Whilst engineers may argue over the way that materials fail, it has to be recognised that there is no universally accepted definition of the manner in which this occurs. Figure 7.1 shows the generally accepted points on the journey to failure.
The initial stage is usually a straight line relationship where stress is proportional to the extension. The graph then curves slightly to the yield point, following which there is irreversible plastic flow. The ultimate tensile strength is the maximum point at which the crack initiates. There is then a propagation stage where a crack develops until finally the material breaks.
1 Limit of proportionality
2 Yield point
3 Ultimate tensile stress (crack initiates)
4 Crack propagates
5 Material breaks
Figure 7.1 Typical phases of failure of a metal
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