New insights of discrepancies in stiffness-equivalent Double-Double and quadriaxial composites via three-point bending tests

Double-Double (DD) laminates have emerged as an advanced development over the traditional Quadriaxial (Quad), offering additional lightweight potential. When converting Quad to DD laminates, equivalence is frequently prioritized for in-plane stiffness alone. This study investigated the converting method from Quad laminate to DD laminates through either in-plane (A-matrix) or bending (D-matrix) equivalence. Three-point bending tests comprehensively compare the bending responses of DD laminates under these equivalence methods. The results show that different equivalence methods lead to significant variation in the bending behaviour regarding both stiffness and strength. The inhomogeneity of thin Quad laminates is the reason for the variation, suggesting Quad cannot work efficiently with thin plies as their bending performance varies with different thicknesses. This highlights the important potential of homogeneous DD laminates in using thin plies for lightweight design. Furthermore, despite the homogenizing characteristics of DD, the strength under bending loads varies in different directions. It is recommended that, when necessary, a combination of both equivalence methods should be considered based on the structural loading conditions to achieve optimal mechanical performance.