  We model and derive the solution for the problem of a Mode I semi-infinite
crack propagating in a discrete triangular lattice with bonds having a contrast
in stiffness in the principal lattice directions. The corresponding Green's
kernel is found and from this wave dispersion dependencies are obtained in
explicit form. An equation of the Wiener-Hopf type is also derived and solved
along the crack face, in order to compute the stress intensity factor for the
semi-infinite crack. The crack stability is analysed via the evaluation of the
energy release rate for different contrasts in stiffness of the bonds.
