Modelling of combined high-temperature creep and cyclic plasticity in components using continuum damage mechanics

A computer-based finite-element viscoplastic damage solver has been developed for the analysis of structural components subject to combined cyclic thermal and mechanical loading. The solver, which is based on continuum damage mechanics, is able to predict the combined evolution of creep and cyclic plasticity damage by solution of the combined boundary-initial value problem. The computational difficulties which arise due to the different timescales, associated with material behaviour at the different temperatures within a thermal field, have been overcome by the use of a normalization technique. The high computer demands associated with the detailed numerical solution of combined thermo-mechanical problems, which have prevented their use in design, have been overcome by the development of a novel ‘cycle jumping’ method which avoids repetitive calculations over those cycles in which the damage fields change insignificantly. Between the ‘jumps’ in the solution technique, the damage and the strain rate fields are coupled and hence allow stress redistribution. The finite-element solver has been used to successfully predict the high temperature behaviour of a slag tap component subjected to cyclic thermal loading generated by infrared heaters and water cooling ducts. The initiation of damage and micro-cracking has been found to occur early in the lifetime at approximately 3000 cycles adjacent to the cooling duct; and the propagation of failure zones has been found to stabilize at 60 000 cycles after which no further damage evolution occurs. A further development of the technique, which requires even less computer resource, is the ‘cycle leaping’ method which neglects the effect of stress redistribution over large numbers of cycles, by leaping from the first few cycles to the final state. With this method good predictions have been made of the fields of damaged and micro-cracked material in the final state of the slag tap component. The technique has potential for use at the early or conceptual stages of design.