13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- It can be seen from Table 1 that in the range of -500C to 2400C, with the temperature increases, the variation of elastic modulus E is not obvious. It has only a little volatility. From Table 2, for the same type of specimen, It can be seen that the critical fracture load reduce with the temperature increase. It is unconcerned with the crack location and the type of rock. Under the same temperature, the value of the critical load is related with the crack position. The shorter the distance between crack and central position of the specimen is, the lower the critical load is. In the rang of -500C to 2400C, for different specimens, with the temperature increase, the deviation rate η between F and F0 gradually reduce. For white linen granite, when L = 30 mm, η gradually reduce from 28.95% down to -21.71%. when L = 15 mm, η gradually reduce from 17.86% down to -33.07%. For red linen granite, when L = 30 mm, η gradually reduce from 37.02% down to -43.39%. When L = 15 mm, η gradually reduce from 27.95% down to -37.82%. Figure 5 Specimen after fracture Figure 6 Divide grid 3. Finite element analysis For abnormal three point bending I-II-III mixed mode fracture specimens in this paper, numerical solution of stress intensity factors for every point in crack tip line were not got in other documents, no more than analytic solutions. In this paper the distribution laws of stress intensity factors KI、KII、 KIII and their critical values in crack fracture initiation point were obtained by FEM calculation under different temperatures. 3.1.Finite element modeling By try of FEM calculations many times, it had been known that if the short sections outside the two lower supports of the specimen were ignore, it did not affect the simulation results obviously, so in this paper, the sections between the two lower supports of specimen were choose into FEM calculation. By this modeling way, unnecessary elements can be reduced. It can improve the calculation speed. Cartesian coordinate system was defined as follows: X axis is along the length of the specimen, Y axis is up along the specimen thickness direction, Z axis is forward along the width direction (Figure 6). The width of edge crack is not more than 1mm. The crack surface is a free surface, and the crack front is a rectangular shape. It is consistent with the crack front in the specimens of experiments. Scanning grid division is used. In order to get accurate stress values near crack tip, refined grids around crack tip are also used. Divided elements situation are shown in Figure 6. Substitute the material parameters E, μ and critical load F into the FEM program, critical values (mixed mode critical fracture toughness) of KI, KII, KIII in failure crack tip can be calculated.
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