Imperfection-insensitive continuous tow-sheared cylinders

Abstract

The design of thin-walled cylinders in axial compression is limited by sensitivity to geometric imperfections. This paper focuses on reducing this imperfection sensitivity from a design perspective. By using variable-angle composites, the load paths 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 entails 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 knockdown factors (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, the optimal straight-fibre laminate for axial cylinder buckling. 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.