Investigation of damage initiation in high-strength dual-phase steels using cohesive zone model

Dual-phase steels have been increasingly used for several vehicle structural parts due to their great combination of high strength and good formability. However, for an effective forming process of such steel sheets, their complex failure mechanism on the microscale plays an important role. In this work, damage initiation occurrences in two dual-phase steel grades were examined by a micromechanics-based final element modeling approach. Two-dimensional representative volume element models were applied to take into account amount, morphologies, and distributions of each constituent phase. Uniaxial tensile tests and fractography of the examined steels were carried out in order to characterize crack formation in the microstructure. According to a dislocation-based theory and local alloys partitioning, stress–strain curves were defined for the individual phases and interphases, where geometrically necessary dislocations were present due to austenite–martensite transformation. Cohesive zone model with extended finite element method and two-dimensional damage locus were applied in the representative volume elements for describing crack initiation induced by martensite cracking and ductile fracture of ferrite, respectively. Parameters of the damage models were identified by means of correlation between experimental and final element simulation results. The states of damage initiation of both dual-phase steels were predicted. Local stress, strain, and damage distributions in the dual-phase microstructures were determined and discussed.