Probabilistic Critical Initial Flaw Size Analysis in the Circumferential Welds of Layered Pressure Vessels

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Abstract NASA operates as many as 300 non-code layered pressure vessels (LPVs) across their research and space flight centers. Many of these mission critical assets are operating beyond their expected service life, but replacement of these LPVs is cost prohibitive. Thus, NASA is interested in assessing fitness-for-service (FFS) and extending the life of these vessels when it is safe to do so. Since FFS of non-code LPVs comes with distinctive challenges, NASA has developed an alternate standard for FFS. Structural analysis under the alternative standard includes logic for performing probabilistic fracture mechanics assessments to estimate the cumulative distribution function (CDF) of critical flaw sizes for known and postulated flaws in key regions of the vessel. The resulting CDFs are predictive distributions that represent the current state of knowledge about the critical flaw size captured by models and data. This paper describes the probabilistic critical initial flaw size method employed by the alternative standard, and demonstrates its use to analyze a surface crack in the circumferential weld of a 9-layer vessel. It also discusses unique aspects of the probabilistic results and presents an approach for interpreting the probabilistic results in light of the deterministic critical initial flaw size solution.