BACK STRESS IN HIGH TEMPERATURE DEFORMATION

The applicability of the following strain rate equation to high temperature deformation is examined. ε=|gfsexp ( -ΔH0 KT) exp ( vτ - τB] KT) Here ε is the tensile or compressive strain rate; φs is a structure factor; ΔH0 is the activation enthalpy in the absence of effective stress; v is the activation volume; τ is the applied stress; τb is the internally-generated back stress; and kT has its usual significance. Published results obtained by creep, hot compression, extrusion and hot torsion covering a wide range of strain rates are analyzed. It is concluded that, as long as σs and v are independent of instantaneous changes in applied stress and temperature, a back stress term must be included if the equation is to be valid for transient as well as steady-state conditions. Possible sources of the back stress are considered, and it is shown that the structure parameter" of empirical strain rate equations arises from the back stress term. © 1969."