The Design of Advanced Composite Structures Using Evolutionary Design Methods

Abstract

The development of an evolutionary optimisation method and its application to the design of an advanced composite structure is discussed. Composite materials are increasingly being used in various fields, and so optimisation of such structures would be advantageous. From among the various methods available, one particular method, known as Evolutionary Structural Optimisation (ESO), is shown here. ESO is an empirical method, based on the concept of removing and adding material from a structure, in order to create an optimum shape. Much work has been done on ESO by various researchers. V. Young, G.P Steven, Y.M. Xie and O.M. Querin extended the basic ESO algorithm to the addition of elements and multiple load cases. S. Savas, M. Ulker, and M.P. Saka utilised ESO to create structures with a uniform stress distribution. Both ESO algorithms were applied to isotropic structures. The basic principle of ESO is to remove material from the model, based on certain criteria, stress being typical. The method can also add material back, where it may become necessary to reinforce the structure, which may happen when excessive material is removed. The model undergoes an iterative process of analysis and modification, the cycle continuing until certain conditions are met, ranging from weight reduction to stress limits. The objective of the current research is to create an ESO method, utilising MSC.Patran/Nastran, to optimise composite structures. The final algorithm created is simple, in order to improve efficiency and reduce total analysis runtimes. The algorithm modifies the properties of the element, rather than removing it from the structure completely. This ensures that the connectivity of the elements remains intact. Elements are selected for removal or addition based on a driving criterion. The composite structures are modelled as a core and shell. The core consists of 3D elements with orthotropic properties, and the skin is represented by non-removable 2D shell elements. These shell elements bear the loads and boundary conditions. They also keep the external shape of the model, which is important for aerodynamic structures. The models were run through the ESO algorithm until the final optimised structure remained. A tailfin of an aircraft was used as an application example. The aim was to reduce weight and create an optimised design for manufacture. The criterion for the analyses undertaken was stress based. The results of this research are presented in the paper.