Optimal Design And Failure Analysis Of Advanced Composite Grid Stiffened Structures

Advanced composite grid stiffened (AGS) structures not only inherit the traditional excellent qualities of fiber-reinforced composite materials, e.g. high specific strength and stiffness, but also exhibit many unique advantages by combining new material technology and new structure design concepts, e.g. environmental robustness and automatic manufacturing. AGS structures appear to be a promising concept and have been widely used in aircraft and spacecraft design. According to the characteristic of composite AGS plate/shell, this dissertation investigates the model, theory, method and failure process for the AGS structures from global-local and macroscopic-microscopic views. In addition, a corresponding software (AGSANS) is developed. This study is a sub-part of the project of National Basic Research Program of China (973 Project) "Study on multi-functional innovation construction of extra-waighted and porous materials & structures" (No.2006CB601205) and National Science Foundation in China "The fracture mechanism of advanced grid stiffened structure (AGS) with damage" (No. 10302004). It is also supported by a grant from the National Key Technologies R&D Program of China Academy of Launch Vehicle Technology during the 10th Five-Year Plan Period "Optimal design and mechanics analysis for composite AGS structures". The main research work can be summarized as follows:1. Hybrid Genetic Algorithm of Composite AGS StructuresConsidering the characteristics for optimal design of AGS structures, such as nonlinear programming, multi-constraints, and mixed discrete-continuous design variables, etc, a new hybrid genetic algorithm (GA) is developed by combining the improving GA and simplex algorithm (SA), which achieves global optimal solutions more quickly. In the method, some solutions are offered for several traditional problems in SA. The present design problem is to minimize the weight of an AGS cylindrical shell under a given load condition with buckling and strength constraints. An equivalent stiffness model (ESM) is used to analyze the AGS structure. Continuous design variables (rib thickness and angle of diagonal ribs) and discrete variables (rib height, diagonal rib spacing and skin laminate stacking sequence) are both considered. Some numerical examples are discussed about the effects of strength constraints and grid configurations upon the optimal results. It is concluded that global buckling constraint is the key factor for identifying the safety of AGS cylindrical shell, and the triangular grid is the ideal configuration under transverse pressure.2. Robust Design for Composite AGS Cylindrical ShellRobust design for the structures is normally necessary, since the size and location of the members generally need high tolerance during manufacturing. Therefore, a robust optimization design method, based on the hybrid GA, for the AGS cylindrical shell with initial-imperfect is proposed, in which both robustness of objective function and constraint functions are considered. To simplify the optimization and reduce the computational cost, the two most sensitive variables, which are introduced in robust analysis, are distinguished from all utilizing sensitivity analysis. In the present analysis, they are rib height and angle of diagonal ribs. By optimizing an AGS cylindrical shell under axial compressure, it can be seen that the optimization results given by the proposed robust design method are quite different from the general deterministic optimization method, and the AGS structures are not sensitive to the initial imperfection.3. Refined Triangular Composite Stiffened Mindlin Plate/Shell ElementSince the AGS structures are so complex and the ribs are normally higher than skin, the present finite element models are all insufficient. A new refined triangular composite stiffened Mindlin plate/shell element is formulated for simulating AGS structures. The corresponding expressions of geometric nonlinearity are also deduced. In the element, the rotations of the ribs and skin are interpolated, respectively, using the same shape functions, while their transverse deformations are regarded as equal. Therefore, the conditions of displacement compatibility are well maintained along the interface of rib and skin, and the rotational constraints of the stiffeners are released. There are no restrictions on the number and orientation of the ribs in establishing element mesh. The numerical results show its good convergence and effectiveness, especially for the case of the AGS structures of higher ribs. The element can be used to calculate the deformation and in-plane stress of stiffened structures accurately. The studies on the buckling and post-buckling behavior for ortho-grid plate and iso-grid cylindrical shell are also carried out.4. Progressive Failure Methodology for Composite AGS StructuresPresenting, literatures on the failure analysis of AGS structures is minimal due to its complexity. In this study, a new progressive failure methodology is developed to simulate the onset and growth of multi-failure for composite AGS plates/shells on the basis of the refined stiffened element model (RSEM). The failure modes considered in this study are inter-laminar failure (i.e. delamination in skin) and intra-laminar failure including fiber failure, matrix cracking, fiber-matrix shear failure in skin, and fiber failure in the ribs. For intra-laminar failure modes, corresponding material degradation rules are introduced. However, a new equivalent degraded stiffness rule is proposed for delamination. Meanwhile, a scheme for calculating inter-laminar shear stresses of the triangular element is developed by employing a mixed approach of finite element and finite difference method, which is confirmed available for thin or moderate thick composite laminate plates. The methodology is validated by some typical examples and is employed to evaluate the progressive failure behavior of a composite orthotropic-grid curved panel with a centrally located cutout under compressive load.5. Mixed Analytical Model of Composite AGS Structures A new mixed analytical model by conjunction of ESM with RSEM is formulated to simulate the composite AGS structures. The strategy is to use SEM to model the local area, where the mechanical features attract more attention, while use ESM to model the non-local area. It can not only simulate the local behavior more exactly but also be more efficient. The numerical simulations show that the accuracy of ESM is remarkably influenced by the configuration of grid, distance and height of the ribs. The limitation of the mixed analytical model is also investigated by analyzing the stress of a special point at the edge of the cutout located at the central of a grid-stiffened plate.