Experimental and numerical studies of NiTi dynamic fracture behaviors under the impact loading

Abstract

The mechanical behaviors of NiTi shape memory alloy under the impact loading raised many interests in the fields of engineering and science, whereas its fracture process has not been well understood so far. In this work, multi-scale experiments and numerical simulations were conducted to comprehensively study the dynamic fracture performances of NiTi alloy. The impact crack experiments were carried out with the utilization of split Hopkinson compression bar, which showed the variation of crack propagation rate versus impact velocity. The Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) were employed to observe the fracture surfaces. The micro images revealed that the fracture behaviors were manifested as quasi-brittle fracture features. By taking the plastic evolution into consideration, a novel constitutive model based on Boyd and Lagoudas phase transition formulation was proposed, which overcomes the limitation of simultaneously describing super-elastic and plastic behaviors of NiTi. With the novel constitutive model, the crack propagation process was accurately reproduced with numerical results in good agreements with our experimental observations. Furthermore, the representative volume element (RVE) with Voronoi tessellation was employed to numerically study the grain size effect on intergranular fracture toughness. It was demonstrated that the reduction of grain size promoted the hinderance effect of intergranular fracture, which led to an evident improvement of intergranular fracture toughness in model with a smaller grain size.