On relating the reference stress, limit load and the ASME stress classification concepts

Abstract

This paper discusses the relationship between the concepts of reference stress, limit load and the ASME stress classification framework, as it applies to common mechanical components and structures. The relationship is based on the underlying notion of load-control. The reference stress method, which has been used mainly in the UK, attempts to correlate creep deformations in a structure with the results of a uniaxial creep test. It has been observed that the reference stress is relatively insensitive to material parameters characterizing creep behaviour. The method has been widely used in the design and life assessment of nuclear as well as conventional components. Specifically, it has been applied to problems pertaining to creep growth, rupture damage, creep buckling, and more recently, elastic-plastic fracture toughness. Determination of the reference stress is not always a simple task. An approximate method of its determination relies on prior knowledge of limit loads for various configurations and loadings. The drawback, of course, is that there is a reliance on the available catalogue of limit loads. Rather than attempting to ‘fit someone into a garment’, this paper discusses a new and a direct procedure for reference stress determination using the GLOSS R-Node method. The reference stress is determined for some pressure component configurations. The ideas are extended to the ASME stress classification framework, and the various stress categories are discussed.