The influence of temperature and strain rate on the upper yield point of ingot iron was studied. Torsion tests were conducted using strain rates of 12.5/sec, 0.25/sec, and 0.0001/sec over the temperature range 77 to 525 deg K. The upper yield point showed a rapid increase as the temperature was lowered. An increase in the strain rate also caused an increase in the yield point. An apparent activation energy can be associated with the strain rate and temperature dependence of the yield point. This energy is influenced by stress level, and it appears from the present study that the relationship can be described by an equation of the form
$ΔH=ΔH¯τ¯−ττ¯b.$
If this relationship is substituted for ΔH in a modification of the Boltzmann relation, the following result is obtained:
$logγ˙γ˙1$

$=MΔH¯RT1τ¯−τ1τ¯b$

$1−T1Tτ¯−ττ¯−τ1b.$
This equation describes the experimental data within ± 3000 psi. The results of this investigation compared with tensile test data from other investigators confirm that state of stress is an important factor in determining whether a material will behave in a ductile or brittle fashion.
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