We use lattice simulations to study the dynamics of cracks under appli
ed strains in brittle materials in the presence of substantial viscous
dissipation. This is probably a physically relevant limit in real bri
ttle materials. In large samples, cracks accelerate in the absence of
viscous effects and become unstable against branching. With viscosity,
cracks have a limiting velocity and long straight cracks can be stabl
e.