A nonassociated constitutive model describing transients in material behaviour induced by strain-path changes

To describe transients in material behaviour induced by strain-path changes (SPCs), including the transients in the stress and r-value versus strain curves, a phenomenological nonassociated constitutive model was proposed. The model was characterised by using a distortional function to define the yield surface based on experimental observations and a simpler nondistortional function to define the plastic potential surface. The prediction accuracy of the simplified plastic potential function was discussed by comparing different plastic potential functions in terms of their prediction of the r-value transient. In addition, anisotropic expansion of the yield and plastic potential surfaces during SPCs was described by a back stress restoration rule. To validate the proposed constitutive model, two-stage loading experiments were conducted on a 6061O aluminium sheet. Stress transients, including early yielding and permanent softening, were observed under both reverse and orthogonal loading. Moreover, a downward transient was observed on the r-value versus strain curve for the prestrained 6061O sheet, which had a constant r-value during monotonic loading. Simulations were conducted in parallel with tests in the subsequent loading stage. The numerical results of the stress-strain response, r-value transient, and force-displacement curves matched well with the experimental outcomes.