3D characterization of failure modes in magnetically driven collapsing cylinders

The formation of shear bands in collapsing thick-walled cylinders (TWC) occurs in a spontaneous manner, allowing to highlight the inherent susceptibility of the material to shear banding, without having prescribed geometrical constraints. Shear bands initiate at the inner boundary of the cylinder, and move outwards, as the cylinder collapses, along the maximum shear stress directions (+/-45 degrees) . The TWC technique has been presented in the literature using explosives or alternatively a magnetic platform to create the loading forces, applying high strain rates of 10⁴-10⁶ sec^-1. The analysis of the multiple shear bands in such an experiment, is done using a post mortem analysis. Optical microscopy of a cross-section of the specimen, allows to track the shear bands’ paths, and analysis is related to their spacing, lengths and distribution. The 2D analysis of a cross section from the middle section of the cylinder is assumed to be representative of the cylinder, for a relatively homogeneous deformation, distanced enough from the boundaries. The 3D actual pattern of the shear bands has not been explored experimentally in previous work, aside than examining adjacent cross sections from the same specimen, along with their similarities and differences. In this work, we used micro-CT analysis to explore the 3D characterization of shear bands in magnetically driven cylinders. Samples with different loading conditions were explored, examining also new samples that undergo tensile states after completing the stage of collapse, resulting with the opening of cracks along the sheared surfaces. Comparison of optical microscopy with to the micro CT cross sections is presented and discussed.