Efficient Design Optimization Methodology For Nonconventional Laminated Thin-Walled Composite Structures

Structural designers seek the best possible design of a structure that optimally meets the requirements of a specific application. The measure of the quality of the final design can often be related to specific stiffness and strength of the structure. Because of their superior mechanical and environmental properties compared to traditional metals, fiberreinforced composite materials have earned a widespread acceptance for different structural applications. The tailoring potential of composites to achieve high specific stiffness and strength has promoted them as promising candidates for constructing lightweight structures. From that aspect, designers have tackled the problem of designing composite laminates, which is inherently challenging due to the presence of non-linear, non-convex, and multi-dimensional problems with discrete and continuous design variables. Witnessing new manufacturing technologies also granted engineers the capability of exploiting the full potential of composites by using nonconventional laminates leading to more complex design problems. To circumvent this difficulty, designers have used lamination parameters as intermediate variables to achieve global optimization. Parameterizing the optimization problem in terms of lamination parameters retains the convex nature of the problem aiming to attain a global optimum design. This thesis aims to demonstrate the use of lamination parameters for efficient multi-level optimization of robust nonconventional laminates by integrating the optimization process with industry design guidelines. In the first optimization step, a theoretical optimum stiffness, parameterized in terms of lamination parameters, is obtained that accounts for optimum v structural performance while maintaining robustness. The second optimization step aims to retrieve the optimal stacking sequence matching the optimum stiffness properties, while accounting for laminate design guidelines to attain adequate industry performance. An important aerospace application incorporates the design of the fuselage in the aircraft, which can be divided into portions of cylindrical shells with a complex array of stiffeners, stringers, and rings that include large and small cutouts. The design of cylindrical shells under bending with a specified cutout is chosen as an application to demonstrate the effectiveness of using nonconventional laminates with arbitrary fiber orientation angles compared to conventional laminates composed of 0°, ±45°, and 90° fiber orientation angles. Constant stiffness laminates are designed for buckling and strength while imposing laminate design rules to achieve robustness. The designed laminates are compared using linear and non-linear analysis with progressive failure analysis to present the performance gains achieved by using nonconventional constant stiffness laminates compared to conventional ones. The presence of the cutout in the cylindrical shell also imposes severe stress concentrations yielding a need to use variable stiffness laminates that have continuously varying fiber orientation angles to redistribute the stresses and obtain a structurally optimal design. The first optimization step of the optimum variable stiffness design is demonstrated in the present study, whereas the optimal fiber angle distribution and fiber path generation are left for future work. A future goal of the research is to also extend the capability to address the design of more realistic fuselage structures including stiffening elements using nonconventional laminates. This aims to prove that structural improvements can be vi achieved by using nonconventional laminates for realistic design problems, which can be a major task towards their industry adoption and certification in the future