Effects of Briquettes with Different Crack Structures on Propagation Characteristics of Ultrasonic Waves under Wetting Conditions

In order to examine the effect of briquettes with different crack structures on ultrasonic characteristics under different wetting conditions, a series of ultrasonic testing are carried out on briquettes at different wetting heights and the ultrasonic characteristics in these coal samples are explored. The results show that ultrasonic amplitude is positively correlated with the emission voltage, whereas ultrasonic frequency is negatively correlated with the emission voltage. Changes in both are closely related to the particle size and density. The ultrasonic velocity is positively correlated with the wetting degree. Sample mass has the greatest effect on the ultrasonic velocity, followed by particle size, and pressure has the smallest effect. At dry stage, ultrasonic velocity in gas coal is less than that in bituminous coal. The opposite is true in the fully wet state. The influence of crack thickness on ultrasonic velocity gradually increases with the wetting degree increasing. At dry stage, the velocity gradually increases with the crack dip increasing, while as the wetting height increasing, magnitude of velocity increase gradually decreases with the dip increasing. The ultrasonic attenuation in the briquettes reduces with the emission voltage enhancing. The attenuation decreases with sample particle size, crack thickness and crack size decreasing and with sample mass, pressure and crack dip increasing. The ultrasonic attenuation shows a trend of increase before decrease with the wetting height increasing. The attenuation of ultrasonic wave increases with wave velocity increasing for intact samples and shows a trend of increase before decrease for cracked samples.