Study On The Effect Of Lode Angle On Sandstone Strength And Acoustic Emission Characteristics

The strength and failure law of rock are the key factors that determine the stability of the surrounding rock of underground space and the mode of oil and gas resource extraction（fracturing mode of reservoir rock）.Due to tectonics,most deep rocks are in true triaxial stress condition（σ₁≥σ₂≥σ₃）.Therefore,it is great practical significance to carry out rock mechanics tests under true triaxial stress conditions.In this paper,the white sandstone of tea plantation in Chongqing,China was used as the test object,strength tests with different hydrostatic pressures and different Lode angles were carried out,to discuss the influence of intermediate principal stress effects on rock strength under different hydrostatic stress conditions.At the same time,using the self-developed acoustic emission monitoring plate suitable for true triaxial instruments,the influence of different Lode angles on the acoustic emission characteristics of sandstone fracture under the same hydrostatic pressure conditions was studied.The main results obtained are as follows:（1）The acoustic emission monitoring plate for multi-functional fluid-solid coupling true triaxial test systems was independently developed.Using the method of embedding the acoustic emission module in the true triaxial indenter,the synchronous acquisition of acoustic emission signals during true triaxial testing is realized.The accuracy of the acquisition signal-to-noise ratio and test data was improved,and 3D sound source positioning was initially achieved.The test results prove that the device is reliable and effective.（2）Using the self-developed multi-functional true triaxial fluid-solid coupling test system,homogeneous sandstone strength test under different hydrostatic pressures and different intermediate principal stress coefficients（Lode angles）were conducted.And using the attenuation coefficient defined in this paper,the experimental results were analyzed.The analysis results show that the Lode angle effect of the rock decreases with the increase of the Lode angle,and the hydrostatic pressure effect also weakens with the increase of the hydrostatic pressure.Meanwhile,the Lode angle effect will be inhibited with the increase of hydrostatic pressure.By analyzing the experimental results,the intermediate principal stress effect is regarded as a comprehensive manifestation of the effects of the Lode angle effect and the hydrostatic stress.It can not only avoid the overlap of the two effects,but also reasonably explain the phenomenon that the rock strength increases first and then decreases with the increase of the intermediate principal stress.（3）The characteristics of acoustic emission signals of sandstone failure under the same hydrostatic pressure（100 MPa）and different intermediate principal stress（Lode angle）conditions were tested.By comparing the acoustic emission event rate curves,it can be shown that there are differences in the form of damage of sandstone under the same hydrostatic pressure and different Lode angles.Due to the different true triaxial lateral stress constraints,sandstone specimens may have the potential to maintain higher strength and higher load carrying capacity under relatively severe damage conditions,this is closely related to the position of sandstone damage,damage modes,nucleation and penetration of sandstone caused by different Lode angle conditions.By comparing the cumulative counts and the energy rates,the differences in sandstone damage,state of critical point before instability and the impact of energy release upon fracture（disaster-proneness）caused by different Lode angle conditions are preliminarily study.Comparing the 3D sound source localization of sandstone damage and the topography of the sandstone CT scan after damage under the Lode angle condition,several major methods to improve the positioning accuracy and reliability were proposed.（4）Based on the IB value and RA-AF value theory,a dynamic evolution monitoring（or detection）map of cracks based on acoustic emission technology is proposed.This map abandons the methods of traditional methods that are meticulous in quantifying the classification of cracks by fuzzy conditions,and focuses on the dynamic propensity evaluation of crack size and type.And the avalanche failure of sandstone under true triaxial conditions was obtained using acoustic emission energy,the frequency of aftershocks is less affected by the Lode angle,but is more significantly affected by the main shock energy range,and the common logarithm ratio of the energy of the main shock and its maximum aftershock is approximately 1.