Pitch catch ultrasonic bond sensor for the in-situ tow placement consolidation of thermoplastic laminates

A nondestructive sensor for on-line inspection of laminated composites was developed. In particular, we have developed a repeatable, low cost method for sensing poorly bonded plies as a thermoplastic laminated composite part is formed. The sensor consists of two piezoelectric transducers mounted in a 'pitch catch' arrangement. One transducer transmits an ultrasonic signal and the other receives the signal. Each transducer is mounted on a Acrylic wedge. The Acrylic wedge, in turn, is mounted on a steel wedge. The Acrylic wedge angle is selected so as to excite a Rayleigh wave in the steel wedge. The steel wedge introduces the Rayleigh wave into the composite via Hertzian contact. There is no couplant at the composite surface. Received signals have been analyzed for frequency shift, frequency filtering, attenuation and changes in wave speed. Of these signal analysis methods, the most consistent indicators of poorly bonded regions are signal attenuation and wave speed. Selected results from an extensive experimental study are described. These results indicate that the Rayleigh wave speed and amplitude are indicative of the bond strength of a ply near the surface of the composite.