Tensile Plastic Instability at an Elevated Temperature and Its Dependence upon Strain Rate

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Abstract
Observations have been made of the changes in shape that develop in initially cylindrical specimens of In–0.5% Pb and In–5.0% Pb alloys extended axially at overall strain rates in the range 1.2×10−7−1.7×10−1 sec−1 at a temperature of 0.69Tm. To deal with the dynamic nature of neck formation and with the possibility that necks once formed may become dormant, the concept of neck activity is introduced. Two types of instability are observed. The first instability is related to the initiation of several active necks in a uniformly deforming specimen. It may occur at small strains (<0.05) and apparently is strain-rate sensitive. The second instability occurs at large strains and involves the unequal rate of growth of concurrently active necks, resulting in the final ascendancy of a single neck. The second instability is related to elevated temperature ductility. At high strain rates, where work hardening is dominant, it corresponds approximately to Considère's criterion, but at low strain rates, where viscous effects are significant, a postponement with respect to the Considère condition is observed, the ductility increasing with decreasing strain rate.

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