Class 2 (2nd Engineer) MEKG 📅 Jan 2023

Exam Question

Explain why a material may fracture when stressed below its yield point. Give examples of components which might fracture in this way if suitable precautions are not taken. Explain how such fractures can be avoided with reference to the materials chosen, careful design and workmanship.

Reference Answer

### **1. Introduction: Fatigue Failure**
A material may fracture when subjected to stresses significantly below its yield point if the load is applied cyclically or in a fluctuating manner. This mode of failure is known as **fatigue failure**. It is a progressive and localized structural damage that occurs when a material is subjected to repeated loading and unloading. Fatigue is the single largest cause of failure in metals, estimated to comprise some 90% of all metallic failures. The critical factor is not the magnitude of a single stress application, but the number of stress cycles the component endures.
### **2. Mechanism of Fatigue Fracture**
The process of fatigue failure occurs in three distinct stages:

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