平台巴西圆盘劈裂和岩石抗拉强度的试验研究

EXPERIMENTAL STUDY ON SPLIT TEST WITH FLATTENED DISK AND TENSILE STRENGTH OF ROCK

在巴西圆盘试样中引进平台作为加载面,可以改善加载处的应力状态。利用有限元计算平台巴西圆盘均匀位移压缩时,随着平台中心角的增大,圆盘中心的拉应力降低,压、拉应力比增大,应用Griffith准则能否确定岩石的抗拉强度以及应该如何选择平台中心角需要研究。对花岗岩、砂岩、石灰岩和辉绿岩,以中心角20°～90°的平台巴西圆盘进行劈裂试验,圆盘不是沿中心线破坏,且抗拉强度随平台中心角增大而增大。原因有两个,一是Griffith准则过高地估计了抗拉强度,二是试验机压头与平台之间存在摩擦。垫入0.5mm厚聚四氟乙烯薄片可使平台圆盘沿中心线破裂,但垫片侧向变形大于岩石,平台受到指向外侧的摩擦力,使得圆盘强度随中心角的增大而降低。单轴压缩强度高达240MPa的花岗岩,其劈裂强度(13MPa)与平台圆盘的劈裂强度相当,其余3种岩石完整圆盘的劈裂强度明显偏低,说明集中载荷造成的影响不容忽视。综合考虑摩擦效应和平台加工质量的影响,圆盘中心角以20°～30°为宜。

The tensile strength can be determined from the split test of a disk specimen,or Brazil test. However the rock near loading points will yield and fracture,which is different from its testing principle. Two flats compressed in uniform deformation are introduced to the Brazilian disk to improve the stress state. From the results of FEM,the tensile stress in the center of disk decreases with the central angle of the flats,and the ratio of compressive stress to tensile stress increases as well. So it must be studied whether the Griffith criterion in tensile region is true or not true,and how to choose the central angle of flattened disk. The flattened disks of granite,sandstone,lime and diabase with central angle of 20°～90° are tested. The disks do not crack along their symmetric lines,and the tensile strength increases with the central angle of flats,which results from that Griffith criterion gives a higher tensile strength and there is friction effect between flats and the ends of testing machine. The disks crack symmetrically when plastic cushions with thickness of 0.5 mm are used. But the plastic cushions produce new friction in opposite direction,which causes that the strength of disk decreases with increment of central angle. The tensile strengths of circle disk and flattened disk are well matched for granite. The Brazil strength is 13 MPa,and the uniaxial compressive strength is up to 240 MPa. Except the granite,Brazil strengths of the other rocks are much lower than that of the flattened disks,which results from the localized load in Brazil test. In order to reduce the effect of friction and the flat quality of specimen on test results,the better choice of central angle of the flat is 20°～30°。