The effect of notches on the fatigue behavior in NiTi shape memory alloys

Fatigue experiments have been performed on superelastic NiTi double-notched plate form specimens with different notch types and sizes. The fatigue loading was controlled by the average stress at the minimum cross-section. The fatigue life as well as the fatigue fracture surface has been investigated in detail. The fatigue life decreases from specimens without notches, to specimens with semi-circular notches followed by v-type notched specimens. Specimens with crack notches show the shortest fatigue lives. The fatigue life decreases with increasing the notch acuity. Not only notch type but also notch size affects the fatigue life under the same macro-average stress. The fatigue fracture surfaces are quite similar to those of the conventional ductile materials with crack initiation, propagation and final fracture area. Typical characteristics of low cycle fatigue surfaces such as multi-fatigue crack initiation sites, weak striations and tyre-like patterns were found. Finite element (FE) simulations were performed to investigate the stress and phase transformation distributions in order to explain and understand the experimental results.