Fracture of Ensis siliqua mollusk shell reveals multiple delaminations as a potential defence and toughening mechanism

Excerpt

Shells display remarkable mechanical properties, particularly their ability to direct crack-growth along boundaries between the calcium carbonate crystals they comprise. This crack deflection acts as a defence mechanism against attack from predators. Some shells, such as Ensis siliqua, are dominated by prismatic layers, a particular microstructural arrangement of calcium carbonate. These layers give the shell a laminated structure, allowing it to resist bending, but also to act as regions that promote delamination. This short communication shows that judicious placement of many prismatic layers close to the innerside of the shell, where the mollusk resides, gives rise to multiple delaminations of shell material. Sections of Ensis siliqua shell are bent in an electron microscope, mimicking the action of an attack by a crab or other predator. It is shown that many cracks form close to the innerside of the shell. Images of these cracks are taken during the deformation process within the electron microscope and later processed to determine work of fracture. Delaminations are shown to significantly increase fracture resistance compared to values obtained for non-biogenic aragonite. These mechanisms are discussed with reference to predation, but also in relation to designing materials that could exploit this property. …