Shaping Anisotropy

Abstract

The cellular, hierarchical nature of living materials such as wood or bone, enables their mechanical properties to vary locally. This variability grants themmulti-functionality and highmechanical performance with low resource consumption. To obtain such desirable features with man-made structures, many manufacturing restrictions need to be overcome. Currently, the most common structural anisotropic materials are fibre-reinforced composites, which allow limited control over material placement, orientation, and directional strength.

In this thesis, we explore how the additive manufacturing of Liquid Crystal Polymers (LCP) can unlock our ability to control these parameters and, therefore, expand the design space to optimize structural performance while limiting material use. To achieve this, we leverage the alignment of nematic domains in LCPs, induced by shear and elongational flow during extrusion through the 3D printer nozzle. By modulating the pressure inside the nozzle to change line width, we observed an axial stiffness range between 3 GPa and 30 GPa, associated with a failure mode shift from ductile to brittle. Demonstrating our ability to print with variable toolpathwidths and sharp curvatures,we created 3D patterns inspired by fluid dynamics. This ombination of shaping freedom and anisotropy control enabled the design of functional objects with embedded crack mitigation strategies, or improved buckling performance.

Anisotropy gradients are also present in the structure of trees. Inspired by the seamless transition of wood fibres from parallel to interlocked at the branch junctions, we manufactured sinusoidal and helical patterns with LCP. The introduction of local waviness decreased axial tensile stiffness, but significantly enhanced toughness. Tensile and interlocking performance were driven by two critical parameters: angle of maximum deviation to the axial direction, and pattern size.Moreover, resistance to normal stresses was improved, as shown by the 88% increase in load-bearing capacity compared to a nonreinforced configuration in a two-strut junction. This opens the possibility to integrate localized interlocking patterns in multi-axis loaded regions of anisotropic parts.

Manufacturing methods which utilize as littlematerial and energy as possible are needed for future space exploration missions. To assess whether the strategies mentioned previously are applicable to this context, 3D-printed LCPs were subjected to four different space environments. High-energy electron radiations created colour-centres, leading to a bright green coloration in the bulk of the specimen. However, no significant decrease in staticmechanical properties was observed, with unidirectional stiffness remaining close to 30 GPa even after intense electron irradiation and thermal cycling. These results indicate that LCPs are a promising alternative to engineering polymers like PEEK or PEI for space applications.

Future research into topology-optimization methods integrating both asymmetry between tension and compression, and mechanical property gradients, will enable to further explore the design space opened by the tunable anisotropy of LCPs.