Issue 36

S.R. Wang et alii, Frattura ed Integrità Strutturale, 36 (2016) 182-190; DOI: 10.3221/IGF-ESIS.36.18 183 the cracks in rocks, which is meaningful to study the deformation and failure laws of rocks [3]. J.P. Liu, et al. observed the changes of AE number in a process of loading test, found that the accelerating release of energy was meaningful to predict the instability or failure of the rock mass [4]. Y.B. Zhang inferred the AE magnitude by the ratio of the whole released energy and AE number in a particular time, found that the higher the ratio, the less AE events had produced the energy [5]. L.C. Jia, et al. monitored the process of the uniaxial compression test for limestone by acoustic emission techniques, and elaborated the crack development law in the rock sample after scanning the destroyed samples by a Computerized Tomography (CT) machine [6]. B.X. Huang, et al. analyzed the stress-strain curves and AE characteristics for coal-bearing rock samples under different stress paths [7]. K. Zhao, et al. conducted the research on AE characteristics of phyllite specimens under uniaxial compression tests [8]. M. Karakus, et al. analyzed the AE signal in the rock drilling process, which would provide the guidance to improve the drilling effect [9]. E. Aker, et al. analyzed the differences of AE between the shear failure and tensile failure in the triaxial compression tests of sandstone and predicted the failure mechanism using the proportion of the isotropic and anisotropic moment tensors [10]. L.P. Frash, et al. conducted the granite tests using AE technique and simulated the evolution of the crack in the geothermal development, providing useful information for the engineering application [11]. The numerical simulation can show some information that the laboratory tests cannot provide. For example, some scholars simulated AE by using Particle Flow Code (PFC) and Rock Failure Process Analysis system (RFPA), which provided a great help to understand AE phenomenon [12, 13]. Additionally, based on the correspondence between AE and cell rupture in FLAC/FLAC 3D code, X.B. Wang and T.C. Han et al. [14, 15] respectively simulated the AE in both laboratory and engineering scales, and they all obtained the good results. And other related works [16-18]. Based on the laboratory test, a further research following the above mentioned results will be conducted by using FLAC 3D technique under the criterion that the rupture of a cell or several adjacent cells was regarded as an AE event. We will analyze the temporal and spatial distribution characteristics of the AE events with large magnitudes and discuss the relationships between AE and natural earthquakes. M ATERIALS AND M ETHODS Samples of Rock Plates he rock plate samples in the tests were Hawkesbury sandstones, which were obtained from Gosford Quarry in Sydney, Australia. The quartz sandstones were formed in marine sedimentary basin of the Mid-Triassic and located on the top of the coal-bearing strata, which contained a small quantity of feldspars, siderite and clay minerals. According to the definition of the thick plate in elastic mechanics, the specimen sizes of the thick plate were designed as 190 mm × 75 mm × 24 mm (length, width and thickness). Equipment and AE Acquisition System The MTS-851 rock mechanics testing machine was selected as the loading equipment, and the load was controlled by vertical displacement and the loading rate was set 1×10 -2 mm/s. The vertical force and displacement in the process of the test were automatically recorded in real time by the data acquisition system. As shown in Fig. 1, the concentrated loading tests were designed to mainly consist of three parts. The top was a point- loading for the concentrated loading. The middle was a loading framework which included four bolts with nuts connecting the steel plates on both sides, and the lateral pressure cell was placed between the deformable steel plate and the thick steel plate so as to monitor the horizontal force. The capacity of the lateral pressure cell LPX was 1000 kg. The bottom was a rectangle steel foundation, and the rotatable hinge support was set on the both sides of the loading framework to maintain the connections with the steel plates. To monitor the cracks initiating and identify the failure location of the rock plate, the USB AE Nodes were used in the test. The USB AE Node is a single channel AE digital signal processor with full AE hit and real time features. In the test there were four USB AE nodes connected to a USB hub for multi-channel operation (Fig. 2). All these AE Nodes were made in MISTRAS Group Inc. USA. Numerical Simulation Scheme AE is due to the internal micro fracture by tension, shearing and compression stress in the rock plate, and this process is accompanied by the release of elastic waves. The rupture of a cell in FLAC 3D is also accompanied by the release of elastic energy, so it can be used to simulate an AE event [15]. In fact, if several adjacent cells rupture in the rock plate in a calculating cycle (step), it should also be regarded as an AE event, which advantage is that we can record the number of T