3D-printed Bio-inspired Overlapping Curl Structure: Toughening Epoxy via Sacrificial Bonds and Hidden Lengths
Z. Xu (TU Delft - Aerospace Engineering)
S. Teixeira De Freitas – Promotor (TU Delft - Aerospace Engineering)
K. Masania – Promotor (TU Delft - Aerospace Engineering)
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Abstract
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....
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