| This dissertation presents a detailed study on the fatigue-crack propagation behavior in 50Ni-50Ti (at. %), and the effect of a stress-induced martensitic transformation on the material's crack-growth resistance. Furthermore, the effect of environment on crack-growth rates was also investigated.; Specifically, fatigue-crack growth was characterized in stable austenite (120°C), superelastic austenite (37°C), and martensite (−65°C and −196°C). In general, as the temperature decreased the fatigue-crack growth resistance increased, where the fatigue threshold was greater and crack-growth rates slower in martensite compared to stable austenite and superelastic austenite. It was found that plane-strain superelastic austenite fatigue specimens did not transform to martensite near the crack tip. The stress-induced transformation was prevented by the existence of the tensile hydrostatic stress-state near the crack tip in fatigue specimens. Plane stress samples, however, did undergo the stress-induced martensitic transformation, as the tensile hydrostatic stress-state was reduced in thinner samples.; Fatigue-crack growth rates in Nitinol at a frequency of 10 Hz were found to be essentially identical in air, aerated deionized water, and aerated Hank's solution (a simulated physiological environment), suggesting that at this frequency, any environmentally-assisted contributions to crack growth are minimal. Specifically, the threshold for the onset of fatigue-crack growth was equal to ∼2 MPa√m for all three environments. Furthermore, the slopes of the mid-growth regime were also similar and equal to ∼3; the maximum applied stress-intensity range at instability prior to failure was 30 MPa√m for all three environments. However, when compared to other biomedical implant alloys, the fatigue-crack growth resistance of Nitinol was the lowest. Specifically, the fatigue threshold, at a fixed load ratio value of ∼0.1, was significantly less (by a factor between 2 and 5) than 316L stainless steel, pure Ti, Ti-6Al-4V, and a CoCr Haynes 25 alloy. Also, the crack-growth rates for any applied stress-intensity range were fastest in NiTi. |
