Failure criterion for aluminium tubes during warm hydroforming and in crash applications

Abstract

A quantitative failure criterion was developed for aluminium (Al) tubes. The developed criterion is based on the critical damage value as calculated from the Cockroft and Latham ductile failure criterion and can be used to determine the forming limit during warm hydroforming or for failure during a crash. As-extruded, fully annealed, and T6-tempered Al 6061 tubes were tested to evaluate the effect of heat treatment conditions and forming temperature. To assess the formability of the Al 6061 tubes, uniaxial tensile tests and hydraulic bulge tests were performed in the temperature range of 25-300°C. The measured flow strength was used in finite element (FE) simulations for the tube hydroforming process. A commercial finite element method (FEM) code, DEFORM2D™, was used to calculate the damage values and strains at various locations in the hydroformed tube. Based on the results of both the experimental and FEM analysis, a failure criterion, which is based on the forming limit and ductile fracture criterion, was developed. Using the critical damage value as a quantitative measure, the form-ability and also the critical damage value of the as-extruded tube is 50 per cent lower than that of the fully annealed tube. The developed failure criterion predicts well the observed failures during hydroforming for a hexagonal-shaped prototype part. Moreover, the failure criterion can be used to predict the crash performance of the hydroformed part, which has variations in the amount of prior deformation and in thickness. The benefit of the proposed failure criterion is that it can be used to predict both the forming limit of the tube during hydroforming and the behaviour of hydroformed tubes during a crash.