# Activity 8.4

Topics: Fatigue, Elasticity, Fracture mechanics Pages: 4 (1098 words) Published: June 18, 2015
﻿Chapter 13 Questions

Name: Lee Jie Min Jasmine, Ke Jun Yong Alan, Lim Kai Kiang, Ong Kim Liang Nicholas, Shankra s/o Krishna Date: May 10, 2015

1. What are the general requirements for a fatigue crack to form and propagate? The general requirements for a fatigue crack to form and propagate are: The presence of a local plastic stress;
The presence of a tension stress;
The presence of a cyclic stress (repeated or fluctuating). Eliminating any one of the above three requirements will stop the fatigue process. 2. Describe the three stages of fatigue in metals.

The three stages of fatigue in metals are:
Stage 1: Crack Initiation – Movements of dislocations resulting into slip line and slip planes. It will begin at the point where the local tensile is a maximum. Stage 2: Crack Propagation (Fatigue Zone) – Cyclic tension stresses spread the crack. If the part breaks in one application of the load, without the cyclic action, it is known as static failure. Stage 3: Final Fracture (Instantaneous Zone) – Crack lengthens, decrease in load support, final failure. 3. How is fatigue in metals recognized?

Fatigue in metals is recognized through the investigation of the fatigue zone. From the investigation, we are able to determine the origin of the crack, type of loading, direction of loading, and stress intensity. The ductility of a metal is recognizable by the shear type fracture surface. The stop marks and herringbone pattern point toward the origin and show the direction of crack progression. A single crack will show that tension or single directional bending was the direction of loading. The relative size of the fatigue zone and the instantaneous zone will reveal the intensity of the stress. If the stress intensity is low, many cycles are necessary for failure and the fatigue zone will be large. Likewise, if the fatigue zone is small, it is typical of a high stress level and failure will occur with fewer cycles. These are the features of a...