FATIGUE USAGE LIFE AND GRADIENT FACTORS FOR ASME CLASS 1 PIPING FATIGUE ANALYSES

摘要

This paper examines conservatisms associated with two aspects of piping fatigue life as applied in NB 3600 fatigue calculations. First, piping component allowable cyclic life is defined by stress-life (S-N) curves based on constant displacement push-pull test results of small diameter (≤ 0.375 inch (≤ 9.5 mm)) smooth test specimens in air at room temperature. These life estimates are subsequently applied to all Class 1 piping components regardless of their actual size or thickness. Secondly, all component cyclic stresses are treated as uniform through-thickness membrane stresses when most transients induce stresses that vary across the thickness of the pipe. The application of fatigue usage correction factors that account for: 1) increased cyclic life associated with the growth of engineering size fatigue cracks in thicker components and 2) the presence of actual through-thickness stress gradients is considered. An example case study indicates that, depending on the nature of the applied cyclic loading and pipe section thickness, considerable reductions in design fatigue usage can be obtained.