Double-Double laminates: A comprehensive assessment of the mechanical performance in comparison to traditional Quad laminates

This study presents a comprehensive evaluation of Double-Double (DD) laminates as a viable alternative to traditional Quad laminates in composite structural applications. DD laminates are a new layup design approach, typically comprising repeated Building Block (BB)s or sub-laminates, formed from two sets of bi-angled plies, e.g. [ Φ / − Ψ / Ψ / − Φ ] r , where r denotes the number of sub-laminate repeats. A Python-based framework is developed to convert any laminate stacking sequence into a stiffness-equivalent DD configuration, incorporating both closed-form and optimisation-based solutions for in-plane and out-of-plane stiffness matching. The framework integrates homogenisation criteria to ensure warpage-free designs and supports both traditional DD and Symmetry-Enhanced Double-Double (SEDD) stacking sequences. Experimental validation of the framework was conducted for both in-plane and out-of-plane conversions. 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 mechanical performance. Results show that DD laminates generally exhibit higher unnotched compressive and OHC strengths, comparable OHT performance, but moderately lower unnotched tensile strength relative to their Quad counterparts. Progressive Failure Analysis (PFA) and Finite Element Analysis (FEA) are employed to investigate failure mechanisms and interlaminar stresses of laminates under unnotched tension, respectively. Digital Image Correlation (DIC) was conducted during OHT and OHC tests, providing insight into the observed failure trends. Flexural behaviour was found to be highly dependent on the specific stacking sequences, with no universal trend favouring either configuration. Overall, DD laminates demonstrate competitive mechanical performance compared to traditional Quad laminates, while offering significant design flexibility and manufacturing advantages.