Investigating the Effect of Thickness on Fracture Toughness of NiTi Shape Memory Alloy

Shape memory alloys (SMAs) can handle large deformations through martensitic phase transformation and reversible transformation from austenite to martensite under temperature and/or stress changes. Due to their amazing properties, SMAs such as NiTi are increasingly used in a variety of applications where integration requires a complete understanding of fracture mechanics and crack growth behavior. Fracture toughness of conventional metals is believed to be dependent on specimen thickness especially due to the ductile fracture behavior of the material. On the other hand, due to the complex fracture behavior and the phase transformation characteristics of SMAs, the desire to explore the thickness effect on fracture toughness of SMAs arises. In this study, the thickness effect on the fracture toughness of NiTi compact tension (CT) specimens is investigated experimentally. Pre-cracked NiTi specimens with nominal thicknesses changing from 12 mm to 5 mm are used in the fracture experiments under mode-I loading conditions at room temperature at which the material is fully martensite and experiences detwinning upon loading. The resistance curves are obtained from load-displacement data and the J-integral values are determined using a recently proposed methodology for SMAs while the crack sizes are obtained implementing elastic compliance method. Critical J-values for specimens with different thicknesses are obtained from the resistance curves and compared to reveal the thickness dependence of J_Ic. The strain fields are measured using the digital image correlation (DIC) technique. This enables investigating the thickness effect by evaluating the zone of nonlinear deformation mechanism near the crack tip. Based on the critical J-values and the strain plots obtained from DIC, the thickness is found to have no obvious effect on the fracture toughness of NiTi SMAs within the considered thickness range