The strain hardening and damage behaviour of isothermally heat treated 6005A aluminium is investigated in order to link the thermal treatment conditions, microstructure and fracture strain. The need for a plastic flow rule involving a stage IV hardening at large strain was found essential to generate quantitative predictions when using an enhanced Gurson type damage model for the material behaviour. This model relies on an explicit description of the three stages of nucleation, growth and coalescence of voids, involving void shape effects. An implementation within a 3D finite element code allows for the simulation of the full tensile response curves up to cracking initiation and final failure. Reasonable agreement is found when comparing the predicted and experimentally measured fracture strains for a wide range of heat treatment conditions using real microstructure based parameters.