چكيده به لاتين
In this study, according to the practical importance of crack growth process in rock materials, the crack propagation in the fracture toughness testing methods of rock materials was discussed using several methods. At first, numerical investigation of crack growth in semi-circular bend specimens with straight through and chevron notches was performed using the finite element analysis. The fracture parameters including dimensionless stress intensity factor and critical crack length that indicates the region of stable crack growth in specimens with V-shaped notch, for a wide range of geometric parameters were calculated. In addition, an analytical method (slice synthesis method-SSM) was performed. Good agreement was observed between the results of two mentioned methods.
Applicability of three experimental methods for studying the crack growth process was investigated. The process of crack growth in specimens with straight through and V- shaped notches were compared based on the number of counts obtained from acoustic emission technique. Also, the images of X-ray CT scanner were also applied to determine the critical crack length with acceptable accuracy. In addition, an optical microscope was used to compare the smoothness of fracture surface in two regions of stable and unstable crack growth. The obtained images showed the smoother surface in stable crack growth region.
The effect of specimen geometry and notch geometry on the value of fracture toughness was studied using the extended maximum tangential strain criterion in that T-stress term is considered. The results showed the direct effect of T-stress on the fracture toughness of rock samples. Moreover, the statistical analysis of rock fracture toughness for several samples (with straight-through and chevron notches) was studied using Weibull probability distribution. More average value of fracture toughness and lower scattering were obtained for chevron notched specimens in comparison with straight notched specimens.
Furthermore, the effect of water vapor pressure on the rock fracture toughness was investigated experimentally. It was seen that by increasing the water vapor pressure, rock strength decreased. In mode II loading condition, this influence is more considerable than that in mode I. In addition, the SCB tests under confining pressure were conducted in order to investigate the effect of the confining pressure on the mode-I fracture toughness of rock. Based on the simulation results, a corrected form of ISRM suggested formulation was introduced for the rock fracture toughness of rocks under confining pressure.
Keywords: rock fracture toughness, critical crack length, slice synthesis method, water vapor pressure, confining pressure
