Linear transformation based orthotropic shear ductile fracture criterion for lightweight metals

Accurate modelling of orthotropic ductile fracture is key to carry out reliable numerical prediction of rupture in plastic deformation of lightweight metals, such as ultra high strength steel, aluminum alloys, titanium alloys and magnesium alloys. Experiments are conducted for an aluminum alloy in shear, uniaxial tension, plane strain tension along rolling direction, diagonal direction and transverse direction as well as the balanced biaxial tension of the Nakajima test. Loading processes are recorded and fracture strain is measured by analysis of deformation with digital image correlation. Fracture behavior is modelled by a shear ductile fracture criterion of DF2016 along different loading directions. It is observed that anisotropy in ductile fracture cannot be correctly described by an isotropic ductile fracture criterion. Thus, an anisotropic ductile fracture criterion is proposed from a shear ductile fracture criterion of DF2014 based on linear transformation of the plastic strain vector into an isotropic equivalent damage strain vector. The anisotropic ductile fracture criterion is applied to model orthotropic fracture strain in shear, uniaxial tension and plane strain tension. The predicted anisotropy in ductile fracture is compared with experimental results for the verification of its accuracy. The comparison indicates that the proposed anisotropic ductile fracture criterion accurately models orthotropic ductile fracture in various loading conditions in shear, uniaxial tension and plane strain tension.