A finite element analysis of evolution of stress-strain and martensite transformation in front of a notch in shape memory alloy NiTi

The evolution of stress-strain and martensite transformation in front of a notch tip in a shape memory alloy NiTi is phenomenologically analyzed by finite element method. The results show that throughout the whole loading process, both the stress and strain near the notch tip increase with an increase in the applied load. The maximum strain occurs always at the notch tip. The stress distribution in front of the notch tip demonstrates four different features due to the martensite transformation and plastic deformation at different loading stages. With increasing the applied load, a partially martensitic zone firstly appears and grows in front of the notch tip, and then a fully martensitic zone, and then a plastic deformation zone occurs at the fully martensitic zone. The martensite volume fraction in the partial martensite zone decreases gradually with the distance away from the notch tip. The fracture initiation in shape memory alloys is predicted to take place in the plastic deformation zone that is very near the notch tip within the fully martensitic zone. The martensite transformation in front of the notch tip is beneficial to improving the fracture toughness of the shape memory alloys.