Thermomechanically transforming Notched NiTi Thin ribbon: Effect of Martensitic Transformation on Stress Gradients

Abstract

In this work, we used numerical simulations to investigate the effect of thermally induced forward martensitic transformation (MT) under constant load on the stress redistribution around stress risers. Finite element simulations of stress redistribution around two-sided notches in a thin NiTi ribbon subjected to constant tensile force and homogeneous cooling from austenite to martensite were performed. Changes in stress gradients around the notches were analyzed with regard to heterogeneity of thermomechanically driven MT. A parametric study was performed in order to understand the effect of transformation strains, Young’s modulus of martensite and notch radius on the magnitude of notch-tip stresses after the completion of MT. We found that forward MT critically amplifies the initial elastic stress gradients around the notch. The increase in magnitude of notch-tip stress is closely related to elastic deformation in martensite needed to accommodate the transformation strain in the surrounding notch unaffected zone. This approximation holds for notch radii being small compared to the ribbon width. For large radii MT is more homogeneous, the notch-tip is more shielded from transforming surroundings and, hence, the notch-tip stress approach the nominal stress.