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Due to increasing use of composites as structural materials, defect characterization in composites has attracted wide attention in recent years. Several undesirable effects in composites include: fiber breakage, inclusions, voids, improper resin distribution, delaminations, and misalignment of fibers. Acoustic methods of nondestructive evaluation (NDE) are proven candidates in identifying these defects to varying levels of success. Prominence has been given to acoustic emission (AE) and ultrasonic inspection techniques in the evaluation of composites; however, the acoustic impact technique (AIT), derived from the coin-tap test, has not been explored in depth. The coin-tap test involves tapping a specific location on a structure with a “coin” or “penny” and judging the quality of the material based on the sound emanated from the localized region. In principle, a “good” region results in an intense sound as compared to a “defective” one. Although coin tapping has been used in this fashion for several years, the most significant drawback has been its operator dependency. AIT attempts to overcome this shortcoming. In AIT, the use of sophisticated electronic instrumentation provides a reliable estimate of the quality of the material. The foundation of AIT lies in sensing and analyzing localized vibrations and disturbances generated by a low-magnitude mechanical pulse input to the structure under test. In this paper, the current status of AIT is reviewed, and new developments are discussed. Results are presented relating to the application of this technique for NDE of graphite/epoxy and graphite/phenolic composites.
acoustic impact technique, NDE, graphite fiber composite, acoustic emission, defect characterization
Professor, Auburn University, Auburn, AL
Assistant professor, Materials Research Laboratory, Tuskegee University, Tuskegee, AL