径向压缩下圆环砂岩样的力学特性研究

Mechanical properties of ring specimens of sandstone subjected to diametral compression

为了研究径向压缩下圆环试样孔壁处的应力特征,利用厚度为34 mm,外径为50 mm的完整圆盘及不同内径(8～30 mm)的圆环砂岩试样,在巴西劈裂试验中测量孔壁的应变变化,分析试样的力学特性。试验结果表明:圆环试样的峰值载荷随着内径的增大而逐渐减少。圆盘和内径较小的圆环试样达到峰值载荷时出现了失稳破坏,载荷迅速跌落:当内径大于16 mm,圆环试样达到峰值载荷后,载荷略有下降,但是试样并没有出现失稳破坏,而是持续压缩一定时间后才破裂。圆环最弱部位拉应力不是材料参数,而是一个结构参数,且随圆环内径而变化,基于弹性理论的Hobbs公式不能用于计算岩石抗拉强度。孔壁岩石的破裂只能是达到拉伸变形极限才会破裂,不能以拉伸应力作为破坏指标。研究结果为理解岩石在压拉组合下的力学特征提供了参考。

An experiment using the rings of 34 mm in thickness, 50 mm in external diameter, and 8-30 mm in internal diameter was designed via the ring test, to analyze the stress characteristics around the hole. Strain gauges were used to record the strain changing around the hole during the test, and the mechanical properties of specimens were studied. The results indicate that the peak load of the specimens decreases with the increase of internal diameter. Both the discs and ring specimens with relatively small internal diameters have abrupt failure when they reach the first peak load. However, when the diameter is greater than 16 mm, there is no clearly defined stress drop after the first peak stress, but a distinct stress drop after the second peak stress is reached. The stress at the weakest point in the ring specimen is a structural parameter rather than a material parameter, and it changes with different internal diameters. The Hobbs’ equation based on elastic mechanics may not be suitable for determining the tensile strength of rock. Rock specimen needs adequate tensile deformation to fail until it reaches the limit stress. The failure of the borehole in the rock may be evaluated by the tensile deformation instead of tensile strength. The results provide important reference for mechanical characteristics of rock under compression-tension stresses.