Volume 27, Issue 3 (May 1999)
Precision in the Extrapolation of Creep Rupture Data
To be able to determine the accuracy in the extrapolation of creep rupture data, expressions for estimating the error are presented. One expression is based on the statistical uncertainty in the least square fit to the data. This estimate can be considered as the minimum possible error that is present in extrapolated values. Expressions for the upper limit of the error based on linear as well as higher-order polynomials are derived. It is demonstrated that the expression based on linear extrapolation has a general applicability and covers most cases of technical interest. Comparison to previously performed extrapolation analyses shows that the actual variation between extrapolation methods lies in general between the least square error and the upper limit estimate. It is proposed that design values derived from extrapolated creep rupture data should be reduced, with an additional data factor if the estimated error in the extrapolation is larger than what is typical in the materials codes and standards. Values for these data factors are given.