Abstract
Compression-tension experiments were conducted to study the effect of pre-compression on the ductile fracture properties of three aluminium alloys. Diabolo-shaped specimens were subjected to five different levels of pre-compression (0%, 10%, 20% 30% and 40%), i.e., the specimens were first compressed to a prescribed strain and then stretched to fracture in tension. The force and the minimum diameter of the specimens were continuously measured during the test until fracture using a laser-based measuring system. Three extruded aluminium alloys, heat treated to peak strength (T6 temper), with different grain structures and textures were tested: AA6060, AA6082.50 and AA6082.25. The AA6060 and AA6082.50 alloys had recrystallized grain structure with equi-axed grains and relatively large elongated grains, respectively, whereas the AA6082.25 alloy had thin pancake-shaped grains, typical of a non-recrystallized extruded alloy. It was found that pre-compression has a marked influence on the ductility of aluminium alloys which depends on the microstructure and strength of the alloy. For the AA6060 alloy, the tensile ductility increased with pre-compression, and was almost doubled for the specimens subjected to 40% pre-compression compared to the ones in pure tension. For the AA6082.50 alloy the tensile ductility was relatively unaffected, while the AA6082.25 alloy exhibited a modest increase in tensile ductility. The tensile ductility is here defined as the strain to failure measured with respect to the compressed configuration of the specimen. To analyse and interpret the experimental results, finite element simulations of the compression-tension tests were carried out using both J2 flow theory and the Gurson model. To study the influence of pre-compression on void growth and coalescence, stress state histories from these simulations were subsequently used as boundary conditions in unit cell simulations.