Tough and stiff composites with simple building blocks
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From bone to dentin to nacre, biomaterials are structurally advanced composites with superior toughness and significant stiffness, based on simple building blocks. Here, using a series of molecular mechanics models with bioinspired topologies, we propose design mechanisms rooted in the simplest mechanical interactions—perfectly brittle linear elastic—which are shown to be sufficient to achieve superior toughness at high stiffness in biological composites. In a two-phase composite system, we show that by adapting the elastic constitutive laws of the matrix phase and by tuning the interactions of the constituents we can realize materials with a large range of combinations of toughness and stiffness. Notably, this can be achieved without changing the fracture energy of the individual composite components. Through a systematic analysis and the development of a simple model, we unveil basic design principles that lead to fundamental insights into the mechanics of natural composites for applications in a range of engineering disciplines.
I. INTRODUCTION
Composite materials are commonly designed from building blocks with contrasting material properties with the goal of combining two attractive properties in one material system. For structural applications, it is of particular interest to understand how to design composites that effectively combine the properties of stiffness and toughness. Experimental investigations have shown that many mineralized biological composites combine these two properties very well, e.g. human bone and nacre.1–3 To exploit the full potential of composite systems in the context of toughness and stiffness, it is imperative to attain a fundamental understanding of the key design mechanisms governing the fracture response of composites. Significant experimental, computational and theoretical work has been performed to explain the superior toughness of biological composite systems and many interesting mechanisms have been proposed. The more common and notable mechanisms cited include, but are not restricted to: energy dissipation and ductility through protein unfolding in the organic matrix phase,4,5 frictional dissipation due to shearing of mineral-organic interfaces,6,7 cooperative deformation mechanisms at a hierarchy of structural levels,8–11 and flaw tolerance at nanoscale attained through nanoconfinement of building blocks.12
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.88 J. Mater. Res., Vol. 28, No. 10, May 28, 2013
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Here we propose that far simpler interactions—linear elastic up to a brittle breaking point—can be sufficient to lay the foundations of superior toughness in two-phase stiff biocomposite structures. Inspired by biological materials we hypothesize that the stiffness ratio, in the linear elastic regime of the constituents, controls the deformation and fracture mechanism of a composite. For a range of mineralized biological composites that combine toughness and stiffness, such as n
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