Effect of joint stiffness on torsional stiffness of open lattice composite structures

In open lattice composite structures, the lattice components are chemically bonded, which affects the overall properties of the structure. This study examines the effects of chemically bonded joints on the torsional stiffness of tubular lattice composite structures. Tubular open lattice structures, known as the open-architecture composite structures, are manufactured by braiding the impregnated carbon fiber tows. Samples were prepared with no chemically bonded and with epoxy joints using braid angles of 35 degrees, 45 degrees, and 67.5 degrees. A finite element model of the open-architecture composite structures is developed to examine the mechanical behavior under torsion. It is shown that there is a significant difference between the samples with no-bonding joints and samples with epoxy joints in terms of torsional stiffness. Torsional stiffness of the structure is retained at 97%, 96%, and 93% of the theoretical limit for 35 degrees, 45 degrees, and 67.5 degrees braid angles, respectively, when joint stiffness is ten times the component stiffness. Torsional stiffness is only 32%, 22%, and 13% of the theoretical limit for 35 degrees, 45 degrees, and 67.5 degrees braid angles, respectively, when joint stiffness is one-10th of the component stiffness. The epoxy bonding at the intersection achieves 72% of the theoretical torsional stiffness of the "perfect joint" for 45 degrees braid angle when joint stiffness is equal to the component stiffness. The finite element model is validated by experimental results. It can be concluded from the finite element analysis and experimental testing that the stiffness of the bonding joints has significant impact on the overall torsional stiffness of the biaxial composite lattice.