銅のクリープ疲労におけるひずみ波形効果と微視損傷

Abstract

Low-cycle fatigue tests were carried out at various temperatures on copper specimens, in order to examine the effect of strain wave shapes on fatigue life and micro damage mode. The fracture surfaces and the internal damage modes of the failed specimens were observed by means of a scanning electron microscope as well as an optical microscope. Results obtained in this study were as follows:(1) The temperature dependence of failure life in fast-fast (\dotε=10−3 s−1) wave shape had good correlation with the hardness and the 0.2% proof stress of the material.(2) The failure lives in the slow-fast and the tensile hold wave shapes were decreased from those in the slow-slow and the fast-slow wave shapes above 473 K. In the tensile hold wave, the failure lives were almost stable above 573 K. The failure lives in the fast-slow wave shape was longer than those in the fast-fast wave above 673 K.(3) At 673 K, detailed metallography revealed that life reduction was due to the accumulation of creep damage during the slow tensile straining. A lot of internal wedge type grain boundary cavities were observed in the slow-fast and the tensile hold wave shapes. In the fast-slow and the compression hold wave shapes, round type cavities were observed. The slow compressive straining recovered most of the damage. Those internal damage modes corresponded to the fatigue lives, that is, the fast-slow wave fatigue life was 20 times as great as the slow-fast strain wave.(4) In the slow-fast wave shape, the failure life was dominated by crack initiation life, and possible mechanism of creep dominated fatigue failure was discussed.