基于巴西劈裂试验的页岩强度与破坏模式研究

Strength and failure modes of shale based on Brazilian test

为了研究页岩横观各向同性性质对劈裂强度及破坏模式的影响,对7组不同倾角下的试样进行了巴西劈裂和声发射测试。研究表明：层理的方向对试样的劈裂强度和最终破坏模式有着重要的影响。随着层面倾角？的增加,页岩试样的劈裂强度逐渐减小。在不同倾角下的破坏模式可分为直线形（？？0°、90°）、月牙形（？？15°）和曲弧形（？？30°、45°、60°、75°）。通过分析不同倾角下圆盘中心点沿层理分解得到的正应力和剪应力,当？？0°时,试样的破坏方式为基质的拉伸破坏;当？？90°时,试样的破坏方式为层理的拉伸破坏,当15°≤？≤75°时,试样的破坏为复合型破坏。观察声发射空间累计定位演化分布情况,直线型的声发射点较均匀地分布在试样中轴线的两侧;曲弧型的声发射点在圆盘的一端较密而另一端较稀疏;月牙型则在试样的中部和一端都累积了部分声发射点;分布特征与宏观破坏方式具有相似性。分形维数D可以较好地反映试样的破坏模式,从倒V形的变化趋势可知,直线形试样D值接近于2,而月牙形和曲弧形试样D值介于2.2~2.7。因此,D值越大,说明裂纹的曲率半径越大;相反D值越小,裂纹越趋于一条直线,破裂形态也就越简单。

This paper aims to investigate the influence of transversely isotropic property on the splitting strength and failure modes of shale. Brazilian tests and acoustic emission tests were conducted upon the shale specimens with seven bedding orientations. It is revealed that the bedding orientation plays an important role in the splitting strength and the final failure modes. With the increase of inclination angle ？, the splitting strength decreases gradually. Three failure modes can be identified under different inclination angles from the tests： straight-like crack（ ？ ？ 0°, 90°）, crescent-like crack（ ？ ？ 15°） and arc-like crack（ ？ ？ 30°, 45°, 60°, 75°）. By analyzing the the normal and shear stress components decomposed along the bedding plane, the failure mode of specimen for ？ ？ 0°is the tensile failure of matrix;the failure mode of specimen for ？ ？ 90°is the tensile failure of bedding plane and the composed failure mode occurres when 15°≤？ ≤ 75°. The spatial evolution and distribution of the acoustic emission（AE） points we have observed is similar to its macroscopic failure modes. The AE points of straight-like crack is evenly distributed at both sides of the specimen axis; the AE points of arc-like crack is dense on the one side and sparse on the other side of Brazilian disk; the AE points of crescent-like crack are distributed at the middle and one end of the specimen. The fractal dimension can reflect the failure modes of the specimen very well. From the inverted V shaped trend of the fractal dimension, the value D of the straight-like failure mode is close to 2; while D of the crescent-like and arc-like failure modes is between 2.2 and 2.7.It can be drawn from the fact that the larger of the value D is, the larger of the crack＇s radius curvature is. The smaller of the value D is, the crack is more close to a straight line and failure mode is simpler.