Quasi-static and dynamic mechanical response of Haliotis rufescens (abalone) shells
Introduction
The study of materials that have evolved through millions of years of evolution and natural selection can provide insights into heretofore-unexploited mechanisms of toughening. Biomimetics is a newly emerging interdisciplinary field in materials science and engineering and biology, in which lessons learned from biology form the basis for novel material concepts 1, 2. This new field of biomimetics investigates biological structures, establishing relationships between properties and structures in order to develop methods of processing and microstructural design for new materials [1].
Many properties of biological systems are far beyond those that can be achieved in synthetic materials with present technology [3]. Biological organisms produce complex composites that are hierarchically organized in terms of composition and microstructure, containing both inorganic and organic components in complicated mixtures 4, 5. These totally organism-controlled materials are synthesized at ambient temperature and atmospheric conditions. The unique microstructures in biological composites and the resulting properties have been, until recently, unknown to materials scientists, but are now beginning to stimulate creativity in the development of future synthetic materials.
The objectives of this work are to evaluate the static and dynamic response and evolution of damage in abalone (Haliotis rufescens). It is already known that the mechanical properties of these “composite” shells are outstanding, if one considers their weak constituents, namely calcium carbonate (CaCO3) and a series of organic binders 6, 7. These mollusks owe their extraordinary mechanical properties to a hierarchically organized structure, starting with single crystals of CaCO3, with dimensions of 4–5 nm (nanostructure), proceeding to “bricks” with dimensions of 0.5–10 μm (microstructure), and culminating in layers approx. 0.2 mm (mesostructure). However, to date their dynamic properties have not been established, and previous mechanical testing has been restricted to three- and four-point bending. Little is known about the mechanisms of compressive failure, as well as about the effect of loading rate on their response.
Section snippets
Mollusk shells
Mollusk shells consist of a proteinaceous matrix in which one or more ceramic phases are embedded 8, 9, 10. These ceramic phases, i.e. calcium carbonate (CaCO3), are not suitable as structural materials because of their brittleness. However, mollusks are known to possess hierarchical structures highly optimized for toughness. These weak constituents, calcium carbonate and a series of organic binders, assembled in a hierarchical fashion, provide the mollusk shells with outstanding mechanical
Experimental approach
The shells studied herein were purchased at a local shell shop (La Jolla, CA) in a dry condition. All abalone samples were cut out of the same specimen to minimize varying test results due to differences in shell history or age. The abalone belonged to the “red” family. The first cuts were made by hand, using a hacksaw with an abrasive blade. After obtaining pieces of appropriate size, a high-speed diamond saw was used for further sectioning. Long flat areas with a minimum length of 40 mm and
Mechanical properties
There are many uncertainties in performing mechanical tests with mollusk shells. Besides the varying layer thickness, there are a considerable number of other natural shell irregularities such as flaws, existing microcracks or even macrocracks, and perforations made by foreign organisms. Additionally, there is some uncertainty about a given shell’s history, including its age and degree of hydration. As such, the determination of the mechanical properties of these shells requires a statistical
Summary and conclusions
Mechanical tests were carried out over a range of stress rates and stress states to assess the mechanisms of damage accumulation in an abalone shell. The strength of this shell shows a considerable variation, and is well represented by a Weibull distribution with parameter m varying between 2.5 and 6.8. The quasi-static compressive strengths [P(V)=0.5] are on the order of 540 and 235 MPa for loading perpendicular and parallel to the layered structures (configurations A and B in Fig. 2),
Acknowledgements
This research was partially supported by the US Army Research Office under the MURI program (Contract No. DAAHO4-96-10376). R. Menig worked at UCSD as part of an exchange program between the University of Karlsruhe (TH) and the University of California, San Diego. Appreciation is extended to Otmar Vöhringer for making this exchange possible. We would like to thank David Harach and Y.-J. Chen for setting up the quasi-static test configurations and A. Strutt for his assistance in the SEM
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