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B425: Structure/property relationships of mollusc shells as natural biocomposite materials (Lead Supervisor: Liz Harper, Earth Sciences)

Supervisors: Liz Harper (Earth Sciences) and Michael Carpenter (Earth Sciences

Importance of the area of research:

The evolution of bivalve shells has been marked by increasing complexity of microstructure and decreasing organic content, while brachiopod shells have remained rather conservative in their construction. The shells of both groups are composite materials in which stiff but brittle calcite and/or aragonite crystals and soft but flexible organic components combine, presumably to optimise strength and flexibility in respect to plastic and elastic deformation which may be induced by predators and the physical environment. Early studies on bivalve shells suggested that many are mechanically strong but that different microstructural arrangements have very different properties. Surprisingly, however, many of the bivalve groups seem to have evolved ‘weaker' shells over evolutionary time. Scientific issues which can now be addressed in this context are (a) the relationship between microstructure/properties, environment and evolutionary history of the organisms, since the physical properties of their shells are likely to be a significant part of their ecology, and (b) the contrast between physical properties achieved in biosystems and those of man-made composite materials.

Project summary:

The purpose of this PhD project will be to apply a combination of new and traditional techniques to determine the elastic properties and brittle failure of a wide range of molluscs.The elastic properties of a wide variety of mollusc shells will be measured by nanoindentation and four point bending, to identify how microstructure determines the bulk properties. Selected samples will then be used for detailed investigation by X-ray tomography and finite element analysis in order to understand why particular microstructures lead to particular advantages in relation to bulk elastic properties which might be beneficial for particular ecological niches.

What the student will do:

The starting point will be taken from recent projects by MSci students who have demonstrated that the elastic properties of bivalve shells can be determined by conventional mechanical testing methods, including four point bending and compression of arched specimens. The Young's modulus, E, of specimens sawn from the shells and of the whole shells themselves will be measured to follow up on the observation that the values differ between species. These results will be complemented by nanoindentation measurements, following a newly devised protocol which ensures that the organic component remains hydrated. In each case microstructures will be characterised by SEM and organic contents by thermogravimetric analysis. The project will then build on recent advances using X-ray tomography which was used for the first time to produce a true 3d representation of the distribution of organic matter and inorganic matrix, in order to undertake finite element analysis of the mechanical properties.

Please contact the lead supervisor directly for further information relating to what the successful applicant will be expected to do, training to be provided, and any specific educational background requirements.

References:

Esteban-Delgado, F.J., E.M.Harper, A.G.Checa, A.B.Rodriguez-Navarro (2008) Origin and expansion of foliated microstructure in pteriomorph bivalves. Biological Bulletin 214, 153-165.

Bignardi, C., M.Petraroli, N.M.Pugno (2010) Nanoindentations on conch shells of Gastropoda and Bivalvia molluscs reveal anisotropic evolution against external attack. Journal of Nanoscience and Nanotechnology 10, 6453-6460.

Merkel, C., J.Deuschle, E.Griesshaber, S.Enders, E.Steinhauser, R.Hochleitner, U.Brand, W.W.Schmahl (2009). Mechanical properties of modern calcite- (Mergerlia truncata) and phosphate-shelled brachiopods (Discradisca stella and Lingula anatina) determined by nanoindentation. Journal of Structural Biology 168, 396-408.

Follow this link to find out about applying for this project.

Other projects available from the Lead Supervisor can be viewed here.

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