Exam Question

### (a) Causes of Fatigue Cracking
Fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum stress values are less than the ultimate tensile strength (UTS), and often below the yield strength of the material. The failure process occurs in three stages: crack initiation, crack propagation, and final catastrophic failure.
1. **Cyclic Loading:**
* **Reason:** Repeated application and removal of stress, even below the material's yield point, causes microscopic plastic deformation at localized points. Over many cycles, these deformations accumulate, leading to the formation of micro-cracks (initiation). Once initiated, the crack propagates with each subsequent stress cycle until the remaining cross-section of the component can no longer support the load, resulting in sudden fracture.