Random Vibration Fatigue Terminology
A reference glossary of key terms in random vibration fatigue — spectral moments, irregularity factor, narrow-band, Dirlik, Rayleigh, damage rate.
Terminology
| Term | Definition |
|---|---|
| Stress PSD | Power spectral density of stress — frequency distribution of mean-square stress [MPa²/Hz] |
| RMS stress | √(area under stress PSD) — standard deviation of stress, not a peak |
| Spectral moments | Integrals of PSD weighted by f^n — m₀ (RMS²), m₂ (zero-crossing rate), m₄ (peak rate) |
| Irregularity factor α | m₂/√(m₀×m₄) — narrow-band (≈1) vs broad-band (→0) |
| Narrow-band method | Assumes single-mode response, Rayleigh cycle distribution — conservative for broadband |
| Dirlik method | Empirical broadband method using spectral moments — widely used, generally reliable |
| Rayleigh distribution | Distribution of peak amplitudes for narrow-band Gaussian process |
| Damage rate E[D] | Expected fatigue damage per unit time — ν × ∫ p(S)/N(S) dS |
| Miner's rule | Linear damage accumulation: D = Σ n_i/N_i, failure at D = 1 |
| S-N curve | Cycles to failure vs stress amplitude — N = C × S^(-b) |
| Rainflow counting | Time-domain cycle counting method — the reference for frequency-domain methods |