| As composite grid structures with unidirectional fibers appear to be a promising concept, an integrated equivalent stiffness model is developed to describe a composite grid structure with or without laminated skins. No extra degree of freedom is introduced to include the in-plane bending and shear of ribs so that Mindlin's plate theory can still be adopted. As a result, the model can be directly incorporated with the existing FEM technique. The model proves to be very accurate for displacements and reasonably accurate for strains or stresses. If high precision is required, the equivalent stiffness model can be incorporated with exact FEM modeling to obtain a refined stress analysis. The proposed model is demonstrated to be rather simple and efficient, especially for dynamic analysis. Hygrothermal behavior and local buckling are also taken into account.; With the equivalent stiffness model as a tool, the performance of composite grid structures compared to traditional laminates, sandwich plates and metal grid structures is clearly demonstrated. Composite grid structures possess several unique properties coming from the combination of the grid configuration and unidirectional fibers. They have excellent strength, flexural stiffness, damage tolerance and low hygrothermal expansion coefficients.; As the equivalent stiffness model provides the basis of analysis, a method of optimum design is established to design a composite grid structure with or without laminated skins. Multiple loads and multiple failure modes are considered. Discrete design is also included. Furthermore, it can be easily incorporated with the FEM to perform structural optimization. The optimization code is demonstrated to be a very powerful design tool.; Finally, a user-friendly computer code is developed to perform the above tasks. |
