An investigation into the fracture mechanisms of twinning-induced-plasticity steel sheets under various strain paths

•The fracture limits of TWIP steel sheets were derived experimentally and numerically.•Various ductile, shear and ductile-shear fracture criteria have been applied.•The ductile-shear fracture models predict the failure limits more precisely.•These models consider nucleation, growth and shear coalescence of voids. The formability of the automotive twinning-induced-plasticity (TWIP) steel sheets is investigated through the hemispherical-punch stretching test. The forming limit diagram (FLD) at fracture is constructed both experimentally and numerically. Numerical studies are also performed to assess the applicability of various uncoupled fracture criteria to predicting the FLD. The results illustrate that the FLD may not be properly predicted using only a ductile or a shear fracture criterion. The principal strains at fracture can be predicted with good accuracy by taking both types of fracture mechanism simultaneously into consideration (e.g. models by Han-Kim (2003) and Lou (2012)). This can be justified by describing the damage process of TWIP steel sheets as the nucleation, growth, and shear coalescence of voids.