(V) Effects of Sample Configuration on High Cycle Fatigue Life of SE Nitinol

Published results show pre-straining a SE Nitinol component can induce compressive residual stresses that significantly increase high-cycle fatigue life. To explore fatigue behaviors under different loading conditions diamond shaped samples were cycled at various R’ ratios (ε_min/ε_max). With sufficient pre-strain and for R’ > 1, data show a positive mean strain dependence of the median high-cycle fatigue limit. This is contrary to the tenet asserting a negative dependence of fatigue life with mean stress but is consistent with previously reported anomalous fatigue behavior of SE Nitinol. For R’ < 1, median high cycle fatigue life exhibits the more traditional negative dependence on mean strain. Fractography supports the effects of compressive (and tensile) residual stresses demonstrating fracture initiation on extrados for R’ > 1 and on intrados for R’ < 1. As further confirmation of the beneficial contribution of residual compressive stresses, flat tensile samples were cut from Nitinol tubing, aged using standard shape setting parameters, electropolished then bent to induce ⁓10% outer fiber strain. Longitudinal surface stresses were calculated from lattice spacing measurements determined from powder diffraction patterns on surfaces of bent samples. Contrary to anticipated compressive stresses, tensile stresses were found on the outer fibers of bent samples. Understanding the etiology and contribution of these complex stress states to the dual nature of the fatigue behavior of this material must include a path-dependent description of the reversible and irreversible material flows subject to constraints imposed during in-vitro testing or in-vivo operation.