3D printing of biomimetic composites with improved fracture toughness

Abstract

Recent researches show that material microstructural designs mimicking biomaterials offer an effective way to produce tougher composites. To reveal the underlying microstructure-toughness relationship, five bioinspired material microstructures are investigated experimentally, including the brick-and-mortar, cross-lamellar, concentric hexagonal, and rotating plywood microstructures. The feature sizes of these microstructures are controlled to be one order smaller than the specimen size, providing better pictures of how crack resistance interacts with heterogeneity. Fracture theories are further used to analyze the toughening mechanisms and find the design criteria. Results show that the rotating plywood structure presents a “J” shaped R-curve, while other structures show “Γ” shaped R-curves. The “J” shaped R-curve gives a larger critical energy release rate and tolerates a longer crack, thus preferable for crack arresting. By contrast, the “Γ” shaped R-curve provides a larger critical failure stress and is beneficial for preventing crack initiation. Moreover, combined experimental results and theoretical analysis suggest that heterogeneity improves toughness by 1) creating stiffness variations to slow down crack propagation and prevent crack penetration and 2) guiding cracks along weak interfaces to promote progressive damage. Our results shed new light on the structure-property relationship that will facilitate the design of tougher and better crack resistant composites.