Sub-Modelling of a Jet Engine Component  and Creation of Stiffness Interval Based on Cast Dimensional Variations

Abstract: While designing jet-engine components information about the loads that the component will be subjected to, is critical. For this, a full system analysis of the engine is often performed with every component put together in a large finite element model, which is called the whole engine model (WEM). This model will mostly be composed of lower order shell elements with a few thousands of elements. At the design level of a component, the FE model is detailed with several hundred thousand higher order solid elements. The detailed model cannot be directly put into the whole engine model due to excessive run times. Therefore there must be a simpler representation of the component -a sub-model- with much fewer elements so that it can be assembled into the whole engine model. This simple model must have the same stiffness (load/displacement) in chosen directions, the same mass and the basic mode shapes and frequencies should also be the same with the detailed model. 4 different structural optimisation schemes were studied to prepare a model: sizing optimisation, optimisation with material properties as design variables, combined sizing and material property optimisation and free-size optimisation. Among these free-size optimisation where each element in the model has its own design variable -thickness- was found to be the most effective method. The stiffness could be matched to the detailed model as close as 5% and so also could the first two fundamental mode shapes and frequencies. Additionally, the initial sub-model prepared was used to do a preliminary study on how variations in casting dimensions would affect the stiffness of the component in a certain direction. This was done by creating a design of experiments (DoE) for the stiffness. A response surface for the stiffness was created in terms of the dimensions that have the most significant effect. This was later used to predict an interval for the stiffness based on variations in the cast dimensions with a confidence level of 99.7%.