Obtaining confidence limits for safe creep life in the presence of multi batch hierarchical databases: An application to 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 many creep data bases, which has been the approach used until now when using the Wilshire equations, leads to a serious and significant underestimate of the predicted safe life for this material. The model allows accurate predictions, with associated levels of confidence, of long-term properties by extrapolation of short-term test results for this steel.