| In situ stress measurement is becoming more and more important in the geological and engineering field for it is the most direct means for people to know about the stress in the crustal rock.Because of its low cost and easy operation,the method of measuring crustal stress by Kaiser Effect is generally being paid great attention to at home and abroad.However,there are some questions about the mechanism and influencing factors of Kaiser effect,such as the effect of Kaiser effect direction,stress level,humidity,temperature,processing stress and other factors on the Kaiser effect.Especially important is that when rock sample is separated from rock mass,its memory of stress whether weakened and disappeared over time or not has great influence on the accuracy of test results.There is always an interval between the rock is separated from rock mass,processed and loading test.If the memory of rock for in situ stress is inaccurate or disappeared during the time,the test results will be meaningless.Existing research results are analyzed,it is inconclusive that whether or not the accuracy of rock’s memory for pre-stress is affected by time,and all of them are experimental conclusions and lack of corresponding theoretical explanations.The WAM-1000 electro-hydraulic servo test system and PCI-2 all-digital acoustic emission signal acquisition system is used to research the characteristics of acoustic emission and signal frequency under the condition of axial compression,three-point bending and disc splitting test based on sandstone.The main contents are as follows:(1)The time delays vary regulations of Kaiser effect under the condition of axial compression,three-point bending and disc splitting test are obtained.It is concluded that there exists time delay of Kaiser effect under the condition of axial compression,there is no time delay of Kaiser effect under the condition of three-point bending,and it is still not clear whether there’s time delay for disc splitting test.Formula of Filicity Ratio(FR)corresponding to delay time is discussed,and the memory law of sandstone for stress is quantitatively analyzed.Meanwhile,corresponding physical denoising method is given out.(2)The Kaiser effect of cylinder specimen is more obvious when slenderness ratio is less than 2.5 than the one whose slenderness ratio is greater than 2.5 under uniaxial compression test.The immersed specimen can’t accurately reflect the initial maximum stress(the inflection point is obvious)in the uniaxial compression test,thus the Kaiser effect is related to the moisture content of specimen.The Kaiser effect of granite is not influenced by the time delay in the uniaxial compression test.(3)In axial compression test,three-point bending test and disc splitting test,the strain curve has obvious corresponding relationship with the cumulative ring curve of acoustic emission,which indicates that the phenomenon of acoustic emission is related to the development of crack.During the cyclic loading process for the uniaxial compression of sandstone,the cumulative acoustic emission curves increase sharply at the vertical section of the longitudinal strain(crack compaction stage)and the oblique section of the longitudinal strain(crack development).The phenomenon shows that the acoustic emission of sandstone under uniaxial compression test has a corresponding relationship with the crack compaction and development.During three-point bending test and disc splitting test,the cumulative acoustic emission curves increase sharply at the rising section of longitudinal strain curve(crack development),it is shown that the acoustic emission phenomenon of sandstone under three-point bending and disc splitting has a corresponding relationship with the re-development of crack.(4)Waveform date of acoustic emission test was analyzed by time-frequency method,the relationship between the spectrum distribution of acoustic emission signal waveform and the development of internal crack was established,which could explain the physical meaning of stress-memory phenomenon of sandstone.At the same time,a Matlab program considered the waveform characteristics of major and minor difference was compiled to judge Kaiser point automatically,and the Kaiser point of the uniaxial compression was recognized successfully.(5)From the aspect of cracking difficulty of the original crack cracking along the original path under cyclic loading,two-dimensional mechanical models for uniaxial compression test,three-point bending test and disc splitting test were established,and the time delay of Kaiser effect was explained theoretically,whose calculation results agree with the experimental results.The time delay of Kaiser effect is reappeared with the particle flow software PFC,and the variation of friction coefficient at micro-cracked contact surface and the change of the re-loading stress path are incentives for the time delay of Kaiser effect. |
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