Experimental Investigation of Heterogeneous Strain Fields Created in Metallic Materials

This study reports experimental evaluation of inhomogeneous strain fields created in NiTi structures during stress-induced martensitic transformation. A superelastic NiTi thin plate was used to create samples with uniform and non-uniform geometries. Digital image correlation technique was employed to measure the local strain variation in the NiTi samples under mechanical loading. Clear Lüders band formation and propagation were observed during experiments. The local strain contours revealed that the deformation was localised and spatially inhomogeneous in all samples. In the geometrically non-uniform sample, the transformation initiation and propagation were controlled by geometrical gradient. Also, the global deformation behaviour exhibited stress-gradient during forward and reverse transformations because of geometrical non-uniformity.