Experimental investigation and numerical analysis: fracture mechanics and microstructural development in Ni55.8Ti superalloy under mixed-mode loading at different temperatures

Shape memory alloys (SMAs) and specifically NiTi superalloys have gained significant attention in different industries for their excellent mechanical properties and superior material characteristics including processability, corrosion resistance, cyclic stability and bio-compatibility. Static and dynamic properties of NiTi SMAs, mainly under tensile loadings, have been widely evaluated in pseudoelastic and pseudoplastic states in many studies. However, there is a gap in the literature concerning the fracture failure of these materials under mixed-mode loadings, as these types of loadings are responsible for the most of the material failures in practical applications. This study is aimed at investigating the fracture toughness and microstructure of NiTi ASTM F2063 shape memory superalloy under mixed-mode fracture conditions and various temperatures, ranging from below the A/M phase transformation temperature to above the temperature at which the material behaves pseudoplastically. The material has been characterized using differential scanning calorimetry (DSC) and uniaxial tensile test apparatus under varying temperature to determine the mechanical and metallurgical characteristics before the mixed-mode fracture experiments. The fracture tests have been conducted using a tensile test machine equipped with a temperature control chamber and an in-lab developed special fixture to make the application of mixed-mode loading on a butterfly shaped specimen, known as Arcan sample, feasible. X-ray diffractometry and fractography using scanning electron microscope have been conducted to characterize the mechanism of failure and development of microstructure under different loading modes and temperatures. Finite element analyses were conducted to determine the fracture toughness and strain energy release rate of NiTi superalloy at different loading conditions.