  The two-chain Frenkel-Kontorova model is applied for an analytical
description of the energy and structure of the network of domain walls in
bilayer graphene. Using this approach, the commensurate-incommensurate phase
transition upon biaxial stretching of one of the graphene layers is considered.
We demonstrate that formation of the equilateral triangular network of domain
walls becomes energetically favourable above the critical relative biaxial
elongation of the bottom layer of $3.0\cdot 10^{-3}$. It is shown that the
optimal period of the triangular network of domain walls is inversely
proportional to the difference between the biaxial elongation of the bottom
layer and the critical elongation as long as it is much greater than the width
of domain walls. Quantitative estimates of the contribution of a single
dislocation node to the system energy and the period of the network of domain
walls are obtained. Experimental measurements of the period could help to
verify the energy of the fully incommensurate state (such as obtained by
relative rotation of the layers) with respect to the commensurate one.
