测试岩石抗拉强度双圆环试样构型和尺寸优化研究

Research on the optimization of the configuration and size of double-ring specimens used in rock tensile strength test

针对岩石抗拉强度测试所采用的双圆环试样易产生应力集中导致测试值偏小的问题,提出了一种新构型试样.采用ANSYS有限元软件模拟计算了121个含有不同内外圆环切割深度的双圆环试样的受载应力分布情况,分析了切割深度对试样受拉区和应力集中区的影响;在GCTS岩石力学试验机上测试了16个双圆环砂岩试样的抗拉强度值,并与巴西劈裂法的测试值进行了对比研究.结果表明:试样受拉区轴向变化曲线呈现出“双峰值现象”,两应力集中区的极值差随切割深度变化的等高线图为扇形分布图,零点线的斜率约为0.3;环形受拉区和应力集中区的峰值应力受切割深度影响显著,区域内峰值差异大是测试值偏低的主要原因;当双圆环试样的尺寸采用内圆环切割深度为4~8 mm,外圆环切割深度为14~16 mm时,应力集中区的平衡因子不超过2%,此时,额外弯矩对受拉区的影响较小,应力分布较均匀,能够实现最佳压拉转换,基于该尺寸双圆环试件得到的抗拉强度测试值接近巴西劈裂法,但标准差更小,能够相对准确地测试岩石的抗拉强度.

To solve the problem that the double-ring specimens used in rock tensile strength test tend to produce stress concentration,which results in the small test value,a new configuration specimen is proposed.The stress distribution of 121 double-ring specimens with different cutting depths of inner and outer rings was simulated and calculated by ANSYS finite element software,and the influence of cutting depth on the tensile and stress concentration areas of the specimens was analyzed.The tensile strength values of 16 double-ring sandstone samples were measured on GCTS rock mechanics testing machine and compared with those measured by Brazilian splitting method.The results show that the axial variation curve of the tensile area of the specimens presents a“double peak phenomenon”,the contour map of the extreme difference between the two stress concentration areas is fan-shaped distribution with the variation of cutting depth,and the slope of the zero line is about 0.3.The peak stress in the annular tensile and stress concentration areas are significantly affected by the cutting depth,and the large difference of peak values in the area is the main reason for the low test value.When the cutting depth of the inner ring is between 4 mm and 8 mm,and the cutting depth of the outer ring is between 14 mm and 16 mm,the balance factor of the stress concentration area does not exceed 2%.At this time,the additional bending moment has little effect on the tensile area,the stress distribution is uniform,and the optimal compression-pull conversion can be realized.The tensile strength test value obtained by the double-ring specimens of that size is close to the Brazilian splitting method,but the standard deviation is smaller.This method can test the rock tensile strength relatively accurately.