Lüders-like band front motion and fatigue life of pseudoelastic polycrystalline NiTi shape memory alloy

We conducted tensile fatigue tests on polycrystalline NiTi strips with in-situ optical observation on the evolution of the localized deformation (Lüders-like band) in order to trace the deformation history of the fatigue-failure material point in the strips under various loadings. It is found that the band front motion triggering localized cyclic phase transformation leads to a short fatigue life while the "elastic" deformation (without the band front motion) results in a much longer life whose failure crack nucleates away from the immobile band front. The results provide hints to derive a material fatigue criterion from the structure fatigue tests.