A visual perspective of creep rupture extrapolation

Methods of extrapolating creep rupture data to meet the needs of the engineering industry have evolved from subjective visual-graphic projections to sophisticated procedures based upon the statistical analysis of mathematical models. But the more complex procedures now available can involve difficulties in the selection or creation of satisfactory mathematical models, and the reliability of model extrapolation can therefore remain uncertain. This paper demonstrates an alternative approach that both resolves those problems and is considerably simpler to apply. Mathematical modelling is confined to the description of temperature dependence only and isothermal stress dependence is represented, with unrestricted flexibility, in numerical form. The application of this approach is described with reference to data for three alloy steels and a nickel alloy. It is argued that the exercise of visual judgement by an analyst, coupled with simple criteria of acceptability, has advantages over methodologies based on the statistical ranking of mathematical equations. The paper discusses the rationale of extrapolation, and how that leads to the logical definition of a limit to extrapolation validity.