Buckling Load Optimization of Straight-Fibre Variable Stiffness Composite Cylinders

The mechanical performance of laminated composite structures can be improved significantly by spatially varying the stiffness properties within the laminate, resulting in a so-called Variable Stiffness (VS) laminate. Typically, the stiffness variation in VS laminates is achieved by steering the fibres within the laminae, resulting in an in-plane variation of stacking sequence and thus an in-plane variation of mechanical properties of the laminate and subsequent load-redistribution within the laminate. The authors use a different approach to design VS composite cylinders, which does not rely on fibre steering. Instead, in-plane variation of the stacking sequence is achieved by stacking patches of unidirectional material or fabric, each of different in-plane dimension. Such a laminate can be designated as straightfibre variable stiffness (SFVS) composite laminate. A method developed at TU Delft to design SFVS laminates is modified to optimize the buckling load of composite cylinders. In this method, the laminate is divided into a discrete number of regions, for each of which the stacking sequence is optimised individually. Stacking sequence continuity is enforced by an algorithm inspired on cellular automata (CA) which relies on the application of a set of simple design rules. The modified method is applied to optimize the buckling load for composite cylinders. The buckling load of the optimised designs is evaluated using commercial finite element software. The effect of the number of design regions into which the cylinder is divided is considered. To conclude an outlook is given on how the modified method can be applied to the design of aerospace structures.