A probabilistic fatigue assessment diagram to get a guaranteed lifetime with a low probability of failure

Abstract Structural integrity assessments typically aim to calculate the integrity of a component under nominal or best estimate conditions. To account for potential variability and uncertainty present in the system, safety factors are often applied to assessment inputs and outputs. This approach does not allow the level of conservatism present to be quantified, often leading to over-conservatism or inadvertent non-conservatism. Probabilistic assessments explicitly calculate the probability of failure based on distributions of the input parameters and hence quantify the margin present in the assessment, leading to a greater understanding of the system. In this study a creep-fatigue damage assessment of a transiently loaded piping component is used as a vehicle to investigate some of the challenges and benefits of probabilistic assessments. A probabilistic assessment of the component life is compared to a lower-bound deterministic calculation to identify the mismatch in margin between the two results. The potential inaccuracies introduced when reducing the computational burden of Monte Carlo simulations with response surface methodologies are explored and tested. Finally, two challenges when attempting to underwrite a very low probability of failure are tackled: the inference of the shape of a distribution’s tails from limited experimental data and the uncertainty of extreme percentiles of finite Monte Carlo samples.

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Benchmarking Probabilistic Code for LBB Analysis for Circumferential Cracks

Volume 1B: Codes and Standards ◽

10.1115/pvp2017-65384 ◽

2017 ◽

Author(s):

Robert E. Kurth ◽

Cédric J. Sallaberry ◽

Bruce A. Young ◽

Paul Scott ◽

Frederick W. Brust ◽

...

Keyword(s):

Probability Of Failure ◽

Primary System ◽

Degradation Mechanisms ◽

Detection Systems ◽

System Pressure ◽

Piping Systems ◽

Primary Water ◽

Low Probability ◽

Assessment Procedures ◽

Leak Before Break

NRC Standard Review Plan (SRP) 3.6.3 describes Leak-Before-Break (LBB) assessment procedures that can be used to assess compliance with the 10CFR50 Appendix A, GDC-4 requirement that primary system pressure piping exhibit an extremely low probability of rupture. SRP 3.6.3 does not allow for assessment of piping systems with active degradation mechanisms, such as Primary Water Stress Corrosion Cracking (PWSCC) which is currently occurring in systems that have been granted LBB approvals. There are several codes available for addressing the requirements of GDC-4. This paper addresses three of these codes: (1) xLPR 2.0; (2) PROLOCA; and (3) PROMETHEUS. Each of these codes is described and applied to a representative plant where active degradation mechanisms have been found. Conclusions about the design, results, and interpretation of the results is then provided. In all cases the probability of failure of the pipe is found to be extremely low when the crack inspections and leak detection systems are modeled.

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Determination of fatigue life for low probability of failure for different stress levels using 3-parameter Weibull distribution

International Journal of Fatigue ◽

10.1016/j.ijfatigue.2020.106080 ◽

2021 ◽

Vol 145 ◽

pp. 106080

Author(s):

Przemysław Strzelecki

Keyword(s):

Fatigue Life ◽

Weibull Distribution ◽

Probability Of Failure ◽

Stress Levels ◽

Low Probability

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Reliability Analysis in the Presence of Aleatory and Epistemic Uncertainties, Application to the Prediction of a Launch Vehicle Fallout Zone

Journal of Mechanical Design ◽

10.1115/1.4034106 ◽

2016 ◽

Vol 138 (11) ◽

Cited By ~ 9

Author(s):

Loïc Brevault ◽

Sylvain Lacaze ◽

Mathieu Balesdent ◽

Samy Missoum

Keyword(s):

Computational Cost ◽

Rare Event ◽

Launch Vehicle ◽

Probability Of Failure ◽

Subset Simulation ◽

Sequential Approach ◽

Variance Estimate ◽

Low Probability ◽

Reliability Assessments ◽

Refinement Strategy

The design of complex systems often requires reliability assessments involving a large number of uncertainties and low probability of failure estimations (in the order of 10−4). Estimating such rare event probabilities with crude Monte Carlo (CMC) is computationally intractable. Specific numerical methods to reduce the computational cost and the variance estimate have been developed such as importance sampling or subset simulation. However, these methods assume that the uncertainties are defined within the probability formalism. Regarding epistemic uncertainties, the interval formalism is particularly adapted when only their definition domain is known. In this paper, a method is derived to assess the reliability of a system with uncertainties described by both probability and interval frameworks. It allows one to determine the bounds of the failure probability and involves a sequential approach using subset simulation, kriging, and an optimization process. To reduce the simulation cost, a refinement strategy of the surrogate model is proposed taking into account the presence of both aleatory and epistemic uncertainties. The method is compared to existing approaches on an analytical example as well as on a launch vehicle fallout zone estimation problem.

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