Fatigue Life Prediction Workbook (Excel w/ SN Curve Library & Miner’s Rule Solver)

Fatigue failure remains a dominant mode of structural degradation in heavy-duty off-road vehicles like tractors, where chassis members experience complex, non-uniform loading histories—including towing, hitch loads, terrain-induced vibrations, and transient impacts. This workbook operationalizes classical fatigue theory by embedding a curated library of S–N curves for common chassis materials (e.g., ASTM A572 Grade 50 steel, high-strength low-alloy steels, and welded joint classifications per IIW or BS 7608), each parameterized with fatigue strength coefficient (σ'_f), exponent (b), and endurance limit thresholds. Users input time-history stress data—either as raw time-series or grouped into stress-range bins—and the solver automatically applies rainflow cycle counting to extract amplitude-mean cycles, then maps each cycle to the appropriate S–N curve segment (e.g., finite-life or infinite-life regime). Miner’s Rule is then applied iteratively across all stress ranges to compute total damage D = Σ(n_i / N_i), where n_i is the number of cycles at stress range Δσ_i and N_i is the corresponding cycles-to-failure from the S–N curve; failure is predicted when D ≥ 1. The workbook also supports safety factor adjustment, mean stress correction (e.g., Goodman or Gerber), and sensitivity analysis via sliders or scenario tables—making it suitable for design iteration, warranty life estimation, and FEA validation.