On the ductile failure of thin-walled aluminum alloy tubes under combined shear and tension
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The aim of this thesis is to establish the extent to which materials can be deformed under shear-dominant loadings. Custom Al-6061-T6 tubular specimens are loaded under radial and corner paths of tension and shear to failure. During the experiments, the deformation is monitored in a test section designed to have nearly uniform stress and deformation at large strains while providing minimum constraint to the development of localization that precedes failure. The recorded shear stress-rotation and axial stress-displacement responses exhibit maxima beyond which deformation localizes in a narrow band that is of the order of the 1 mm wall thickness of the test section. For the mainly shear dominated stress paths followed, deformation remained nearly planar allowing for the establishment of both the true stresses and the local deformation strictly from measurements. Results from thirteen radial path experiments as well as from four corner path experiments show the strain at failure to monotonically increase as the mean stress decreases, a result that is in contrast with previously reported results for Al alloys. Also, the measured failure strains are significantly larger than previously reported values. Analysis of corner stress paths investigates the path dependence of localization and failure. Results show little path dependence on the failure strains, but some path dependence on stress maxima and failure stresses. Furthermore, statistical grain-level strain estimates from five of the stress paths revealed a significant variation in strain across the macroscopically observed localization zone. In the neighborhood of the crack tip strains with 25-100% higher levels than the macroscopic values were recorded. This indicates that localization also occurs at a smaller scale than hitherto understood. The difference between the macro strain at failure and the average grain level values increased as the axial/shear stress ratio increased.