The 0.8-0.25 mm diameter aluminum inclusions, bonded on the bottom surface of the epoxy casting insulator with a thiclrness of 52 mm, were tested quantitatively using ultrasonic technique. The root mean square deviation (RMSD) which is based on the power spectrum of the reflected wave at the original and several damage states was introduced as damage index. The results show that when aluminum inclusions exist in the insulation, the time delay between the initial and reflected waves increased in the time domain, and the RMSD values decreased with the decreasing of aluminum inclusion diameter. The aluminum inclusion with the diameter of 0.25 mm can be effectively tested with ultrasonic technique. This study provides experimental data accumulation for the application of ultrasonic testing technique in the epoxy casting insulation.
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