A mechanical model for simulating intergranular stress corrosion cracking is presented. The model has been developed to address the limitations of percolation-like models, which do not account for the mechanical crack driving force and cannot capture experimentally observed phenomena such as the formation of ductile bridging ligaments by resistant boundaries. The model is based on a regular representation of material microstructure and a categorisation of grain boundaries as susceptible and resistant to corrosion. Crack propagation in two-dimensional microstructures with several fractions of experimentally observed susceptible boundaries is studied. Monte Carlo-type simulations with random distributions of boundaries and a range of susceptible and resistant boundary failure strengths are performed. The effects of crack bridging and crack branching are quantified. It is concluded that together with the fraction of susceptible boundaries, the resistant boundary failure strength is the significant parameter controlling the shielding effect of bridges on crack propagation.