Fracture of Nitinol under quasistatic and dynamic loading

Owing to the potential application of Nitinol as an advanced structural material, it is essential to thoroughly understand the deformation and fracture behavior of Nitinol under various loading conditions. The present study explores the fracture behavior of Nitinol under quasistatic and dynamic loading, with emphasis on the fracture toughness and fracture mechanism of Nitinol. To this end, the precracked bend sample was employed to perform dynamic fracture testing using a modified (pulse-shaped) Hopkinson-pressure-bar-loaded fracture-testing system. The dynamic fracture initiation toughness was measured under stress-state equilibrium conditions at a loading rate of similar to 10(6) MPa root m/s. To further investigate the fracture mechanism, additional dynamic fracture tests were performed using double-crack, four-point bend samples. The experimental results indicate that the dynamic fracture toughness of Nitinol is higher than it is under quasistatic loading, and that the loading rate influences the fracture mechanisms of Nitinol. The interplay between the dynamic strength of Nitinol and the activation stress for stress-induced martensite (SIM) transformation plays an important role in the fracture behavior of Nitinol.