13th International Conference on Fracture June 16–21, 2013, Beijing, China proportional to confining pressure from 0 to 28MPa. In addition, they compared their experimental results with some similar results proposed by other researchers and found that the tendency of fracture toughness increased with the increasing confining pressure was basically consistent but the corresponding relation of fracture toughness for different rocks was not the same, and some relationship were nonlinear. It is necessary to point out that this method belongs to Brazilian disk experiment under both confining pressure and diametrical forces loading condition. The validity and reliability of the results obtained by using the stress intensity factor formulas which only consider concentrated load regardless of confining pressure to calculate the stress intensity factors(or fracture toughness) under both the confining pressure and diametrical forces loading condition are debatable. And, whether or not the critical loading condition (or critical loading angle) for pure mode II crack under both the confining pressure and diametrical forces is the same with the critical loading condition (or critical loading angle) for pure mode II crack under diametrical forces has to be discussed. Lou et al.[9] designed a test system of the mode I and mode II fracture toughness for rocks using Brazilian disk specimens and hydraulic fracturing technique on MTS multifunctional material testing machine, and calculated the mode I and mode II stress intensity factors by using the approximation formulas proposed by Atkinson et al.[1]. They analyzed the effects of specimen size, confining pressure and other factors on the mode I and mode II fracture toughness of rocks according to the experimental data. The results showed that: both of the mode I and mode II fracture toughness increased with the increasing confining pressure in a certain pressure range, but the trend was not obvious under high confining pressure. Their experiment principle and calculation formula of stress intensity factors are similar to Al-Shayea’s method and formula [8], and have similar problems. Therefore, how to evaluate effects of confining pressure on the stress intensity factors and critical loading angle for pure mode II crack is an important problem, and is worth of further investigating. It will provide the theoretical foundations for conducting fracture toughness tests under confining pressure. In the present paper, based on the formula of the stress intensity factors for central cracked circular disk subjected to diametrical forces, the closed-form expressions of the stress intensity factors for central cracked circular disk subjected to confining pressure was obtained firstly and then the explicit expressions of the stress intensity factors were achieved under both the confining pressure and diametrical forces loading condition. Finally, the effects of confining pressure on the stress intensity factors and critical loading angle was analyzed. 2. Stress intensity factors for Cracked Brazilian Disk under confining pressure 2.1 Stress components for uncracked Brazilian Disk under confining pressure The schematic diagram of an uncracked Brazilian disk under confining pressure is shown in Fig.1. The thickness of the disk isBand the radius isR. Fig.1 Uncracked Disk under confining pressure
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