Extended double–double (DD) laminates: expanding design space and advancing mechanical performance beyond standard DD laminate constraints

This study introduces Extended Double-Double (ExDD) laminates as an extension of the standard Double-Double (DD) concept, enabling laminate stiffness beyond the feasible domain of four ply DD sub-laminate configurations. ExDD laminates employ five- or six-ply sub-laminates, such as [0/±Φ/±Ψ], [90/±Φ/±Ψ], and [±Φ2/±Ψ], complementing the standard DD approach by providing enhanced design flexibility and higher orthotropy. Closed-form solutions for converting to ExDD ply angles through lamination parameters were derived and integrated into a Python-based conversion framework, validated experimentally for both in-plane and out-of-plane stiffness equivalence. Homogenisation grids were developed to identify optimal stacking sequences and minimum sublaminate repeats. Mechanical testing, including unnotched tension/compression, Open-Hole Tension (OHT), Open-Hole Compression (OHC), and flexural tests, revealed nuanced performance trends. While ExDD laminates achieved stiffness equivalence, they generally exhibited lower unnotched tensile strength compared to Quad laminates. However, ExDD laminates outperformed Quad laminates in compression and demonstrated competitive flexural and open-hole performance, achieving up to 29% higher OHT strength in specific cases. Where comparable to standard four-ply DD, ExDD laminates exhibited superior OHT properties but demonstrated reduced unnotched tensile strength. Overall, ExDD laminates broaden the applicability of the DD methodology to scenarios demanding higher orthotropy while preserving the core benefits of the methodology.