Innovative multi-level methodology incorporating the techniques of finite element modelling and multimodal optimization for concept design of advanced grid stiffened composite panels against buckling.

Abstract

Since the Second World War fiber reinforced polymer (FRP) composites have become more attractive as a structural material in a variety of engineering practices, such as infrastructure construction, automobile industry and aerospace engineering, due to high specific strength and stiffness as well as flexibility in tailoring the structural performance. On the other hand, a stiffened panel always performs better in resisting loads compared to an unstiffened panel of same weight. Thus a combination of lightweight composite materials and stiffened structural forms can efficiently enhance the load resisting capability that can be buckling strength of a structure. Stiffened composite panels are subjected to any combination of in-plane, out-of- plane and shear load conditions during service life. These types of thin-walled structures are vulnerable to lose global and local stability under compression loadings. Consequently, buckling-resistant design is one of the most critical issues of stiffened composite panels applied in real practices. Moreover, the buckling optimization design of composite panels is usually a typical multimodal optimization problem, in which there exist multiple global optimal solutions with identical or closely comparable optima of structural performance. Recently, with the development of manufacturing techniques, advanced grid stiffened (AGS) composite panels have increasingly emerged and gained more attention as these grid-stiffening configurations help to enhance the structural efficiency in a more effective way in complex loading conditions compared with conventional unidirectionally-stiffened composite panels. These grid-stiffening configurations provide more available options to select outstanding concept designs of AGS composite panels against buckling for the final appropriate design development at the final construction stage. In this PhD thesis, a novel multi-level optimization methodology for concept design of advanced grid stiffened composite panels against buckling has been developed. Furthermore, an efficient finite element (FE) modelling component for buckling analysis and a robust particle swarm optimization (PSO) algorithms for multimodal optimization have been presented, in order to further consolidate the performance of the proposed methodology. The thesis is divided into six chapters, which are briefly described below: In Chapter 1, a general background along with the objective and originality of the present research is presented. An efficient FE modelling technique is presented in Chapter 2, for the prediction of buckling response of grid stiffened composite panels having different stiffening arrangements. The laminated skin of the stiffened structure is modelled with a triangular degenerated curved shell element. An efficient curved beam element compatible with the shell element is developed for the modelling of stiffeners which may have different lamination schemes. The deformation of the beam element is completely defined in terms of the degrees of freedom of shell elements and it does not require any additional degrees of freedom. Chapter 3 aims to extend conventional unimodal optimization to challenging multimodal optimization of composite structures, by means of newly emerged multimodal PSO using niching techniques. It has shown that the ring topology based PSO without any niching parameter is more robust and efficient for multimodal optimization of composite structures, compared with the species-based PSO (SPSO) and the fitness Euclidean-distance ratio based PSO (FERPSO). In Chapter 4, a random reflection boundary is proposed to replace the conventional fixed absorption boundary for the range-exceeding particles, in order to eliminate/reduce the significance and sensitivity of an empirical parameter of particles’ maximum velocity in PSO. Based on the results obtained from the experimentation on the abovementioned test functions, empirical guidelines for appropriately using the half-range/full-range random reflection boundary are further proposed. Chapter 5 presents an efficient methodology to conduct concept design of AGS composite panels, based on a multi-level approach where an inner 3- stage optimization process is nested within an outer 3-step optimization process. The proposed methodology is applied to a design optimization problem of an AGS composite plate against its buckling resistance, by incorporating a ring topology based multimodal PSO algorithm with an improved FE buckling analysis model. Finally, the conclusions of the present research are summarized in Chapter 6. The limitations and the future development directions of the present study are also described in this chapter.