Cyclic plastic behaviour of high-strength steel Q690 : stress state dependence

Abstract

High-strength steel (HSS) Q690 demonstrates significant potential in steel construction projects owing to its superior strength-to-weight ratio. However, it is prone to ultra-low cycle fatigue (ULCF) failure during seismic events, leading to significant cyclic plastic deformation. This study investigates the influence of stress state on the cyclic plasticity of HSS Q690. First, the stress state dependence on the plastic behaviour of structural steels under monotonic loading and cyclic loading was carefully revisited. It was demonstrated that the conventional definition of stress state parameters, such as stress triaxiality and Lode angle parameter, derived from a yield criterion without backstress, may be inadequate for describing cyclic plasticity, where backstress plays a critical role. To address this, a new definition of stress state parameters, incorporating the effects of backstress, was proposed. Subsequently, a series of cyclic tests of HSS Q690 specimens covering a wide range of the proposed stress state parameters were conducted, and finite element (FE) simulations were performed to evaluate the impact of the new parameters. Results showed that stress triaxiality has no effect on the cyclic plasticity of HSS Q690 while the new Lode angle parameter shows a considerable influence. A new constitutive model was developed to account for Lode angle dependence and calibrated using cyclic test results. The model accurately captures the stress state dependence of HSS Q690's cyclic plastic behaviour.