The Laboratory Study Of The Rock's Viscoelastic Properties

Attenuation, velocity and its dispersion, Load/Unload response ratio are closely related to the rock's viscoelastic properties. From the three acpects, this paper studied the viscoelastic properties of the rock.Many studies on the attenuation has been done-mainly concentrated in the frequencies of seismic wave, compared with this, the researches of attenuation under a longer period driven by a certain cycle of stress (strain) amplitude were less. This paper focused on the relationship among the attenuation, the stress amplitude and cycle time under different initial stress levels (the baseline of the amplitude).On this basis, in order to obtain the attenuation in a wider frequency domain, under unchanged initial stress, a series of creep and ultrasonic tests were performed, then the results were compared with. As the two stress amplitudes were zero with the same pressing process, the effect from stress amplitude on the attenuation was eliminated. Both results were only concerned with the keeping time and could be comparable. Because of testing time corresponding to the amount of creep, after changing the relationship between attenuation and creep time into the relationship between attenuation and the amount of creep, the attenuation was extrapolated in the smaller strain (small amount of creep) direction, it was found that the attenuation value of zero creep was close to the attenuation measured by ultrasonic. Combining the existing constant-Q under 0.1～1MHz, the correlation between the attenuation frequency relationship and attenuation-strain was suitable. The creep attenuation was extrapolated in the bigger strain direction, the attenuation of creep rupture could be found and the creep time could been estimated according to the amount of rupture creep, once the correctness of extrapolating in the direction of creep rupture was tested by experiments, then the attenuation for one sample within the lowest frequency to MHz could be obtained.Specifically, to find out the relationship between attenuation and the initial stress, amplitude and cycle time of the stress, a series of uniaxial stress cycling tests performed in triangle waveform loading with equivalent amplitude but different cycle time or equivalent frequency but different amplitude under different initial stress levels were conducted on dry Taching sandstones and sandstones from the Zipeng mountain of the Tan-Lu fault zone using MTS testing machine. And then, the relationships among Q, the initial stress level, the strain (or stress) amplitude and the frequency were obtained. With the help of Endochronic theory, linear viscoelastic theory and P-M model theory, I gave a suitable reason for the results.Using the experimental results I calculated the parameter of Endochronic theory, and the attenuation of unknown stress amplitude and frequency were well predicted compared with experimental values.In order to obtain attenuation under a long period, according to the results, except the effe(?) of stress amplitude on the attenuation, the attenuations from the different creep time which respectively continued 1 hour,2 hours,4 hours,12 hours,1 day,2 days and 3 days under certain initial stress level were jointly considered with the results from the ultrasonic frequencies 470KHz,700KHz. The results showed that the attenuation of creep was higher than that of ultrasonic, the smallest difference between them was about 40%, which might be related to the strain of different methods. After changing the relationship between attenuation and creep time into the relationship between attenuation and strain, the linear correlation coefficient was above 95%. The result of creep attenuation extrapolated in the smaller strain (small amount of creep) direction was very close to the ultrasonic attenuation. Taking the strain into account, the results of two methods were in line with. At the same time, from the point of strain, why the attenuation in 0.1～1MHz almost was unchanged could be explained. The rock underground in a certain state of stress for a long time, the results of attenuation measured by creep is closer to the attenuation of layer rock than other methods.Many studies also have been done on velocity, but often emphasized the experimental results and the regularity, the discussion of the influence from experimental conditions on the results was less. For different measurement conditions would result in deviation of experimental results, even the velocity values from the same sample by the same person might be different, because of different measurement conditions. So it was necessary to study the impact of various factors on the velocity for standardizing the measurement conditions.Transducers with different frequencies (550KHz,50KHz) were used to measured the velocities of nearly a hundred samples(sandstone, marble, steel, aluminum, plexiglass) with different lengths to quantitatively understand the effects of the length of the samples, the transducer's frequency, signal amplification, coupling agents, coupling strength on the testing results. The results revealed the effects of sample sizes, intrinsic attenuation on the velocity dispersion. The length caused stable velocity was smaller than the recommended length, which meant the recommended standards could be relaxed.Base on the results, I measured the attenuation by the dispersion method and corrected the coefficient of rock mass integrity tested by seismograph and acoustic instrument.Load/Unload response ratio(LURR) theory has been achieved well in processing the rock fracture data, but the study on the change of LURR of the whole creep rupture process was relatively lack. In order to study the whole process of creep rupture in the point view of LURR, experiments on the granite sample at uniaxial stress were performed by using grade loading method to obtain the creep curves at grade loading process and creep fracture curve. After decomposing the creep fracture curve, the Young modulus, creep value and creep rate during the fracture process were analyzed by Load/Unload response ratio method. It was found that the law of the load/unload response ratio's variation in the fracture were consistent with that during the rupture process of earthquake, and the response curve of the Young modulus changing with the stress was different from the typical constitutive curve, which was probably because of the weakening of the rock and the transition from the steady creep to the accelerating creep.