Nondestructive evaluation of laminated composites using rayleigh waves

The design and implementation of a nondestructive method for evaluating int erlaminar bonding during the in-situ thermoplastic tow placement process wa s developed. This method utilizes two piezoelectric transducers in a "pitch -catch" arrangement. Each transducer is mounted on an acrylic wedge which, in turn, is mounted on a steel mediator wedge. This "double wedge" assembly produces Rayleigh surface waves in a composite specimen without a gel coup lant. The received signals are analyzed for changes in the Rayleigh wave sp eed. Poorly bonded top most plies cause the Rayleigh wave velocity to decre ase by as much as 30%. Verification of this bond quality sensor is demonstr ated under various tape laying conditions. In each specimen, the Rayleigh w ave velocity was measured. The bond quality was assessed quantitatively wit h a peel test and qualitatively by visual inspection (photomicrographs). Th e Rayleigh wave velocity was compared with the strength and thickness of th e bond between the top ply and substrate. The Rayleigh wave speed increases linearly with the top ply peel strength. Photomicrographs indicated that p oorly bonded specimens have a resin rich region at the top ply/substrate in terface. These tests demonstrate that the double wedge sensor can measure b ond quality during in-situ consolidation of thermoplastic matrix composites .