Analysis of forming limit behaviour of high strength steels under non-linear strain paths using a micromechanics damage modelling

Abstract

The formability characteristics of advanced high strength (AHS) steel grades 780 and 1000 subjected to varying forming histories including strain path changes were described by means of a damage modelling approach. The Marciniak tests of the examined steels were firstly carried out to apply pre-strains under various deformation modes. Afterwards, tensile, stretch-bending and hole expansion test of the pre-strained samples were performed. FE simulations coupled with the Gurson-Tvergaard-Needleman (GTN) ductile damage model were conducted to predict failure occurrences of AHS samples in the two-stage forming tests. The damage model parameters were determined under consideration of pre-strain effects on void evolution mechanisms and its interdependencies with microstructure constituents of steels, for which hybrid methods of metallographic analyses, local strain measurement and representative volume element (RVE) simulations were employed. Then, forming limits of the investigated steels in different forming procedures with non-proportional loading paths were characterized. It was shown that the micromechanics damage simulations in combination with the proposed parameter identification scheme could precisely represent the fracture states of steels under such complex forming conditions.