Abstract:
The shape and structure of the shells of molluscs has attracted considerable attention. One aspect of interest is the comparatively high
resistance to fracture of these shells. It is known that they are composite structures of aragonite, other calcereous materials, and up to 5%
by volume of protein ‘glue’. A large component of their toughening derives from crack tip blunting, deflection and closure, concepts wellknown
from the field of fracture mechanics. However, the possibility that they might also derive a measure of toughening from a residual
stress distribution has been generally overlooked, although Illert first raised this over a decade ago. The optimum situation would be
when the inner surface of the shell is maintained in a state of tensile stress, while the outer layers are in the necessarily counter-balancing
compressive state. We have examined this hypothesis using a combination of neutron diffraction and scanning electron microscopy and
find that it is certainly feasible. However, a definitive proof will require a diffraction study at higher resolution.
