, , , Marvin J. Cohn
Uncertainty in modeling the creep rupture life of a full-scale component using experimental data at microscopic (Level 1), specimen (Level 2), and full-size (Level 3) scales, is addressed by applying statistical theory of prediction intervals, and that of tolerance intervals based on the concept of coverage, p . Using a nonlinear least squares fit algorithm and the physical assumption that the one-sided Lower Tolerance Limit ( LTL ), at 95 % confidence level, of the creep rupture life, i.e., the minimum time-to-failure, minTf , of a full-scale component, cannot be negative as the lack or "Failure" of coverage ( Fp ), defined as 1 - p , approaches zero, we develop a new creep rupture life model, where the minimum time-to-failure, minTf , at extremely low "Failure" of coverage, Fp , can be estimated. Since the concept of coverage is closely related to that of an inspection strategy, and if one assumes that the predominent cause of failure of a full-size component is due to the "Failure" of inspection or coverage, it is reasonable to equate the quantity, Fp , to a Failure Probability, FP , thereby leading to a new approach of estimating the frequency of in-service inspection of a full-size component. To illustrate this approach, we include a numerical example using the published creep rupture time data of an API 579-1/ASME FFS-1 Grade 91 steel at 571.1 C (1060 F) (API-STD-530, 2007), and a linear least squares fit to generate the necessary uncertainties for ultimately performing a dynamic risk analysis, where a graphical plot of an estimate of risk with uncertainty vs. a predicted most likely date of a high consequence failure event due to creep rupture becomes available for a risk-informed inspection strategy associated with a nuclear powerplant equipment.
Proc. of ASME Symposium on Elevated Temperature Application of Materials, April 3-5, 2018,
Seattle, WA, U.S.A.
April 3-5, 2018
Aging component, API 579-1/ASME FFS-1 Grade 91 steel, coverage, creep rupture, dynamic risk analysis, failure of coverage, failure probability, fracture mechanics, full-scale component, in-service inspection, least squares fit, lower tolerance limit, mathematical modeling, multi- scale, nonlinear least squares fit, nuclear powerplant, prediction intervals, risk-informed inspection strategy, statistical analysis, tolerance intervals, uncertainty quantification.