EFFECTS OF CYCLIC LOADS ON LBB ASSESSMENT OF HIGH ENERGY PIPING

摘要

The experimental investigations reported in this paper clearly illustrate the need of addressing the cyclic fracture/tearing and number of safe load cycles in the fracture analysis of NPP piping components. Following conclusions have been drawn from the investigation: 1. The load-controlled tests have highlighted the importance of the number of load cycles in the fracture assessment of piping, subjected to a cyclic loading event. The cyclic fracture tests have shown, that the pipes will fail in limited number of load cycles with the load amplitude sufficiently below the monotonic fracture collapse loads. The displacement-controlled cyclic tearing tests have shown, significant reduction in the fracture resistance in comparison to corresponding monotonic. Hence cyclic effects should be accounted for in the LBB qualification of the piping components. 2. Based on experimental results, a Master Curve was developed to quantify the damage (loss in load carrying capacity and loss in fracture resistance) due to cyclic loading as a function of number of load cycles and monotonic fracture load of circumferentially through wall cracked straight pipes made of ductile material such as Low Carbon Steel or Stainless Steel. The curve is independent of crack sizes as well as pipe sizes, since all experiment data lie within a narrow scatter band. This curve gives the load reduction factor as a function of the number of load cycles and is applied on the monotonic critical load. 3. Based on Master Curve "Additional Safety Margins" were evaluated to account forthe cyclic tearing damage and the same has been recommended for LBB qualification of PHT piping of NPPs. It was deciced to use a safety factor of 4/3 for SSE loading and 3/2 for OBE loading over the monotonic critical load, for LLB assessment. For an OBE, the factor is relatively higher in view of its higher frequency of occurrence than a SSE. These additional safety margins would be applied to Indian nuclear power plants, since none of the current international standards or regulatory guides account for these effects. More details on certain aspect of the above investigations can be referred to in Gupta et al [6].