On the contribution of buried flaws to piping failure probabilities

Probabilistic fracture mechanics models conservatively assume that all cracks are surface breaking flaws at the pipe inner diameter. In contrast, data on welding defects indicate that only a small fraction of flaws are inner surface breaking versus the majority of flaws which are buried within the wall of the pipe. This paper describes a probabilistic fracture mechanics model that simulates fatigue crack growth of randomly located buried flaws. The fracture mechanics model focuses on the growth of flaws which are initially buried but are located near to the inner or outer surfaces. These flaws grow to become surface flaws, and their growth rates become enhanced by environmental effects of the fluid within the pipe. Calculations are performed for a range of pipe sizes, cyclic stress levels, and piping materials to compare failure probabilities for buried versus surface flaws. These results provide a basis to estimate the fraction of buried flaws which can be treated as equivalent to surface flaws within the framework of simplified fracture mechanics models. The results of the calculations showed that conservative fracture mechanics models can significantly overestimate failure probabilities.