Inhomogeneous Strain/Stress Profiles in the Nacre Layer of Mollusk Shells
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I.
INTRODUCTION
INORGANIC/ORGANIC composites produced by organisms reveal superior mechanical properties, which are greatly influenced by the hierarchical ultrastructures of these materials on various length scales.[1–4] Sophisticated arrangement of different structural blocks results in the three-dimensional distribution of forces acting within a material. These forces are under intensive studies aimed at better understanding of the mechanics of biogenic structures.[5–9] One of the most investigated microstructures is the nacre in mollusk shells.[10,11] The nacre layer is built of oriented ceramic lamella of calcium carbonate, the interfaces between them being filled by organic substance (Figure 1). Such a structure demonstrates greatly enhanced resistance to fracture mainly due to crack retardation in the regions containing the inorganic/organic interfaces. These interfaces are the source of internal forces and related inhomogeneous stress/strain fields. Additional forces are imposed on ceramic lamella by those organic macromolecules, which are located within the ceramic crystallites themselves.[12] These so-called intracrystalline organic molecules[13,14] cause anisotropic lattice distortions of the unit cells in biogenic aragonite[15–21] and calcite,[22,23] and, being converted to stresses, correspond to about 200 MPa.[12] Thus, the mechanics of biogenic materials, in general, and mollusk shells, in particular, is determined by a complicated interaction at different length scales between the organic substance and the mineral phase. Specifically, this interaction can influence the shape of biogenic structures via internal bending forces[24] originating in the organic/inorganic interfaces. Elastic bending of the shell, when it occurs, should lead to the specific distribution of the bending strain component B. POKROY, Doctor, V. DEMENSKY, Student, and E. ZOLOTOYABKO, Professor, are with the Department of Materials Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel. Contact e-mail: [email protected] Manuscript submitted December 6, 2009. Article published online March 25, 2010 554—VOLUME 42A, MARCH 2011
across the shell thickness, equaling zero at the bending axis and reaching the maxima (of the opposed signs) at the outer and inner surfaces. In order to probe the elastic strain/stress distributions across the shell thickness, we have developed the depth-resolved strain measurement technique, in situ under controlled etching in selected areas.[25] Etching-mediated gradual stress release, which starts at the outer shell surface, is revealed as the time-dependent change of the shell curvature. The latter is measured by a strain gage glued to the inner (lustrous) shell surface. In our previous article,[25] we focused on the curvature analysis and developed a static model, in which the shell curvature is explained in terms of depth-dependent interface forces. However, the strain release curves themselves, measured as a function of etching time, remained untouched since their treatment is beyond th
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