Localised prior strain-hardening increases the tearing resistance of ductile steel

In structural materials, ‘intrinsic’ toughening originates from plastic dissipation of strain energy at the tips of cracks. This depends on a material's microstructure and its stress–strain response. By introducing a spatially-varying distribution of prior strain-hardening into a material, we can modify the stress field which develops around a crack as it is loaded, producing an increased resistance to ductile tearing. We demonstrate this toughening effect using synchrotron X-ray diffraction and digital image correlation measurements of the crack tip region in a ductile ferritic steel. Localised strain-hardening also introduces a residual stress, but this is shown not to contribute significantly to the initiation of tearing in this material.