Failure mode transition in natural mineralized composites
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Failure mode transition in natural mineralized composites Reza Rabiei, Sacheen Bekah and Francois Barthelat1 1 Department of Mechanical Engineering, McGill University 817 Sherbrooke Street West, Montreal, Quebec H3A 2K6 Canada ABSTRACT Mineralized biological materials such as nacre and bone achieve remarkable combinations of stiffness and toughness through staggered arrangements of stiff components bonded by softer materials. These natural composites are therefore substantial source of inspiration for emerging synthetic materials. In order to gain new insights into structureperformance relationships of these staggered structures, nacres from four species were compared in terms of fracture toughness and damage propagation pattern. Fracture tests revealed that all nacres display rising crack resistance curves, but to different extents. Using in-situ optical and atomic force microscopy, two distinct patterns of damage propagation were identified in columnar and sheet nacre respectively. These two different patterns were further confirmed by means of large scale numerical models of staggered structures. Similar mechanisms possibly operate at the smallest scales of the microstructure of bone. INTRODUCTION Structural biological materials like seashells or mineralized skeletons are composed of relatively weak, small scale structural components, assembled in intricate ways that lead to remarkable combinations of stiffness and toughness. In some cases the degree of “amplification” of mechanical performance from the base components is unmatched by any synthetic material [1, 2]. A well known example of this performance is nacre from mollusk shells. Nacre is made of microscopic mineral tablets (95% vol.) closely packed by means of an organic glue (5% vol.) to form a three-dimensional brick wall structure [3]. The mineral tablets are arranged either in distinct columns forming columnar nacre, or in a random fashion resulting in sheet nacre [4]. Despite its highly mineralized structure, nacre is 3,000 times tougher than its mineral ingredient [1], a remarkable material property amplification which stimulates further research to understand the mechanisms involved. The staggered arrangement of the tablets in nacre dictates a wellknown load transfer mechanism, whereby the applied tensile load is transferred through tensile stress in mineral tablets and shear stress at the softer interfaces between them. With sufficient applied load the tablets “slide” on one another, this “sliding” being mediated by thin layers of softer organic materials capable of accumulating large deformations while dissipating energy. In case of columnar nacre where a quasi-periodic structure predominates, sliding only occurs in the overlap regions between the tablets, generating deformation bands perpendicular to the loading direction [4, 5]. Cracks eventually also follow the overlap regions [6]. However, for sheet nacre no clear deformation pattern has been reported. It is also not clear which of the columnar and sheet nacre arrangements results in a tougher
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