Calculator Input
Example Data Table
These sample values help users compare likely composite fatigue outcomes across different laminate loading conditions.
| Case | σmax (MPa) | σmin (MPa) | σu (MPa) | σref (MPa) | Nref | b | Predicted Life (cycles) |
|---|---|---|---|---|---|---|---|
| Laminate A | 160 | 20 | 600 | 220 | 100,000 | 0.12 | 839,032.68 |
| Laminate B | 180 | 30 | 600 | 220 | 100,000 | 0.12 | 368,166.16 |
| Laminate C | 220 | 50 | 600 | 220 | 100,000 | 0.12 | 77,053.90 |
Formula Used
1) Stress range: Δσ = σmax − σmin
2) Stress amplitude: σa = (σmax − σmin) / 2
3) Mean stress: σm = (σmax + σmin) / 2
4) Stress ratio: R = σmin / σmax
5) Mean stress correction: Km = 1 / (1 − σm / σu)
6) Combined modifier: Kc = Ko × Kt × Kh × Kq × Kk
7) Equivalent stress: σeq = (σa × Km) / Kc
8) Design stress: σd = σeq × SF
9) Fatigue life relation: N = Nref × (σref / σd)^(1 / b)
This calculator uses an empirical composite fatigue approach. It combines a Basquin-style S-N relation with mean stress correction and laminate reduction factors.
Use laboratory fatigue data to calibrate reference stress, reference cycles, and exponent values for the exact material system, layup, and loading direction.
How to Use This Calculator
- Enter maximum and minimum cyclic stresses for the laminate.
- Add ultimate strength from validated material data.
- Set reference stress and reference cycles from fatigue testing.
- Enter the fatigue exponent matching your material model.
- Adjust orientation, temperature, moisture, quality, and knockdown factors.
- Apply the desired safety factor and target service cycles.
- Press calculate to view life, utilization, and serviceability.
- Download the summary as CSV or PDF for reporting.
Frequently Asked Questions
1) What does this calculator estimate?
It estimates fatigue life for a composite member under cyclic loading. The tool uses stress range, mean stress, reference S-N data, and several reduction factors to predict expected cycles to failure.
2) Why is mean stress included?
Mean stress influences composite fatigue damage. Higher mean stress usually reduces life. The correction factor raises equivalent fatigue stress when average loading approaches the laminate strength limit.
3) What is the fatigue exponent?
The fatigue exponent controls how quickly life changes with stress. Small exponent values make fatigue life very sensitive to stress increases. Use measured material data whenever possible.
4) What do the modifier factors represent?
They represent orientation mismatch, temperature effects, moisture exposure, manufacturing quality, and conservative design knockdowns. Together they reduce effective fatigue capacity before the life calculation.
5) Can this replace coupon testing?
No. This is a screening and comparison tool. Final engineering decisions should rely on validated coupon, element, or full-scale fatigue test data for the actual laminate and loading environment.
6) What does the damage fraction mean?
Damage fraction compares target service cycles against predicted fatigue life. Values below one indicate acceptable demand relative to calculated life. Values above one suggest redesign or lower loading.
7) Why might predicted life change sharply?
Fatigue relationships are strongly nonlinear. Small stress increases can produce large life reductions, especially when the fatigue exponent is low or the mean stress correction is high.
8) Which units should I use?
Use consistent units throughout the calculator. The page labels stresses in MPa, but any consistent stress unit works if ultimate strength and reference stress use the same unit.