A Code of Practice for the determination of cyclic stress-strain data

Abstract

There are no procedural standards for the determination of stress-strain properties where a reversal of stress is involved. The purpose of this Code of Practice is to detail the requirements for cyclic stress-strain (CSS) testing on uniaxial testpieces. CSS testing may entail the use of a single testpiece to produce data over several strain ranges. Alternatively, data from a number of constant strain range tests may be obtained, for example as the by-product of a series of low cycle fatigue (LCF) endurance tests. Procedures for LCF testing are covered by a number of existing Codes of Practice and Standards [1–6], and this document does not recommend any alteration to these. This Code of Practice has been prepared by the CSS Working Party of the ESIS TC11 High Temperature Mechanical Testing Committee.Historically, CSS results have been reported in terms of a relatively simple power law. However, engineers involved in design and assessment activities are now increasingly tending to use more advanced constitutive relations such as the Chaboche equations [7]. Hence, the model equations available to characterise CSS behaviour cover a range of complexities, with the approach selected being determined by the requirements of the end-user application. These will be influenced by such factors as the type and history of loading, the operating temperature and presence of thermal gradients, the variation of cyclic plastic strain within the component, and the need to determine absolute magnitudes or ranges of stress and strain. The laboratory test procedures defined in this Code of Practice are capable of generating the CSS data required for the full spectrum of model equations currently used in engineering assessment.In addition to recommending best laboratory practice, this document includes sections on engineering requirements, test data analysis (including the connection between alternative forms of model equation), and the exploitation of existing data. Advice is also given for those circumstances where testpiece material is limited, thus requiring quick methods of data acquisition using block loading techniques. In all cases, the use of cylindrical testpiece gauge lengths is recommended, and only isothermal testing at appropriate temperatures under strain-controlled conditions is covered.