crack propagation laws were studied by means of observing marble specimens
which were conducted uniaxial static and cyclic loading tests
with electron-microscope. The studying results showed: (1) Different parts of rock specimen suffer differential degree damage; (2) Failure models and characteristics of rock specimens change with loading paths; (3) Rock cracks are to be closed
when amplitude of cyclic stress is smaller than “fatigue threshold stress” of rock. This is mainly testimony of “rock fatigue threshold stress” existence.