  Using a two dimensional lattice model we investigate the crack growth under
the influence of remote tensile forces as well as due to an internally applied
pressure (hydraulic fracturing). For homogeneous elastic properties we present
numerical finite-size scalings for the breaking stresses and pressures in terms
of crack lengths and lattice sizes. Continuum theory predicts for the tensile
and for the pressure problem identical scaling functions. Our findings for the
tensile problem are in very good agreement with continuum results. However, for
the hydraulic fracture problem we observe a different finite-size scaling. We
explicitly demonstrate that the modified scaling is a consequence of the
discrete structure of the lattice (micro structure).
