Advancing double-double composite laminates as next-generation alternatives to traditional quad laminates

Introduced by Prof. Stephen Tsai in 2017, the DD concept offers simplified structural design, reduced manufacturing complexity, and inherently straightforward thickness tapering, providing significant opportunities for structural mass reduction. DD laminates are constructed from repeated sub-laminate building blocks composed of two bi-angled ply pairs which, when repeatedly stacked, lead to through-thickness homogenisation.

However, as an emerging area of research, uncertainties remain regarding the extent to which homogenisation can replace conventional laminate symmetry, as well as the resulting in-plane and out-of-plane mechanical performance and impact damage tolerance of DD architectures.<br><br>Sufficient through-thickness homogenisation of the laminate eliminates undesirable out-of-plane distortions commonly observed in asymmetric composite laminates.

While DD homogenisation criteria have been proposed in the literature, no robust, experimentally validated criteria have been established. In this work, updated homogenisation criteria are proposed and evaluated through laminate manufacture using Carbon Fibre Reinforced Polymer (CFRP) prepreg material. Two new DD stacking-sequence approaches, termed Symmetry-Enhanced Double–Double (SEDD) laminates, are introduced to reduce the minimum laminate thickness required to meet these criteria.<br><br>A Python-based framework is developed to convert arbitrary laminate stacking sequences into stiffness-equivalent DD configurations.

The tool integrates closed-form and optimisation-based algorithms to match both in-plane and out-of-plane stiffnesses, enforces homogenisation criteria to ensure warpage-free designs, and supports both conventional DD and SEDD architectures. Unnotched tension, unnotched compression, Open-Hole Tension (OHT), Open-Hole Compression (OHC), and flexural tests were performed on a diverse set of laminates to comprehensively assess the in-plane and out-of-plane mechanical performance.<br><br>While the standard four-ply DD building block, [±Φ/±Ψ], provides reasonable coverage of the feasible design space, it cannot achieve the high degrees of orthotropy required in many aerospace structures.

Extended Double–Double (ExDD) laminates employ five- or six-ply sub-laminates, such as [0/±Φ/±Ψ] and [±Φ2/±Ψ], complementing the standard DD approach by providing enhanced design flexibility and higher orthotropy. The homogenisation and mechanical performance of these laminates are investigated.<br><br>Composite airframes are susceptible to low- and high-velocity impacts during fabrication, service, and maintenance, including runway debris and dropped tools.

To address these concerns, the study presents a comprehensive experimental evaluation of the Low-Velocity Impact (LVI), pristine compression, and Compression After Impact (CAI) behaviour of DD and ExDD laminates. Performance is benchmarked against Quad laminates designed to match in-plane or out-of-plane stiffness, enabling a rigorous assessment of the damage resistance and residual strength of DD laminates.<br><br>Through the combined assessment of homogenisation, mechanical performance, and impact damage, this thesis demonstrates that DD and ExDD laminates constitute viable and competitive alternatives to traditional Quad laminates in modern composite structures.

Collectively, the results position Double–Double-based laminate architectures as flexible, high-performance design solutions that enable lighter, more damage-resistant structures while significantly simplifying laminate design and manufacturing, supporting their adoption in next-generation aerospace and other advanced engineering applications.<br><br><i>Thesis is embargoed until 31 July 2027.</i>