Exam Question

### (a) Causes of Fatigue Cracking of Engineering Components (4 Marks)
Fatigue is a failure mechanism where a component fractures under repeated or fluctuating (cyclic) stresses that are significantly lower than the material's ultimate tensile strength (UTS) or even its yield strength. The primary causes are:
1. **Cyclic Loading:** This is the fundamental prerequisite for fatigue. Stresses fluctuate between a maximum and minimum value over time. This cyclic action initiates microscopic cracks at stress concentration points, which then propagate with each cycle until the remaining cross-section of the component can no longer sustain the load, leading to sudden, catastrophic failure. This process occurs in three stages: crack initiation, crack propagation, and final fracture.
2. **Stress Concentration (Stress Raisers):** Most components have geometric discontinuities such as holes, keyways, threads, or sharp corners. These features, known as stress raisers, cause a localized amplification of stress. The stress at the root of a notch can be several times higher than the nominal stress in the bulk of the component. Micro-cracks invariably initiate at these points of high stress concentration.