| The structure of tectonic coal is broken and rich in pores and fissures,which leads to the difficulty of sampling,which makes it difficult to test the mechanical properties of tectonic coal and limits the development technology of tectonic coal seam.Therefore,it is of great significance to study the mechanical properties of tectonic coal and the characteristics of resistivity and acoustic emission parameters in the process of instability and failure for the correct evaluation of mechanical characteristics and structural changes of tectonic coal.This paper takes remolded tectonic coal as the research object.Based on the establishment of a test system that includes loading methods and resistivity and acoustic emission testing,uniaxial compression,cyclic loading,and graded loading tests are carried out on remolded tectonic coal samples.The resistivity and acoustic emission parameters of the samples are tested during the test,and the mechanical properties of remolded tectonic coal samples under different loading modes are obtained.The characteristics of resistivity change and acoustic emission response during loading and the corresponding relationship between them and the failure process of the specimen are obtained.The effects of different loading methods on the resistivity and acoustic emission characteristics are discussed.The results are as follows:(1)The stress-strain curves of remolded tectonic coal samples under different loading modes were obtained,and hysteresis loops appeared in the stress-strain curves under cyclic loading.Secondly,the peak strength of remolded coal samples increases with the increase of molding pressure,holding time,initial moisture content and loading speed.Thirdly,under uniaxial compression,the failure mode of remolded coal samples is mainly(nearly)parallel to the axial direction,while under cyclic loading and graded loading,the failure mode is along an angle of about 45° to the axis.(2)The relationship among resistivity,acoustic emission energy,cumulative energy,time and stress of remolded coal samples under uniaxial compression was obtained.Firstly,under uniaxial compression,the resistivity of remolded structural coal sample shows a trend of rapid decrease-the decrease rate tends to be gentle-the resistivity drops to the minimum-the resistivity increases with the loading.The changing process of resistivity corresponds well with the failure process of the sample,and the time when the resistivity decreases to the minimum value is slightly ahead of the time when the sample reaches the peak strength.Secondly,the AE energy released reaches the maximum before and after the specimen failure.Thirdly,the cumulative energy of AE is relatively small at the initial stage of loading,while increases rapidly in the yield stage and failure stage.(3)The relations between resistivity,acoustic emission energy,cumulative energy and time and stress of remolded coal samples under cyclic loading were obtained.Firstly,resistivity decreases during loading while increases during unloading.Secondly,under cyclic loading,the AE activity is mainly concentrated in loading stage.Thirdly,under cyclic loading,the AE cumulative energy curve of remolded coal samples shows obvious steps.The cumulative AE energy increases with the increase of the number of cycles.With the increase of loading speed,the cumulative AE energy decreases.(4)The relationships among resistivity,acoustic emission energy,accumulated energy,time and stress of remolding coal samples under graded loading are obtained.Firstly,the resistivity of the remolded coal samples decreased obviously in the loading stage,but in the stabilized pressure stage,the resistivity of the samples remained unchanged.Secondly,during the graded loading process,the emission of acoustic emission energy is mainly concentrated in the loading stage.Thirdly,under the graded loading,a platform will appear on the curve of AE cumulative energy.With the increase of grading times,the cumulative energy of AE increases.With the increase of loading speed,the cumulative acoustic emission energy of the sample decreases.(5)The mechanical properties,resistivity characteristics and acoustic emission characteristics of remolded tectonic coal samples are affected by different loading methods.Firstly,the peak strength of the remolded coal samples under cyclic loading and graded loading is higher than that under uniaxial compression.Secondly,under uniaxial compression,the failure mode of remolded coal samples is mainly(nearly)parallel to the axial direction,while under cyclic loading and graded loading,the failure mode is along an angle of about 45° to the axis.Thirdly,the resistivity of the samples under cyclic loading and graded loading will decrease in the loading stage,while in the unloading stage of cyclic loading,the resistivity of the samples increases.The resistivity of the samples is basically unchanged during the steady-pressure stage of the graded loading.Fourthly,under cyclic loading and graded loading,the emission of acoustic emission energy mainly concentrated in the loading stage,and the cumulative energy curve produced steps.There are 68 figures,7 tables and 86 references in this paper... |
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