On the evaluation of high temperature creep-fatigue responses of structures

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This thesis details the research and development steps undertaken to improve and enhance the Linear Matching Method Framework (LMMF) for high temperature structural integrity assessment. Firstly, a more accurate interpolation method is developed to calculate the revised yield stress, which is necessary to calculate the creep rupture limit. Convergence issues are also investigated to improve the numerical difficulties often suffered by the "uctuating" revised yield stress. Secondly, the extended Direct Steady Cycle Analysis (eDSCA) is used to solve two complex case studies and a comprehensive theoretical study is undertaken on the effect of combined creep and cyclicplasticity. This subroutine is enhanced by adding many new features, including the possibility to calculate the fatigue damage using only few tensile constants.;Multiple creep dwells within the load cycle can be considered and improvements are made to the cyclic loop construction. In order to make the eDSCA more accurate for creep damage assessment, the stress modifed ductility exhaustion method is implemented combining the advantages of both stress and strain based methods. Thirdly, several case studies on creep-fatigue interaction are discussed thanks to the flexibility of the LMMF including the detrimental effect of creep-ratchetting. Furthermore, a new potentially dangerous mechanism involving the sudden accumulation of plastic strain during the creep dwell is identified and studied by numerical means.

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