produce a flaw indication. For ferromagnetic materials (e.g., steel), permeability values are high; as a result, the standard skin depth is very small, typically less than 1-3 mm depending on the frequency used. ECT in steel can detect only surface-breaking cracks or near surface-breaking cracks. Non-ferromagnetic materials (e.g., aluminum) have a relative permeability of approximately 1 mm; therefore, skin depths are larger and subsurface flaws may be detected.
ECT is sometimes used to determine the conductivity of materials or to differentiate one metal from another based on the electrical properties of the material. For this application, the test frequency is often set such that three times the standard skin depth calculated from the preceding equation is less than the thickness of the material. This ensures the material boundaries will not affect the ECT because current density is reduced to 1/e3 (~5 percent) in the area of the boundary.
The density of current in the material depends on the intensity of the magnetic field inducing the current. Magnetic fields attenuate exponentially in air, such that the distance from the probe coil to the surface of the material under test is an important test parameter. This effect is referred to as the “lift-off factor”—the loss in signal strength resulting from the distance between the probe coil and the surface of the material under test. In general, uniform and very small lift-off values are preferred for achieving better detection sensitivity to flaws. For steel bridges, coating on the steel causes lift-off of the sensor, decreasing signal intensity relative to uncoated steel. This affect can be mitigated by proper calibration that includes similar lift-off of the probe from the surface of the material.(1,2)