Study On Dynamics Features Of Ultrasonic Velocities In Loading Coal

Ultrasonic testing technology is a new technology which has developed for nearly 40 years and an important research tool to understand the physical characteristics and structural features of coal and rock.In this paper, the dynamic characteristics of coal samples were systematically studied with the methods of theoretical analysis combined with laboratory experiment. Firstly, the ultrasonic amplitude and velocity were tested in conditions of normal temperature and pressure conditions with loading conditions(uniaxial compression and splitting tension). Then, the attenuation law of propagation of ultrasonic in coal was theoretically analyzed and the comparison between the results of theoretical analysis and the results of laboratory experiment was made, which achieved a good conclusion. Finally, the frequency values variation under the condition of normal temperature and pressure conditions with loading conditions.By means of theoretical analysis and laboratory experiment, the main conclusions were as follows in this paper:(1) Under normal temperature and pressure conditions, the ultrasonic attenuation law of coal samples with different lengths was analysed, and ultrasonic diffusion attenuation formula was presented from the perspective of theoretical derivation and experiment. The difference of ultrasonic attenuation in different ranks of coal samples under the same length was ultrasonic viscous absorption attenuation. In the study on attenuation of ultrasonic in coal samples under different frequencies, absorption was the mainly ultrasonic attenuation at 50 KHz, 100 KHz and 500 KHz three frequencies, and ultrasonic attenuation dominated by scattering at both 800 KHz and 1000 KHz frequencies.(2) In the whole of uniaxial loading and splitting tensile stages, ultrasonic attenuation coefficient of coal sample reduces earlier and increases later with increasing of load variation. Compared with the changes of ultrasonic wave velocity in the whole stage, the change of attenuation coefficient was more obvious. The equation describing the relationship between ultrasonic attenuation coefficient and coal sample loads was established under different loading stages, and the ultrasonic attenuation law was analyzed theoretically in loading conditions.(3) Under normal temperature and pressure conditions, with the decrease of the first amplitude and the increase of the attenuation coefficient the main frequency of coal sample reduced. The ultrasonic main frequency value with different emission frequency would be with the increase of different emission frequency. Under loading condition,The ultrasonic main frequency value would change dramatically at the point in time when the rate of change about the ultrasonic attenuation coefficient took place. The main frequency value was the maximum while the attenuation coefficient was the minimum and after the rupture of coal sample, the attenuation coefficients increased dramatically, and at the same time the main frequency value reduced to the minimum.The maximal amplitude of coal sample's ultrasonic frequency increased and then decreased, which was consistent with the attenuation coefficient of variation in the process of loading: the attenuation coefficient was the minimum while the corresponding maximal amplitude of frequency domain was the maximum. After the rupture of coal sample, spectrum curve would appear typical double-peak.In summary, through the experimental study on dynamics features of ultrasonic velocities in loading coal, this paper provided theoretical basis and technical support for the prediction of coal and rock dynamic disaster using non-contact geophysical methods.