Correlation between reliability and safety factor based methods in characterizing uncertainty of creep rupture properties

Abstract

Reliability and safety factor based methods are usually employed to characterize uncertainty of creep rupture properties, but the correlation between these two methods is rarely reported. In this work, a framework of correlation between reliability and safety factor based methods was proposed, and application of this framework in representation of minimum stress to creep rupture was reported. Effect of standard deviation on correlation results was discussed and comparisons of minimum stress to creep rupture in codes and standards were conducted. Results indicated that for a given reliability, the safety factor included in the minimum stress to creep rupture is higher at a small stress level and a high temperature. Accordingly, the minimum stress to creep rupture by reliability based method in ASME code (i.e. 95 %) is lower than that by safety factor based method in RCC-MRx code (i.e. 1.25), presenting a smaller allowable stress value and more conservative evaluation. Contrarily, a smaller safety factor is gained at a high stress level and a low temperature for a given reliability. The opposite conclusion is drawn when the reliability at a given safety factor is analyzed. In addition, the safety factor of 1.5 is larger than that at the reliability of 99 % for a wide range of stress levels, inducing an over-conservative design and excessive manufacturing costs.