Some Experiments on the Alternating Stress Fatigue of a Mild Steel and an Aluminium Alloy at Elevated Temperatures

Abstract

Alternating-stress-fatigue tests have been carried out at several speeds of between 10 and 8,000 cycles per min. on a normalized 0.17 per cent carbon steel at air temperature, 400, 450, and 500 deg. C. and on the aluminium alloy R.R. 58, in a fully softened condition, at 200 deg. C. It has been shown that for the mild steel the fatigue strength at air temperature depends mainly on the number of cycles of stress irrespective of the cyclic speed, but at 400–500 deg. C. failure at a given stress range occurs after a certain time which is approximately independent of the speed. For the aluminium alloy at 200 deg. C. there is evidence of a small speed effect, but fatigue failure depends mainly on the number of stress cycles applied and not on the time of the application. In addition, a study has been made of the dynamic stress-strain relations, and these are shown to be useful as a guide to the variations of fatigue strength with temperature and speed. They also provide a means for the determination of the stress redistribution when plastic strain occurs in bending fatigue, and it is shown how this, together with the effect of size of test piece may account for the difference between bending and direct-stress fatigue strengths. A description is given of the special apparatus designed for the work.