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When a material is subjected to a stress that is greater than or equal to its yield stress, the material deforms plastically. When the stress is below this level, then in principle it should only deform elastically.

However, provided the temperature is relatively high (see later for the meaning of this), plastic deformation can occur even when the stress is lower than the yield stress. This deformation is time-dependent and is known as creep.

During loading under a constant stress, the strain often varies as a function of time in the manner shown below:

Plot of time versus strain showing the 3 stages of creep

This TLP focuses primarily on steady-state creep. In practice, this often dominates the creep behaviour – for example, the period during which it occurs is usually much greater than those for primary or tertiary creep.

There are two broad mechanisms by which steady state creep takes place: diffusion creep and dislocation creep.

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