A multi-level set gradient based algorithm for buckling optimization of blended composite structures

An approach is presented for the optimization of stiffened composite skins, which guarantees the continuity (blending) of plies over all individual panels. To fulfill design guidelines with respect to symmetry, balance, contiguity, disorientation and percentage rule of the layup, first a stacking sequence table is generated. Next, a novel multi-level set gradient based method is introduced for the global optimization of the location of ply drops. The method aims to turn the discrete optimization problem associated with the integer number of plies into a continuous one. It gives the optimum thickness distribution over the structure in relation to the specific table of stacking sequence. The proposed method is applied to the optimization of the layup of a composite stiffened skin of a structure resembling a wing, considering constraints on the local buckling load. During gradient based buckling optimization it is crucial to take switching of the critical buckling mode into account. An effective method is presented to achieve this and the point of implementation is discussed in detail. Optimum design of a stiffened structure resembling the upper panel of a wing torsion box subjected to different in-plane normal and shear loads is optimized subjected to no local buckling constraint.