About the determination of the thermal and athermal stress components from stress-relaxation experiments

The determination of the thermal and athermal stress components using relaxation experiments along a stress–strain curve is critically evaluated. Short-term stress-relaxations are performed along the stress–strain curve of single crystals of Ge at 850 K, Cu, and Ni3Al at 300 K. They are analyzed by three different equations with two or three parameters including the athermal stress. The stress components obtained are compared to the values determined by stress-reduction experiments considered as the reference method. The relaxation rate is considered successively to be a power function or a hyperbolic sine function of the effective stress or a hyperbolic decrease of stress with time is assumed. It is shown that the three methods overestimate or underestimate the stress components depending on the material and deformation conditions. The error can be as large as about 100%. Reasons for the inadequacy of short-term relaxation experiments for the determination of the stress components are discussed.