Shear cracking in an Al powder compact studied by X-ray microtomography

An experimental study is presented of the compaction of an aluminium alloy powder to determine the sequence of physical processes that leads to shear cracking. The powder was compacted in a die containing a long central vertical pin. Image correlation was used to map out the evolution of the displacement and strain patterns in the compact from the motion of tin markers in the powder captured in a sequence of X-ray tomographic 3D volumes. Compaction is observed to occur most extensively above the pin, creating an essentially rigid domed region as the powder approaches full density locally. As the compaction force is increased further, intense shear deformation occurs around the dome, smearing out the particles and leading to the formation of a shear crack. Optical micrographs taken destructively at different stages of compaction show clearly the deformation of the particles either side of the shear plane and the morphology of the crack.