Abstract
An experimental program was conducted in which the etch pit technique was used for the direct observation of dislocation configurations at various stages of yielding. Poly-crystalline tensile specimens of 3 per cent silicon-iron were loaded in tension at constant strain rate and by load pulses. A new model of the delay-time for yielding at constant applied stress is presented. Three assumptions used are (a) no dislocation motion occurs below a critical resolved shear stress, (b) the yielding rate is dependent upon the velocity of mobile dislocations, and (c) the end of the delay period occurs when yielding of the grains has spread continuously through the thickness of the specimen. This model is consistent with the experimental observations and explains the true static upper yield point and the shape of the strain vs. time curve at constant applied stress. The model also yields reasonable values for the stress concentration factor on grains in the critical cross-section that are least favorably oriented for slip.
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