The adiabatic correction factor for deformation heating during the uniaxial compression test

The isothermal uniaxial compression test is a common method to determine the flow stress of metals. For accurate flow stress data at strain rates > 10 exp -3 s exp -1 , the data must be corrected for flow softening due to deformation heating. The first step in the correction is to determine the increase in temperature. An adiabatic correction factor, eta , is used to determine the temperature between strain rates of 10 exp -3 to 10 exp 1 s exp -1 . The adiabatic correction factor is the fraction of adiabatic heat retained in the workpiece after heat loss to the dies, eta = ( Delta T sub ACTUAL )/( Delta T sub ADIABATIC ), where Delta T sub ADIABATIC = (0.95 f sigma d epsilon )/( rho C sub p ). The term eta is typically taken to be constant with strain and to vary linearly (0 to 1) with log ( epsilon ) between 10 exp -3 and 10 exp 1 s exp -1 . However, using the finite element method (FEM) and a one-dimensional, lumped parameter method, eta has been found to vary with strain, die and workpiece thermal conductivities, and the interface heat-transfer coefficient (HTC). Using the lumped parameter method, an analytical expression for eta was derived. In this expression, eta is a function of the die and workpiece thermal conductivities, the interface heat-transfer coefficient, workpiece heat capacity, strain, and strain rate. The results show that an increase in the HTC or thermal conductivity decreases eta . (Example material: TiAl.)