Miner's Rule Cumulative Fatigue Damage
Miner's Rule Cumulative Fatigue Damage
Real components rarely see a single constant stress amplitude over their life — they experience a mix of load levels, each contributing its own share of fatigue damage. Miner's rule offers the simplest way to combine these contributions: at each stress level, the fraction of life used up is just the cycles applied divided by the cycles that level would take to cause failure alone, and these fractions simply add. Despite its simplicity and known limitations — it ignores load sequence and interaction effects between high and low stress cycles — Miner's rule remains the default first-pass method across mechanical and aerospace fatigue design because of how well it balances simplicity against reasonable accuracy for many loading histories.
Miner's rule gives cumulative damage as D = n1/N1 + n2/N2, summed over each load level. where n_1c and n_2c are the number of cycles actually applied at load levels 1 and 2, and N_1c and N_2c are the cycles to failure at those same load levels (from the S-N curve).
Inputs
n_1c := 10000=10000
N_1c := 50000=50000
n_2c := 5000=5000
N_2c := 20000=20000
Each ratio is the fraction of that stress level's total fatigue life that has been consumed, and Miner's rule simply assumes these fractions add linearly regardless of the order the loads occurred in.
D_damage := n_1c/N_1c + n_2c/N_2c=0.45
Results
A cumulative damage index below 1.0 indicates the component has fatigue life remaining, while D = 1.0 is the classical (if simplistic) failure criterion — this result shows meaningful damage accumulated but with some life remaining under this loading history. Because Miner's rule ignores sequence effects, engineers often apply a design damage limit below 1.0 (such as 0.5-0.7) as a margin against the known non-conservatism of the linear damage assumption under variable-amplitude loading.
Assumptions & limitations
- Linear damage accumulation — load sequence and interaction effects are ignored.
- Each load level's N is taken from the same S-N curve/mean stress condition.
- Failure is defined at D = 1.0, a simplification of real cumulative damage behavior.
- Applicable to two (or more) discrete load levels — not a continuous load spectrum.