Buckling optimization of steering stiffeners for grid-stiffened composite structures

Grid-stiffened composite structures, where the skin is stiffened by a lattice of stiffeners, not only allow for significant reduction in structural weight but are also competitive in terms of structural stability and damage tolerance compared with sandwich composite structures. As the development of Automated Fiber Placement (AFP) technology matures, integrated construction of skin and stiffeners is easily manufacturable. Optimization of grid-stiffened structures is needed to fully take advantage of the expanded design possibilities. In this paper, a steering/curved stiffener layout is optimized for grid-stiffened composite structures in order to enhance the structural buckling resistance. A homogenization method is used to calculate the equivalent material properties. Global and local buckling loads are determined by a global/local coupled strategy. A linear variation of stiffener angles is assumed resulting in the formation of a locally rhombic lattice pattern by the stiffeners. Moreover, manufacturing constraints are considered in the optimization by setting a lower bound on the stiffener spacing. Since the calculation is implemented on an equivalent model with a fixed mesh, it is possible to use a gradient-based optimization algorithm. A comparison between the performance of grid-stiffened composite structures with curved stiffeners, with straight stiffeners, and with variable-stiffness skins with curved fibers, reveals the potential of curved stiffener configurations in improving structural efficiency