Study On B-value Of Acoustic Emission Of Rock After High Temperature

Based on the research background of underground engineering involving high temperature rock,rock mechanics and acoustic emission tests were carried out by using three typical rocks: sandstone,limestone and granite.The changes of acoustic emission and b-value in each stage of rock deformation during loading are studied.At the same time,taking granite after cold and heat cycle as an example,the effects of temperature and cycle times on acoustic emission and b-value of rock are studied.In this paper,the variation of b-value with temperature of three kinds of rocks is analyzed emphatically,and the change of rock interior is analyzed in detail from the microscopic point of view.The research results of this paper provide theoretical and experimental support for the high temperature problems encountered in the general engineering construction and the stability of surrounding rock in the process of geothermal development or tunnel fire rescue.The main results of this paper are as follows.(1)In this paper,sandstone and limestone are treated at high temperatures ranging from 25 ? to 900 ?.The temperature range of granite is 25 ?~650 ? and the number of cycles is 0~20.The variation of acoustic emission and b value of three kinds of rocks under different temperatures is systematically analyzed.It is found that the temperature threshold of sandstone is 550 ?,the temperature threshold of limestone is 600 ?,and the temperature threshold of granite is 450 ?.(2)The b-values of the three kinds of rocks change dynamically during loading.The b-value increases when the micro-fracture in the rock is in stable propagation;the b-value is almost unchanged when the failure state of microcracks is relatively stable;the b-value decreases with the increase of the proportion of large-scale cracks;the b-value changes in a large range when the micro-crack state suddenly changes,which represents a sudden instability propagation.(3)The b-value of sandstone increases slowly with the increase of temperature,decreases rapidly after 550 ?,and increases rapidly after 700 ?.The b-value of limestone decreases gradually with the increase of temperature,and there is a peak value about 600 ?.After 700 ?,the b-value increases rapidly.The b-value of granite increases slowly with the increase of temperature and cycle times.450 ? and 5 cycles of heating and cooling are a sudden change point where b-value increases rapidly.(4)High temperature mainly affects the change of mineral composition in rocks,and causes thermal damage to rocks,thus affecting the change of b-value.Sandstone and granite are mainly quartz expansion and contraction at high temperature,while limestone is mainly calcite expansion and contraction.