Fracture behavior of the compact tension specimens in NiTi shape memory alloys

In the present work, the fracture behavior of a NiTi pseudoelastic alloy has been investigated. The fracture toughness is obtained by experiments on compact tension (CT) specimens, which are pre-cracked at a low fatigue load. The fracture toughness of the studied material is K_IC = 39.4 MPa m^1/2. The fracture surface of selected specimen is studied by scanning electron microscope (SEM). It is found that the fracture mechanism is dominated by cleavage fracture. With the increasing crack velocity, the fracture surface at the fast fracture regime exhibits a more brittle response. The finite element simulation on both specimens with and without pre-crack is carried out to better understand the fracture mechanisms of the NiTi alloys. It is shown that a pre-crack results in phase transformation at the crack tip under very low load. However, due to the low fatigue load and short pre-crack length, the material is always partly phase transformed, even at the crack tip, during the fatigue pre-crack process. There is always a small area around the crack tip fully transformed during the fast fracture process.