Wood-inspired interlocking junctions using 3D-printed liquid crystal polymers

Abstract

Where the trunk of a tree splits into co-dominant branches, wood fibres are highly interlocked. Such an arrangement of fibres has been shown to impart superior strength and toughness to this critical junction. Here, wavy patterns are 3D printed with a liquid crystal polymer (LCP) to evaluate the potential of wood-inspired localized adaptations to improve the robustness of junctions between orthotropic struts. The highly anisotropic, fibrillar microstructure of LCPs is harnessed by Fused Filament Fabrication, yielding Young's modulus and tensile strength reaching 30 GPa and 500 MPa respectively. However, unidirectional 3D-prints subjected to normal tensile stresses show weak interfaces, like in wood. To overcome this weakness, sinusoidal, helix and saw-tooth patterns are 3D-printed to create interlocking between layers. A trade-off is established between uniaxial tension and short-beam shear with increasing interlocking angle of the sinusoidal pattern. We find that the work associated with crack propagation in Mode I is increased three-fold compared to a unidirectional pattern, through extrinsic toughening. When applied to a more complex load case in a curved beam in four-point bending, helix-patterning at the junction zone increases the maximum load by 88 %. By locally controlling anisotropy via waviness, this method opens the possibility of improving toughness and transverse properties where the stress state is multi-axial without adding mass in future recyclable structural materials.