Answer
Fatigue failure in wave springs typically initiates at the inner or outer edges of the wave crests where the tensile stress is highest. Under cyclic loading, the stress range $\sigma_r = \sigma_{max} - \sigma_{min}$ must be evaluated against the Modified Goodman Criterion. If the calculated stress $S = \frac{3 \cdot \pi \cdot P \cdot D_m}{4 \cdot b \cdot t^2 \cdot N^2}$ exceeds the endurance limit of the material (e.g., approximately 30-40% of tensile strength for 17-7PH), micro-cracks will propagate. Failure is often accelerated by surface imperfections such as pits or tool marks from the coiling process. To mitigate this, shot peening can be applied to induce compressive residual stresses on the surface, effectively shifting the mean stress downward and extending the fatigue life from $10^5$ to over $10^6$ cycles.
