| The paces of some infrastructure construction,such as deep resource exploitation,transportation,water conservancy and underground space development,are promoted as the national economic construction rapidly develops.However,the opportunities usually company with the challenges.The instability of rock mass heavily threatens the life safety of technical staff and leads to the incalculable economic loss during the process of engineering construction.The damage failure characteristics,the fracture mechanism and the instability warning of brittle flawed rocks are the basic scientific issues for the instability failure of rock mass.This study closely focuses on the key scientific issues of the cracking damage of flawed rocks and further investigates.Some methods,including acoustic emission(AE),high-speed photography,peridynamics and mathematical analysis,are employed to further study the mechanical responses,the cracking behaviors,the fracture mechanism,the fracture propagation mechanism,the quantitative characterization of damage and the stability warning of flawed rocks.The main study contents and conclusions are as follows:(1)A series of uniaxial compression experiments are conducted on flawed sandstone specimens by using both AE and high-speed camera techniques.Based on the evolution characteristics of two AE indexes of RA value(rise time/amplitude)and AF(AE counts/duration)value,the fracture modes of flawed rocks during the cracking process are classified as two fracture modes of tensile cracks and shear/mixed mode cracks.The different types of fracture modes of flawed rocks under uniaxial compression are effectively evaluated by the method of two AE indexes and qualitatively revealed the fracture mechanism of flawed rocks.The kernel density estimation(KDE)is used to visually evaluate the cracking modes during the compression process of flawed rocks.KDE is a kind of parametric model,which can overcome the distribution randomness of data set obtained by AE data and can well visualize and identify the high concentration regions of RA and AF values.The ultimate failure modes of flawed rocks are all triggered by shear fracture modes.The evolution characteristics of continuous increase in RA values(more than 400 ms/v)can play as an early warning information for the ultimate failure of rocks.(2)AE waveforms generated during the cracking process of flawed rocks are processed by using the method of Fast Fourier Transformation(FFT)to obtain its two-dimension spectrum and to further gain the evolution characteristics of dynamic dominant frequency and its corresponding amplitude.During the deformation damage process of flawed rocks under uniaxial compression,four modes of AE signals,including low-frequency and high-amplitude(LF-HA),low-frequency and low-amplitude(LF-LA),intermediate-frequency and low-amplitude(IF-LA)and high-frequency and low-amplitude(HF-LA),are found.Two concentration bands,i.e.,low dominant frequency(L-type waveform)band and high domiant frequency band(H-type waveform),in the distributions of dominant frequency are observed.The micro-tensile cracking event and the micro-shear cracking event correspond to the L-type waveform and the H-type waveform,respectively.Based on the distribution characteristics of different types of dominant frequency bands,the micro-cracking mechanism of flawed rocks under uniaxial compression are analyzed and discussed quantitatively and statistically.Micro-tensile cracking events mainly occurs before the ultimate failure.Micro-shear cracking events mainly domianate the ultimate failure process of flawed rocks.AE signals with LF-HA usually accompany with the occurrence of large-scale macro-cracking events.(3)The pre-existing single flaw with the different wavilness angles obviously affects the mechanical properties of rocks.The mechanical parameters of flawed rocks,including elastic modulus,initiation stress and peak strength,decrease as the wavilness angles increase.The peak strain of flawed rocks increases as the as the wavilness angles increase.Tensile wing crack I dominants the first crack initiation modes of all flawed rocks.However,tensile wing crack V as the first crack initiation modes also occurs in flawed rocks with the wavilness angles of 20° to 90°.The extended non-ordinary state-based peridynamics is carried out to simulate the deformation failure process of flawed rocks with different wavilness angles.The numerical results are roughly consistent with the experimental results.The distribution characteristics of stress field during the deformation failure process are utilized to effectively analyze the fracture propagation mechanism of flawed rocks with different wavilness angles.The multifractal theory is employed to study the dynamic nonlinear characteristics during the cracking process of flawed rocks.The time-varying evolution process of multifractal parameters can well characterize the damage failure process of flawed rocks.Based on the dynamic evolution characteristics of multifractal parameters,a quantitative criterion is developed to evaluate the damage process of flawed rocks.The coupling application of multifractal parameters can be viewed as a precursor in the prediction of fracture damage of flawed rocks.(4)The filling material in pre-existing flaw markedly enhances the compressive capacity of flawed rocks.Compared with the unfilled rocks,the mechanical parameters of filled rocks,including the peak strength,crack initiation stress and elastic modulus,are all significantly increased.The filling material and the inclination angle of flaw have obvious effects on the behaviors of crack initiation and propagation,mainly including the cracks initiation location,initiation cracks type and cracks propagation path.The theoretical fracture angles of flawed rocks obtained by Mohr Coulomb criterion is basically consistent with the fracture angles observed by experimental results.The influencing mechanism of the filling material on the fracture propagation of flawed rocks is revealed by the distribution characteristics of strain field during the deformation process of flawed rocks.Based on the AE time series,entropy theory is used to analyze the chaotic characteristics of the system of filled and unfilled rocks under compression.The evolution characteristics of entropy-value can effectively evaluate and quantitatively characterize the damage process of flawed rock.The reasonable selection of AE time series parameters obviously affects the calculation and analysis of entropy theory.(5)The spatial-temporal AE technique is employed to forecast the time-of-instability of flawed rock under complex stress conditions.The behavior of damage acceleration near the instability point of rocks can be characterized by time-reversed Omori law.The instability point of flawed rocks is effectively identified by the evolution characteristics of post peak stress with time.The starting point of damage acceleration is reliably identified by using the evolution characteristics of spatial-temporal AE events.Based on AE localization technology and time-reversed time Omori law,a pseudo-prospective forecast way is established.The spatial AE events in the damage localization band obtained by AE localization technique can be used to calculate the precursor AE event rate of power-law acceleration behavior in the approach to the instability point.The prediction of time-of-instability of rocks is employed by pseudo-prospective forecast way.A high-quality prediction results of time-of-instability are achieved as the data points constantly updates towards the instability point of rocks.Based on the time-reversed Omori law,the reliability of the forecast results obtained from the spatial AE event rate is higher than that obtained from the temporal AE event rate.The reliability of prediction results obtained by using pseudo prospective way(changeable power exponent p)is higher than that obtained by using retrospective way(power exponent p=-1). |
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