A new parametric approach, termed the Wilshire equations, offers the realistic potential of being able to accurately life materials operating at in service conditions from accelerated test results lasting no more than 5000 hours. These Wilshire equations contain discontinuities that have in the literature been interpreted either in terms of changing deformation mechanisms or changes in where deformation occurs within a material (i.e., within boundaries or … Continue reading Constraints Imposed by the Wilshire Methodology on Creep Rupture Data and Procedures for Testing the Validity of Such Constraints: Illustration Using 1Cr-1Mo-0.25V Steel

This paper highlights the observation that the Wilshire equations for failure times and times to various strains, as reported in the original literature, may not be the most appropriate ones for all materials—including the one selected in this study. Further, such appropriateness can be determined by looking at the consistencies between the parameter estimates obtained using minimum creep rates in comparison to using failure times. … Continue reading The importance of creep strain in linking together the Wilshire equations for minimum creep rates and times to various strains (including the rupture strain): An illustration using 1Cr-Mo-V rotor steel

Given the strong reliance on long-term datasets containing multiple heats of the same material (i.e., different bathes), there is a pressing need to obtain reliable predictions of creep properties that are representative of all such batches. This paper presents a new estimation framework for the theta methodology which enables a distribution of creep properties to be predicted which should be representative of any batch of … Continue reading Accounting for Batch to Batch Variation when Predicting the Safe Life of Materials Operating at High Temperatures: An Application to 1Cr-1Mo-0.25V Steels Using the Theta Methodology

Recently there has been renewed interest in assessing the predictive accuracy of existing parametric models of creep properties, with the recently develop Wilshire methodology being largely responsible for this revival. Without exception, these studies have used multiple linear regression analysis (MLRA) to estimate the unknown parameters of the models, but such a technique is not suited to data sets where the predictor variables are all … Continue reading A partial least squares solution to the problem of multicollinearity when predicting the high temperature properties of 1Cr–1Mo–0.25V steel using parametric models

The existing length of the materials development cycle is currently far too long. Validation of a new methodology to reduce this time requires evidence of its applicability to a wide range of materials. This study focuses on 18Cr–12Ni–Mo steel because of its importance to future power generation. The methodology is shown to accurately predict the strengths of numerous batches of this material obtained from different … Continue reading Prediction of long-term creep rupture data for 18Cr–12Ni–Mo steel

The creep and creep rupture properties of 18Cr–12Ni–Mo steel tubes have been analysed using the Wilshire equations. The observed behaviour patterns are then briefly discussed in terms of the dislocation processes governing creep strain accumulation. A suitable statistical framework for analysing both the single and multi batch data available on this material is then specified. It is shown that ignoring the hierarchical nature present in … Continue reading Obtaining confidence limits for safe creep life in the presence of multi batch hierarchical databases: An application to 18Cr-12Ni-Mo steel

This paper develops a modification of the shared frailty model that allows for the distribution of the response and the random effects to be determined by the data (from within the exponential family of distributions). A simulated maximum likelihood and a computationally simpler simulated linear least squares approach to estimating this model are also presented. These generalizations are important for modelling high-temperature deformation because they … Continue reading A frailty model for the analysis and prediction of creep and creep fracture: with application to high temperature plant life assessment using 1Cr–1Mo–0.25V