Enhanced energy of parallel fractures in nacre-like composite materials

Nacre, often found inside sea shells such as oysters, has a spectacular resistance to fracture, which results from their construction out of alternating layers of soft and hard material. The fracture resistance is greatest for cracks moving perpendicular to the layers, but cracks parallel to the layers also have difficulty in progressing, and this fact has not been well explained. In this letter, we consider small parallel fractures in nacre-like material and show the possibility that the fracture energy is increased over a value due to the near-tip processes by a factor lambda that is given by the ratio of short- and long-time viscoelastic moduli of the soft matter. Small cracks running along the layers take a scaling form we call a "lenticular trumpet" that has not previously been observed.