Imperfection-Insensitive Continuous Tow-Sheared Cylinders

The design of thin-walled cylinders in axial compression is limited by sensitivity to geometric imperfections. This paper focuses on reducing the imperfection sensitivity of cylinders from a design perspective. By using variable-angle composites, the load paths within the cylinder are tailored to reduce the effective area over which imperfections can initiate buckling. Continuous Tow Shearing (CTS) is one such variable-angle manufacturing technique. It does not cause manufacturing defects associated with Automated Fibre Placement and there is a fibre angle-thickness coupling that results in a local thickness build-up --- used as a design feature to embed stiffeners in the cylinder. Nonlinear finite element models with seeded imperfections are used to calculate a knockdown factor (KDF). It was found that there is an inverse trend between embedded stiffener frequency and the KDF. The best performing CTS cylinder has a KDF 30% greater and a specific buckling load 4% greater than a QI cylinder. It was also found that the smaller the effect of geometric imperfections on the
pre-buckling strain field the greater the KDF of the cylinder. This novel finding shows that a computationally inexpensive nonlinear analysis can provide a first-order approximation of the imperfection sensitivity of a cylinder in axial compression.