Hardness testing is among the oldest material tests – Rockwell since 1908, Vickers since 1921. Highly accurate but destructive, hence sample-only. Acoustic resonance delivers a 100 % hardness statement – indirect, but inline-capable.
Why hardness and natural frequency correlate
Hardness correlates with Young's modulus: harder = stiffer. From the vibration equation: higher stiffness → higher natural frequency.
A 1 % harder steel batch typically raises natural frequency by 0.3–0.5 %. A too-soft batch (faulty heat treatment) is immediately visible.
Comparison matrix
| Criterion | Hardness test | Acoustic resonance |
|---|---|---|
| Probe depth | near-surface | integral |
| Non-destructive | no (indent) | yes |
| 100 % capable | no (1–10 % sample) | yes |
| Speed | 20–60 s per measurement | 0.3–1.5 s per part |
| Hardness value direct | yes (HRC, HV, HB) | no (relative) |
| Inline integration | difficult | easy |
| Cost per inspection | 0.30–1.50 € | 0.02–0.10 € |
When to pick which
Hardness testing remains essential for material characterisation and for hardness values mandated by standards.
Acoustic resonance is the first choice for 100 % sorting in series – the "heat-treatment guard". Detects under-hardened parts, faulty tempering, mix-ups.
Hybrid standard
100 % resonance inline + hardness test once per shift/batch. Complete hardness monitoring with documented sample verification.
Practical example: roller bearing rings
Replaced 8 % destructive hardness sampling (12 s/part) with 100 % acoustic resonance (0.8 s/ring). In the first quarter we detected 14 hardness deviations from a misadjusted tempering line – sampling would have missed 12 of them.
