The role of residual stress in the fracture properties of a natural ceramic

The role of residual stress in enhancing the fracture properties of the shell Ensis siliqua (razor shell) is reported. Using energy-variable synchrotron X-ray diffraction it is shown that the calcium carbonate crystals are preferentially orientated as a function of depth. Using scanning electron microscopy the structure is found to be mainly divided into two regions, namely crossed lamellar and prismatic. The latter are shown to be at a higher residual compressive stress than the former which gives rise to delaminations of the structure upon mechanical deformation. The role of residual stress in the structure is also shown to be critical to the fracture properties of the material and this has implications for the fabrication of man-made laminate ceramics.