封圧下における大理石試料の曲げ破壊に対する破壊力学的考察

封圧下における岩石の曲げ破壊に関する研究 (第2報)

抄録

Fracture propagation in the flexural test under confining pressure is discussed theoretically from the viewpoint of linear fracture mechanics and compared with the experimental results on Akiyoshi marbles.
Taking the effect of pre-existing cracks into account, the differences in the test results between fine grained and coarse grained marbles with respect to the bending moment-outer fiber strain diagram and the stability of fracture propagatior are explained by the stress intensity factor as a function of fracture crack length provided that the external forces are kepi constant after fracture initiation (Fig. 8 (a) and (b)).
Fracture toughness and the effective length of pre-existing crack for fine and coarse grained marbles are evaluated fronthe relationship between the confining pressure and the outer fiber bending stress at fracture initiation. This is summarizec in Table 1 which shows that the effective length of pre-existing crack is approximately 8 times grain size for fine grainec marble and approximately 3 times for coarse grained marble, and that fracture toughness increases with the confining pressure, especially when the confining pressure is greater than 120 kg/cm².
Also, it is shown that the number of fracture cracks initiated between the two loading points is determined by the relaxation zone created by the initiation of fracture crack and, therefore, the number of fracture cracks depends directly or the average length of arrested fracture crack (Figs. 9 and 10).