Higher strength of double-double over traditional quad laminates in equal-stiffness design of single lap joint

Double-Double (DD) is a novel laminate design concept that not only reduces design variables and simplifies the manufacturing process but also exhibits unique homogenization properties. While theory suggests that for any QuaDriaxial (QUAD or QD) laminate, DD can provide a corresponding laminate configuration with equivalent stiffness, the mechanical behavior of their Single-Lap Joint (SLJ) still requires further investigation. In this study, a DD SLJ equivalent to the quasi-isotropic QD SLJ commonly used in fuselage structures is designed by ensuring equivalence in the in-plane stiffness matrix. While the experimental results show nearly identical in-plane stiffness, the tensile strength of the DD SLJ is approximately 13 % higher than that of the Quad SLJ. Additionally, DD SLJ exhibit reduced out-of-plane deformation compared to Quad SLJ. Both theoretical and numerical analyses are conducted to elucidate the underlying mechanisms responsible for the superior mechanical behavior of DD SLJ. The findings indicate that the observed differences are primarily due to variations in bending stiffness, despite identical in-plane stiffness matrices. The results further demonstrate that the smaller bending deformation of DD laminates leads to reduced interfacial peeling forces, making DD SLJ less susceptible to failure. Finally, a comparison of different staggered ways of DD laminates indicated no significant differences in strength or stiffness. The study demonstrates that DD SLJ, designed with equivalent in-plane stiffness matrices, not only achieve the same tensile stiffness as QD SLJs but also exhibit higher strength. This highlights the great potential of DD laminates for application in the single-lap joints of aircraft structures.