Ductile failure behavior of polycrystalline Al 6061-T6 under shear dominant loading

Ductile failure in polycrystalline aluminum alloys under pure shear as well as with superposed tension and compression loading is explored through the modified Arcan shear experiments. Specimens obtained through tests interrupted at various stages of deformation and failure evolution are examined through quantitative microscopy to discern the mechanisms of failure and to evaluate the local strain evolution quantitatively. Fractographic observations are used to identify the onset and evolution of damage processes during deformation and failure of these aluminum alloys. Local strain levels are estimated from measurements of the change in grain size with deformation and used to indicate that the local values of failure strains are likely to be much larger than that estimated from strains averaged over characteristic specimen dimensions such as the gage length or the specimen diameter. Lower bound estimates of the failure strain in low triaxiality conditions are obtained from the experiments. It is shown that strain-to-failure decreases monotonically with stress triaxiality in stark contrast with recent works where a reverse behavior in low stress triaxiality levels was reported. Eventual failure that occurs through void growth and coalescence is shown to be restricted to a very small region within the localized deformation band.