Electromagnetic Testing (ET): Physics, Probes, and Signal Interpretation
Core physics
ET relies on Faraday's law and eddy currents induced in a conductive test object by a time-varying coil field. The currents concentrate near the surface; standard depth of penetration (the depth where current density falls to about 37%, or 1/e) is δ ≈ 1/√(π f μ σ). Increasing frequency, permeability, or conductivity reduces penetration. This single relationship drives most ET questions.
Impedance plane
Understand that the coil's impedance changes with the test object, and that the impedance-plane diagram separates lift-off, conductivity/permeability, and defect signals by phase. Phase angle is the key discriminator; amplitude alone is ambiguous. Know that lift-off produces a characteristic phase that must be nulled or compensated.
Probes and techniques
Know absolute, differential, and reflection probes and when each is used: absolute for gradual property changes and setup, differential for suppressing gradual variations and detecting localized flaws, reflection for confined access. Understand through-coil testing for tubing and the role of fill factor. Know that magnetic saturation is used for ferromagnetic tubing to suppress permeability variation.
Traps
- Assuming a bigger signal means a bigger flaw - phase and depth matter.
- Forgetting that permeability variations in ferrous material swamp defect signals unless saturated.
- Confusing standard depth of penetration with actual detection depth.
- Ignoring edge effects and geometry changes as false-signal sources.
Worked example
If frequency is quadrupled, δ scales as 1/√f, so penetration halves. Use this to reason about whether a proposed frequency can reach a given flaw depth.