13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- the critical crack length is derived from Eq. 6. Average IC K values for 3D model of CCNSCB considered were confirmed to converge to constant values with Twenty-five concentric rings of crack front mesh elements. Here, fracture toughness is considered from the middle point of crack front. Table 1. Summary of fracture tests conducted on CCNSCB specimens manufactured from the crystalline rock max( ) p kN Test No. 1.914 1 1.82 2 1.8 3 1.871 4 1.876 5 1.981 6 1.877 Average Figure 4. A typical finite element mesh used for simulating CCNSCB specimen. The experiments conducted on CCNSCB showed that before the crack reaches its critical length, no significant resistance is observed with crack extension. Indeed, in the chevron-notched specimens, the crack is allowed to extend sub-critically, due to the high stress concentration. The chevron notch causes the crack propagation to initiate at the tip of the V alignment and to extend radially outwards in a stable fashion until the point where the fracture toughness is obtained. V or chevron shape generates a process of stable crack growth under increasing load, from the initial crack length ( 0a ) to its critical length ( ma ). Beyond ma , the crack extension takes place in an unstable fashion. At the critical length, the pre-crack has fully developed in the rock sample and the fracture toughness is evaluated. This procedure also provides a smaller fracture process zone (FPZ), compared to a straight crack. The experimental results also show that V-shaped grooves provide sharper critical cracks and lower scatter in fracture loads. Therefore, this specimen can be a good alternative for measuring fracture toughness of rock masses and for investigating the process of crack growth in brittle materials.
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