THE EFFECT OF SIZE OF CRACK ON ENERGY OF SHEAR FRACTURE

This paper deals with an experiment on cylindrical marble samples in which artificial unlinked cracks making angles of about 30?with the cylindrical axis are cemented by epoxy resin. The effect of crack size and filling material on energy of shear fracture are studied experimentally. The results are that as the relative area of the cracks increases from zero to 90% the shear fracture energy decreases gradually from (6.92.1) 103J/m2 to(0.60.5)103J/m2. The theoretical values of stress drop obtained by Knopoff's method and the method of slip-weakening model are almost the same. However, the ratio of these values of stress drop to that observed experimentally increases gradually and reaches about 10 as the relative crack area increases from zero to 90%. At the same time, some possible factors causing the large difference of stress drop during earthquakes and rock failure and the large difference of values of shear fracture energy obtained by various methods, are discussed. It has been proposed that the effect of the honeycomb-contact structure of fault surfaces may be important.