Fatigue resistance of branching phase-transformation fronts in pseudoelastic NiTi polycrystalline strips

Inhomogeneous martensitic phase transformation induced by tensile stress can trigger Lüders-like band formation in pseudoelastic NiTi polycrystalline which has influence on its fatigue behaviors. It is often intuitively conjectured that the phase-transformation front (band front) has lower fatigue resistance than the regions inside or outside the band (i.e., the Martensite or Austenite domains), but no experiment in literature can confirm or disprove this conjecture. In this paper, a systematic fatigue study is performed on NiTi strips with nucleated bands under cyclic “elastic” loading, where the nominal strain amplitude is set as a small value (0.25%) to keep the band front immobile. Meanwhile, a close relation between the front morphology and the strip's fatigue failure is revealed with the help of a “global camera” to achieve the full-field strain distribution (by Digital Image Correlation) and a “local camera” to observe the local front morphology. It is found that the band front branches into many small-scale narrow bands in thin strips while the front has no branching and just becomes diffusive in thick strips, and the fatigue life of the thin strip is shorter than the thick strip. Moreover, for the strips cut along the rolling direction, the fatigue failures of the strips with different thicknesses always occur at the phase-transformation front. The reason is attributed to the stress concentration in the front due to the branching front morphology and geometry necking, which depend on the structure geometry (strip thickness here), and the dissipative microscopic interface motion of Austenite-Martensite phase mixtures.