Frequency effect on low-cycle fatigue behavior of pseudoelastic NiTi alloy

This study presents experimental results on the effect of loading frequency during low-cycle fatigue on the fracture mechanism and properties of pseudoelastic NiTi wire. The uniaxial tensile tests were performed in stress-controlled mode over the 0.1-10 Hz frequency range. It was shown that the number of cycles before specimen failure decreased with increasing frequency from 0.1 Hz to 1 Hz but increased at frequencies above 1 Hz. It was fractographically shown that fatigue striations were observed on the fracture surfaces of specimens tested at all loading frequencies. A new type of fatigue striations was described on fracture specimens tested at frequencies up to 1 Hz. They are characterized by different spacing between successive striations with a change in the slope of the fracture at the transitions between them. This feature may be associated with forward and reverse phase transformations during half-periods of loading and unloading of nitinol. Dissipated energy and residual strain of nitinol decrease with increasing frequency. The residual strain and fatigue life of nitinol is frequency dependent. A new type of fatigue striations of various spacing was found at frequencies below 1 Hz. The spacing of striations changes due to alternate crack growth in martensite or austenite.