Abstract:
"A Statistical Approach to Estimating a 95% Confidence Lower Limit for the Design Creep Rupture Time vs. Stress Curve When the Stress Estimate Has an Error Up to 2%"
Recent experimental results on creep-fracture damage with minimum time to failure (minTTF) varying as the 9th power of stress, and a theoretical consequence that the coefficient of variation (CV) of minTTF is necessarily 9 times that of the CV of the stress, created a new engineering requirement that the finite element analysis of pressure vessel and piping systems in power generation and chemical plants be more accurate with an allowable error of no more than 2% when dealing with a leak-before-break assessment. A statistical approach, based on the non-linear least square fitting algorithm and the 4-parameter logistic function, is proposed here to calculate a 95% confidence lower limit for the design curve of time-to-failure (TTF) versus stress as a function of the statistical uncertainty of the data and the measurement uncertainty of stress (up to 2%). Implications of this approach to a better and more robust prediction of TTF for components subject to creep damage are discussed with examples.
