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Air-coupled Non-Contact Ultrasound for SHM of Composites

The use of ultrasound for structural health monitoring SHM and Nondestructive Evaluation NDE provides a well-known effective method for evaluation of materials and structures for inspection and maintenance. However, conventional ultrasound has certain limitations, primarily the need for coupling liquid or direct contact with the test material. An alternative is air-coupled ultrasound, which however suffers from substantial attenuation in air, especially at frequencies in the MHz frequency range. Two recent developments have greatly furthered this method, i.e., (1) development of the gas matrix composite piezoelectric transducer and (2) enhancements in signal processing. Here we present three unpublished application examples of the inspection of (1) composites plates with impact damage, (2) flawed helicopter rotor blades and (3) a partially thawed frozen food product. Whereas the air-coupled ultrasound method shows promise there are inherent limitations at this time, such as inability for operation at high temperatures because of thermal fluctuation of the air above the specimen and limited flaw resolution because of the high attenuation in air at frequencies above about 5 MHz.


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