Class 2 (2nd Engineer) MEKG 📅 Oct 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. (16)

Reference Answer

### **1. Introduction: Fracture Below Yield Point - Fatigue Failure**
A material can fracture when stressed below its normal yield point due to a phenomenon known as **fatigue failure**. This type of failure is not caused by a single, steady overload but by the repeated application of cyclic or fluctuating stresses. Even if the peak stress in each cycle is well within the material's elastic limit, microscopic cracks can initiate and grow over time, leading to an eventual catastrophic fracture.
Fatigue failure is a progressive process that occurs in three distinct stages:
* **Stage I: Crack Initiation**

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