Eddy current testing is one of the oldest electromagnetic NDT methods – its strength is sensitivity to near-surface cracks in conductive materials. Acoustic testing covers the complementary territory. Knowing both lets you combine them into a complete inline strategy.
How they work
Eddy current (ECT): coil generates AC magnetic field, induces eddy currents in conductive parts. Defects alter eddy currents, coil senses the impedance change. Penetration via skin effect, typically 0.1–2 mm in steel.
Acoustic resonance: excitation, full natural-frequency image of the entire part.
Comparison matrix
| Criterion | Eddy current | Acoustic resonance |
|---|---|---|
| Detection zone | near-surface, ≤ 2 mm | full volume |
| Surface crack sensitivity | ★★★★★ (≥ 0.1 mm) | ★★★☆☆ (≥ ~3 mm) |
| Volume defect detection | not possible | ★★★★★ |
| Material variety | conductive only | nearly all solids |
| Heat treatment sorting | ★★★★☆ | ★★★★★ |
| Material mix-up | limited | very good |
| Geometry sensitivity | high (lift-off) | moderate |
| Inline speed | 0.1–1 s per position | 0.2–2 s per part |
When to pick which
Eddy current: near-surface cracks on rotationally symmetric steel/non-ferrous parts, surface heat treatment defects.
Acoustic resonance: unknown defect location, volume defects, complex geometry, non-conductive materials.
Hybrid solution
For a roller bearing customer we currently combine both: eddy current on the ground surface (near-surface micro cracks), acoustic resonance for heat treatment quality (integral hardness). Both stations in < 3 s, full coverage.
