13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- * 0 α ∂ = ∂ Y The related numerical results will be presented and discussed later in section 4. 3. Sample preparation and fracture toughness tests A crystalline rock was selected for fracture toughness tests. This type of rock contains very few discontinuities and can be assumed to be isotropic and homogeneous. Figure 3. Loading set-up utilized for conducting fracture tests in CCNSCB specimen The rock samples were prepared from a rock sheet of 20mm thick using water jet to form semi-circular disks of diameter 80mm. A rotary saw of radius = s R 50mm and thickness 0.6mm was used to generate the chevron notch in the specimens. The penetration of rotary saw into specimens was considered 11mm. In the experiments, the half-distance between the two bottom supports (S) was 20mm. The standard 3-point bend loading was used for fracturing the SCB specimens (see Fig. 3). After creating the chevron notch, the specimen was placed inside the loading set-up and the failure loads were recorded. SANTAM/STM-150 machine with a capacity of 15000N was utilized for conducting the fracture tests on the CCNSCB specimens. 4. Dimensionless stress intensity factor 4.1. Numerical and experimental results It was observed in the entire experiments that when the load reached its critical level, sudden failure took place for the test samples. The load-displacement curve was almost linear up to the maximum load. Thus, using the critical failure load in each experiment, the fracture toughness values of CCNSCB specimen can be computed from Eq. 3. The details of experimental results obtained from each test including, the fracture loads are listed in Table 1. Using the maximum load obtained from the experiment, the corresponding fracture toughness is calculated using finite element modeling. The singular elements were considered in the first ring of elements surrounding the crack tip for producing the square root singularity of stress/strain field. A J-integral based method was used for obtaining the stress intensity factors. Fig. 4 shows a typical mesh pattern generated for simulating the CCNSCB specimen. The average value of maximum load was 1.877kN and using this value in the finite element model, mode I fracture toughness IC K is obtained 1.208 1 2 MPa.m and by its substitution in Eq. (3), the dimensionless stress intensity factor in the CCNSCB specimen is found 3.64. During the simulation,
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