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THE governing relation between strain η and stress σ for the behaviour of metals at temperatures above about half the melting point seems to be of the form : where E is the elastic modulus and M is a function of temperature1. The quantity σ0, which has the dimensions of stress, is a constant for a metallurgically stable material and is independent of temperature. Its value does, however, vary considerably, depending on the presence of impurities or of alloying material.