Abstract：Based on the principle of correspondence between velocity field of incompressible viscous fluid in steady state and displacement field of incompressible elastic solid in equilibrious state，the stress formulae expressed in stream function are deduced and the equivalent relation between streamline and principal stress trajectory is derived. Using the solution of streamlines of the flat-plate flow，the extension law of cracking path for rock-type materials is predicted under compressive loading，which is in good consistence with the experimental results.
