Testing Model Structure Through a Unification of Some Modern Parametric Models of Creep: An Application to 316H Stainles
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I.
INTRODUCTION
THE selection of materials for high temperature applications in power plants is typically based on the requirement that creep failure should not occur under the prevailing operating conditions during plant lives of approximately 30 years. Although complex stresses and temperatures are often encountered by materials used in power generation, design decisions are generally made on the basis of allowable tensile creep strength. This strength is commonly taken to be 67 pct of the average stress (up to 1088 K).[1] At present, protracted and expensive test programs lasting 12-15 years are necessary to determine the required long-term strengths and lives. A reduction in this 12-15 years of ‘materials development cycle’ was therefore defined as the No. 1 priority in the 2007 UK Energy Materials—Strategic Research.[2] With the aim of reducing this development cycle, a new group of parametric creep models has been developed in recent years that are characterized through their use of a normalized stress (defined as the ratio of stress to tensile strength, r/rTS) for the determination of safe life. The rationale behind this new group of creep models is that by definition, failure will be instantaneous
M. EVANS is with the College of Engineering, Swansea University, Fabian Way, Crymlyn Burrows, Wales, SA1 8EN, UK. Contact email: [email protected] Manuscript submitted May 23, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
when stress is equal to a material’s tensile strength. Then when the material is subjected to a stress that is an infinitesimally small fraction of its tensile strength, the material should remain intact for a very long period of time. That is, tf must vary from 0 towards ¥ as r/rTS varies from 1 towards 0. Unfortunately, the rate at which this happens is not fully understood, and so this group of models assumes that the relationship between r/rTS and tf (at a fixed temperature) is given by an inverted S-shaped curve.[3,4] The models within the group are then differentiated by the mathematical function used to describe this inverted S-shaped curve, and so consequently these models can end up producing very different safe life predictions. This paper aims to tackle this problem by specifying a generalized model that nests the creep model first put forward by Wilshire and Battenbough[3] and the model proposed by Yang et al.,[4] i.e., these two models are special cases within this more general model. Within such a framework, it is then possible to use some basic statistical tests to allow experimental creep data to determine the correct shape of the inverted S-shaped relationship between r/rTS and tf. Once identified, this shape can be used to obtain safe life predictions that are compatible with experimental data, rather simply using some ad hoc functional form for the S-shaped relationship. To achieve this aim, the paper is structured as follows. The next section gives a brief review of old and new parametric creep models and this is followed by a section deriving the generalized creep model to
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