Yielding at Varying Load Rates

A. H. Cottrell and B. A. Bilby have proposed a theoretical relationship for predicting the yield strength of certain metals as a function of stress rate and temperature. This relationship contains a number of undetermined coefficients and is based on a dislocation model consisting of a Frank-Read source, a dislocation segment with fixed ends lying on a slip surface bounded by a grain boundary. The dislocation segment is bound along its length by a Cottrell “atmosphere” of solute atoms. In this study, the yield strength of several selected aluminum alloys were determined experimentally under rates of stress application ranging from approximately 4 x 105 psi per sec to 19 x 106 psi per sec. For this phase of the investigation, all data was obtained at room temperature (290°K) so that the effect of stress rate on the yield strength could be established. An explosive, impact testing apparatus was designed and constructed specifically for this purpose. The test apparatus permitted the simultaneous measurement of load, strain, and velocity as a function of time. Certain experimental data are used to evaluate the undetermined coefficients that appear in the theoretical expression for yield strength. The yield strengths predicted by the theory are found to be in good agreement with the experimental values, indicating that the dislocation theory provides an accurate means of predicting yield strength.