  In our previous paper (Eur. Phys. J. E 4, 121 (2001)) we proposed a
coarse-grained elastic energy for nacre, or stratified structure of hard and
soft layers. We then analyzed a crack running perpendicular to the layers and
suggested one possible reason for the enhanced toughness of this substance. In
the present paper, we consider a crack running parallel to the layers. We
propose a new term added to the previous elastic energy, which is associated
with the bending of layers. We show that there are two regimes for the
parallel-fracture solution of this elastic energy; near the fracture tip the
deformation field is governed by a parabolic differential equation while the
field away from the tip follows the usual elliptic equation. Analytical results
show that the fracture tip is lenticular, as suggested in a paper on a smectic
liquid crystal (P. G. de Gennes, Europhys. Lett. 13 (8), 709 (1990)). On the
contrary, away from the tip, the stress and deformation distribution recover
the usual singular behaviors (sqrt(x) and 1/sqrt(x), respectively, where x is
the distance from the tip). This indicates there is no enhancement in toughness
in the case of parallel fracture.
