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SUMMARY .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 percent 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.

The stress dependence of screw-dislocation velocity in single and polycrystalline specimens of an iron-3.14% silicon alloy was measured by observation of slip-band growth. An electrolytic etching technique was used to reveal dislocation intersections with the specimen surface, and slip bands were observed to form from fresh scratches and from grain boundaries as a result of pulse loading. Screw dislocation velocity on the {110} 〈111〉 system in single crystals at room temperature followed the relation ν = (τ/τ0)n, where n = 30.1. A plot of screw-dislocation velocity vs nominal resolved shear stress in individual grains of polycrystalline specimens shows considerable scatter which is attributed to the effects of stress variations due to elastic anisotropy. Observation of slip-band growth in scatched and unscratched grains indicates that the stress required to activate grain boundary sources is greater than the stress required to propagate fresh dislocations.