The present study was motivated by a need to model and understand creep in an ageing power plant particularly when residual stresses are present. A number of methods of generating residual stress in laboratory specimens are presented. Both numerical and experimental studies were conducted. The finite element (FE) analysis used in the numerical study was validated with an extensive experimental study of residual stress field characterisation in the test specimens. Two novel means of generating highly triaxial residual stress field in the laboratory specimens were identified and were followed up with design and manufacture. The specimens with internal residual tensile stresses of high triaxiality were used in the subsequent creep study. Using specimens containing residual tensile stress field with variable triaxiality and plastic strain the influence of prior straining in the creep cavitation initiation was also highlighted. A mechanical strain relief deep-hole (DH) drilling residual stress measurement technique used in residual stress measurement of specimens with high triaxiality highlighted a potential difficulty in using a simple elastic approach to data reduction in the DH measurement method. A comprehensive FE study modelling the DH using an elasto-plastic analysis and an incremental trepanning process in contrast to a previous elastic analysis with a single-step trepanning process [George 2000] was carried out to study further the influence of DH process on the residual stress field. The reconstructed residual stress distribution obtained in the DH FE analysis considering the influence of the electro-discharge machining (EDM) trepanning showed a good correlation with the DH measured residual stress distribution in the components with residual stress field with high triaxiality. A further DH FE study of the repair welded pipe three material models were considered and the result was found to be highly sensitive to the material model selected. The DH FE study of repair weld revealed that the original residual stress distribution was generated even considering the cutting process of the DH method. Consequently the deep hole drilling technique was used to characterise the residual stress field in a number of selected locations in practical components including thick-section steel welds consisting of a welded nozzle component which was thermally aged at 550�C for 19,644 hours and a pipe with repair weld.
|Date of Award||2005|
|Supervisor||David Smith (Supervisor)|