Effects of Lüders-like bands on NiTi fatigue behaviors

Although the formation of Lüders bands in NiTi polycrystalline structures (strips/tubes) has been well reported and is believed to have considerable influence on the material's fatigue behaviors, there is no direct experimental evidence showing the relation between the band formation and the material's fatigue failure. For each NiTi polycrystalline strip in this study, a strain-controlled isothermal test was firstly conducted to check the pseudoelastic properties and then a stress-controlled fatigue test was performed with synchronized optical observation on the evolution (cyclic nucleation/annihilation) of Lüders bands in the strip. It is found that the fatigue failure crack always occurs within the band or at the band front, because the band formation is an indicator of the weak zones of the samples and the cyclic band nucleation/annihilation accelerates microstructure degradation in those zones. Under the same loading scheme of the fatigue tests, the samples with bands have much shorter fatigue life than the samples without bands. The experiments provide the bases for future theoretical study on the interaction between the martensitic phase transformation and the plasticity/defects and give some hints for developing a proper fatigue criterion for the softening material in engineering applications.