DIRECT MEASUREMENT OF DISPLACEMENT AND ESTIMATE OF SHEAR FRACTURE ENERGY

Based on the equation used to estimate the shear fracture energy in slipweakening model, the method of displacement measurement in triaxial test is analysed. Displacement measured across the rock fracture by using a transducer mounted directly on the rock specimen is compared with piston displacement measured outside the pressurized vessel. We show that the former method is more precise and reliable than the latter. As the sampling rate is much faster compared with the stick slip process, the relation between stress and displacement can be obtained directly. Both internal and external transducers are used at same time to measure the same stick slip event. There are important differences between the results from these two types of tranc-ducers. The shear fracture energy (G) estimated by the former is much higher, from three to ten times, than the latter, and therefore is much closer to the seismologically inferred values. The reason of the lower value obtained by the external fransducer is also briefly discussed.