The onset of unstable fracture and crack deflection of AA7085 in mixed mode I + II loading

Abstract

The current study focuses on the planar mixed mode fracture of AA7085. A loading device based on the concept given by Richard et al. has been developed for inducing planar mode-mixity in compact tension and shear (CTS) specimens. The CTS specimens extracted from the AA 7085 plate were subjected to mixed mode quasi-static loading after pre-cracking the specimens by fatigue loading under mode-I conditions. The desired pre-cracking length was achieved after performing an extended finite element method (XFEM) based fatigue crack growth simulation. Different mode-mixity has been introduced in the specimens by changing the orientation of the loading device with respect to the loading axis of the machine. Experimentally observed crack deflection was compared with the angles predicted by different planar mixed mode fracture criteria, namely MTS, Richards, and strain energy density. Equivalent stress intensity factors proposed at the onset of unstable fracture have been determined using different criteria and compared with each other. A new equivalent stress intensity factor for mixed mode has been proposed from Shah's equation of fracture locus. It has been found that the equivalent stress intensity factor based on Shah's equation of fracture locus is suitable for the alloy being investigated. The fracture morphology of the fractured specimen was inspected in a field emission scanning electron microscope. Both inter-granular and dimpled ductile fracture was observed in the specimens irrespective of the mode-mixity. Extended finite element simulation was performed to determine tangential stress distribution at a distance around the crack tip.