Development of continuum damage mechanics models to predict the creep deformation and failure of high temperature structures
The use of classical creep continuum damage mechanics, constitutive and damage equations is restricted, to model certain types of creep deformation and fracture mechanisms, under isothermal conditions; and, to extend their predictive capabilities for a wider range of problems they have to be modified. The constitutive and damage equations are modified to represent the bi-linear, log. stress vs. log. rupture, and the log, stress vs. minimum strain rate, characteristics of materials; so that the change in material behaviour, as a mechanism change occurs, is represented in the constitutive model, by a change in the slope of these characteristic lines. Uni-axial creep tests of as-cast (OFHC) Copper have been performed at 150°C, 250°C and 500°C; and, an anisothermal constitutive model has been developed for the temperature range 150°C to 500°C, which highlights how the constitutive equations may be modified, to model creep behaviour under varying temperature conditions. The model predictions are in good agreement with the test results. A compact tension specimen has been studied, which has shown the importance of modelling the effects on rupture, of the high tri-axial stress-state present at the crack-tip, which accelerates void growth. Modified constitutive equations, have been used to model the mechanism of constrained cavity growth, and has enabled improved damage distribution and. lifetime predictions to be obtained for the compact tension specimen, similar to those expected from experimental tests. Non-local damage techniques are developed to model the effects of grain size characteristic dimension, on the failure of large and small cracked tension specimens. Non-local damage techniques are shown to be necessary to give accurate, physically related, finite element solutions. Suitably modified constitutive and damage rate equations are used to model the high temperature failure of a circumferential weld, in a thick steam-pipe, operating at a constant temperature and pressure. The models developed predict, the growth of damage in certain microstructural regions of the weld, and the lifetime of the component; which are observed to be in close agreement with the results from a fullsize pressure vessel tests. It will be shown that it is essential to use creep constitutive and damage equations in computer models, which accurately represent the underlying physics of IX the predominant creep mechanisms present. The implications of the research work on future computer modelling and on design are discussed.