Effect of compression–tension loading reversal on the strain to fracture of dual phase steel sheets

The effect of loading direction reversal on the onset of ductile fracture of DP780 steel sheets is investigated through compression–tension experiments on flat notched specimens. A finite strain constitutive model is proposed combining a Swift-Voce isotropic hardening law with two Frederick–Armstrong kinematic hardening rules and a Yoshida-Uemori type of hardening stagnation approach. The plasticity model parameters are identified from uniaxial tension-compression stress–strain curve measurements and finite element simulations of compression–tension experiments on notched specimens. The model predictions are validated through comparison with experimentally-measured load–displacement curves up to the onset of fracture, local surface strain measurements and longitudinal thickness profiles. In addition, the model is used to estimate the local strain and stress fields in monotonic fracture experiments covering plane stress states ranging from pure shear to plane strain tension. The extracted loading paths to fracture show a significant increase in ductility as a function of the compressive pre-strain. A Hosford-Coulomb damage indicator model is presented to provide a phenomenological description of the experimental results for monotonic and reverse loading.