The conch shell as a model for tougher composites

The conch shell is 95% by volume CaCO3 (chalk), yet its toughness is 10 3 times greater than that of monolithic CaCO3. In this review paper we look at how this increase in toughness is achieved and what lessons can be learnt for designing new tough composites. Essentially, we find that the CaCO3 is finely divided into single crystals whose relevant dimensions are below the Griffith flaw size for the anticipated stresses; thus upon failure intergranular cracking dominates. Furthermore, failure is encouraged to proceed in a controlled way, which frustrates crack growth and maximises crack surface area. This strategy of maximising damage can only be successful in combination with self-healing properties. Examples are given of synthetic analogues, so-called biomimetic materials.

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