Considerations Relevant to Lower Yield Stresses

The significance of the macroscopically inhomogeneous nature of yield propagation and of various features of the yield region of load/elongation curves is discussed. It is shown that the important rate parameter during yield propagation is the velocity of the Lüders front and not the overall applied strain rate. Hence, comparison of yield and general flow stresses should be made only after due consideration of the different local strain rates resulting from differences in homogeneity of deformation. The yield stresses of Armco iron have been determined at constant Lüders front velocities for various grain sizes. When plotted against (grain-size)^−½, the intercepts at infinite grain size (σ_i) are smaller and not so strain-rate-dependent, and the slopes of the relationships (k_y ) are much more strain-rate-dependent, than for yield stress/(grain-size)^−½ relationships found using yield stresses determined at constant (applied) elongation rate.