A fatigue calculation can report several plausible numbers that answer different questions. A stress range describes the applied cycle. A mean-stress correction compares that cycle with a reference fatigue strength. A damage sum combines blocks using supplied lives. Keeping those steps separate makes assumptions easier to review.
1. Reconstruct the cycle
σm = (σmax + σmin) / 2σa = (σmax − σmin) / 2Δσ = σmax − σmin = 2σaR = σmin / σmax, when σmax ≠ 0For illustrative extrema of 180 MPa and 20 MPa, the mean is 100 MPa, amplitude 80 MPa, range 160 MPa and ratio 0.1111. Supplying the range where an amplitude is expected doubles that input. If the maximum is zero, the ratio is undefined; the extrema and amplitude can still describe the cycle.
2. Read the Goodman screen as a screen
Take an ultimate strength Su = 500 MPa, yield strength Sy = 350 MPa and fully reversed fatigue strength Se = 200 MPa for the target life and conditions. These are illustrative inputs, not properties assigned to a named alloy. Altair’s mean-stress documentation describes the Goodman correction and its limitations.
Equivalent amplitude = σa / (1 − σm / Su)Utilization U = σa / Se + σm / SuProportional load factor = 1 / UHere the equivalent amplitude is 80 / (1 − 100/500) = 100 MPa. Utilization is 80/200 + 100/500 = 0.60; its reciprocal is approximately 1.67. That factor scales mean and amplitude together. It is different from scaling only the alternating component at a fixed mean.
The peak-to-yield ratio is 180/350 ≈ 0.514. Passing that separate screen does not establish fatigue life. Our implementation gives no beneficial correction for negative mean stress, and it rejects a tensile mean at or beyond the selected strength intercept. Notch, size, surface, environment and reliability effects require justified inputs or a more suitable analysis.
3. Keep Miner’s inputs traceable
D = Σ(ni / Ni)Suppose one block applies 20,000 cycles at a condition with reference life 100,000 cycles; another applies 50,000 cycles with reference life 1,000,000 cycles. The contributions are 0.20 and 0.05, giving D = 0.25. Repeating this entire block from zero modeled damage would reach unity after four blocks.
The reference lives must match each block’s amplitude, mean stress and conditions. As described in Siemens’ Miner-rule reference, the linear sum omits sequence effects. Unity is a model threshold, not a guaranteed failure or safety boundary. The calculator does not count rainflow cycles or infer an S–N curve, and “four blocks to unity” is not a remaining-service-life claim.
Review these inputs in the Fatigue Load Calculator ↗