Fatigue Analysis in CAE
Fatigue life prediction in CAE: S-N curves, strain-life approach, cycle counting, and critical plane methods.
Definition & Context
Fatigue analysis predicts crack initiation and life under cyclic loading, where a component fails far below its static strength. The two dominant approaches are stress-life (S-N) for high-cycle fatigue (>10⁴ cycles, elastic) using Wöhler curves, and strain-life (ε-N) for low-cycle fatigue where local plasticity dominates.
Variable-amplitude histories are reduced to cycle counts with rainflow counting, then accumulated with Miner's rule (Σ nᵢ/Nᵢ = 1 at failure). Mean-stress corrections (Goodman, Gerber, Soderberg) account for the damaging effect of tensile mean stress. Tools include nCode DesignLife, ANSYS nCode, fe-safe, and FEMFAT.
Why It Matters
Most in-service mechanical failures are fatigue, not overload. Getting fatigue life right governs warranty, safety, and weight — over-design wastes material, under-design causes field failures.
Best Practices
- Use rainflow counting on real measured or simulated load histories, not a single peak load.
- Apply the correct mean-stress correction — Goodman is conservative for brittle materials, Gerber less so for ductile.
- Account for surface finish, size, and stress-concentration (Kf) factors that dramatically reduce life.
- Use strain-life for low-cycle/thermal-mechanical fatigue where plasticity occurs.
- Correlate against test data or field returns to calibrate the damage model.
Common Pitfalls
- Applying an S-N (high-cycle) method to a low-cycle plastic problem.
- Ignoring mean stress — tensile mean stress can halve life.
- Using idealised-smooth S-N data without surface-finish and notch knockdown factors.
- Assuming Miner's linear damage is exact; sequence effects can matter.
Standard Engineering Workflow for Fatigue Analysis in CAE
4. GD&T Detailing & CNC Toolpath Export
Author 2D fabrication sheets with complete ASME Y14.5 / ISO 1101 geometric tolerances (Position, Flatness, Runout) and export STEP AP242 / Parasolid models for 5-axis CAM.
Common Failure Scenarios & Troubleshooting
| Failure / Geometric Issue | Root Cause & Mitigation Strategy |
|---|---|
| Sketch breaks or flips geometry when adjusting dimensions | Sketch was under-constrained. Always apply geometric constraints (tangency, horizontal/vertical) before adding driving numerical dimensions. |
| Shell or Fillet feature fails on complex curved topology | Curvature radius is tighter than fillet radius or minimum wall thickness. Inspect surface curvature using Zebra stripes and eliminate zero-radius sharp corners. |
| Assembly performance severely lags during rotation | Large assembly mode was disabled. Suppress non-essential cosmetic features (threads, knurls) and use lightweight component representations. |
Industry Standards & Compliance Codes
- ISO 1101 (Geometrical Product Specifications)
- ISO 2768 (General Tolerances for Machining)
- ASTM / DIN Material Specifications