Optimal Design of Tow-Placed Fuselage Panels for Maximum Strength with Buckling Considerations

The introduction of advanced tow-placement machines has made it possible to fabricate novel variable-stiffness composite structures where the fiber orientation angle varies continuously within each ply and throughout the structure. This manufacturing capability allows designers of composites to use the fiber orientation angle as design variable in their analysis, not only for each ply as with conventional composites, but at each point within a ply. Consequently, beyond the improvements that can be accomplished with traditional composites with straight fibers, the directional material properties of composites can be fully exploited to improve the laminate performance. In this paper, design tailoring for the pressure pillowing problem of a fuselage panel bounded by two frames and two stringers is addressed using tow-placed steered fibers. The panel is modeled as a two-dimensional plate loaded by out-of-plane pressure and in-plane loads. A Python-ABAQUS script is developed to perform the linear and geometrically nonlinear finite element analyses of variable-stiffness panels. The design objective is to determine the optimal fiber paths within each ply of the laminate for maximum load carrying capacity and for maximum buckling capacity. Simulated-annealing algorithm is used to solve the optimization problems. Optimal designs are obtained for different loading cases and boundary conditions. As a basis ofcomparison, a practical constant-stiffness quasi-isotropic design is used. Numerical results indicate that by placing the fibers in their optimal spatial orientations within each ply, the load carrying capacity and the buckling load of the structure can be substantially improved compared with traditional straight fiber designs. It is shown that laminates optimized for maximum failure load have buckling loads that are higher than those for quasi-isotropic laminates. On the other hand, laminates optimized for maximum buckling load fail at load levels lower than laminates optimized for maximum failure load. However, the failure loads of those laminates may still be higher than those for their quasi-isotropic counterparts.