Influence of Strain Ratio and Environment on Bending Fatigue Life and Form of Fracture Surface in TiNi Shape-Memory Alloy Wire

The fatigue properties of a TiNi shape-memory alloy wire were investigated by both a rotating-bending test and a pulsating-bending test. The influence of strain amplitude, strain ratio, frequency and environment upon fatigue life was discussed. The results are summarized as follows. (1) The fatigue curves in the rotating bending and the pulsating bending intersect. In the region of low-cycle fatigue, the slope of the fatigue curve and the number of cycles to failure in rotating-bending are smaller than those in pulsating-bending. (2) The fatigue crack initiates at a certain part on the surface of a wire and grows with the number of cycles, resulting in final rupture. The larger the strain amplitude, the smaller the ratio of a fatigue fracture surface and the higher the average fatigue-crack growth rate. (3) In the pulsating-bending test, the smaller the strain ratio, the shorter the fatigue life. The smaller the strain ratio, the larger the ratio of a fatigue fracture surface and the higher the average fatigue-crack growth rate. (4) In the rotating-bending test, the fatigue life in water and in silicone oil is longer than that in the air.