Peculiarities of Fatigue Fracture of a Shape Memory Alloy at Different Load Frequencies

The fracture features of shape memory alloy samples (nitinol) under low-cycle fatigue with different load frequencies (0.1; 1 and 10 Hz) were investigated. The frequency dependences on the number of cycles before the samples fracture, the maximum, and the residual deformation before and after their fracture, respectively, were analyzed. The maximum value of the dissipated energy in the load cycle decreases with an increase in frequency from 0.1 to 10 Hz. Fractography showed that the mechanism of samples fatigue failure depends on the loading frequency. At high frequency classical fatigue striations decorated with secondary cracks dominated at the nitinol fracture surface. While at low frequency, a special type of striated relief with alternate changes in the striation spacing and the orientation of shear planes, in which a local crack growth occurred in the loading cycle, were found. The appearance of fatigue striations was explained by phase transformations inherent in nitinol, due to the rearrangement of its crystal lattice at the crack tip with multiple direct and reverse phase transformations (austenite→martensite→austenite), respectively which occur in half-cycles of loading and unloading of samples.