Study on Fatigue Properties of Shape Memory Alloys

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Fatigue tests have been made in order to investigate the fatigue life, fatigue crack nucleation and fatigue crack propagation in Ti-Ni shape memory alloys. The following results were obtained.(1) The data of fatigue lives can be expressed by two lines, the slope of the line in the shorter life region being steeper than that in the longer life region. The former line expresses the fatigue life under cyclic superelastic deformation, while the latter line that under cyclic elastic deformation.(2) The fatigue life was almost the same irrespective of melting method.(3) Fatigue crack uncleated at TiC inclusions in specimens which were made by high frequency induction melting, while they nucleated along grain boundaries in specimens which were made by electron beam melting.(4) Fatigue crack propagation rate increases with increasing test temperature in a temperature range between Ms and Td; above Td the stress-induced transformation does not occur or the critical stress for inducing martensites is too high to relax the stress concentration at a crack tip effectively.