A high-resolution real-time ultrasonic imaging system for NDI applications

Ultrasonic NDI methods have an impressive record of applications on metallic and composite structures. However, limitations arise from the need for a wet couplant between the specimen and the transducer and the rather long inspection times necessitated by point-by-point scanning of large structures. To overcome these constraints, a dry-contact large-area ultrasonic imaging system is being developed for real-time high-resolution NDI applications. This system includes the following: a large ultrasonic source, either piezoelectric or laser-based, a polymer dry-couplant, and a commercially available real-time ultrasonic CCD camera displaying easy-to-interpret images rather than A-scans. Applications of this real-time high-resolution ultrasonic imaging system on metallic and composite structures, using either PZT or laser-based ultrasound generation as the source, are presented. Aluminum and unidirectional and woven composites have been investigated. Images acquired in both through-transmission and pulse-echo modes are presented. Images of artificial defects of different types and shapes in the investigated materials will be demonstrated. The latest developments of the imaging system, with laser-based ultrasound generation as the source, are also reported. The laser-based source provides an efficient solution for some applications of the imaging system. In this configuration, the ultrasound is generated in a 1in. diameter area by an expanded laser beam which heats a constrained absorbing polymer layer. The soft polymer layer is also used as dry couplant to transmit the ultrasound between the test sample and the imaging system.