An ultrasonic inspection method for a graphite ingot was developed to detect internal plane flaws that orient in various directions; this is needed to perform quality assurance of throat inserts of solid rocket motors. Some of the problems unique for this graphite inspection were solved. An ultrasonic beam in graphite shows uneven propagation behavior both in an individual ingot and among them. The individual unevenness results in variation in echo heights of flat-bottomed holes, which can be compensated through probe scanning by varying incident angles of the two independent axes. This probe scanning procedure is, thus, necessary to detect internal plane flaws that orient in various directions. The latter unevenness can be compensated by measuring the wave velocity and attenuation coefficient in the test block itself before the inspection. A test block which included artificial internal cracks was fabricated and was inspected by the developed method. It was, then, sliced into several thin blocks, and the sliced blocks were inspected by the conventional ultrasonic testing method using a normal beam technique. The two methods detected identical flaws, which proved the validity of the developed method. The technique described here has been enacted as JIS Z 2356 under the title, "Method of automatic ultrasonic inspection for graphite ingot".

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