Yield (engineering) - Factors Influencing Yield Stress

Factors Influencing Yield Stress

The stress at which yield occurs is dependent on both the rate of deformation (strain rate) and, more significantly, the temperature at which the deformation occurs. In general, the yield strength increases with strain rate and decreases with temperature. When the latter is not the case, the material is said to exhibit yield strength anomaly, which is typical for superalloys and leads to their use in applications requiring high strength at high temperatures.

Early work by Alder and Philips in 1954 found that the relationship between yield stress and strain rate (at constant temperature) was best described by a power law relationship of the form

where C is a constant and m is the strain rate sensitivity. The latter generally increases with temperature, and materials where m reaches a value greater than ~0.5 tend to exhibit super plastic behaviour.

Later, more complex equations were proposed that simultaneously dealt with both temperature and strain rate:

where α and A are constants and Z is the temperature-compensated strain-rate - often described by the Zener-Hollomon parameter:

where Q_HW is the activation energy for hot deformation and T is the absolute temperature.