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Multiscale toughness amplification in natural composites Francois Barthelat, Reza Rabiei and Ahmad Khayer Dastjerdi Department of Mechanical Engineering, McGill University 817 Sherbrooke Street West, Montreal, Quebec H3A 2K6 Canada ABSTRACT Hard biological materials such as bone and nacre exhibit remarkable mechanical performance, particularly in terms of fracture toughness, despite the weakness of their constituents. Mechanical performance of nacre and bone can largely be explained through their staggered microstructure where stiff inclusions of high aspect ratio are embedded in a softer matrix. The mineral inclusions provide hardness and stiffness while the organic matrix introduces ductility. The high performance of these natural structures is unmatched by any synthetic ceramic, which therefore makes them a substantial source of inspiration for development of new artificial materials. While the modulus and strength of these structures are well understood, fracture toughness remains unclear and controversial. In this work, chevron double cantilever beam fracture tests show that the interfaces in nacre have a low toughness, comparable to that of the tablets (in J terms). This highlights the important role of structural design on fracture toughness. At the next step, a fracture model is presented to explain the toughness amplification observed in natural staggered structures based on two essential extrinsic toughening mechanisms: crack bridging and process zone. The modeling results show that toughness can be further amplified by incorporating high concentrations of small inclusions with high aspect ratio. This conclusion is applicable to construction and optimization of natural and biomimetic composites. INTRODUCTION The remarkable performance of structural biological composites has made the mechanics of these materials an increasingly interesting subject for researchers and engineers in the past few decades [1]. To achieve high mechanical performance, natural materials employ a large variety of structures, mechanisms and ingenious designs. Beyond this apparent diversity, however, common structural patterns or “universal motives” can be found across biological materials at smaller length scales [2]. An excellent occurrence of universal motives is the staggered structure, where stiff inclusions of high aspect ratio are embedded in a ductile organic matrix with some overlap. A well known example of such structure is nacre from mollusk shells, where aragonite tablets form a three dimensional staggered structure with softer protein and polysaccharide layers at the interface (Figure 1a) [3]. Under tensile stress the tablets can “slide” on one another, a key mechanism that generates significant deformation and energy dissipation. Another example is collagen fibrils, composed of staggered collagen molecules. In bone the fibrils are mineralized by nanometers size crystals, intercalated with the softer collagen molecules in a staggered fashion (Figure 1b) [4]. In turn, those fibrils assemble into a staggered structure to fo