Investigation on fatigue behaviors of NiTi polycrystalline strips under stress-controlled tension via in-situ macro-band observation

One of the challenging issues in studying the fatigue behaviors of NiTi shape memory alloy is the interaction between the material damage and material's martensitic phase transformation which takes place in a macroscopic homogeneous mode or a heterogeneous mode (forming macro-domains like Lüders bands). In this paper, we conducted systematic experiments on NiTi polycrystalline strips under stress-controlled cyclic tensile loadings with in-situ optical observation on the Lüders-band evolution. It is found that the applied stress level and frequency influence the band formation changing the material's fatigue behaviors significantly. The band evolution divides a specimen into "non-active zones" (with the "elastic" deformation of Austenite or/and Martensite) and "active zones" (with the cyclic phase transformation) where fatigue failure occurs. Based on the local strain measurement by Digital Image Correlation, the local residual-strain (damage) accumulation rate is determined and found to have a good correlation with the material's fatigue life. Our results would be helpful for further understanding and modeling the material's stress- and frequency-dependent fatigue behaviors.