Competitive Meso-Damage Model Dependent on Stress State for Advanced High-Strength Steels

Advanced High-Strength Steel (AHSS) exhibits stress-state-dependent competing shear–tensile fracture modes that limit the applicability of conventional ductile fracture criteria based solely on equivalent plastic strain accumulation, such as the Forming Limit Diagram (FLD) approach and the classical Gurson–Tvergaard–Needleman (GTN) model. This paper proposes an extended GTN damage model incorporating Hill’48 anisotropy and the Nahshon–Hutchinson shear mechanism, regulated by a stress-state-dependent weighting function. The experimental program comprised uniaxial tension tests for constitutive calibration, notched plate specimens with shear angles ranging from 0° to 90° (spanning pure shear to tensile–shear stress states), and tension-bending tests. The fracture initiation point was identified from the abrupt load drop on the experimental force–displacement curve and further located in the finite element simulation to extract the corresponding stress-state history. SEM fractography was employed to characterize the microscopic damage mechanisms, revealing a continuous transition from shear-dominated to void-dominated damage at a critical stress triaxiality of approximately 0.35. A weighting function dependent on both stress triaxiality and the normalized Lode angle was formulated to couple void evolution with shear band localization. Following calibration via finite element inverse fitting, the model, implemented as an ABAQUS VUMAT subroutine, successfully reproduced fracture strains and crack paths across stress states ranging from pure shear to high hydrostatic tension. Comparative simulations indicate that this approach yields improved prediction accuracy over the classical GTN model, particularly under mixed-mode conditions, thereby offering a practical numerical tool for analyzing AHSS formability.