| Under the action of cyclic loading,the internal structure of rock has different degrees of damage,its strength characteristics and mechanical behavior will change with different stress paths.Acoustic emission(AE),microseismic(MS)and electromagnetic radiation(EMR)are closely related to rock deformation and fracture process.AE mainly reflects the micro-crack propagation information in the process of rock fracture,MS mainly reflects the macro-crack propagation information,and EMR mainly captures the electrical signals released by rock fracture.The three reflect the deformation and fracture behavior of rock from different perspectives.At present,the monitoring methods and signal parameters obtained in the process of rock damage and failure under cyclic loading are relatively single,which is difficult to support a comprehensive and thorough analysis of rock damage and failure process.Therefore,this article combines three acoustoelectric technologies to monitor the rock loading process,and explores the acoustoelectric characteristics of the dry and saturated sandstone failure process under cyclic loading,which has a certain guiding role in judging and predicting the stability of the rock.The main research contents and results are as follows :(1)The cyclic loading and unloading tests of dry and saturated sandstones with loading and unloading rates of 0.2k N/s～0.6k N/s were carried out.The AE,MS and EMR signals of rock samples from loading to failure were monitored.The influence of loading and unloading rate on the acoustoelectric characteristics of sandstone are studied.The results show that the AE and MS energy curves of dry and saturated sandstone under different loading and unloading rates appear double-peak phenomenon.With the increase of cyclic loading and unloading rate,the double-peak phenomenon of AE and MS becomes less obvious.The faster the loading and unloading rate is,the higher the AE,MS,EMR energy release rate of dry and saturated sandstone at each level of circulation and loading to the failure process,and the corresponding increase rate of saturated sandstone is smaller than that of dry sandstone.(2)Nine equal amplitude cyclic loading and unloading tests were carried out on dry and saturated sandstone with the loading upper limit of 35MPa～55MPa and the unloading lower limit of 0MPa.AE,MS and EMR signals of rock samples from loading to failure were synchronously monitored,and the influence of loading upper limit on the acoustoelectric characteristics of sandstone was explored.The results show that the larger the loading upper limit,the greater AE energy value of each cycle,the greater the AE energy value of sandstone in the early cycle.The smaller the loading upper limit is,the stronger the AE signal in failure stage is,and the higher AE energy is.The relative law of MS is similar to AE.(3)The damage evolution process of dry and saturated sandstone from loading to failure under different grading cyclic loading and unloading rates and different equal amplitude cyclic loading upper limits was explored.The results show that when the loading and unloading rates are the same,the five damage parameters characterized by dissipation energy,AE energy,MS energy,EMR energy and total acoustoelectric energy in the graded cycle of sandstone increase nonlinearly with the cycle stage.The larger the loading and unloading rate,the higher the damage corresponding to the cyclic loading and failure process of sandstone.The damage evolution curve of dry sandstone is in arc shape,and the damage evolution curve of saturated sandstone is in S shape.Five kinds of damage evolution curves with nine cyclic of sandstones are approximately straight lines under constant amplitude cyclic loading with the same loading upper limit.The five damage parameters characterized by dissipation energy,AE,MS,EMR,and total acoustoelectric energy all increased sharply during loading to failure,and the growth rate of dry sample was greater than that of saturated sample.In addition,the damage evolution trend of total acoustoelectric energy and dissipated energy is the most consistent,and the damage parameter characterized by total acoustoelectric energy is the best.(4)The uniaxial reloading of dry and saturated sandstone after 9 equal amplitude cyclic tests was carried out.The acoustoelectric signals of sandstone under different loading upper limit are compared and analyzed from various angles.The differences and connections of AE,MS and EMR signals during sandstone loading were explored.The results show that there are some differences in AE,MS and EMR energy release rate,three kinds ratio of acoustoelectric energy to total acoustoelectric energy in failure process,three kinds damage parameters characterized by acoustoelectric energy,three kinds of acoustoelectric ringing(pulse)of dry and saturated sandstone during the stage of compaction,elastic,stable rupture,unstable rupture under different loading upper limit conditions.The three kinds of acoustoelectric energy rates of dry and saturated sandstone change less during the compaction stage to elastic stage,and the larger the loading upper limit is,the smaller three kinds of acoustoelectric energy rates from stable fracture stage to unstable fracture stage in the process of sandstone failure.In the four stages of sandstone loading to failure,the ratio of AE and EMR energy to the total acoustoelectric energy in the failure process gradually decreases,and the ratio of MS energy to the total acoustoelectric energy in the failure process gradually increases.The larger the upper limit value of loading is,the more the AE energy ratio is reduced,the larger the MS energy ratio is and the smaller corresponding growth rate is,and the smaller EMR energy ratio is and the larger the corresponding reduction rate is.When the rock sample is in the compaction stage,the increase of damage parameters characterized by AE and EMR energy is greater than that of MS and the total acoustoelectric energy.In the early process of elastic and stable fracture stage,damage parameters characterized by AE,EMR,MS and total acoustoelectric energy increased slightly.During the process of stable fracture to unstable fracture stage,the damage parameter characterized by MS energy of sandstone increases most.The AE,MS,EMR ringing(pulse)number of dry and saturated sandstone decreased from compaction stage to elastic stage,and increased from elastic stage to stable fracture stage.Compared with saturated samples,the growth rate of dry samples was smaller.In the unstable fracture stage,three kinds of ringing(pulse)of dry sandstone increase,and three kinds of ringing(pulse)of saturated sandstone decrease. |
