Evaluation of internal microcrack evolution in red sandstone based on time–frequency domain characteristics of acoustic emission signals

The freezing method is an effective approach for constructing coal mine shafts through water-rich soft rock strata. The frozen wall produced by this technique stops the movement of water and offers temporary support, ensuring the safety and stability of the shaft working face. However, most frozen rock masses generally comprise ice-filled flaws and frozen intact rock. A frozen fissured rock mass undergoing the long-term effects of excavation unloading under in situ stress gradually accumulates damage or creep deformation over time; this damage is mainly responsible for the significant deformation, cracking and even instability of the frozen rock wall. To study the creep characteristics of ice-filled fissured rock masses under unloading conditions, a series of conventional triaxial compression tests and triaxial unloading creep tests were performed on ice-filled flawed red sandstone specimens from the Shilawusu mine in Northwest China using a self-developed DRTS-500 subzero rock triaxial testing system. In conjunction with the experimental data, the instantaneous strength and deformation characteristics of the rock specimens were analyzed, and their creep deformation characteristics and damage evolution characteristics were discussed. In this paper, based on the nonlinear creep characteristics of frozen rock, fractional calculus theory and damage theory, a new nonlinear creep damage model of frozen fissured red sandstone was defined in series with the improved Burgers model and elastoplastic damage model. The proposed creep model can reasonably describe the creep deformation of frozen fissured red sandstone. Classical elastoplastic mechanics and creep theory were used to derive the three-dimensional creep damage constitutive equation, and the results of creep experiments and simulation results of the creep damage constitutive model were very consistent in this study. The new creep model not only reflects the whole creep deformation process of frozen fissured red sandstone but also exhibits better performance than the classical Burger model and improved Nishihara model in describing the primary creep stage and accelerating creep stage. Therefore, the proposed nonlinear creep damage model is suitable for studying the mechanical creep properties of frozen fissured rock under unloading conditions. The results can provide an important reference for the long-term stability assessment of frozen rock walls of coal mine shafts.

Study on the Triaxial Unloading Creep Mechanical Properties and Damage Constitutive Model of Red Sandstone Containing a Single Ice-Filled Flaw

To investigate the creep and instability properties of a cemented gangue backfill column under a high-stress area, the uniaxial compression creep tests were conducted by single-step and multi-step loading of prismatic samples made of cemented gangue backfill material (CGBM) under the high stress-strength ratio The creep damage was monitored using an electrical resistivity device, ultrasonic testing device, and acoustic emission (AE) instrument The results showed that the CGBM sample has a creep hardening property The creep failure strength (CFS) is slightly larger than the uniaxial compressive strength (UCS), ranging in ratio from 1089% to 1165% The instantaneous strain, creep strain, and creep rate increase with increasing stress-strength ratio in the single-step loading creep tests The instantaneous strain and creep strain decrease first and then increase during the multi-step loading creep process The axial creep strain of the CGBM column can be expressed by the viscoelastic-plastic creep model Creep instability is caused by the accumulation of strain energy under multi-step loading and the continuous lateral expansion at the unconstrained middle position during the creep process The creep stability of a CGBM column in a high-stress area can be monitored based on the variation of electrical resistivity, ultrasonic pulse velocity (UPV), and AE signals

Creep properties and resistivity-ultrasonic-AE responses of cemented gangue backfill column under high-stress area

Cracking and deformation of cuboidal sandstone with a single nonpenetrating flaw under uniaxial compression

Understanding the mechanism of progressive debonding of bolts is of great significance for underground safety. In this paper, both laboratory experiment and numerical simulation of the pull-out tests were performed. The experimental pull-out test specimens were prepared using cement mortar material, and a relationship between the pull-out strength of the bolt and the uniaxial compressive strength (UCS) of cement mortar material specimen was established. The locations of crack developed in the pull-out process were identified using the acoustic emission (AE) technique. The pull-out test was reproduced using 2D Particle Flow Code (PFC2D) with calibrated parameters. The experimental results show that the axial displacement of the cement mortar material at the peak load during the test was approximately 5 mm for cement-based grout of all strength. In contrast, the peak load of the bolt increased with the UCS of the confining medium. Under peak load, cracks propagated to less than one half of the anchorage length, indicating a lag between crack propagation and axial bolt load transmission. The simulation results show that the dilatation between the bolt and the rock induced cracks and extended the force field along the anchorage direction; and, it was identified as the major contributing factor for the pull-out failure of rock bolt.

Experimental and numerical studies on progressive debonding of grouted rock bolts

Acoustic emission characteristics of creep fracture evolution in double-fracture fine sandstone under uniaxial compression

摘要: 疲劳是岩石的重要力学特性之一，与工程的安全稳定密切相关.在循环荷载作用下，岩石初始损伤不断累积、加剧，最终导致岩石的失稳破坏，从而诱发工程灾变.阐述了近年来基于声发射、红外辐射技术开展岩石疲劳损伤与破坏方面的研究进展.首先论述了研究人员采用声发射时域参数、声发射空间演化以及Felicity比开展岩石疲劳损伤特性的研究进展.进而，针对研究人员采用声发射时域参数、损伤变量、分形维值、Felicity比、加卸载响应比、RA以及b值等参数开展岩石疲劳破坏前兆的研究进展进行讨论.在此基础上，论述了研究人员采用红外辐射技术开展岩石疲劳损伤与破坏前兆的研究进展.最后，提出了今后岩石疲劳损伤与破坏前兆研究中需要进一步深入解决的几点问题.本文可对深入认识岩石疲劳损伤机理、破坏前兆特征起到积极的促进作用. 关键词: 疲劳损伤 / 破坏 / 声发射 / 红外辐射 / 岩土工程 / 工程地质 

岩石疲劳损伤及破坏前兆研究现状与展望

Experimental investigation on the effect of preexisting orthogonal cross flaws on cracking behaviors and acoustic emission characteristics of red sandstone

This study investigates the effect of prefabricated fissures on the creep mechanical strength and acoustic emission (AE) characteristics of sandstone. Under the same test conditions, indoor uniaxial compression creep AE tests were performed on intact and artificially fissured sandstone specimens using a rock rheometer and a PCI-Ⅱ AE tester controlled with an RLJW-2000 microcomputer. The variation characteristics of strain, long-term strength, number of AE events, AE ringing count, AE energy, and RA value of the two specimens were then analyzed to determine the effect of prefabricated fissures on the creep mechanical properties and AE characteristics of sandstone. The results show that under the same stress level, the instantaneous strain, creep strain, and total strain of the prefabricated fissures specimen was larger than the corresponding values of the intact specimen. The prefabricated fissures exhibited the most significant influence on the creep strain of the specimen, followed by the total strain. The fissures showed the least influence on the instantaneous strain of the specimen. Furthermore, the long-term strength of the prefabricated fissures specimen was only 37.5% of that of the intact specimen. Owing to the influence of the prefabricated fissures, the long-term strength of the sandstone was significantly reduced. In engineering practice, fissures should be noted and analyzed for the potential failure of a formation. Moreover, as the stress level increases, the number of AE events of the specimen also increases. The number of AE events and cumulative AE events generated by the prefabricated fissures specimen at various stress levels is significantly higher than the corresponding values of the intact specimen. The maximum ringing count of the prefabricated fissures specimen was 7.3 times that of the intact specimen, and the maximum AE energy was 3.8 times that of the intact specimen. The cumulative AE ringing count was 1.5 times that of the intact specimen, and the cumulative energy was 4.5 times that of the intact specimen. The prefabricated fissures significantly affect the maximum AE ringing count, maximum AE energy, cumulative AE ringing count, and cumulative AE energy of the specimen. Finally, tensile fissures were the main microcracks generated inside the intact specimen during creep, and shear cracks were secondary. The microcracks generated inside the prefabricated fissures specimen consisted of mixed tension-shear cracks, but mainly consisted of shear cracks with fewer tension cracks. The RA value correlates well with the observed internal microcracks, indicating that the RA value can be used to characterize fractures in sandstone specimens. The RA value has a guiding significance for characterizing the type of internal microcracks in the process of rock creep.

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Experimental study on the effect of prefabricated fissures on the creep mechanical properties and acoustic emission characteristics of sandstone under uniaxial compression

Monte-Carlo random finite-difference analysis (MCRFDA) can incorporate the spatial variability of soil properties into the analysis of geotechnical structures. However, two factors, namely, the fineness of the generated elements (reflected by the number of elements, N e ) and the number of MC simulation iterations, N MC , considerably affect the computational efficiency of this method, creating a barrier to its broad use in real-world engineering problems. Hence, an MCRFDA model of a circular underground cavern is developed in this study. The convergent deformation of the cavern is analyzed while considering the spatial variability distribution of the elastic modulus. Moreover, the effects of N MC and N e on random FD calculations are investigated. The results show the following. An N MC greater than 500 is desirable for the FD analysis of a conventional structure. For a specific structure, N e does not have a significant impact on the mean of the simulated values but appreciably affects the standard deviation (SD) of the simulated values, where reducing N e increases the SD of the simulated values.

/pdf/effects-of-the-number-of-simulation-iterations-and-meshing-7z1z6k99.pdf

Huaichang Yu

Papers

Acoustic emission characteristics of creep fracture evolution in double-fracture fine sandstone under uniaxial compression

岩石疲劳损伤及破坏前兆研究现状与展望

Experimental investigation on the effect of preexisting orthogonal cross flaws on cracking behaviors and acoustic emission characteristics of red sandstone

Experimental study on the effect of prefabricated fissures on the creep mechanical properties and acoustic emission characteristics of sandstone under uniaxial compression

Effects of the number of simulation iterations and meshing accuracy in monte-carlo random finite-difference analysis