저주파 결함 탐지기를 이용한 하중단계에 따른 암석의 상태 평가

Abstract

The study is to evaluate quantitatively the flaws in the rock with low-frequency ultrasonic flaw detector method, one of the non-destructive test methods through load steps. For this purpose, the velocities by the CND tester and low-frequency ultrasonic flaw detector was performed on the square granite samples with various sizes. Then, applicability as the evaluation of rock flaws was identified with the low-frequency ultrasonic flaw detector method. In addition, the flaw state in granite was defined to analyze the velocities to three directions with the load steps, which was 50%, 60%, 70%, 80% of uniaxial compressive strength for granite. The results are as follows. By the results of the velocities by the CND tester and low-frequency ultrasonic flaw detector on the square granite samples with various sizes, an average velocity by CND was about 1890.8 m/s, and one by low-frequency ultrasonic flaw detector was about 1896.7 m/s in the sample of IG-1. In the case of IG-2, an average velocity by CND was about 1975.8 m/s, and one by low-frequency ultrasonic flaw detector was about 900.0 m/s. The difference between the two results was about 2.2 times. In the case of IG-3, an average velocity by CND was about 2730.0 m/s, and one by low-frequency ultrasonic flaw detector was about 1500.0 m/s. The difference between the two results was about 1.8 times. In the case of IG-4 with borehole and IG-5 without borehole in the sample, an average velocity by CND was about 2282.1 m/s and 2228.9 m/s, and one by low-frequency ultrasonic flaw detector was about 1000.0 m/s and 900.0 m/s, respectively. The difference between the two results was about 2.3 and 2.5 times. Considering only the above results, an average velocity by the CND was totally lower than that by low-frequency ultrasonic flaw detector. By the results of the velocity variation with the load steps of uniaxial compressive strength for granite of IG-7 estimated by silver Schmidt hammer test, the velocity tended to decrease at 50% to 70% of the load step, then to increase at 70% to 80% of the load step. This suggests that the microcracks in granite are closed at the low load step, then the new cracks are formed as the load step increases. The velocity are in the order of z-axis>y-axis>x-axis, which means that the new cracks have the greatest effect on x-axis velocity reduction. In conclusion, the low-frequency ultrasonic flaw detector method can be applied to quantitative evlauation of flaws in rock, and it can be used to analyze the variation of velocity in each direction according to the load step.