Mechanical Properties of Nacre Constituents: An Inverse Method Approach
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Mechanical Properties of Nacre Constituents: An Inverse Method Approach Francois Barthelat and Horacio D. Espinosa Mechanical Engineering Department, Northwestern University 2145 Sheridan Road, Evanston, Illinois 60208, U.S.A. ABSTRACT Nacre, also known as mother-of-pearl, is the iridescent layer found inside some mollusk species such as oyster or abalone. It is made of relatively weak materials, but its hierarchical microstructure is so well optimized that its macroscopic mechanical properties are far superior to those of its constituents. For this reason there is a great interest in nacre as a source of inspiration for novel designs of composites. Despite many years of research on nacre, an accurate characterization of its constituents is lacking. In this work nacre was tested as a layered composite material using low depth indentation and uniaxial compression. The first test was modeled using finite element analysis and the second test was modeled as a Reuss composite in compression. A micromechanical model of the interface was also pursued to gain insight on the relevance of the interface features such as tablet roughness and biopolymer hydrated response. The results of the two experiments were combined to solve an inverse problem that yielded the needed properties for tablets and interfaces. These findings are expected to make possible computational models of nacre with a new degree of accuracy and therefore contribute to a better understanding of the mechanisms leading to its remarkable properties.
INTRODUCTION Nacre is a biological structural material found in some seashells such as oyster or abalone. It is a composite material mainly composed of aragonite (95% vol.) arranged in microscopic tablets (Figure 1) which are bonded together by a biopolymer mortar (5% vol.). Aragonite is a brittle ceramic, but by the addition of the organic phase and a well-designed microstructure nacre achieves strength and toughness that are 20 to 30 times that of monolithic aragonite [1]. Its remarkable design and the hierarchical mechanisms responsible for its behavior have attracted much attention in the past few years. Many authors suggested that the microstructure of nacre was optimized for strength and toughness [2-4] which makes it attractive as a source of inspiration for novel composite materials designs. Most of the micromechanics models developed in the past describe nacre as a combination of hard inclusions (tablets) with a much softer interface [3-5]. However accurate material properties for each of these phases are still missing. Tables I and II show some properties for aragonite, nacre tablets and their interface. Some of these values were measured experimentally while others were borrowed from similar materials. Few attempts were made to measure the properties of nacre constituents independently. Katti et al. [5] and Li et al. [6] presented results for indentation tests on nacre, but in interpreting the data they used standard techniques [7] that neglect the effect of roughness and underlying interfaces.
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