Inelastic analysis considering individual material behavior is expected to play a more and more important role in design and fitness-for-service assessment of various pressure-retaining components. Constitutive model is a fundamental element of such an analysis and modeling of stress-strain relations under uniaxial loading constitutes its basis. Some formulae for describing stress-strain relations under monotonically increasing loading have been developed and incorporated in some codes to provide a guidance for elastic-plastic analysis. The author has been trying to find alternative formulae to improve the accuracy and widen the applicability. A simple formula based on the Swift-type equation was derived as a result of systematic analysis of the test data on a number of materials used in nuclear power plants. An alternative expression was also developed in order to circumvent the deficiency observed in ferritic steels. All the constants in these expressions were represented by the functions of temperature, yield stress and tensile strength to make it possible to apply them without further information. Formulae were found to be applicable to various materials such as austenitic stainless steels, high- and medium-strength ferritic steels and some of Nicked based alloys.

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