The Optimal Design For The Stacking Sequences Of Composite Structures With Ply-drops

The fiber reinforced polymer(FRP)composite material has been widely used in aeronautics,astronautics,transportation,wind energy and sport instruments due to its good properties such as the high specific stiffness/strength.The tailoring design of composite structures could improve the structural efficiency and reduce the manufacturing cost.In the design processes of composite structures,ply angles,ply sequences and ply thicknesses are taken as the design parameters,while the symmetry,balance,stability and strength constraints are usually included.Due to the discreteness,very large design space and ply-drop problems existing in the optimal design process,the local convergence problem sometimes makes existing methods unable to find the best design solutions.So,further research should be conducted to address this problem.A comprehensive literature review was made for the studies on the stacking sequence optimization and performance prediction of composite structures.Problems existing in this field were pointed out,and the purpose and significance of this study were also elaborated.The following studies have been completed in this thesis:(1)Optimization study on the stacking sequences of composite laminate.An improved genetic algorithm is proposed to optimize the stacking sequences of composite laminates.A new coding method is used in the construction of chromosomes to realize the odd-symmetry constraint,even-symmetry constraint and balance constraint.To explore the design space more sufficiently in the optimizations,ply-ranking-mutation operator,ply-quantity-mutation operator,ply-type-mutation operator,ply-position-swapping operator and crossover operator are designed.Optimization examples of a typical laminated composite panel with different load cases show that the composite laminate can be well optimized with an acceptable efficiency.More important,better solutions have been stably obtained in the verifications.The results reveal that the proposed optimization method is more effective in finding the optimal stacking sequences of composite laminates.(2)Optimization study on the global stacking sequences of multi-laminate-panel composite structures with ply drops.A new method has been proposed and implemented in the framework of genetic algorithm(GA)to optimize the global stacking sequence of composite structures with ply-drops.To explore the design space more sufficiently in the optimizations,the prerequisites of the continuity between composite substructures have been studied.To implement the continuity constraint through the summarized continuity rules,newly constructed chromosomes are utilized in the evolutionary optimization of the global stacking sequence of composite structures.Genetic operators are also designed to keep the diversity and follow the continuity rules.Optimization examples of a typical laminated composite structure show that the tapered composite structures can be well optimized with an acceptable efficiency.More important,slightly or obviously better solutions have been stably obtained in the verifications.The results reveal that the proposed optimization method is more effective in finding the optimal stacking sequences of composite structures with ply-drops.(3)Optimization study on the global stacking sequences of multi-sandwich-panel composite structures with ply drops.A genetic algorithm-based method is proposed to globally optimize the stacking sequence of multi-sandwich-panel composite structures for minimum weight with strength and buckling considerations.The prerequisites for the continuity between sandwich panels are firstly studied.To implement the summarized continuity rules in the evolutionary optimization,three newly constructed chromosomes are developed to encode the global stacking sequence with no additional repair.Genetic operators,including specialized mutation,swapping and crossover operators,are also developed to effectively explore the design space and keep the continuity rules followed.The Hashin's criterion and maximum stress criterion are used to evaluate the strength of sandwich panels.A typical multi-sandwich-panel composite structure with identical and different core thicknesses is optimized to verify the validity and efficiency of the proposed method.It is found that much lighter solutions have been obtained with an acceptable efficiency in all cases.It is also found that the weight of the multi-sandwich-panel composite structures can be further reduced when the core thicknesses are not identical.(4)Study on the performance predictions and optimizations of composite structures with limited test data.A PNN based approach is proposed to predict the performance of composite structures based on limited test data.As a probabilistic method,the PNN can avoid the over-fitting of experimental data.The key issue of this approach is to utilize the limited experimental results and existing knowledge sufficiently by converting the strength prediction problem into a classification problem.To evaluate the predictive accuracy,predictions were implemented based on three sets of open-hole tensile strength tests of composite plates.Compared with the variability commonly existing in the experimental data,the PNN model produces acceptable predictions.The test-data-based optimization method for composite structures is developed on the basis of the PNN-based prediction model and the improved GA.With the assistance of limited test data and ply-drop designs,the proposed method reduced the weight of an open-hole composite laminate significantly.