An assessment of in-service stress relaxation of a work-hardened aluminum-magnesium alloy

Time-dependent relaxation processes continue after forming of sheet metal components. Mechanical properties and even the shape of the parts may evolve with time, which may promote the failure of the metal components. In-service stress relaxation is generally associated with low strain rate and non-uniform deformation; its mechanism is still not clear because of the difficulties in experimental measurements. The objective of this work is to gain the micro-mechanical insight of stress relaxation, and construct an appropriate constitutive model, which can be applied to predict the time-dependent relaxation process after forming.; The bent beam relaxation test—a simple method to measure relaxation at stress levels well below the usual 0.2% offset yield stress, is outlined first, and the method to analyze the experimental results is also discussed. Several constitutive models are applied to fit the experimental results, such as the mechanical threshold stress (MTS) model, Hart's model and a modified Hart's model. The fitting results show that only the Modified Hart's model can provide a consistent description for stress relaxation, especially the initial transient process. However; with the special phenomenological expression in the plastic element, the Hart's type models are not suitable for simulating the deformation with bulk plastic flow. In order to predict the typical industrial forming and in-service response of sheet metal, such as sheet metal forming and subsequent springback or relaxation, a general model—the Mix model, is proposed. The new model keeps the basic frame of the Modified Hart's model and introduces the expression of the MTS model to represent the macro-plastic deformation. As a result, this model can characterize well both the micro-plastic processes that lead to stress relaxation and springback, and the bulk plastic flow in the process of sheet metal forming. The algorithm of time integration of the Mix model is also discussed and implemented within ABAQUS by a UMAT. The numerical simulations for the bent beam relaxation test and several complex deformation processes, including time-dependent springback, are per formed based on the developed code. The simulated results are encouraging. It is proven that the Mix model is effective to evaluate the time-dependent stress relaxation and springback.