基于直裂纹半圆三点弯曲试验的煤样断裂特性研究

Study on fracture characteristics of coal samples based on notched semi-circular bending tests

为研究煤的变形断裂特性,采用三点弯曲测试法对3组预裂半圆盘煤样开展Ⅰ型断裂试验,探讨了Ⅰ型断裂韧度与单轴抗压强度、单轴抗拉强度以及弹性模量的相关性,得到了煤样三点弯曲试验中的载荷位移特征,利用非接触全场应变测量系统监测分析了煤样变形破坏过程中的应变场演化规律与断裂过程。试验结果表明:3组煤样的Ⅰ型断裂韧度分布在0.14~0.29 MPa·m,煤样断裂裂纹均从预制裂纹顶端起裂,扩展路径近似为直线,呈典型的张拉破坏形式。煤样的Ⅰ型断裂韧度与其弹性模量和单轴抗拉强度正相关,线性关系式分别为:E=10.47K和σ=5.48K,相关性系数R~2分别达到0.99和0.97。煤样的变形破坏过程可分为压密阶段、弹性变形阶段、弯曲变形阶段、抵抗裂纹产生阶段和破坏后阶段。弯曲变形与抵抗裂纹产生阶段是能够表征煤样断裂特性的重要阶段,抵抗裂纹产生阶段载荷的差值直观反映了煤样Ⅰ型断裂韧度的大小。弯曲变形阶段前,水平应力主要集中在直裂纹两侧;进入弯曲变形阶段后,直裂纹顶端出现应力集中并逐步向上发展;进入抵抗裂纹产生阶段后,应力集中程度与范围快速增大,逐步形成张拉裂纹的潜在轮廓与路径,煤样断裂瞬间应变率呈显著的非线性增长特征。

To study the fracture characteristics of coals,the three-point bending method was used to conduct the modelⅠfracture tests of three sets of notched semi-circular bending coal samples.The correlations of modelⅠfracture toughness with uniaxial compressive strength,uniaxial tensile strength and elastic modulus were discussed.The load and displacement characteristics of coal samples in the three-point bending test were derived.By using the non-contact full-field strain measurement system,the strain field evolution law and fracture process of the coal samples were monitored and analyzed.The results show that the model I fracture toughness of the three sets of test coal samples is ranging between 0.14-0.29 MPa·m.The cracks of the samples emerged from the tips of the prefabricated straight crack,and the propagation path is approximately straight,indicating a typical tension failure mode.The modelⅠfracture toughness of coal is positively correlated with elastic modulus and uniaxial tensile strength.The linear relations are respectively:E=10.47Kandσ=5.48K,and their correlation coefficients R~2 are 0.99 and 0.97,respectively.The failure process of coal samples can be divided into five stages:compaction,elastic deformation,bending deformation,crack resistance and post-destruction.The bending deformation and crack resistance are the critical stages to characterize fracture characteristics of the coal samples.The loading difference of the crack resistance stage directly represents the modelⅠfracture toughness of the samples.Before the bending deformation,the horizontal stress of the samples is mainly concentrated on both sides of the straight crack.After the bending deformation,the stress concentration occurs at the tip of the straight crack and gradually develops upward.During the crack resistance stage,the degree and range of stress concentration significantly increase,and the potential configuration and path of the tension crack are gradually formed.The fracturing moment of the coal samples shows significant nonlinear growth characteristics of the strain rate.