Mechanical responses of L450 steel under biaxial loading in the presence of the stress discontinuity

The Lüders plateau behaviour is a phenomenon of stress discontinuity associated with strain localisation which can affect the fracture behaviour of mild steel subjected to plastic strains. Together with experimental studies of biaxial loading tests with digital image correlation (DIC), this work considers the effect of stress discontinuity on the mechanical behaviour of L450 steel subjected to biaxial loading, using the so-called “up-down-up” (UDU) constitutive model in finite element analyses (FEA) of cruciform specimens. The softening modulus of the UDU model and the stress ratio were found to significantly affect the simulated evolution of the Lüders band. A method has been proposed to use appropriate UDU parameters which capture the characters of the Lüders bands in biaxial loading conditions. Based on the selected parameters, the formation of the Lüders bands illustrated in the stress distribution and the Lüders plateau in the force-displacement were studied over the full range of the loading ratio under biaxial loading conditions.