Analysis of formability of advanced high strength steel sheets with phenomenologically based failure criteria with separate treatment of instability, shear and normal fracture

Despite their wide application in sheet metal forming analysis, Forming Limit Diagrams cannot supply reliable results for the cases involving non-proportional strain paths or material classes with reduced ductility such as advanced high strength steels (AHSS). Fracture criteria appear as complimentary tools for assessment of formability in these cases. CrachFEM, as an advanced failure model, merges an instability criterion that includes strain hardening and yield loci effects with fracture criteria which monitor damage accumulation for ductile normal fracture and ductile shear fracture separately where stress triaxiality ratio and maximum shear stress dependence are taken into account, respectively. In the present study, rectangular deep drawing of two AHSS classes is studied both experimentally and numerically. Blanks with different rolling directions and blank orientations with respect to the punch are taken into account. Simulations are conducted using CrachFEM failure model and LS-DYNA where texture of the sheet due to rolling is modeled with Hill’48 type anisotropic yield locus. Experimental studies reveal that the failure occurs mainly due to instability with necking whereas in- plane shear stress state in drawing zone seems to be insufficient to create shear fracture. Numerical results show not only the predictive capability of CrachFEM but also regarding weaknesses which needs improvement for better predictions.