Thermoset epoxies are extensively used in engineering fields such as automotive and aerospace applications owing to their excellent mechanical properties and thermal stability. However, their highly crosslinked molecular structure renders them inherently brittle and prone to fracture, underscoring the need for effective toughening strategies.

Spider silk exhibits remarkable fracture resistance due to its molecular architecture, which incorporates sacrificial bonds and hidden lengths (SBHL). Upon loading, the sacrificial bonds rupture first, followed by sequential unfolding of the hidden lengths in protein chains, dissipating significant energy without catastrophic failure. Inspired by this mechanism, this thesis introduces and validates the SBHL toughening concept for structural epoxy, providing a foundation for future engineering applications......

The research presented in this thesis started with the fabrication of spider silk-inspired structures. To replicate the natural SBHL toughening mechanism, polymeric overlapping curl (OC) fibers with sacrificial bonds and hidden lengths were 3D-printed using the liquid rope coiling effect. Three polymers—polylactic acid (PLA), liquid crystal polymer (LCP), and polyamide 6 (PA6)—were employed. Uniaxial tensile tests characterized the effects of geometry, post-treatment, and material properties on the OC mechanical responses. Results showed that single-sided OC fully unfolded, whereas double-sided curls failed prematurely. Post-treatments combining heat and pressure enhanced the load-capacity of sacrificial bonds by up to 77 %, but introduced defects in the fibers that caused premature failure and reduced toughness by up to 67 %. To ensure the complete hidden length unfolding and improved toughness, polymers with either high fracture strength (e.g., LCP, 311MPa) or high fracture strain (e.g., PA6, >2) were found essential, achieving toughness increases of 32% and 46 %, respectively....

Bio-based epoxy adhesives face significant challenges due to their relatively poor mechanical properties compared to their petroleum-based competitors, including low fracture toughness and abrupt failure. By mimicking the molecular structure of spider silk, which is one of the toughest materials in nature, 3D-printed polymer overlapping curls consisting of coiling fibers with sacrificial bonds and hidden lengths, were impregnated into a bio-based epoxy adhesive to improve its mode I fracture toughness. Such bio-inspired structures were designed specifically to toughen and improve the crack resistance of adhesive joints. These overlapping curls were embedded in the bio-based epoxy bondline with various adhesion patterning strategies, aiming to architect the fracture scenario and increase mode I energy dissipation. Double cantilever beam test results show that an extrinsic bridging is triggered by the embedded curls that promote progressive failure and delay crack growth, which improved the mean energy release rate by 133% and enhanced the mean peak energy release rate up to 313%. The proposed 3D-printed coiling fibers successfully improved the mechanical performance of the bio-based epoxy and retarded the crack growth within the bondline, opening new horizons for their use as carriers of bondlines in structural applications to control crack growth in adhesively bonded joints.

In this study, a 3D-printed biomimetic overlapping curl structure inspired by spider silk molecular structure, containing sacrificial bonds and hidden lengths, is studied as a toughening mechanism for a bio-based epoxy. Experimental results of the fracture phenomena of the overlapping curl-reinforced bio-based epoxy identify three toughening mechanisms triggered by the overlapping curl: (1) crack re-initiation, (2) overlapping curl bridging, and (3) epoxy ligament. First, the integrated overlapping curl creates a void within the epoxy matrix. As the crack tip reaches the end of this void, the crack re-initiates. Then, as the hidden length of overlapping curl unfolds, it leads to a bridging effect in resisting crack growth. In addition, for the smallest hidden length, an epoxy ligament is formed due to crack branching, significantly improving the energy release rate. The epoxy fracture energy release rate increased by 13 %. The overall modest improvement is attributed to the large plastic dissipation energy of the epoxy and the relatively low overlapping curl load-capacity. However, when expanding the design space numerically, it was shown that as the failure load of the overlapping curl increases, the bridging effect increases progressively. The introduction of the bio-inspired overlapping curl structure into bio-based epoxy proves the concept of a toughening strategy for developing high-performance sustainable composite materials.

Aiming to aid the sustainable transition to fossil fuel-free epoxy materials and enhance the toughness of bio-based epoxies, here we integrate an overlapping curl microstructure consisting of coiling fiber with sacrificial bonds and hidden lengths into a bio-based epoxy matrix. Inspired by natural material, where exceptional properties are achieved at low environmental cost, the microstructure mimics the molecular structures of spider silk, known for its exceptional fracture resistance. The 3D-printed overlapping curl shows a saw-tooth mechanical response with continuous load-carrying ability thanks to the break of sacrificial bonds and unfolding of the hidden lengths. By embedding the overlapping curl into the compact-tension configuration of the bio-based epoxy, an extrinsic toughening mechanism is triggered as the hidden length unfolds. Experimental results show that a single-sided overlapping curl structure is able to improve the toughness of bio-based epoxy by 19%.

Spider silk is known for its excellent strength and fracture resistance properties due to its molecular design structure, characterized by sacrificial bonds and hidden lengths. These structures have inspired reinforcements of synthetic polymer materials to enhance toughness. In this study, we mimic these natural toughening mechanisms by designing and manufacturing 3D-printed polymeric structures incorporating overlapping curls consisting of coiling fiber with sacrificial bonds and hidden lengths. Utilizing the liquid rope coiling effect, we manufactured overlapping curls using three polymers: polylactic acid (PLA), liquid crystal polymer (LCP), and polyamide 6 (PA6). Uniaxial tensile tests were performed to characterize the mechanical properties of overlapping curl as a function of geometries, post-treatments, and material constitutive parameters. Our results show that single-sided overlapping curls can fully unfold while double-sided curls are prone to premature failure. Heat-pressure post-treatment was found to significantly increase the load-capacity of the sacrificial bonds by up to [Formula presented] due to increased contact area. However, the defects introduced in the fibre after the break of the sacrificial bonds, make the structure more susceptible to premature failure, limit the complete unfolding of the hidden length, and lead to a decrease up to [Formula presented] of the toughness. To guarantee the complete unfolding of the hidden lengths and improve the toughness, we demonstrate that selecting a polymer material with either high fracture strength (e.g., LCP, [Formula presented]) or high fracture strain (e.g., PA6, >2) is crucial, and increase toughness up to [Formula presented] and [Formula presented], respectively.

Bio-based epoxy materials face major challenges in their relatively poor mechanical properties compared to their petroleum-based competitors, including low fracture toughness and abrupt failure. By mimicking the molecular structure of spider silk, which is one of the toughest materials in nature, we manufactured polymer overlapping curls consisting of coiling fibers with sacrificial bonds and hidden lengths through 3D printing. These curls were embedded in a bio-based epoxy aiming to improve its toughness. The bio-based epoxy adhesive layer integrated by such 3D-printed coiling fibers was tested under mode I opening load using Double Cantilever Beam tests. The results show an extrinsic bridging triggered by the embedded curls that promote progressive failure and improve the mode I fracture toughness by 285%. The proposed 3D-printed coiling fibers can improve the performance of biobased epoxies and retard crack growth, opening new horizons for their use in structural applications and the use of these bio-inspired overlapping curls to control crack growth in adhesively bonded joints.