A Comparison Between Dislocation Theory and Experimental Measurements of Delayed Yield in Steel

A dislocation model of a yield nucleus for materials, such as annealed low-carbon steel, which exhibit a distinct yield point, is presented. On the basis of the model an analytical expression is derived which relates the time delay for yielding under constant applied stress to the magnitude of the stress and the temperature. This expression involves the ratio of the binding energy of a dislocation with a Cottrell "atmosphere" to the total energy of a dislocation. The expression also involves the frequency of thermal fluctuations associated with the release of a dislocation from an "atmosphere". The numerical values of these two constants are chosen to fit the analytical expression to experimental data. The value of the ratio of the binding energy to the total energy so determined is essentially the same as the value obtained by Fisher (1)*. However, this value disagrees markedly with the theoretical estimate made by Cottrell (3). The value of the frequency of thermal fluctuations is shown to be in substantial agreement with the frequency to be expected from consideration of the length of the thermally activated portion of a dislocation during the process of its release from an "atmosphere".