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0898-L02-03.1

Mechanical Properties of Biological Nanocomposite Nacre: Multiscale Modeling and Experiments on Nacre from Red Abalone Pijush Ghosh, Devendra Verma, Bedabibhas Mohanty, Kalpana Katti, Dinesh Katti North Dakota State University Fargo, ND, 58105, USA. ABSTRACT Nacre, the inner iridescent layer of mollusks shell is a bio-nanocomposite with the mineral aragonite as a major constituent and 2-5% of organics mainly in the form of proteins. Our multiscale modeling and experimental studies reveal that the microstructure and the small weight percent of organics are the key parameters attributed to the extreme toughness of nacre. We report that the presence of platelet interlocks nacre have a significant role in the enhancement of mechanical properties. Molecular simulation study is conducted to understand the behavior of aragonite-organic interface. The mechanical behavior of organics and inorganics in presence of each other is described using steered molecular dynamics simulations. This provides some understanding on the deformation mechanisms of the protein present between the aragonite layers. Our nanoindentation results indicate that the elastic modulus and hardness of nacre decreases as it is exposed to a denaturing temperature for proteins. The changes in the organic inorganic interaction have been experimentally described using Fourier Transform Infrared Spectroscopy. This work gives insight into the contribution of the various factors existing at different length scales on the overall mechanical behavior of nacre. INTRODUCTION Nacre, the shiny inner layer of molluscan seashells is a natural biocomposite with exceptional engineering properties. The major constituent of nacre is aragonitic CaCO3, with 2-5% by weight of organics mainly in the form of proteins. In literature, the structure of nacre is widely investigated [1-3]. Nacre consists of highly organized polygonal shaped aragonitic platelet layers of approximate dimension of 0.5 µm. These aragonitic platelets are separated by organics layers of approximately 20 nm thickness. This structure of nacre with alternate layers of aragonite and organics is similar to “brick and mortar” structure [3-6]. Nacre has fracture toughness which is 3000 times more than that of pure aragonite [7]. Further nacre exhibits a highly hierarchical morphology which is a characteristic of biological materials. Due to its exceptional mechanical properties, nacre is a source of inspiration for biomimetic design. We have developed 3D finite element (FE) models that incorporate the details of the nano and microstructure of nacre. Our FE simulations indicate that the average modulus of the organic interphase is of the order of 15 GPa.[8], which is much larger than expected for bulk structural proteins. Subsequently, this prediction was validated with results from nanoindentation experiments. Further, our simulations also indicated that for a purely laminated “brick and mortar” architecture, the yield stress of the organic phase has to be extraordinarily high [9]. We incorporated