Material response, localization, and failure of an aluminum alloy under combined shear and tension: Part I experiments

This two-part series of papers presents complementary experiments and analyses of ductile failure of an aluminum alloy in the challenging regime of moderate to low stress triaxiality. Part I uses a custom tension-torsion experimental setup to establish the response and the onset of failure under proportional and non-proportional stress histories that form the basis for the numerical simulations in Part II. The test section of the adopted tubular specimen allows large localized strains to develop unimpeded, which were captured using high-resolution digital image correlation (DIC). The measured failure strains exhibit a monotonic decrease as triaxiality increases, similar to the results of Scales et al. (2016). A difference is that the present failure strains are calculated by integrating the work-compatible equivalent plastic strain increments over the loading history, which necessitates the use of a constitutive model. It is shown that for triaxialities greater than 0.2 the integrated failure strains are very close to the values measured directly from the last DIC image. Furthermore, the choice of constitutive model does not significantly affect the measured strains. For the four corner-path experiments, the state of stress and equivalent strain at failure were found to be close to the values of the corresponding radial paths. Microscopic evaluations indicated that the material exhibited limited void growth until just prior to rupture. Part II of this series investigates the extent to which properly-calibrated plasticity models can reproduce the high strains in the localized zones.