Fracture behaviour analysis of AA7075 aluminium alloy by finite element and boundary element methods

Abstract

Fracture mechanics is a field of study that focuses on the behaviour of materials containing cracks under stress, particularly for high-strength aluminium alloys like AA7075 series that are used in aerospace as well as automobile industries because of their high strengthto-weight ratio. This study focuses on fracture test experiments and the analysis of the behaviour of AA7075 under various loading conditions through finite element analysis (FEA) and boundary element techniques (BET) to determine life estimation, stress distribution, stress intensity factors (SIF), and energy release rates (ERR) during fracture. The FEA model is developed for a standard edge-cracked specimen of dimensions w= 50 mm, thickness 5 mm, and initial crack length, a= 10 mm. Under tensile loading of 10 kN, the stress intensity factor is computed as approximately 18.2 MPa√m, which agrees well with experimental observations. The corresponding energy release rate obtained from the simulation is 82.5 J/m², indicating a predominantly brittle fracture mode due to the alloy’s T6 temper. Using BET analysis, crack propagation trajectories are simulated, showing a stable crack growth phase until the critical crack length ac= 18.4 mm, beyond which unstable fracture occurs. The predicted fracture toughness for the alloy is found to be 25.6 MPa√m, consistent with literature values for AA7075-T6, ranging between 24 - 28 MPa√m. The simulated fatigue crack growth rate follows the Paris law with constants, C = 1.2 x 10-10 and m = 3.5, producing a fatigue crack growth rate between 1×10⁻⁶ and 5×10⁻⁴ mm/cycle for ΔK values from 10 to 20 MPa√m. The meshing and loading properties of Ansys 2024R2 are utilized for the boundary conditions, while the FEA inbuilt functions in MATLAB incorporate experimental data to predict behaviour under Mode I fracture. From the analysis, it is determined that stress intensity factors depend on both crack length and specimen geometry. The energy release rates and stress distribution at the crack tip are largely influenced by the applied stress, with peak von Mises stresses reaching 460 MPa near the crack tip region. These analysed data aid engineers in determining life estimation and crack propagation limits in AA7075 components. The study concludes that components made from AA7075 should maintain service stresses below 70% of the yield strength (≈350 MPa) to prevent premature crack propagation, improving fatigue life and reducing economic and safety risks.