Residual stress in mollusc shells

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Australian Institute of Nuclear Science and Engineering
The aragonite-protein composite material out of which the shells of most molluscs are made has a fracture toughness of about 8 MPa√m. This is surprisingly high considering that the aragonite phase has a bulk fracture toughness of only 0.9 MPa√m. One reason for the improved performance of the shell relative to pure aragonite is that it is comprised of a strongly textured array of aragonite platelets in an organic matrix. It is well known that cracks that initiate in an aragonite platelet are deflected, blunted or arrested when they reach the more ductile organic phase. We speculate that a compressive residual stress at strategic locations in the shell may further improve its resistance to crack propagation. To investigate this hypothesis we used neutron diffraction, X-ray diffraction and Raman spectroscopy in an attempt to identify whether a detectable stress distribution exists in a large mollusc shell and, if so, whether this stress state can provide enhanced fracture toughness. Freshly collected shells of the gastropod Ninella torquata (family Turbinidae), which has a diameter of about 10 cm, were used. The texture of the samples was readily extracted using neutron diffraction and an apparent residual stress gradient of several MPa identified. This effect was not evident in X-ray diffraction of powder samples taken in layers spaced through the wall thickness. The possible existence and implications of a non-uniform stress distribution through the shell are analyzed and discussed.
Composite materials, Molluscs, Aragonite, Fracture properties, Residual stresses, Neutron diffraction, X-ray diffraction
Cortie, M. B., Dowd, A., He, K. Y. H., Choi, A., & Luzin, V. (2016). Residual stress in mollusc shells. Paper presented at 13th AINSE-ANBUG Neutron Scattering Symposium, Sydney, NSW, Australia, 29-30 November 2016.