Stress Indices Evaluation of Piping Ratchet Fatigue From Extreme Dynamic Loading

Piping systems are subjected to a wide range of anticipated and/or postulated dynamic loading. With the exception of steam propelled water slug flow transient, thee dynamic loading result in response reversals that have not the characteristics of sustained loading. Dynamic test to failure did not exhibit the classical collapse catastrophic failure mode. The dynamic response reversals exhibited gradual ratcheting and fatigue crack growth that concurrently lead to the failure. The stress intensity classifications provided by the piping stress indices, in conjunction with Edmunds and Beer or Bree Plate ratchet models, are utilized to estimate the accumulated ratchet strains in the pipe component. The allowed maximum accumulating local ratchet strain is set arbitrarily to 10%, which is less than half of the measured ratchet strains found in the dynamic tests. Simple criteria for evaluating the degradation level in the piping components subjected to the concurrent ratchet-fatigue failure mechanisms are presented. The approach utilizes a methodology similar to that used in the creep-fatigue interaction damage provided in the ASME Section III, Subsection NH, Appendix-T. Replacing the creep strain term by that of the ratchet strain, and replacing the bilinear damage function with the square root function, yields the ratchet-fatigue degradation criteria formulations presented in this paper. Considerations for the plastic buckling due to the accumulated compressive ratchet strains as well as the primary component of the piping thermal expansion stress due to elastic follow-up are also presented.