Evolution of internal stress variables during cyclic deformation of copper

Continuum mechanical and microstructural theories of cyclic plasticity are analysed with regard to the development of internal stress variables and their techniques of measurement. Annealed polycrystalline commercial copper specimens are deformed in strain-controlled low cycle fatigue at various strain rates and strain amplitudes. The internal stresses are measured by unloading and reloading the specimens within the primary hysteresis loop by assuming that the strength of the material is composed of an isotropic stress component and a kinematic stress component. The evolution of internal stress variable as a function of strain rate, strain amplitude and number of cycles is determined. A number of features such as stress asymmetry in cyclic flow stress, back stress and the Bauschinger effect are observed and their dependence on strain rate and strain amplitude during cyclic hardening and saturation is determined. The results are discussed in terms of dislocation dynamics in the dislocation cell interior during fatigue.