The role of miniature specimen creep tests in power plant life management

The current Thesis describes for the first time based on extensive industrial data how small specimen creep testing techniques can be applied within a practical and deployable life assessment framework and in conjunction with other assessment techniques. The current state of the art for small specimen creep testing is critically reviewed; a review of traditional techniques used on site for the metallurgical assessment of material condition is also included, with examples from site investigations and assessment campaigns in both conventional and nuclear plant applications. The work describes how small specimen creep testing methods and other complementary tools can be used in a new and structured approach to life management. This specifically refers to the potential to develop and implement novel life assessment models that take advantage of the significant amount of site data currently routinely acquired during plant outage overhauls. A novel predictive lifing model for the use of hardness data is developed. In fact, a novel, phenomenological, relationship between room temperature hardness and creep data, obtained by uniaxial creep and impression creep tests, has been found and used for an innovative lifing approach that includes hardness data in a modified Liu and Murakami creep damage model. The latter is discussed with a description of how it could be practically implemented and validated in-service. The capability of impression creep testing method in determining the minimum creep strain rate data by use of conversion relationships that relates uniaxial creep test data and impression creep test data is demonstrated. Consequences of possible geometry inaccuracies in the position of the indenter were investigated and some general comments on the conversion relationships are also provided. The creep damage evolution of an ex-service CrMoV pipe section is investigated in order to demonstrate how normally acquired industry data and data obtained by small specimen creep tests could be used in a real situation. The study emphasises the importance of correlating the operating conditions (temperature and stress) of power plant components with the results from metallurgical examinations and small specimen creep tests. The current research also reports a novel investigation of the applicability of Chakrabarty’s theory, for membrane stretching of a circular blank over a rigid punch, to small punch creep test and determines new ranges of applicability of the CEN Code of Practice CWA 15627