Effects of Nanostructure on Bulk Mechanical Properties of Nacre - 3D Finite Element Modeling of the Segmented/Layered Bi
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Effects of Nanostructure on Bulk Mechanical Properties of Nacre 3D Finite Element Modeling of the Segmented/Layered Biocomposite Dinesh R. Katti,1 Kalpana S Katti, 1 Jingpeng Tang,1 Jeffrey M. Sopp2, and Mehmet Sarikaya2 1 Department of Civil Engineering, North Dakota State University, Fargo, ND 58105, USA 2 Materials Science and Engineering, University of Washington, Seattle, WA 98195 ABSTRACT Nacre is a segmented layered composite containing both nanoscale-thick organic and submicrometer thick scale inorganic phases. In addition to controlling the formation of the intricate architecture, the organic plays a significant role in the mechanical properties of the biocomposite. In our previous work, three dimensional finite element models of nacre were constructed to design ‘brick and mortar’ micro-architecture to study effects of nonlinear response of the organic component. Recently, nanomechanical properties such as hardness and elastic moduli of the individual components of nacre have been determined using nanoindentation techniques. In this work, we used these actual properties of the components to perform mesoscale finite element models to quantitatively evaluate nanoscale effects. Specifically, we studied the effect of the solid contacts between the platelets through the organic layer on bulk properties under tensile and compressive loading. In the new 3D finite element model, we also incorporated the pseudo-hexagonal platelet morphology to more accurately represent the nacre microstructure. The multiscale approach in our study involves incorporation of experimentally obtained nanoscale parameters into meso-macroscale numerical models. Our simulations imply that mineral contacts in the aragonitic platelets have marginal impact both on bulk elastic behavior of nacre and yielding. Stress concentration in contact regions were high enough for the contacts to break long before yield started in nacre. These results have important implications in the design of biomimetic segmented-layered composites for improved mechanical properties.
INTRODUCTION Nacre, the inner section of some mollusk shells, is a laminated composite consisting of 0.5 µm thick aragonitic platelets separated by a thin 10-30 nm thick layer of organic composed mainly of proteins and polysaccharides. This microarchitecture is often referred to as ‘brick and mortar’ [1-5] because of the staggered organization of the platelets in successive layers. The platelets are pseudo-hexagonal, but they can also be trigonal, square, and pentagonal, all containing multiply twinned domains. The organic matrix is generally uniform throughout the thickness, and is made of ordered proteins and polysaccharides within a matrix. Nacre has a combination of high strength and toughness both in compression and bending. The 4-point bend fracture toughness and specific flexural strength nacre (of, e.g., red abalone and pearl oyster) are 20 to 50 times those of synthetically produced monolithic CaCO3 polycrystals and are also better than most monolythic ceramics (such as Si3N4, Si
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