Contribution of stress triaxiality and Lode parameter in preloading to the subsequent plastic deformation behavior of aluminum sheet

This work is dedicated to the influence of preloading modes on the stress-strain response under nonproportional loading, with a special focus on the effects of stress triaxiality and Lode parameter in preloading. Two-stage loading experiments were conducted using a 3 mm-thick 1060O sheet. In the preloading stage, two types of tests, namely plate compression and biaxial tension, were designed because they could provide the stress state with the same Lode parameter but different stress triaxiality. By adopting specimens with different shapes and compression tools, as well as adjusting the tensile force ratio in two directions of the biaxial tension specimens, two kinds of Lode parameter, i.e. 1 and 0, associated with both negative and positive stress triaxiality, were achieved. Then small-scale uniaxial tension specimens cut from the preloading ones were tested. Macro stress curve and r-value curve against strain were analysed. Finally, numerical simulation using Chaboche hardening model was performed to evaluate the model’s description ability about the mechanical properties of preloaded material. Result shows that for two kinds of preloading with the same Lode parameter but different stress triaxiality, the stress and r-value evolution can be very different. Conventional constitutive models defined on the basis of deformation strain are unable to distinguish the influence of stress triaxiality in preloading.