Variable Stiffness Lay-up Optimization Of Composite Laminated Structures

Fiber-reinforced composite materials are widely used in thin-walled structures due to their excellent mechanical properties and flexible designability,especially for weight-sensitive aerospace applications.However,the features such as multiple variables,discrete nature of variables and multiple values that accompany with design flexibility,result in more complex and more difficult design of composite laminated structures than metal material structures.Therefore,modern optimization methods are employed to accomplish the task of lay-up optimization.In order to maximize the potential of weight saving,the optimization is needed to find adequate stacking configurations,based on the load condition and the constraints on stiffness,strength and so on.The ideal laminated structure is characterized by variable stiffness stacking configurations.Therefore,in the premise of meeting the requirements of manufacturing,some lay-up optimization methods are developed and proposed in this thesis to design variable stiffness laminated structures which are reinforced by straight or curvilinear fiber.The main contents of this thesis are as follows:(1)A new lay-up optimization method based on fiber continuity model is proposed.Referring to the problem of fiber continuity in multi-zone laminated structures,based on the study of the existing fiber continuity models,the concept of ply drop sequence(PDS)is proposed.Furthermore,a PDS based fiber continuity model is proposed,through which stacking configurations in all design regions of the structure can be parameterized by the guide,the PDS and the thickness distribution of the structure.According to the new parameterization method,a genetic algorithm(GA)is improved and a fiber continuity model based lay-up optimization method is established.Then,the effectiveness of the method is verified by a classical example.Using adjustable drop-off rule,the new lay-up optimization method ensures the fiber continuity and the flexibility of stacking configuration,and provides a platform for the subsequent design of variable stiffness laminated structures.(2)Effects of the trajectory and manufacturing technology on the buckling behavior of curvilinear fiber(or variable angle tow,VAT)composite laminates are studied.Aiming at breaking the limitation of the existing linear variation method of fiber angle,a new description which defines angle variations by segmental linear functions is developed.The parametric analysis of the buckling characteristics of the ideal VAT laminates whose fiber angles are defined by segmental linear functions,are carried out under the uniaxial and bi-axial axial compression conditions respectively.It is shown that the new description method is beneficial to the buckling of laminates with complex loads.On the other hand,in order to solve the problem that the stiffness distribution of the ideal VAT laminates is not consistent with the distribution of the actual VAT laminates,a general method of angle distribution determination and overlapped defect localization in finite element models is deduced for VAT laminates manufactured by auto fiber placement(AFP),according to the manufacturing technology of AFP.Based on this modeling method,the influence of AFP and continuous tow shearing(CTS)on the buckling characteristics of the actual VAT laminates is studied through finite element analysis.It is shown that CTS laminates are more suitable for variable stiffness design.This part of the study lays the foundation for better design of actual VAT laminated structures.(3)The lay-up variable stiffness optimization of a composite missile wing is carried out by using parallel computing technique.In order to improve the optimization efficiency,a parallel GA based on fiber continuity model is constructed.In addition,the lay-up of the straight fiber-reinforced composite wing is optimized by this method.On the other hand,thickness variation can improve VAT laminate's performance,therefore the drop-off mechanism is introduced into the VAT laminated structures.In combination with the fiber continuity model,the manufacturing technology of the curved fiber and the parallel GA,a new hybrid variable stiffness optimization method is proposed.At the same time,in order to map the fiber trajectories defined on the plane to the skin surface,the trajectory planning method and corresponding transverse curvature calculation method of the curvilinear fiber on the wing skin are constructed.Finally,based on the manufacturing technology of CTS,the hybrid variable stiffness optimization is carried out to design the lay-up of curvilinear fiber-reinforced composite wing.By comparing the above two design schemes,it is shown that the new hybrid variable stiffness method can further increase material utilization,and provide useful information for the application of curvilinear fiber-reinforced variable stiffness structure in the future.(4)A lay-up optimization method is proposed based on surrogate model.In view of the problem that lay-up optimization of the actual structure needs a large amount of calculation,the advantages and disadvantages of the existing approximation optimization methods are summarized.And a new surrogate-model-based lay-up optimization framework is proposed for straight fiber-reinforced laminated structures.In order to construct a surrogate model based on laminate parameters(LPs),according to the particularity of LPs' feasible space,a design of experiment method based on space filling is established.In view of the complexity of the lay-up optimization problem of laminated structure,the intelligent adding point technology and the dynamic surrogate model are integrated into the optimization framework to construct a lay-up optimization method with precision and efficiency.Finally,the approximation optimization method is applied to the lay-up design of two numerical examples and composite propeller respectively.It is shown that the new method is suitable for the optimization design of multi-zone straight fiber-forced laminated structures,and can significantly reduce the amount of calculation under the premise of satisfying the design requirements.In conclusion,this thesis investigates the lay-up optimization methods for multi-zone straight fiber-reinforced laminates and VAT laminates by employing the fiber continuity technique and curvilinear fiber forming process.The validation examples and engineering application show that these methods can be utilized as guidance for the lay-up optimization of the similar composite laminated structures,which has certain engineering application value.