Failure parameter identification and validation for a dual-phase 780 steel sheet

A hybrid experimental-numerical procedure was implemented to determine the failure surface of a dual-phase 780 steel sheet as a function of the effective plastic strain, triaxiality, and Lode parameter using butterfly specimens with in situ digital image correlation strain measurement and supporting finite element calculations. A butterfly-type test specimen was employed to experimentally obtain stress states ranging from simple shear to plane strain tension including mixed tensile and shear loading. The numerically-derived failure surface was implemented into the phenomenological GISSMO damage model in the commercial finite element code LS-DYNA and the accuracy of the failure surface was determined using finite element predictions of the characterization experiments. A series of independent validation experiments related to sheet metal forming were performed including a hole tension test, a conical and flat punch hole expansion test, and a hemispherical punch test. The finite element models utilizing the damage model were able to accurately reproduce the load-displacement and surface strains of the sheet material for both the characterization and validation experiments. Prediction of the failure orientation and location compared favorably to each of the validation tests.