A modified Lemaitre damage model phenomenologically accounting for the Lode angle effect on ductile fracture

Dual phase (DP) steels consisting of two phases, ferrite and dispersed martensite, offer an attractive combination of strength and stretchability, which is a result of the strong distinctions of these constituents in mechanical properties. To properly describe the new features of these steels, various phenomenological uncoupled models or criteria are proposed in the past decade. These models or criteria determine more accurately the ductile fracture dependency on stress states in terms of stress triaxiality and Lode angle. However, they miss the softening effect caused by damage for materials in which damage is very much pronounced during large plastic deformation. In this study, a conventional Lemaitre damage model with isotropic hardening is adopted and modified in a phenomenological way to account for the Lode angle effect on ductile fracture. To demonstrate the necessity of incorporating the damage effect to stress-strain response, interrupted tests are performed to examine the damage evolution history. Notched dog-bone specimens with different radii are subsequently tested under the tensile loading to calibrate the material parameters and validate the model after material parameter calibration. With only five independent damage material parameters, the proposed model achieved a good accuracy on the prediction of the force displacement response of deformation from uniaxial tension to plane-strain tension. The prediction of the final ductile fracture is also in a good agreement with the experimental data, which is a considerable improvement from the conventional Lemaitre damage model. (C) 2014 Elsevier Ltd.