A hybrid non-contact ultrasonic system for sensing bond quality in tow-placed thermoplastic composites

A novel non-contact nondestructive evaluation system is proposed for sensing bond quality in tow-placed graphite/PEEK thermoplastic composites. The inspection tool consists of a hybrid ultrasonic system which employs laser generation of narrow-band surface waves with fiberoptic light delivery, and signal detection by air-coupled transduction. The fiberoptic link enhances flexibility and remoteness of the laser source from the test piece. The system output is not influenced by the poor light reflectivity of the test graphite/PEEK material. Signal narrow-banding by light spatial modulation enhances the measurement sensitivity and avoids detrimental material ablation while allowing control of thr surface wave penetration depth. The theory of the laser generation of narrow-band surface waves is reviewed and the generation efficiency as a function of the wave propagation direction in the composite is discussed. The inspection capabilities of the proposed system are demonstrated by successful detection of a series of subsurface defects in a tow-placed unidirectional AS-4/APC-2 cylinder provided by Boeing Commercial Aircraft. The defects examined include regions of poor consolidation, inter-laminar linear porosity (voids elongated along the fibers with 1.5% volume fraction), and delamination or interlaminar weakness. The ultrasonic inspection unit is non-contact, flexible, light-weight and allows for high inspection rates. These characteristics, coupled with a large signal-to-noise ratio of the measurements, show promises for the use of the proposed sensor in on-line control of the thermoplastic tow-placement process with in situ consolidation.