Optimization of a composite cylinder under bending by tailoring stiffness properties in circumferential direction

A fiber-reinforced cylindrical shell with given geometry and material properties is optimized for maximum load-carrying capability under bending. The shell is assumed to be built using an advanced fiber-placement machine, which allows in-plane steering and overlapping of fibers, resulting in a so-called variable-stiffness shell. The design methodology for strength and stiffness variation in circumferential direction by means of fiber placement is explained and restrictions on the manufacturability are specified. Implementation in the commercially available finite element package ABAQUS® for structural analysis is described. Subsequently, the cylinder is optimized to carry a maximum buckling load under bending, while applying a strength constraint. Constraints on the global stiffness are imposed by means of comparison with a baseline quasi-isotropic shell, while a matrix dominated lay up is avoided at all locations in the laminate in order to ensure that the laminate is strong enough in all directions in case a hole is present. Optimization is done using a surrogate model in order to minimize the amount of finite element analyses. Improvements of up to 17% are obtained by changing the load path. The tension side is made stiffer and the compression side softer in longitudinal direction by changing the fiber orientation from near zero at the upper (tension) side to higher fiber angles at the lower (compression) side, such that load is relieved from the compression side. This results in a higher load-carrying capability of the cylinder.