Investigation of fatigue crack growth in superelastic NiTi alloy by using DIC method

In recent years, shape memory alloys, specially NiTi, as a group of smart materials, have received more attentions in industrial applications. Martensitic phase transformation in shape memory alloys is the most important factor in their unique behavior. In this paper, the formation of stress induced martensite phase in the crack tip of superelastic NiTi (50.8% Ni) samples was investigated by using the DIC method. In particular, SEC specimens were subjected to fatigue mechanical loading, then the crack length and also displacement fields at the crack tip of specimens were measured by DIC technique. Control of the crack length was performed using a high magnification camera during the fatigue test. In the following, stress intensity factors were calculated according to ASTM standard E647-15. Obtained results from the fracture analysis show that fatigue threshold values are decreased with increasing the load ratio (R). In the present paper, for a load ratio of R=0.05, during the crack propagation, the fatigue threshold value is 〖∆K〗_th=17 MPa√m, while stress intensity factor is estimated about 35 MPa√m before the final failure. Also, as a new method in observation of the phase transformation, DIC pictures indicated the formation of stress induced martensite at the specimens crack tip.