Ductile failure of aluminum alloy tubes under combined torsion and tension

Recent work on ductile failure under shear-dominant stress states has challenged the long-accepted premise that the strain at failure monotonically decreases with increasing triaxiality. This paper presents results from experiments in which custom Al-6061-T6 tubular specimens are loaded to failure under radial paths of shear and tension, spanning a range of triaxiality from 0.07 to 0.58. A long, thin-walled test section machined into the specimens has nearly uniform stress and deformation until a load maximum is reached and provides minimum constraint to the localized deformation that follows. Localization takes the form of a circumferential band with width the order of the wall thickness. Stereo digital image correlation (DIC) is used to monitor the deformation inside these localization zones up to failure. The specimen geometry, experimental setup, and use of DIC allow the true stresses and strains to be established directly from experimental measurements. The measured strains at failure are found to monotonically decrease as the triaxiality increases, a trend that is in concert with long accepted theory. However, the failure strains are significantly larger than previously reported values, approaching measurements based on statistical grain-level measurements. The results highlight the important role of localization in ductile failure, and the need for a diagnostic technique with sufficient resolution to accurately establish the strain at failure. In incorporating the reported failure strains in the modeling of ductile failure, the observation that this alloy deforms to rather large strains free of damage must be taken into account. In other words, plasticity governs this behavior until very close to the end of life of the material.