Abstract
The bending fatigue strength of Ti-6Al-4V alloy was investigated in the temperature range of 20°C to 450°C. A small artificial defect was introduced in the form of a small hole drilled in the surface of an unnotched specimen. The diameter of the hole was about 0.1mm and its depth was virtually the same as the diameter. The fatigue strength σF of the unnotched specimen was compared with the σFN of specimens with a small hole. The following results were obtained.(1) The fatigue strength is remarkably decreased even by such a small hole. The decrements of the fatigue strength caused by the hole, (σF-σFN)/σF are about 40%, 32%, 27% and 47% at 20°C, 250°C, 350°C and 450°C, respectively.The values of σFN predicted by the Murakami's √area parameter model are about 3%-17% smaller than the experimental results. The prediction seems to give a conservative estimation.(2) In the temperature range of 350°C to 450°C, both σF and σFN are increased by solution hardening, dynamic strain aging and so on. Since the static yield strength is hardly influenced by such a strengthening factor, the temperature dependence of the yield strength may be used as the reference for evaluating the increments in the fatigue strength. It is concluded that the maximum increments of σF and σFN come to about 60%.
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