真三轴试验中的端部摩擦效应分析

端部摩擦效应是三轴试验的常见问题之一。相对于常规三轴试验，由于在茂木式真三轴试验中方形试样4个端部需设置两对端部垫块，从而导致端部摩擦问题更加突出。运用FLAC3D，针对Mohr-Coulomb材料，即无中间主应力影响材料，模拟真三轴加载过程中端部摩擦对试样强度和变形行为的影响。计算结果表明，端部摩擦也可以产生虚假中间主应力效应，即使对于无中间主应力效应材料，中间主应力也会导致最大破坏主应力的增加，且摩擦系数越大，这种趋势则越明显。分析了端部摩擦产生这种趋势的原因，指出了真三轴试验中减小端部摩擦的重要性。

Analysis of end friction effect in true triaxial test

End friction effect is one of common problems in triaxial test. In the Mogi-type true traxial test, two pairs of loading platem need to beset on four ends of quadrate sample, which means that the end friction effect is more significant than that in conventional triaxial tests. This study aims at revealing the extent to which the end friction influences rock strength and deformation behavior. True triaxial tests are simulated using FLAC3D where Mohr-Coulomb ideal yield criterion is adopted. The simulation results indicate that the end friction can also lead to illusive effect of intermediate principal stress, even though the rock has no intermediate principal stress effect, the intermediate principal stress will still cause the increase of maximum principal stress, and this tendency become more obvious with the increasing of friction coefficient. Then, the reasons of the results and tendency are analyzed; and the importance of reducing end friction effect in the true triaxial test should be pay much more attention to.