Crack coalescence and brittle failure characteristics of open rock bridges
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摘要: 端部开裂的裂隙岩体内部岩桥贯通是导致岩体突发失稳破坏的主要原因,脆性破坏特征明显。利用声发射仪和岩石力学刚性试验机,对不同长度开放岩桥开展单轴压缩试验,结合高速摄像机记录信息,分析了开放岩桥裂纹起裂、扩展、贯通规律及脆性破坏过程力学特性和声发射特征,并通过三维断裂力学理论揭示了开放岩桥裂纹起裂扩展贯通机理。研究结果表明：与闭合裂纹相比,开放裂纹岩桥主裂纹从下部预制裂纹内尖端起裂,往上部拐折扩展贯通岩桥和上端面,贯通过程岩桥发生逐次多级破坏,次裂纹主要沿主裂纹拐折处起裂扩展贯通岩桥及下端面；应力曲线表现出峰前“波动上升”和峰后“滞留–突降”特征,声发射参数“多峰值”现象明显,阶段性和突发性特征突出；同时,三维理论分析表明裂纹起始扩展方向与岩桥长度无关,沿最大压应力方向扩展。研究所得开放岩桥裂纹扩展贯通特征可为裂隙岩体突发失稳破坏过程提供理论依据。Abstract: The coalescence of rock bridge of fractured rock mass with end cracking is the main cause of the sudden damage of the rock mass, and the brittle failure is obvious. The uniaxial compression tests are conducted on the open rock bridges with different lengths by using the acoustic emission tester and the rock mechanics rigidity tester. By use of the information recorded by high-speed camera, the crack coalescence laws and acoustic emission characteristics of open rock bridges are analyzed. The mechanism of crack initiation of open rock bridges is revealed by the theory of three-dimensional fracture mechanics. The results show that compared with the closed crack, the main crack of an open rock bridge starts from the tip of the crack at the lower part, and the main crack turns to the upper part expanding through the rock bridge and the top end. During the coalescence of the main crack, the rock bridge presents successive multi-stage failure. The secondary crack propagates along the inflection of the main crack and extends through the rock bridge and the lower end. The stress curve shows the "fluctuation rise" before the peak and "stagnation-sudden" after the peak, and the "multi-peak" phenomenon of the acoustic emission is obvious, with obvious stage and sudden features. Simultaneously, the three-dimensional theoretical analysis shows that the crack initiation direction is independent of the length of the rock bridge and extends along the direction of the maximum compressive stress. The obtained crack propagation characteristics of the open rock bridge may provide a theoretical basis for the sudden failure of the fractured rock mass.
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Keywords:
- fractured rock mass /
- open rock bridge /
- crack coalescence /
- brittle damage /
- acoustic emission
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图 1 不同长度花岗岩岩桥试样示意图
Figure 1. Schematic diagram of granite bridge samples with different lengths
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图 2 不同长度岩桥单轴压缩轴向应力–轴向应变曲线
Figure 2. Stress-strain curves of rock bridges with different lengths under uniaxial compression
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图 4 不同长度典型岩样破坏面及裂纹扩展角θ0三维示意图
Figure 4. Three-dimensional diagram of failure surface and crack propagation angle θ0of typical rock samples with different lengths
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图 6 开放裂纹起裂扩展破坏特征示意图
Figure 6. Schematic diagram of crack initiation and propagation failure characteristics of open cracks
表 1 应力曲线分界点对应强度与单抗强度比
Table 1 Strengths and uniaxial compressive strength ratios of stress curve demarcation point
岩桥长度/mm 强度比/% A/D B/D C/D 50 36.05 72.09 87.21 60 45.63 74.76 95.15 70 41.77 81.00 83.54 
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表 2 单轴压缩下不同长度花岗岩岩桥力学参数均值
Table 2 Average mechanical parameters of granite bridges with different lengths under uniaxial compression
岩桥长度/mm 高度/mm 直径/mm 弹模/GPa 强度/MPa 50 99.95 50.04 11.23 88.86 60 99.92 50.02 13.65 114.11 70 100.01 50.05 16.95 147.82
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表 3 不同长度典型岩样裂纹起裂贯通过程及脆性破坏形态
Table 3 Crack initiation coalescence process and brittle failure morphology of typical rock samples with different lengths
岩样编号 裂纹起裂 裂纹扩展–贯通模式 脆性破坏形态 破坏特征描述 G-50-2 
①主裂纹从下部裂纹尖端起裂,产生2条张拉裂纹向上部扩展贯通岩桥；②主裂纹贯通过程有多个拐折点,说明岩桥不是一次性贯通,而是多次张拉-剪切逐级破坏,贯通后大块崩落,次裂纹沿主裂纹发育；③次裂纹快速扩展贯通为宏观裂纹,试件发生脆性破坏,伴随较小声响,岩块轻微弹射。 G-60-2 
①主裂纹从下部裂纹尖端起裂,产生1条张拉裂纹向上倾斜扩展贯通岩桥上端面,贯通过程发生多次剪切拉裂；②主裂纹贯通后大块崩落,次裂纹沿主裂纹拐折处萌生扩展；③次裂纹向下扩展贯通岩桥下端面,向内扩展贯通岩桥,伴随较大响声,岩桥脆性破坏,岩块弹射远。 G-70-2 
①主裂纹从下部裂纹尖端起裂,产生1条张拉裂纹向上倾斜扩展贯通岩桥上端面,贯通路径拐折处明显减少；②主裂纹贯通后大块崩飞,尖端处小岩块弹射,次裂纹发育；③次裂纹快速扩张,部分贯通试件下端面,部分向内扩展贯通岩桥,试件发生脆性破裂,响声大,岩块弹射很远。
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表 4 单轴压缩下不同长度开放岩桥裂纹扩展角
Table 4 Crack propagation angle of open bridge with different lengths under uniaxial compression
试件编号 扩展角θ0/(°) 试件编号 扩展角θ0/(°) 试件编号 扩展角θ0/(°) G-50-1 65 G-60-1 71 G-70-1 70 G-50-2 70 G-60-2 68 G-70-2 76 G-50-3 71 G-60-3 — G-70-3 69 G-50-4 69 G-60-4 72 G-70-4 72 G-50-5 62 G-60-5 69 G-70-5 68 均角 67.40 均角 70.00 均角 71.00
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表 5 不同长度岩桥AE特征信息统计
Table 5 Statistics of AE feature information of rock bridges with different lengths
试样编号 微增 第一稳定期起止时间/s 突增 第二稳定期起止时间/s 剧增（最大值） 计数/(106次) 能量/(106aJ) 计数/(106次) 能量/(106aJ) 计数/(106次) 能量/(106aJ) G-50-2 0.81 0.92 298~342 1.36 1.83 360~380 5.10 7.22 G-60-2 2.33 2.16 375~405 3.75 4.14 415~450 7.08 10.6 G-70-2 0.82 0.73 361~372 1.18 1.71 375~440 8.24 14.1
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