Analysis of time and temperature dependence of upper yield point in iron

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., .25/sec., and .001/sec. over the temperature range 77°K to 525°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 ∆H= ∆̅H(τ- τ/τ)^b. If this relationship is substituted for ∆H in a modification of the Boltzman relation, the following result is obtained: log⁡(γ ̇/γ ̇_1 )=M ¯∆H/〖RT〗_1 〖((¯τ- τ_1)/¯τ)〗^b [1-T_1/T 〖((¯τ- τ)/(¯τ-τ_1 ))〗^b]. This equation describes the experimental data within ±3000 psi. The results of this investigation compared with tensile test data from other investigators show that state of stress is an important factor in determining whether a material will behave in a ductile or brittle fashion.Wright Air Development Center, Contract No. AF 33(616)-5153, Project No. 702