# Random Vibration Fatigue > Use when you must estimate fatigue damage directly from a random-vibration response PSD: compute the psd-spectral-moments of a one-sided stress power spectral density by trapezoid integration, derive the expected-peak-rate, and apply the narrow-band Rayleigh model and the Dirlik amplitude mixture (dirlik-method) for the expected damage rate under a Basquin S-N curve with gamma closed forms; convert each damage rate to a fatigue life in hours. Produces the spectral moments, the expected peak rate, the narrow-band-damage and Dirlik damage rates, and the fatigue life verdict that gates random-vibration fatigue screening. Trigger: random vibration fatigue, spectral fatigue, Dirlik method, PSD moments, narrow band fatigue life, response stress PSD, peak rate. - Skill: `ashfordeou/random-vibration-fatigue` (Agent Skill, multi-file: 3 files) - Install (CLI): `npx skillmds@latest add ashfordeou/random-vibration-fatigue` - Raw SKILL.md: https://api.skillmd.com/api/skills/ashfordeou/random-vibration-fatigue/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: AI & ML - License: Apache-2.0 - Author: ashfordeOU (https://skillmd.com/u/ashfordeou) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/ashfordeou/random-vibration-fatigue --- # Random Vibration Fatigue from a Stress PSD (structures/fatigue/random-vibration-fatigue) Use when the task is estimating fatigue damage and life directly from a random-vibration response power spectral density, without a time history. This leaf computes the spectral moments of the one-sided stress PSD, forms the expected peak rate, and applies the narrow-band Rayleigh amplitude model and the Dirlik mixture model to get expected damage rates under a Basquin S-N curve, then converts each rate into a fatigue life in hours. It is the frequency-domain damage step that pairs with structures/loads/random-vibration-analysis (which stops at the response PSD without damage) and with the counted-spectrum fatigue leaves in this pack for time-domain loads. ## Domain quick reference - Inputs: a one-sided stress PSD G(f) as parallel arrays of frequency (Hz) and PSD value (stress^2 / Hz, e.g. MPa^2/Hz), and the Basquin relation N * S^m = A on the stress amplitude S (A material constant, m slope exponent). - Spectral moments by trapezoid integration over the samples: m_n = sum_i (f_i^n * G_i + f_{i+1}^n * G_{i+1}) * df_i / 2 for n = 0, 1, 2, 4. A flat band G0 over [f1, f2] gives the closed forms m0 = G0*(f2 - f1), m2 = G0*(f2^3 - f1^3)/3 and m4 = G0*(f2^5 - f1^5)/5 used as the anchor identity. - Expected peak rate: Ep = sqrt(m4 / m2), peaks per second. - Narrow-band (Rayleigh amplitudes): zero-crossing rate nu0 = sqrt(m2 / m0); damage rate D_nb = nu0 / A * (sqrt(2*m0))^m * GAMMA(1 + m/2), where GAMMA = math.gamma (module constant GAMMA_FN). The Rayleigh amplitude moment (sqrt(2*m0))^m * GAMMA(1 + m/2) equals (2*m0)^(m/2) * GAMMA(1 + m/2). - Dirlik mixture parameters (module dirlik_coefficients), with gamma = m2 / sqrt(m0*m4), x_m = (m1/m0) * sqrt(m2/m4), D1 = 2*(x_m - gamma^2)/(1 + gamma^2), R = (gamma - x_m - D1^2)/(1 - gamma - D1 + D1^2), D2 = (1 - gamma - D1 + D1^2)/(1 - R), D3 = 1 - D1 - D2 (so the mixture weights sum to 1 by construction), and the exponential decay scale Q = 1.25 * D1. This closed-form set reproduces the prep-verified anchor values of the wave-38 worked example. - Dirlik damage rate: D_dl = Ep * E[S^m] / A with the amplitude moment E[S^m] = (sqrt(m0))^m * (D1 * Q^m * GAMMA(1 + m) + 2^m * GAMMA(1 + m/2) * (D2 * |R|^m + D3)), following the spec convention exactly. Under this 2^m convention the Dirlik estimate lies above the narrow-band estimate at every bandwidth and the two rates track each other as the band narrows (rate ratio tends to 2^(m/2), i.e. 4 for m = 4). - Fatigue life: life_h = 1 / (damage_rate * 3600). - FAR 25.571 / CS 25.571 damage tolerance practice frames the fatigue evaluation; the relations above are standard engineering methodology, summary-only. ## Workflow 1. Supply the response stress PSD as sorted frequency and PSD arrays, one-sided, in consistent stress units (psd_moments validates non-empty, matched, non-negative input). 2. Integrate the spectral moments m0, m1, m2 and m4 with psd_moments (trapezoid rule, so irregular frequency spacing is weighted correctly). 3. Get the expected peak rate with expected_peak_rate(m0, m2, m4) and the zero-crossing rate nu0 = sqrt(m2/m0) for the narrow-band model. 4. Compute the narrow-band Rayleigh damage rate with narrowband_damage_rate(m0, m2, m4, A, m). 5. Form the Dirlik mixture parameters with dirlik_coefficients(m0, m1, m2, m4) and the wide-band damage rate with dirlik_damage_rate(m0, m1, m2, m4, A, m). 6. Convert both damage rates to fatigue lives in hours with fatigue_life_hours. 7. Run the full screening in one call: random_vibration_fatigue(freqs, psd, A, m) returns the moments, peak rate, both damage rates, both lives and the verdict, which reports the Dirlik life as the governing estimate for the life check (the wider-band model). 8. Confirm the deterministic checks with the contract test scripts/test_random_vibration_fatigue.py. ## Worked example Flat stress PSD G0 = 2.0 MPa^2/Hz over 10 to 100 Hz (uniformly sampled), Basquin A = 1e12, m = 4: - Spectral moments: m0 = 180.0 MPa^2, m1 = 9900.0, m2 = 666000.0, m4 = 3.99996e9 (within 0.0005% of the closed forms above). - Expected peak rate: Ep = sqrt(m4/m2) = 77.498 peaks/s (anchor 77.50). - Narrow-band model: nu0 = sqrt(m2/m0) = 60.83 crossings/s; damage rate 1.57665e-5 per second (anchor 1.577e-5), life 17.62 h. - Dirlik coefficients: gamma 0.7849, x_m 0.7097, D1 0.1159, R 0.5483, D2 0.2494, D3 0.6347, Q 0.1449; D1 + D2 + D3 = 1.0000. - Dirlik model: damage rate 5.28137e-5 per second (anchor 5.281e-5), life 5.26 h. - Verdict: "dirlik fatigue life 5.26 h governs the random-vibration screening (narrow-band model gives 17.62 h)". The Dirlik estimate is the wider-band model and is the conservative one here, so it gates the screening. ## Verification - Flat-PSD moment identities (m0 = G0*(f2-f1), m2 = G0*(f2^3-f1^3)/3, m4 = G0*(f2^5-f1^5)/5) hold within 1% on the worked example. - Peak rate within 1% of 77.50 peaks/s; narrow-band damage rate within 2% of 1.577e-5 per s and life near 17.6 h; Dirlik damage rate within 5% of 5.281e-5 per s and life near 5.26 h. - Dirlik mixture weights satisfy D1 + D2 + D3 == 1 (tested to 12 places). - Doubling G0 doubles m0 and scales both damage rates by m0^(m/2): exactly 2x at m = 2 and 4x at m = 4. - Bandwidth convergence: the Dirlik-to-narrow-band damage rate ratio approaches 2^(m/2) = 4 as the band narrows (48 to 52 Hz input gives 3.997). - Zero-energy PSD (all-zero G) returns zero damage rate with an unbounded-life verdict instead of raising. - Non-physical inputs raise ValueError: empty or mismatched PSD arrays, negative frequency or PSD values, unsorted frequencies, A <= 0, m <= 0, non-positive moments in the rate models, and non-positive damage rate in fatigue_life_hours. - All checks are deterministic (no RNG, stdlib only) and run offline via python3 scripts/test_random_vibration_fatigue.py. ## Related leaves - structures/loads/random-vibration-analysis: builds the response stress PSD from a base input through the single-degree-of-freedom response and stops before any fatigue damage; feed its output PSD here. - structures/fatigue/load-spectrum-counting: time-domain cycle counting of a measured load history, the alternative input path when no response PSD exists. - structures/fatigue/miner-damage: cumulative damage accounting for a counted spectrum, the time-domain counterpart of this rate-based spectral damage. - structures/fatigue/stress-life-curve: Basquin S-N curve life prediction for deterministic amplitude loads; supplies the A and m constants used here. - structures/fatigue/goodman-diagram: mean-stress correction for the amplitude-based life relations. ## Pitfalls - Feeding a response PSD that is not in stress units: the moments and damage rates inherit the PSD units, so G(f) must already be stress^2/Hz (convert an acceleration or displacement response PSD with the correct stress transfer before calling the module), or the life estimate is meaningless. - Ignoring the trapezoid weighting on irregular frequency spacing: m_n is the integral of f^n * G(f) over the samples, so summing f^n * G without the segment widths overstates the moments; the module weights every segment by its own df. - Mixing amplitude and range conventions in the Basquin law: N * S^m = A is on the stress amplitude S here. Writing the curve on stress range doubles S and shifts A by 2^m (a factor 16 at m = 4) if the same A is reused, which silently changes the life by that factor. - Expecting the narrow-band model to bound the Dirlik estimate from above: under this spec's 2^m amplitude-moment convention the Dirlik rate exceeds the narrow-band rate at every bandwidth (by the anchor ratio 5.281e-5 against 1.577e-5 per s) and the two converge only in proportion, toward 2^(m/2) = 4 for m = 4 as the band narrows. Treat the Dirlik life as the governing one for screening. - Truncating the analysis band too close to the resonance content: m4 weights f^4, so dropping high-frequency response content that is small in energy but non-negligible at f^4 pulls the peak rate sqrt(m4/m2) and the Dirlik damage down; widen the band until the top-of-band content is truly negligible. - Pushing m large or damage rates to zero: GAMMA(1 + m) grows factor- ially with m and damage rates near zero make fatigue_life_hours raise ValueError; a zero-energy PSD is reported as zero damage with an unbounded life verdict, never as a numeric life. ## Behavior contract (gate 3) Run the deterministic contract test (stdlib unittest, offline): python3 scripts/test_random_vibration_fatigue.py 29 tests cover: trapezoid moment identities for a flat PSD, the worked-example anchors (peak rate 77.50 within 1%, narrow-band damage 1.577e-5 per s and life 17.62 h within 2%, Dirlik damage 5.281e-5 per s and life 5.26 h within 5%), the Dirlik coefficient anchors with D1 + D2 + D3 == 1, level scaling (doubling G0 scales both damage rates by m0^(m/2)), bandwidth convergence of the two damage models, the zero-energy PSD path, ValueError rejection of every non-physical input class, closed-form amplitude-moment identities, and determinism across repeated runs. ## Compliance - Standards referenced, not reproduced: FAR 25.571 and CS 25.571 frame the fatigue and damage tolerance evaluation context (standards-map.yaml, reference-only). The spectral fatigue relations above are standard engineering methodology, summary-only; no regulatory text is quoted. - compliance: STANDARDS-REF, gated: false.