An Investigation of Thermal-Stress Fatigue as Related to High-Temperature Piping Flexibility

Abstract

Abstract This paper is a summary report of a co-operative research investigation extending earlier work on the thermal-stress-fatigue resistance of AISI Type 347 stainless steel. The investigation was to provide additional qualitative experimental support of the concept of fatigue design based on an allowable stress range which is embodied in the ASA-B31.1 Code Rules for Piping Flexibility Design. Two specific objectives were established, together with the findings of this research: (a) Investigation of the effect of cycling which produces tensile rather than compressive stress under the hot condition. It was found that there was no significant difference. (b) Investigation of the effect of reducing the net mechanical strain range for a given temperature range to a point approaching the design strains for a piping system. While the temperature range was held constant, the trend of reducing the net mechanical strain range was to increase the number of cycles to failure similar to that obtained when the net mechanical strain range was reduced by lowering the temperature range under full constraint. In addition, for the same mechanical strain range, widening the temperature range decreased the number of cycles to failure.