JOURNAL OF COAL SCIENCE & ENGINEERING (CHINA)

ISSN 1006-9097

pp 631–633

Vol.14 No.4

Dec. 2008

Effect of size on characteristic of acoustic emission in rock specimen∗ YUAN Rui-fu(袁瑞甫), LI Xiao-jun(李小军) ( School of Energy Science and Engineering, Henan Polytechnic University, Jiaozuo

454000, China )

Abstract A series of uniaxial-compression tests were conducted on some granite specimens. A multi-channel, high-speed AE signal acquiring and analyzing system was employed to acquire and record the characteristics of AE events and demonstrate the temporal and spatial distribution of these events during the failure process. The test results show that in the primary stage, many low amplitude AE events are developed rapidly and distributed randomly throughout the entire specimens. In the second stage, the number of AE increases much slower than that in the first stage, while the amplitude of most AE events became greater. Contrarily to the primary stage, AE events clusteres in the middle area of the specimen and distributes vertically conformed to the orientation of compression. The most distinct characteristic of this stage is a vacant gap formed approximately in the central part of the specimen. In the last stage, the number of AE events increases sharply and their magnitude increases accordingly. The final failure location coincidently inhabited the aforementioned gap. The main conclusion is that most macrocracks are developed from the surrounding microcracks existed earlier and their positions occupy the earlier formed gaps, and the AE activity usually becomes quite acute before the main rupture occurs. Keywords

rock specimen, acoustic emissifon, size, gap area, rockburst

Introduction Acoustic emissions (AE) are transient elastic waves generated by the rapid release of energy from localized sources within materials such as metal, rock and concrete, when they undergo changes in the mechanical, thermal and hydraulic environment. They have provided plentiful information regarding the failure process in materials. The temporal and spatial distribution of microcracks formed during the rock failure process is significant to know the characteristic of rock. This is quite valuable for predicting rock bursts and mine failures. Through the AE location studies, Mogi pointed out that the pre-mainrupturing microcracks tended to cluster along the main rupture plane[1]. Despite the comparatively poor locating accuracy of their

experiments, the guideline used in their study had attracted tremendous attention in the seismological circles. Byerlee and Lockner introduced the multi-channel tape recorder and computer into their locating studies, and identified the clustering of AE events coincided with the onset of creep[2]. With the view to keep away from the noise in seismic phase recognition controlled by the threshold value, Sondergeld adopted the operator-computer dialogue technique in his seismic phase recognition. He greatly enhanced the locating accuracy, and displayed his data in a series of stereographic projections[3 4]. Subsequently, Japanese seismologists undertook a number of studies in the same field and yielded results like the formation of microcrack gaps and the migration and concentration of microrupturings[5], etc. However, few studies could depict the behavior

∗ Supported by the Special Subject of 863 Programm(2007AA06Z107) and the Younth Foundation of HPU(Q2008-51) Tel: 86-391-3987235, E-mail: [email protected]

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of spatial distribution of AE events in detail because of locating accuracy of their equipments, and none of them pointed out the critical value of AE distribution of rock failure which is very significant to the application of AE technique in mine and other geoengineering. The purpose of this paper is to illustrate the spatial distribution of microcracks during the whole failure process of rock with different size under uniaxial compression by using a multi-channel, high-speed AE signal acquiring and analyzing system. Furthermore, we also aim at analyzing the relationship between the characteristics of AE distribution and the damage level of rock based on fractal-damage mechanics.

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Experiment specimens and facilities

Four groups of granite specimens were employed in the experiment. The sizes of specimens in four groups are about 70 mm×70 mm×75 mm, 70 mm×70 mm × 100 mm, 70 mm×70 mm×150 mm, and 70 mm×70 mm×200 mm respectively. The velocity of P-wave transmitted in the specimens is about 4 000 m/s. A servo-controlled hydraulic testing machine was used in the experiment. It could record the value of load and displacement, and draw the curves of load-displacement and stress-strain instantaneously. A multi-channel, high-speed AE signal acquiring and analyzing system called HUS (Hyperion Ultrasonic System) was employed to acquire the AE signals. It could record the temporal and spatial distribution of AE events in the specimen during loading and visually display them by the post processor in 3D model. The specimens were uniaxially loaded; the loading rate (displacement) was 0.002 mm/s. Two rubber cushions were matted between pressing machine and specimen to eliminate the noise generated by friction.

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length groups orderly. Therefore, in the elasticity stage, the more length of the specimen the less AE signals occurred in the same stress level. After elasticity stage, the occurring of AE signal change to irregular along with the increase of stress, so in this stage, the size of specimen have no effect on the AE signals.

Relationship of stress and AE in specimens with different size

The stress-AE curves of different size specimens are sketched in Fig.1. There are almost not AE signals in the initial stage which is also called crack closing stage, the effect of specimen size is very small in this stage. In the second stage, namely elasticity stage, the microcracks in the specimen become generating and propagating and the AE signals also occurring continuously. AE signals first occur in the 75 mm length group and then occur in the 100, 150 and 200 mm

Fig.1

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3.1

Curves of stress with AE numbers for different size specimens

Comparison of locating result and failure position in the specimens with different size

Locating result of AE events in the specimens The locating result of AE events in different size specimen was analyzed in Fig.2. In the initial loading stage, AE events occurred at the two ends of specimens; along the increase of load, the AE events move to the center of specimens gradually. In the 75 mm length group, the AE events began to occur in the center area of the specimens; in the 100 mm length group, the AE events occurred in the right center area of the specimens in this stage; while in the 100 and 150 mm length groups, the AE events still distributed in the two ends of specimens, but the distributing areas were more close to the center. Then the center area became a gap area where almost no AE events occurred, and this area was usually the dangerous position because some macrocracks would occur here. Same phenomenon was observed in the forecasting research of earthquake. This result is significant to forecasting the position of rockburst in deep mine by microseismic monitoring technique. The AE events locating results indicate that the microcracks initiate at the two ends of specimens, and then will extend to the center area form the ends, a gap area contains few AE events will be formed in the center area and this area will be the final failure position because the macrocracks always occur here. The phenomenon is more evident with the increase of spe-

YUAN Ruifu, et al. Effect of size on characteristic of acoustic emission

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cimen size.

Fig.3 Actual failure results for different size specimens

only few microcracks generated and some intrinsic microcracks close or extend, so the number of AE events is very few. Along with the increase of loading, many new microcracks begin to occurring, and then AE events increase continuously. Therefore, the acoustic emission reflect the failure process and damage level of the specimen, each AE signal contains plentiful information of the structure changing inside the rock. (2) The spatial distributing of AE events well reflects the development of microcracks inside the rock. The microcracks initiated at the two ends of the intact specimen, and then extend to the center area along with the increase of loading. A gap formed during the loading process and when the specimen reached to final failure, macrocracks occurred in the gap area. This conclusion is very significant to forecasting the rockburst in deep mine. References Fig.2 AE events locating process and result for granite specimens with different length by AE monitoring system ( a )~( d )specimen length are 76, 100, 148, 200 mm

3.2

Failure character of the specimens with different size Fig.3 shows four destroyed specimens with different size. All the four specimens are split by axial cracks. The failure positions are consistent in the AE locating results, so the accuracy of AE events location is credible.

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Conclusions (1) In the initial stage of failure process, there are

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