The mechanical design of nacre

Abstract

Mother-of-pearl (nacre) is a platelet-reinforced composite, highly filled with calcium carbonate (aragonite). The Young modulus, determined from beams of a span-to-depth ratio of no less than 15 (a necessary precaution), is of the order of 70 GPa (dry) and 60 GPa (wet), much higher than previously recorded values. These values can be derived from ‘shear-lag’ models developed for platey composites, suggesting that nacre is a near-ideal material. The tensile strength of nacre is of the order of 170 MPa (dry) and 140 MPa (wet), values which are best modelled assuming that pull-out of the platelets is the main mode of failure. In three-point bending, depending on the span-to-depth ratio and degree of hydration, the work to fracture across the platelets varies from 350 to 1240 J m -2 . In general, the effect of water is to increase the ductility of nacre and increase the toughness almost tenfold by the associated introduction of plastic work. The pull-out model is sufficient to account for the toughness of dry nacre, but accounts for only a third of the toughness of wet nacre. The additional contribution probably comes from debonding within the thin layer of matrix material. Electron microscopy reveals that the ductility of wet nacre is caused by cohesive fracture along platelet lamellae at right angles to the main crack. The matrix appears to be well bonded to the lamellae, enabling the matrix to be stretched across the delamination cracks without breaking, thereby sustaining a force across a wider crack. Such a mechanism also explains why toughness is dependent on the span-to-depth ratio of the test piece. With this last observation as a possible exception, nacre does not employ any really novel mechanisms to achieve its mechanical properties. It is simply ‘well made’. The importance of nacre to the mollusc depends both on the material and the size of the shell. Catastrophic failure will be very likely in whole, undamaged shells which behave like unnotched beams at large span-to-depth ratios. This tendency is increased by the fact that predators act as ‘soft’ machines and store strain energy which can be fed into the material very quickly once the fracture stress has been reached. It may therefore be advantageous to have a shell made of an intrinsically less tough material which is better at stopping cracks (e. g. crossed lamellar). However, nacre may still be preferred for the short, thick shells of young molluscs, as these have a low span-to-depth ratio and can make better use of ductility mechanisms.

Hierarchical structure observation and nanoindentation size effect characterization for a limnetic shell

“…Its stiffness is on the same order as mineral, but its strength is two to three times higher than that of mineral. Specifically, the fracture toughness of a nacre structure is 3000 times higher than that of a monolithic mineral [16][17][18][19]. Thus far, the mechanical properties of even man-made ceramic cannot reach such a high level [20,21].…”

Blood-clotting-inspired reversible polymer–colloid composite assembly in flow

“…Nature, on the other hand, can assemble to perfection complex composite materials for a variety of functions. For example, the abalone shell is made out of calcium carbonate and organic matrices, or bone that is composed of calcium phosphate and collagen fibres, both of which display excellent mechanical properties 8,9 . In most cases, these materials are constructed very slowly, and are perhaps not viable for technological applications.…”

A novel biomimetic approach to the design of high-performance ceramic–metal composites

“…These structural biological materials are sophisticated hierarchical composites based on brittle minerals and ductile polymers, which can exhibit mechanical properties that are far beyond those that can be achieved using the same synthetic compounds (Meyers et al 2008a). In particular, the principles of biologically controlled self-assembly in seashells have generated much recent interest (Jackson et al 1988;Kamat et al 2000;Evans et al 2001;Wang et al 2001;Rubner 2003;Mayer 2005); indeed, it has been suggested that their sophisticated structures can offer guidelines for the development of new generations of structural materials (Ortiz & Boyce 2008;Aizenberg & Fratzl 2009). …”