Experimental Finding Of Initiation Fracture Toughness And FEM Simulation Of Fracture Behaviour Of UFG 7075 Al Alloy

Abstract

The present work describes an experimental evaluation of yield strength, tensile strength, initiation fracture toughness and Finite element simulations of fracture behaviour for both bulk and ultrafine-grained (UFG) 7075 Al Alloy. The 7075 Al alloy has been rolled for different thickness reductions (40%, 70% and 90%) at cryogenic (liquid nitrogen) temperature, and its mechanical properties and microstructural morphology have been investigated. Rolling of the Al alloy at cryogenic temperature suppresses the dynamic recovery and grain growth, which leads to grain fragmentation. Dislocation cells formed during consecutive rolling passes, transformed into fully formed UFG (600 nm) up to 70% thickness reduction. Grain size gets reduced further when 90% thickness reduction is achieved. Incremental crack growth simulations have been carried out by commercial software ABAQUS under quasi-static loading using deformation plasticity theory based on Griffith energy concept. J-integral, stress along crack path, effect of crack and specimen size over J-integral, stress distribution and plastic dissipation ahead of the crack tip have been investigated for some practical crack problems under mechanical and thermo-mechanical loading. The numerical examples indicates a significant enhancement in crack arrest capabilities of UFG alloys for the same boundary conditions because of decreasing J values with increasing % thickness reduction. This is attributed to the improved mechanical properties (UTS: 625 MPa and YS: 610 MPa) of the cryorolled alloy which hinders the onset of plasticity, results from ultrafine-grain formation.