A Hybrid Procedure for Ratchet Boundary Prediction

Methods exist in today’s published literature which establish proximity to the ratchet boundary of a given load set by decomposing a cyclic load history into constant and cyclic components. Such methods operate by calculating the utilisation of yield capacity throughout the structure in response to the cyclic load. The remaining yield capacity is then available to support the constant load. In this paper, a hybrid procedure is described which uses established finite element techniques to obtain a stable response to the cyclic load component, followed by a limit load analysis based on the remaining yield capacity, to calculate the maximum primary load. This approach is particularly useful in conjunction with Fourier based cyclic procedures which, although capable of predicting the existence of a stable cyclic response, are not based on classical shakedown theory and are therefore unable to predict proximity to ratchet, unless a search procedure is used. The hybrid approach provides the combined benefit of an efficient cyclic response calculation scheme with a measure of proximity to the ratchet boundary. In this paper, the hybrid method is applied to the Bree case before application to a more complex thermo-mechanical transient, typical of nuclear power plant loading. The generation of interaction diagrams for both cases is considered.