Evaluation of uncoupled ductile fracture criteria for the dual-phase steel Docol 600DL

The modified Mohr–Coulomb (MMC) fracture criterion recently proposed in the literature and novel extended versions of the Cockcroft–Latham (ECL) and Rice–Tracey (ERT) fracture criteria, explicitly accounting for Lode dependence, are evaluated for the cold-rolled, dual-phase steel Docol 600DL. In addition, two special cases of the ECL criterion as well one special case of the ERT criterion are assessed. To this end, experimental data from uniaxial tension, plane-strain tension, in-plane shear and modified Arcan tests on 2 mm thick steel sheets are utilized. The experimental results display only moderate variation of the fracture strain as a function of the stress state. The criteria are calibrated by using two of the tests, while the remaining three tests are used for validation purposes. The shape of the fracture strain surface as function of stress-state is similar for the MMC, ECL and ERT fracture criteria, but the MMC criterion tends to give a larger range in predicted fracture strain, and the ERT criterion does not distinguish between axisymmetric stress states having Lode parameter equal to +1 and −1. The damage evolution of the MMC and ECL criteria are linear functions of the equivalent plastic strain, while the damage evolution is exponential with the equivalent plastic strain for the ERT criterion. The damage evolution of the ERT criterion displays a similar shape as the evolution of void area fraction of a comparable steel material. The fracture strains obtained with the fracture criteria are compared with experimental data. In order to extend the range of the experimental validation, additional data from formability tests conducted on material from the same batch are employed. The ECL criterion gave good predictions on the equivalent strain at fracture initiation, while a somewhat larger spread was observed for the ERT and MMC criteria.

► The fracture characteristics of the 2 mm thick Docol 600DL steel sheet are presented.
► Modified versions of the Mohr-Coulomb, Cockcroft-Latham and Rice-Tracey criteria are assessed.
► The experimental fracture strain is better predicted by the extended Cockcroft-Latham criterion.