Experimental Study On Macroscopical Mechanical Properties And Crack Propagation Of Marble By Thermal Cycling

The study of the strength and deformation properties of brittle rock after high-temperature action and high-temperature cycling,and the meso-evolution laws are key scientific and technical issues in geotechnical engineering.Only by accurately grasping the changes in mechanical properties of brittle rocks in complex geological environments can we accurately evaluate the deformation and stability properties of rock masses and ensure the construction safety and long-term stability of large-scale geotechnical engineering.Therefore,this article selects fine-grained marble as the research object,adopts a combination of laboratory experiments and theoretical analysis,conducts uniaxial compression experiments,and monitors the acoustic emission phenomena during the entire process of compression failure to study the strength and deformation properties of rocks.Including peak intensity,Young's modulus,brittle transition characteristics,failure mode,acoustic emission timing parameters,characteristic stress,damage evolution model,etc.Meticulous testing methods are used to analyze the evolution law of crack propagation during thermal damage in order to reveal the mechanism of macroscopic mechanical properties change.The content of this paper is as follows:(1)The uniaxial compression experiments were performed on fine-grained marbles at room temperature and on high-temperature one-time,two-times,four-times,eight-times,and sixteenth thermal cycles at temperatures ranging from 0 to 400?,and the acoustic emission signals of the whole process were monitored.Meticulous observations were used to observe the development of microcracks.The results showed that:The physico-mechanical properties of the rock undergoing thermal cycling are degraded,and the longitudinal wave velocity,peak strength,and elastic modulus are significantly reduced,while the peak strain and crack density parameters are significantly increased.The marble stress-strain curve tends to be flattened,and the initial compaction stage becomes longer.It shows that the brittleness of the rock after heat treatment is weakened and the ductility is enhanced;The rock samples that have undergone heat treatment produce more acoustic emission signals during the initial compaction stage,but the acoustic emission activity from the elastic stage is not as severe as at room temperature,and the peak value of the acoustic emission signal lags behind the peak intensity,which is associated with an increase in ductility of rock samples after heating cycle;The fracture pattern of rock samples undergoing different thermal cycle times changes from a single fracture failure to a multi-crack fracture to a shear failure,this is related to the different forms of cracks,the different changes in the mineral structure within the rock sample and the different energy release;With the increase in the number of thermal cycles,a large number of microcracks have been observed in the interior and boundary of the rock-like mineral crystals by means of microscopic observations,and the parameters such as number,opening,length and linear crack density have gradually increased,the direction of crack development is random,and the development of grain boundary cracks is the main reason.The reason is that the thermal expansion coefficient of mineral particles is not uniform,and thermal stress is generated at the boundary after heating,and exceeds the yield strength of the mineral boundary.(2)Using uniaxial compression experimental data,using different calculation methods to determine the characteristic stress of thermal cycle marble,including closed stress,crack initiation stress and damage stress,the results show that:The characteristic stress values calculated by using different calculation methods have similar laws,that is,they all decrease with the increase of the number of thermal cycles,and the normalized characteristic stress increases with the increase of the number of cycles;The volumetric strain model algorithm and the acoustic emission parameter method are convenient to determine the characteristic stress,but they all depend on the subjective judgment of the experimenter.The calculation process of ASR method,LSR method and CAEH method is more complicated,but they both overcome the influence of human factors and make the results more objective;The degree of dispersion of the cracking initiation stress calculated by different methods decreases as the number of thermal cycles increases;Because of the anisotropy of rock,or the limitations of experimental instruments and experimental operations,only the use of multiple methods to determine the characteristic stress value can increase the credibility.(3)According to the macroscopic experimental data,the damage variables are defined based on acoustic emission parameters and deformation parameters respectively,and the damage evolution law of progressive damage of rock samples is obtained.Finally,the acoustic emission constitutive model and Logistic function constitutive model are used to fit the two variables respectively.The results show that:The evolution law of two kinds of damage variables based on acoustic emission parameters and deformation parameters can well reflect the changes of crack closure,linear elasticity,steady crack propagation,non-stable crack propagation,and peak damage in all stages of progressive failure of rock;With the increase of the number of thermal cycles,the two types of damage variable increase with the increase of axial strain,indicating that thermal damage increases the ductility of the rock;The acoustic emission model can describe the curve shape of the experimental data more accurately,but the simulated peak intensity is lower than the measured value.