Fatigue Crack Growth Behavior of TiNi Alloy with Shape Memory Effect
Details
Tension test, S-N fatigue test and fatigue crack growth test for Ti-55.3mass%Ni alloy with shape memory effect were conducted at room temperature. As the result of tension tests, Young's modulus E_A in the austenite phase is 28.5 GPa, E_M in the martensite phase is 13.4 GPa, the transformation stress σ_r is 69.2 MPa and breaking strain δ is 18.9%. S-N diagram obtained by the S-N fatigue tests was divided into three regions; Region I, II and III. In Region I where the applied stress was above 200MPa, the fatigue life was the low cycles less than 6.0×10^4 cycles. Then, main crystal structure was martensite phase. In Region II from the fatigue limit(about 153MPa) to 200MPa, fatigue life was varied by the rate of austenite and martensite phase. Region III is bellow the fatigue limit. In this region, the austenite phase mainly occupies. Fatigue crack growth test was conducted under three kinds of maximum stresses, 80, 90 and 110MPa with the constant stress ratio. For each maximum stress, the linear relation was obtained when the crack growth rate daldN was plotted against effective intensity factor ΔK_<eff> on double logarithmic plot. After the crack initiation, crack growth rate was constant or decreases. After the fatigue tests, fractgraphy was conducted by SEM. The striations were observed in fracture surfaces of fatigue crack growth test. There space was equal to the crack growth rate.
