LENTICULAR FRACTURE OF A SMECTIC LIQUID-CRYSTAL

We discuss the Griffith problem, where a thin cavity (parallel to the smectic layers) is extended by a tensile stress sigma-0 (normal to the smectic planes). The shape of the cavity (two spherical caps intersecting at a fixed angle 2-theta-0) is very different from the Griffith ellipsoid, obtained with simple elastic media: the shape can be understood in terms of the renormalized surface tension of smectics. This <<lenticular>> fracture mode is observable only when the layers are stiff, e.g., with alternate layers of two homopolymers A and B where A is molten, but B is glassy. With soft materials, undulation instabilities should prevail.