The characterisation of residual strain in Ensis siliqua shells

Abstract

This study reports the variation of residual strains within the posterior ventral area of the Ensis siliqua mollusc shell, as determined using glancing incidence synchrotron X-ray diffraction. The outer layer of this structure exhibits a tensile strain, in contrast to a compressive strain observed within the inner layer. Fluctuations in unit cell parameters for the inner layer have been determined, showing that the microscopic prismatic layer of the structure exhibits a compressive strain orientated parallel to the surface of the shell. This is thought to enhance the crack deflection properties of this layer, and aid in resisting catastrophic failure. Further analysis of residual strains has been performed using the same method, throughout several stages of compressive testing of the anterior dorsal region of the shell. This identified no variation in residual strains at various levels of loading, and it is therefore proposed that load may be transferred via the organic matrix of mollusc shell structures. A Raman spectroscopic investigation, comparing whole and powdered shell with non-biogenic aragonite, has shown that residual strains are also present in this analagous material which is devoid of organic content. This indicates that the observed strain is not entirely due to the organic matrix.