硬脆性页岩断裂韧性二维数值模拟研究

Two-dimensional numerical simulation of fracture toughness of hard brittle shale

以四川盆地长宁地区龙马溪组的页岩为研究对象,采用真实破裂过程RFPA^(2D)数值分析软件,模拟人字形切槽巴西圆盘试件渐进破裂的整个过程,研究不同层理密度、层理角度以及层理力学性质等因素与页岩断裂韧性的关系,以及这些因素对裂缝前缘形态的影响,并探究断裂韧性与岩石脆性的关系。研究结果表明:断裂韧性随层理角度增加而先减小后增大;在层理角度相同的条件下,随着层理面力学性质增强,断裂韧性逐渐增大;当层理角度较小时,裂缝前缘随层理面发生转向,并且沿层理发生脆性剪切破坏;当层理角度大于40°时,层理面作用显著减弱,裂缝在延伸时前缘随层理出现转向和分叉,并在试样端部产生次生裂纹;层理密度较小时,裂缝在预置裂缝方向开裂后,受层理面影响转向层理面方向,最终贯穿圆盘,若角度增大,伴随有次生裂纹出现,若增大至80°以上时,裂缝穿过层理面,会出现转向和分叉;层理密度较大时,裂缝不均匀扩展现象增多,圆盘端部伴随产生的次生裂纹数量也逐渐增加;随着岩石的脆性指数增大,表现的断裂韧性减小,脆性指数有随角度先增大再减小的趋势。

Taking the shale of Changning area in Sichuan Basin as a research object, the effect of different bedding density, bedding angle and bedding mechanical properties on the fracture toughness of the RFPA 2D was simulated by using the numerical analysis software of the real fracture process and simulating the progressive rupture of the Brazilian disc specimen with Herringbone notch. It was discussed the influence of these factors on the shape of fracture front, and it was investigated the relationship between fracture toughness and rock brittleness. The results showed as follows: the fracture toughness decreased with the angle change first and then increased, the minimum value occurred at 20°, and the fracture toughness increased with the increase of the mechanical properties of the bedding plane under the same bedding angle;when the bedding angle was small, the fracture front turned with the bedding surface, and the brittle shear failure along the bedding layer occurred;when the bedding angle was greater than 40°, the bedding effect was obviously weakened;when the crack was extended, the leading edge turned and forked with the bedding, and a secondary crack was produced at the end of the specimen;when the bedding density was small, the crack in the direction of the crack in the preset fracture was affected by the bedding surface, and the direction of the steering layer was influenced by the layer, with secondary cracks appearing, if increased to above 80°,when the cracks passed through the bedding surface, the steering and bifurcation would appear, and when the bedding density was large, the uneven expansion of the crack increased and the number of secondary cracks accompanying the disc end increased gradually;the fracture toughness decreased with the increase of brittle index of rock, and the brittleness index had the tendency of increasing and decreasing with the angle first.