DEVELOPMENT OF CONJUGATE-SHEAR FRACTURE FROM A CRACK IN MARBLE

Laser holographic interferoinetry technique is used for measuring the strain displacement field in the uni-axial compression testing of marble samples. The localization of deformation caused by the augmentation of microcracks can be revealed by the hologram.The rock sample used is a thin marble plate (105806 mm) with a alot 25mm long, and less than1 mm wide. It is stressed to failure under uni-axial compression in a servo-controlled testing machine. The sample is thought to be in a plane-stress state.The experiment shows:1. The strength of the rock sample is related to the angle 9 between the slot and the compression axis, when =45, it is lowest, in agreement with the theory of fracture mechanics).2. When is less than45, crack starts to grow from points initially under tensile stress near the slot tip. The crack extensions initially make an angle 90 with the slot and gradually bend towards the axial direction and then become stable.3. When angle is larger than45, secondary crack growth can happen near the ends of the slot after the primary cracks. These secondary cracks initially make an angle 60 with the slot. They also bend towards the axial direction and then become stable.4. For the tested, especially when less than 45, at the peak or over the peak loading, with the primary and secondary cracks already stabilized, there appears to have mierofractures springing up around slot tips, and developing into shear fractures in the direction conjugate to the slot and opposite to those of the two previous tensile cracks. These shear fractures seem to be the result of strain softening during the adjustment of stress and deformation near the crack tips under increasing axial loading.