Viscoelastic fractures in stratified composite materials : ”

– We consider fractures in a stratified composite material with solid layers separated by thin slices of extremely soft matter. Viscoelastic effects associated with the soft layers are taken into account via the simplest model for weakly cross-linked polymers. We find that certain small cracks running along layers take a new " trumpet " shape quite different from previously known shapes. Introduction. – Nacre is a composite material with solid layers (aragonite) separated by thin slices of soft organic matter (proteins) (Fig. 1). It has a spectacular toughness where a fracture propagates normal to the layers [1, 2, 3, 4]. This toughness can be explained from the absence of stress concentration in this structure [5, 6]. In the present note, we consider a different problem, where the fracture plane is parallel to the layers. The fracture properties have some similarity with those of a smectic liquid crystal, which have been discussed under the name of lenticular fracture [7, 8]. Our aim here is to consider the dynamics, when the soft layers behave like a viscoelastic fluid. But we shall start with a reminder of the statics, using a simple scaling argument. The crucial feature is that, for a fracture cavity of size X along the layers, the elastically distorted zone has a size Y (perpendicular to the layers) which does not scale like X, but rather like Y ≃ X 2 /l (1)