CRYSTALLOGRAPHIC TEXTURES FROM THE EXOSKELETON OF THE LOBSTER HOMARUS AMERICANUS AND CALCULATION OF THE MECHANICAL PROPERTIES OF THE CALCITE PHASE

摘要

The exoskeleton of the crustacean Homarus americanus, the American lobster, is a biological multiphase composite consisting of a crystalline organic matrix (chitin), crystalline biominerals (calcite), amorphous calcium carbonate and proteins. Variations in the composition and in the crystallographic arrangement of the crystalline phases of this tissue can lead either to a rigid material serving as a protective armor layer or it can render the material highly flexible serving as a constructional element as in articular membranes at joints. Beside these properties, the exoskeleton has a rather low density in view of the high stiffness it can provide in parts. One special structural aspect which has not yet been studied in detail is the occurrence of pronounced crystallographic orientations and resulting directional anisotropic mechanical properties. So far only separate pole figures have been interpreted [1, 2]. The crystallographic textures of chitin and calcite have been measured by wide angle Bragg diffraction, calculating the Orientation Distribution Function (ODF) from pole figures by using the series expansion method according to Bunge. A general strong relationship can be established between the crystallographic and the resulting mechanical and physical properties. In biological matter this aspect seems to be of particular importance since natural constructions exploit the presence of structural anisotropy of its components in a much more efficient and elegant way than usually encountered in man-made materials.