Fracture of Nitinol under Quasistatic and Dynamic Loading

Abstract

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 $ 10 6 MPa ffiffiffiffi m p /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.

The effect of strain rate on trigger stress for stress-induced martensitic transformation and yield strength in Ti–18Al–8Nb alloy

“…Yet another factor, which may influence the trigger stress for SIMT is strain rate. Although few studies relating the effect strain rate [11,[18][19][20][21][22] on the trigger stress for SIMT in NiTi based alloys have been carried out, no information is available in the literature on the effect of strain rate on trigger stress for SIMT in Ti-Al-Nb alloys.…”

Fracture toughness of Ceramic-Fiber-Reinforced Metallic-Intermetallic-Laminate (CFR-MIL) composites

“…For example, it was employed for delamination strength and toughness measurement, crack initiation, propagation and arrest behavior in an array of materials [32][33][34][35][36]. A variant of the 4PB specimen, double-notched 4PB specimen, has been employed to investigate the fracture mechanism of materials under both low [37,38] and high [39] loading rates. In this work, an attempt was made to validate the 4PB test with the detection of crack initiation in measuring the fracture toughness of the CFR-MIL composites that are more ductile than ceramics.…”

Influence of cold work and texture on the high-strain-rate response of Nitinol

“…The effect of triaxiality has been confirmed through quasi-static tensile testing of notched specimen and plane-strain fatigue crack growth studies [70,71], which showed that the triaxial stress constraint suppresses SIM transformation. However, higher strain-rates tend to produce mixed-mode (ductile void and cleavage) type failure mechanism [44,72]. Recent high-strain-rate fracture toughness studies of Ni-Ti [72] have also shown that the superelastic material exhibits a high amount of ductile void growth ( Fig.…”

“…However, higher strain-rates tend to produce mixed-mode (ductile void and cleavage) type failure mechanism [44,72]. Recent high-strain-rate fracture toughness studies of Ni-Ti [72] have also shown that the superelastic material exhibits a high amount of ductile void growth ( Fig. 12(g)) even though the quasistatic fracture surface exhibits only quasi-cleavage type failure (see Fig.…”

“…Dynamic and quasi-static tensile fracture surfaces of annealed Ti-50.8Ni rod at (a, b) −196 • C; (c, d) 300 • C, (e, f) 20 • C. (g, h) Fracture surfaces under three-point bend loading under dynamic and quasi-static loading conditions[72].…”