Abstract
This paper describes that the difference between out-of-phase and in-phase thermal fatigue lives of gray cast iron results from the difference in maximum tensile stress between two phases, which is caused by the unsymmetry in the stress-strain relation of gray cast iron.
Thermal fatigue tests were performed in the temperature range, 150∼500°C, in which the growth of cast iron was negligibly small, under strain controlled conditions. The results obtained in this study are summarized as follows;
(1) The life of in-phase thermal fatigue, in which tensile stress appears at the high temperature side, was about 30 times longer than the life of out-of-phase thermal fatigue, in which compressive stress appears at the high temperature side.
(2) In the hysteresis loop of the first cycle, the maximum tensile stress in in-phase thermal fatigue was smaller than that in out-of-phase thermal fatigue. This fact is considered to result from that the unsymmetry of the stress-strain relation of gray cast iron, in tensile and compressive sides, is emphasized further by the disparity in temperature at which the maximum tensile stress appears.
(3) The ratio of this tensile stress to tensile strength at the maximum temperature in in-phase, was also smaller than the ratio at the minimum temperature in out-of-phase. Therefore, the former life was longer than the latter life. And, after Nf/2 strain cycles, this tendency became more remarkable.
