Exam Question

### (a) Causes of Fatigue Cracking of Engineering Components (4 Marks)
Fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum stress values that cause such damage may be much less than the ultimate tensile strength, and even below the yield strength of the material. The primary causes are:
1. **Cyclic Stress/Strain**: This is the fundamental prerequisite for fatigue failure. Repeated application and removal of load (or fluctuating load) creates cyclic stresses. Even if the peak stress is below the material's yield point, microscopic plastic deformation can occur at a local level (e.g., at stress raisers). Over many cycles, these micro-cracks coalesce and grow, leading to failure. The relationship is often described by an S-N (Stress vs. Number of cycles) curve, which shows that a lower stress amplitude can be endured for a greater number of cycles.
2. **High Stress Concentration**: Most components have geometric features that cause a localized increase in stress. These 'stress raisers' or 'stress concentrations' include sharp corners, holes, keyways, screw threads, or abrupt changes in cross-section. The stress at these points can be several times the nominal applied stress, causing the local stress to exceed the material's endurance limit and initiate a crack, even when the overall component stress is low.