SELECTION OF DEFORMATION MEASUREMENT DATA OF ROCK UNDER MTS COMPRESSION TESTS

This paper investigates the accuracy and applicability of axial strain obtained by axial extensometer, LVDT and press-plate displacement. We carried out the uniaxial and triaxial compression tests of rock with MTS testing machine, and compared the stress-strain curves obtained with the axial extensometer, press-plate displacement and LVDT. Furthermore, we analyzed the effects of the results under three different measurement methods on the shape, strength and deformation parameters of the stress-strain curves of rock samples. The results show that in the pre-peak strength stage, the deformation data measured by the plate displacement and LVDT, the stress threshold value and deformation parameters have large differences. So axial strain should adopt the axial extensometer to obtain the deformation data. In addition, when the deformation data measured by the axial extensometer reaches the peak strength, the axial strain can decrease, and this curve can be easily mistaken as a class Ⅱ curve. It is recommended to install a LVDT sensor at the fixed head of the MTS testing machine, and calculate the axial strain after peak strength with the axial deformation measured by LVDT. When the LVDT sensor is not allowed to be installed, we should adopt the pressure plate displacement measurement to represent the deformation of the sample after the peak strength.