EFFECT OF LOADING TIME ON HIGH-CYCLE RANGE IMPACT FATIGUE-STRENGTH AND IMPACT FATIGUE CRACK-GROWTH RATE

In the previous study, the authors developed a Hopkinson bar-type, high-speed impact fatigue testing machine, and carried out impact fatigue tests on two types of carbon steels. In this study, after some modifications and improvements to the testing machine, a new method was introduced to generate impact stress patterns with various loading times at maximum stress levels, and impact fatigue tests were carried out on S10C (0.1%C) steel by changing the loading time from 470 to 940-mu-s. Fatigue crack growth tests were also carried out on the same material using another Hopkinson bar-type testing machine, with the loading times of 780 and 1050-mu-s. It was confirmed that the impact S-N property in the high-cycle range was dependent on the loading time and there was a relationship of sigma(max)(N(f)T)m = D, where sigma(max) is maximum stress, N(f) the number of cycles to failure, T the loading time, and m and D are material constants. It was also confirmed that the fatigue crack growth rate was affected by the loading time, and NT (the product of the number of stress cycles and the loading time) was an important parameter governing the crack growth rate.