Allowable Stress Criteria Based on the Onset of Third Stage Creep: An Historical Perspective

Abstract

This paper documents the basis for the current time dependent allowable stress criterion based on onset of third stage creep. The criterion is one of three that are used to develop allowable stresses for elevated temperature, Class 1, nuclear components*. The recommendation to consider third stage creep is part of a 1970 position paper prepared by one of the authors for the Task Force on Material Behavior of the Subgroup on Elevated Temperature Design. The position paper recommended that allowable stress/strain limits be set to preclude operation in third stage creep. This was based on the difficulty (at the time) of predicting the effects of multiaxial stresses on creep behavior and the possibility of through-wall cracks appearing before rupture. The principal references supporting this recommendation reported experimental results in which pressurized tubes failed almost exclusively before the onset of third stage creep. Also included are the reported results from a 1993 long term creep and rupture test of a nozzle-to-sphere model in which failure was also by small pinhole leaks prior to observed onset of third stage creep. In addition to excerpts from the above references, commentary from a current perspective is also provided.