Improved buckling performance of fibre-steered laminated plates with embedded pseudo-stiffeners

Tow-steered laminates, those in which the fibre angles vary spatially across a ply, have been investigated for improving structural performance under various loading conditions, such as buckling, bending and vibration. When steering tows by the Continuous Tow Shearing (CTS) process, the material is sheared in-plane along a curvilinear reference path. This in-plane shearing generates a non-linear orientation-thickness coupling which can be exploited in design of laminated structures. The thickness change generated by this mechanical coupling allows embedded stiffeners to be laminated into the structure. These embedded stiffeners are termed ‘pseudo-stiffeners’ and have been considered for reducing the imperfection sensitivity of a thin-walled CTS tow-steered cylinder [1]. However, the design and use of these pseudo-stiffeners to act as conventional stiffening features has not been investigated. Previous studies have steered tows continuously across a ply in a chosen direction. This continuous and global dependence of fibre angle on in-plane position has been a barrier for wider adoption of the tow steering concept in industry as it requires specialised and new modelling approaches. For example, as the fibre angles varies continuously, it is not clear how is what the most optimal tow-steered reference paths are for a specific load case. In this work, the concept of ‘local’ tow steering is proposed, where tows are steered across a comparatively shorter length than conventionally defined. This ‘local’ steering approach in conjunction with the CTS process allows for pseudo-stiffeners to be embedded in a tow-steered laminated structure. Hence, instead of attempting to define a non-intuitive curvilinear reference path, one can instead define pseudo-stiffeners through definition of design parameters, such as pseudo-stiffener orientation, in-plane width, out-of-plane thickness and count. Hence, this work utilises the potential of pseudo-stiffener design to propose several novel tow-steered structures of improved structural performance than that possible with conventional straight fibre design. To quantify performance benefits of these pseudo-stiffeners on structures manufactured by the CTS process, a study into the potential improvements
of buckling performance for laminated plates is conducted. Following a classic load case, a simply-supported CTS plate is analysed under uniaxial compression. The effects of pseudo-stiffeners, embedded by locally increasing the laminate thickness through tow steering, are directly compared in performance to an optimum straight fibre design. However, by embedding pseudo-stiffeners through shearing tows a mass increase is also generated. Hence, to allow for a fair comparison of performance between straight fibre and CTS plates, the mass of the tow-steered structure shall be conserved. Firstly, this work will embed pseudo-stiffeners into the optimum straight fibre design to explore the expected further increased structural performance that this design can achieve. Next, an investigation into other more novel designs will be considered. As laminated structures are comprised of multiple layers of material, a CTS laminate can be readily designed such that stacked plies of multiple different steering directions can embed grid-like stiffening patterns, e.g., ortho- and iso-grids. It is expected that these embedded stiffening patterns can allow for increased structural performance. REFERENCES: [1] Lincoln, R. L., Weaver, P. M., Pirrera, A., and Groh, R. M., “Imperfection-insensitive continuous tow-sheared cylinders,” Composite
Structures, Vol. 260, 2021, p. 113445. https://doi.org/10.1016/j.compstruct.2020.113445,
URL https://doi.org/10.1016/j.compstruct.2020.113445.