Fatigue properties of a TiNi shape-memory alloy wire subjected to bending with various strain ratios

A fatigue-test machine for alternating bending of a wire under strain-controlled conditions was developed. Bending-fatigue tests on a TiNi shape-memory alloy wire were then performed for various strain ratios. The results obtained can be summarized as: (1) the fatigue life curves under alternating bending and pulsating bending, as expressed by the relationship between maximum strain and the number of cycles to failure, systematically follow the order of strain ratio; (2) the larger the strain ratio, the longer the fatigue life; (3) the fatigue life under rotating bending is shorter than that under alternating bending; (4) the increase in temperature during cyclic bending becomes larger in the order: rotating bending, alternating bending, pulsating bending. The fatigue life decreases in proportion to the increase in temperature; and (5) the fatigue limit of strain for alternating bending, pulsating bending and rotating bending is in the region of R-phase transformation.