PREDICTION OF RATCHET LIMIT UNDER THERMOMECHANICAL LOADINGS

Abstract eng:
The mechanical components of machineries, like nuclear reactors, aircraft gas turbine propulsion engines and piping systems, usually operate under high levels of cyclic thermo-mechanical loads. As a result of these loading conditions, the material usually exceeds the elastic regime, plastic strains occur and a final state appears, which may be either elastic shakedown (safe state), or plastic shakedown (unsafe state), or ratcheting (unsafe state). This state may be derived either using a time consuming step by step analysis or adopting modern Direct Methods. These methods are capable to estimate the limits of each cyclic state depending on the levels of the variable loads, without tracing the whole loading history. Towards this direction, the Residual Stress Decomposition Method for Shakedown (RSDM-S) has been developed, which can successfully predict the elastic shakedown limit [1]. In the present work, this method will be extended so that it may predict the ratchet limit. The method is formulated using three dimensional brick elements and is incorporated in FEAP [2]. It is assumed that a von Mises elastic perfectly plastic material holds. Finally applications of the approach to structures, subjected to combined cyclic thermo-mechanical loadings, are presented. ACKNOWLEDGEMENT This project has been financed by the State Scholarships Foundation (IKY), through the program “Research Projects for Excellence IKY/SIEMENS”.

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