EVALUATION OF ELECTROSTATIC AIR-COUPLED ULTRASONIC TECHNOLOGY FOR MEASURING CHANGES IN RESIN CONTENT IN CARBON PREPREG TOW MATERIALS

A feasibility study on the use of air-coupled ultrasonic transducers t o sense resin content variation in carbon prepeg tow materials in a no n-contact mode has been undertaken. Electrostatic transducers capable of generating and detecting ultrasound in air over the frequency range 50-500 kHz were utilized. The basis of the non-contact technique was to excite plate waves in a tow sample with air-borne ultrasound, and d etermine the velocity of the plate waves by measuring ultrasound radia ted back into the air couplant. Changes in resin content would manifes t themselves in the velocity of the plate waves propagating in the tow sample. Air-coupled excitation and measurement of plate wave velocity was successfully performed on a 35 percent resin content tow sample. The measured plate wave velocities were approximately 15 percent lower than a simple theoretical estimate, the difference presumably due to factors not included in the theoretical estimate.