A novel biomimetic material duplicating the structure and mechanics of natural nacre
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Nacre from mollusk shell is a high-performance natural composite composed of microscopic mineral tablets bonded by a tough biopolymer. Under tensile stress, the tablets slide on one another in a highly controlled fashion, which makes nacre 3000 times tougher than the mineral it is made of. Significant efforts have led to nacre-like materials, but none can yet match this amount of toughness amplification. This article presents the first synthetic material that successfully duplicates the mechanism of tablet sliding observed in nacre. Made of millimeter-size wavy poly-methylmethacrylate tablets held by fasteners, this “model material” undergoes massive tablet sliding under tensile loading, accompanied by strain hardening. Analytical and finite element models successfully captured the salient deformation mechanisms in this material, enabling further design refinements and optimization. In addition, two new mechanisms were identified: the effect of free surfaces and “unzipping.” Both mechanisms may be relevant to natural materials such as nacre or bone. I. INTRODUCTION
Materials produced by nature exhibit remarkable properties that are attracting the attention of engineers, material scientists, chemists, and zoologists in search of inspiration for novel material designs.1 Often made of materials with relatively poor structural qualities, biological materials achieve their performance through intricate microstructures finely tuned over millions of years of evolution.2–4 Elucidating the structure– properties relationships in these materials is a challenging task, but is nevertheless an essential step for a successful biomimetic “transfer of technology” from natural to synthetic materials and systems. Nacre from mollusk shells, also known as mother of pearl, is now identified as an excellent model for high-performance materials offering attractive combinations of stiffness, strength, and toughness.5 Nacre is a highly mineralized material made of 95% of the mineral calcium carbonate, which comes in the form of microscopic polygonal tablets closely stacked to form a dense, three-dimensional (3D) brick wall.6 The remaining constituents are proteins and polysaccharides, which form an organic matrix concentrated at the interfaces between the tablets.7 Nacre is relatively stiff and hard compared to other biological materials,2 which is easily explained by its high mineral content. The most remarkable and less obvious property of nacre is actually its toughness, which is 3000 times that of the mineral it is made of (in JIC terms).2 Much of this toughness is explained by nacre’s ability to deform past a “yield point” and to develop large a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.65 J. Mater. Res., Vol. 26, No. 10, May 28, 2011
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inelastic strains over large volumes around defects and cracks. This mechanism generates tremendous toughness amplification,8 effectively making nacre damage tolerant.9 Failure strains in nacre e
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