Investigation of fracture behaviour and low cycle fatigue properties of cryorolled Al-Mg alloy

Abstract

In this work influence of cryogenic forming on mechanical and fatigue properties are investigated for Al-Mg (AA 5754) alloy. The received alloy is subjected to rolling at liquid nitrogen temperature (cryorolling) for 40% thickness reduction. The material’s performance against crack growth for as received and cryorolled alloys are evaluated by fatigue crack growth (FCG) test at the stress ratios (R-Ratios) of 0.1, 0.2 and 0.5. Brittle fracture toughness (KIc) and ductile fracture toughness (JIc) are experimentally evaluated for both as received and cryorolled alloys. A series of uniaxial strain controlled low cycle fatigue tests (LCF) at room temperature are conducted to evaluate fatigue performances of the alloys. Significant improvements in yield and tensile strength are observed for cryorolled AA 5754. FCG tests exhibit lower value of the fatigue threshold and higher value of the fatigue life for cryorolled alloys. Increase in brittle fracture toughness (KIc) and decrease in ductile fracture toughness (JIc) value is observed for cryorolled alloys. Cryorolled alloys show mild hardening in the initial cycles followed by noticeable cyclic softening at all strain amplitudes during LCF tests. The hysteresis loops and fatigue lives have been investigated for both as received and cryorolled alloys. The extended finite element method (XFEM) has been utilized to simulate the elastic-plastic crack growth during fracture toughness evaluation. Heaviside function and asymptotic crack tip enrichment functions are used for modelling of cracks in compact tension and three point bend specimens. LCF behaviour of the material is modelled by finite element method (FEM) using Chaboche kinematic hardening model. The fracture surface morphologies obtained from different tests are studied by scanning electron microscopy (SEM).