Effects of notch geometry on stress–strain distribution, martensite transformation and fracture behavior in shape memory alloy NiTi

In this paper, the stress–strain distributions and martensite transformation in three specimens with different notch geometries in a shape memory alloy NiTi are calculated by using finite element method (FEM), and their effects on fracture behaviors are analyzed. The results show that three characteristic loads, that is, starting load and finishing load of the martensite transformation and the beginning load of the plastic deformation occurring in the transformed martensite at crack/notch tips can be determined on the load–displacement curves calculated by the FEM. With increasing the notch acuity from blunt notch through acute notch to crack, the characteristic loads and the differences between them decrease. The stress–strain distributions exhibit different features at four load sections divided by the three characteristic loads. With the increase of the notch acuity the stress and strain at the crack/notch tips and their growing rate both increase. For producing the same full martensite zone in front of the crack/notch tips, the loads needed decrease with the increase of the notch acuity. The partial martensite zone size of the blunt notch specimen is larger than that of the acute crack/notch specimens. For the crack specimen, the plastic zone in the full martensite zone can form at a very low load, while for the blunt notch specimen a large load is needed to form the plastic zone. With increasing notch acuity from blunt notch through acute notch to crack, the sizes of the “fracture process zone” in front of the crack/notch tips decrease rapidly, and the crack propagation will change from unstable to stable manner.