Laboratory Investigation on the Effects of Natural Fracture on Fracture Evolution of Granite Exposed to Freeze-Thaw-Cyclic (FTC) Loads

The natural fractures in rock mass are susceptible to damage evolution when subjecting to repeated freeze-thaw (F-T) weathering in cold regions, which can lead to the instability of rock engineering and even occurrence of geological hazards. Knowledge of how natural fracture impacts the overall fracture evolution of freeze-thawed rock is important to predict the stability of rock structure. In this work, we reported uniaxial experimental measurements of the changes in strength, deformation, acoustic emission (AE) pattern, and Felicity effect during increasing amplitude stress-cycling conditions on granite. The results show that the change of fracture aperture is related to the fracture openness and filling characteristics, open-type fracture is sensitive to F-T treatment, and its aperture increases faster than the close-type and fill-type fracture. In addition, strength decreases, and the damping characteristics first decrease and then increase with increasing natural fracture volume. AE activities also present different responses during sample deformation. The proportion of AE signals having low-frequency characteristics increases with increasing natural fracture volume, and the shear sliding along natural fracture results in the surge of AE activities. Moreover, the Felicity effect indicates that the Felicity ratio presents a fluctuation decreasing trend, and the preexisting fractures alter the stress memory characteristics of rock. It is suggested that the changes of the geomechanical and AE pattern are the interactions between the natural fracture and the newly stimulated fracture. The testing results are expected to improve the understanding of the influence of natural fractures on rock fracture evolution and can be helpful to predict the stability of rock structures and rock mass in cold regions.

Experimental Research on Acoustic Emission Characteristics and Felicity Effects during Coal Fatigue Failure under Cyclic Loading

In order to study the acoustic emission characteristics and Felicity effect in the process of coal fatigue failure and reveal the internal relationship between the fatigue damage evolution law and the acoustic emission activity, with the help of MTS815.02 electrohydraulic servo rock mechanics test system and PCI-2 acoustic emission detection and analysis system, a triaxial cycling loading acoustic emission test was carried out on the coal samples. The results show that the higher the upper limit stress is, the more obvious the degree of fatigue damage will be caused by coal samples. At the same time, the more active acoustic emission signal will appear. The coal samples under linear loading are on the initial damage state, and slight fatigue, moderate fatigue, deep fatigue, and ultimate fatigue failure under cyclic loading. The acoustic emission shows the “L-” type development evolution law in any previous stress level range, while at the last stress level, it shows the obvious “U-” type development evolution law. The higher the frequency of the cyclic loading is, the higher the rate of initiation and expansion of the microcrack will be, while the more obvious acoustic emission phenomenon will appear. Furthermore, the ringing counting rate is basically the same as that of the energy counting rate. Under triaxial cyclic loading, a shear failure mode that extends along different directions of fracture surface will be presented. The acoustic emission in the range of different stress levels shows a different degree of Felicity effect. In contrast, it is more reasonable to use the principal stress difference as a parameter to study the Felicity effect of coal under cyclic loading.

Impacts of Cyclic Loading and Unloading Rates on Acoustic Emission Evolution and Felicity Effect of Instable Rock Mass

Uniaxial cyclic loading-unloading compression experiments with schemes of six loading rates and six unloading rates were carried out on the combined testing platform. Impact of loading and unloading rates on rock AE characteristics was revealed. Results show that increasing loading and unloading rates resulted in decreasing total AE rings in the entire rock deformation and failure process. Increasing loading rate decreased the AE rings at loading stages, and increasing unloading rate decreased the AE rings at unloading stages in the same cycle. Total AE counts had a negative linear relationship to the loading and unloading rates. The loading stage was the active period of the AE phenomena, and impacts of the loading rate on the AE characteristics were more apparent. Especially when the loading rate was greater than 2.0 kN/s, brittle failure of rock specimens became noticeable. After the cyclic load reached the uniaxial compressive strength of 1/3∼1/2 times, the rock Felicity effect became more obvious. With the increase of the loading rate, the Felicity ratio decreased in the elastic stage and increased a little in the plastic stage, whereas with the increase of the unloading rate, the Felicity ratio decreased gradually in the elastic stage and remained almost the same in the plastic stage.

Research Status and Prospect of Concrete Acoustic Emission Technology

By discussing the current research status of the concrete acoustic emission technology from four aspects as: the concrete acoustic emission of different conditions; the characteristics of the concrete acoustic emission; the Kaiser effect and Felicity effect of the concrete acoustic emission and the analysis of the concrete acoustic emission data. The concrete acoustic emission research should be done with the practical project, and establish effective processing and analyzing technology of the concrete acoustic emission, to apply the concrete acoustic emission technology better to the measurement of real project.

Experimental Verification and Theoretical Analysis on Felicity Effect of Acoustic Emission in Concrete

The experiment of acoustic emission (AE) on concrete specimens under uniaxial cyclic loading was conducted. The Kaiser effect of acoustic emission in concrete and the Felicity effect, which manifest the memorizing ability to the maximum previous stress level of Kaiser effect, were validated by the experiment. The mechanism of Felicity effect was analyzed based on the theory of statistical damage mechanics and a tentative AE factor constitutive model of brittle material under uniaxial cyclic loading was suggested. The curve of constitutive model is in good coincidence with the curve from the experiment. The experimental results showed that the Felicity effect became clearer along with the increasing of stress level. Each loading cycle would cause new damage inside the material, and the response of material to the new loading cycle is different from the previous cycle.