Experimental investigation of the effect of phase transformation on fracture parameters of NiTi shape memory alloy

This thesis aims to investigate the effect of martensitic phase transformation on fracture parameters of Nickel-Titanium (NiTi) SMA. NiTi specimens were characterized using optical microscopy and uniaxial tensile testing. 1 mm thick compact tension (CT) coupons were cut to determine critical stress intensity factor (Kc) and crack tip opening displacement (CTOD) under Mode-I loading at room temperature. To observe the effect of martensitic phase transformation, tests were also carried out above martensite desist temperature (Md) at which the stress induced phase transformation is suppressed. The results revealed that the martensitic phase transformation ahead of the crack tip, improves the fracture behavior of NiTi SMA. Finite elements, ABAQUS was also used to solve the 2-D cracked CT specimen problem under Mode-I loading. Shape and size of the transformation at the crack tip were examined, displacement and stress distribution near the crack tip were obtained. Kc and CTOD values were also calculated using J-integral for both superelastic and austenitic NiTi. FE analysis also revealed that the phase transformation enhances the toughness of Nitinol. The numerical results were compared to those obtained from tests and a good agreement was observed.