Ti-Ni계 형상기억 합금의 피로특성에 관한 연구

A Study on the Fatigue Properties of Ti-Ni Shape Memory Alloys

The effects of strain amplitude. test temperature and stress on the fatigue properties for Ti-Ni wires were investigated using a rotary bending fatigue tester specially designed for wires. The fatigue test results were discussed in connection with the static tensile properties. The DSC measurement was conducted after fatigue test in order to clarify the change of transformation behavior due to the progress of fatigue. Under the temperature below or near the Af, the strain amplitude(<TEX>$varepsilon_a$</TEX>)-failure life (Nf) curve showed to be composed of three straight lines with two turning points. Of the 2 turning points, the upper one was coincident with the elastic limit strain and the lower one with the proportional limit strain. With rising of the test temperature above Af, the pattern of <TEX>$varepsilon_a$</TEX>-Nf curve changed gradually to composition of 2 straight lines. The <TEX>$varepsilon_a$</TEX>-Nf curve shifted depending on test temperature. In the short and medium life zones, the higher the temperature was, the shorter the fatigue life. However, in the long life zone, above the Af temperature, the fatigue life was not affected by the temperature. The transformation enthalpy measured after fatigue test was dependent on Nf, <TEX>$varepsilon_a$</TEX>, and test temperature.