Development of nondestructive-evaluation techniques for high-temperature ceramic heat-exchanger components

An assessment was made of ultrasonic techniques developed to date for nondestructive evaluation of SiC heat exchanger tubes. The results suggest that ultrasonic inspection is a useful and valuable technique for inspecting these tubes, including ceramic butt joints. However, this method alone is currently not sufficiently effective to detect all critical flaws because of (1) the difficulties in following the surface of an out of round tube; (2) the high velocity and thus the large angle of refraction of sound in SiC, which for small diameter tubes leads to significant beam distortion; and (3) insuffient resolution, relative to the small critical flaw size in ceramics, at conventional ultrasonic testing frequencies. The experiments show that higher frequencies (up to 35 MHz) are required for effective wall thickness measurements and detection of laminar type flaws.