Electromagnetic Testing (ET) - Level II Study Guide
Core concepts
Eddy current testing induces circulating currents in a conductive part with an alternating magnetic field from a coil. The technician reads the coil's electrical impedance, displayed as amplitude and phase angle on an impedance plane; discontinuities, geometry, and material properties each perturb that signal differently. You must know coil configurations (absolute, differential, encircling, surface/pancake), test frequency selection, fill factor, lift-off, edge effect, and the separation of conductivity from magnetic permeability responses.
Skin depth governs everything. At one standard depth of penetration, current density has fallen to about 37% of its surface value and the signal phase lags by one radian, about 57 degrees. Depth decreases as frequency, conductivity, or permeability increases.
Traps to expect
- Ferromagnetic materials such as carbon steel are dominated by permeability, so ET normally targets nonferromagnetic tubing or uses saturation techniques.
- Lift-off changes both amplitude and phase; the statement that lift-off causes no phase shift is false.
- Differential coils reject slow, gradual changes but respond strongly to abrupt ones; absolute coils are the opposite.
- A small coil in a large tube gives poor fill factor and reduced sensitivity.
- Higher frequency means shallower penetration, not deeper.
Rule of thumb
Penetration depth varies inversely with the square root of frequency, so quadrupling frequency halves the depth. Going from 100 kHz to 400 kHz halves it.
Worked example: a tube is inspected at 50 kHz with a standard depth of 1.0 mm. At 200 kHz the standard depth becomes 0.5 mm, so a flaw at 1.5 mm depth is now far less detectable.