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 sensin g bond quality in tow-placed graphite/PEEK thermoplastic composites. The in spection tool consists of a hybrid ultrasonic system which employs laser ge neration of narrow-band surface waves with fiberoptic light delivery, and s ignal detection by air-coupled transduction. The fiberoptic link enhances f lexibility and remoteness of the laser source from the test piece. The syst em output is not influenced by the poor light reflectivity of the test grap hite/PEEK material. Signal narrow-banding by light spatial modulation enhan ces the measurement sensitivity and avoids detrimental material ablation wh ile allowing control of thr surface wave penetration depth. The theory of t he laser generation of narrow-band surface waves is reviewed and the genera tion efficiency as a function of the wave propagation direction in the comp osite is discussed. The inspection capabilities of the proposed system are demonstrated by succ essful detection of a series of subsurface defects in a tow-placed unidirec tional AS-4/APC-2 cylinder provided by Boeing Commercial Aircraft. The defe cts examined include regions of poor consolidation, inter-laminar linear po rosity (voids elongated along the fibers with 1.5% volume fraction), and de lamination 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 measure ments, show promises for the use of the proposed sensor in on-line control of the thermoplastic tow-placement process with in situ consolidation.