A method for the transformation of creep constitutive equations

• Published in: The International journal of pressure vessels and piping 1996-10, Vol.68 (3), p.299-309
• Main Authors: Perrin, I.J., Hayhurst, D.R.
• Format: Article
• Language: English
• Subjects: Applied sciences; Creep; Estimation; Exact sciences and technology; Failure analysis; Functions; Mathematical models; Mathematical transformations; Mechanical properties and methods of testing. Rheology. Fracture mechanics. Tribology; Metals. Metallurgy; Steel; Strain rate; Welds
• ISSN: 0308-0161; 1879-3541
• DOI: 10.1016/0308-0161(95)00069-0

A method has been developed that allows the material constants of complex creep constitutive equations to be determined from the knowledge of the constitutive equations of a base material, and a set of normalized material property ratios. These ratios are based on the global features of creep behaviour, such as minimum creep strain rates, lifetimes and failure strains, which may be determined, or estimated, without the need for detailed records of the creep strain which are not available in many circumstances due to the expense, or difficulty, of testing. The normalized material property ratios are formed between the global features of the creep behaviour for a non-base material, for which the material constants are to be determined, and a base material, for which the constitutive equations and their constants are accurately known. It is shown how these normalized material property ratios may be combined into a mathematical function that, when minimized, yields the constitutive constants of the non-base material. The method has been developed for a set of constitutive equations which incorporates three state variables to describe the creep behaviour of a ferritic steel. Two extrapolations of these constitutive equations to different temperatures are presented as a verification of the method. As an example, the method is used to determine the constitutive equations for the Type IV material region of a ferritic steel weldment from a knowledge of the constitutive equations for the parent ferritic steel. Finally discussed are the wider uses and implications of the method.