Scaling up the Single Transducer Thickness- Independent Ultrasonic Imaging Method for Accurate Characterization of Microstructural Gradients in Monolithic and Composite (1998)

Don J. Roth, Dorothy V. Carney, George Y. BaakliniNational Aeronautics and Space AdministrationLewis Research CenterCleveland, Ohio 44135andJames R. Bodis, Richard W. RauserCleveland State UniversityCleveland, Ohio 44115SUMMARYUltrasonic velocity/time-of-flight imaging that uses back surface reflections to gauge volumetric material qual-ity is highly suited for quantitative characterization of microstructural gradients including those due to pore fraction,density, fiber fraction, and chemical composition variations. However, a weakness of conventional pulse-echo ultra-sonic velocity/time-of-flight imaging is that the image shows the effects of thickness as well as microstmcturalvariations unless the part is uniformly thick. This limits this imaging method's usefulness in practical applications.Prior studies have described a pulse-echo time-of-flight-based ultrasonic imaging method that requires using a singletransducer in combination with a reflector plate placed behind samples that eliminates the effect of thickness varia-tion in the image. In those studies, this method was successful at isolating ultrasonic variations due to material mi-crostructure in plate-like samples of silicon nitride, metal matrix composite, and polymer matrix composite. In thisstudy, the method is engineered for inspection of more complex-shaped structures--those having (hollow) tubular/curved geometry. The experimental inspection technique and results are described as applied to (1) monolithic mul-lite ceramic and polymer matrix composite proof-of-concept tubular structures that contain machined patches ofvarious depths and (2) as-manufactured monolithic silicon nitride ceramic and silicon carbide/silicon carbide com-posite tubular structures that might be used in real world applications.BACKGROUNDPrior studies have shown that ultrasonic velocity/time-of-flight imaging that uses back surface echo reflectionsto gauge volumetric material quality is well suited (perhaps more so than is the commonly-used peak amplitudec-scanning) for quantitative characterization of microstructural gradients. Such gradients include those due to porefraction, density, fiber fraction, and chemical composition variations (fig. 1) (refs. 1 to 5). Variations in these micro-structural factors can affect the uniformity of physical performance (including mechanical [stiffness, strength], ther-mal [conductivity], and electrical [conductivity, superconducting transition temperature], etc. performance) ofmonolithic and composite components (refs. 2 and 6 to 8). A weakness of conventional ultrasonic velocity/time-of-flight imaging (as well as to a lesser extent ultrasonic peak amplitude c-scanning where back surface echoes aregated (ref. 16)) is that the image shows the effects of thickness as well as microstructural variations unless the part isuniformly thick. This limits this type of imaging's usefulness in practical applications. The effect of thickness iseasily observed from the equation for pulse-echo waveform time-of-flight (2x) between the first front surface echo(FS) and the first back surface echo (Bl), or between two successive back surface echoes where:NASA/TM--1998-206625 1