NONDESTRUCTIVE INSPECTION OF GRAPHITE-EPOXY COMPOSITES FOR HEAT DAMAGE USING LASER-INDUCED FLUORESCENCE

Laser-induced fluorescence has been applied to the nondestructive insp ection of graphite-fiber-reinforced polymer-matrix composites. Upon cu ring, the epoxy resin in these materials forms a highly cross-linked n etwork of aromatic sub-units. This result yields a material with signi ficant absorptivity from the ultraviolet through the near-infrared and a fluorescence bandwidth covering 200 to 300 nm. When subjected to te mperatures beyond the recommended upper service temperature, these mat erials experience thermally induced degradation and a reduction in mec hanical strength. Furthermore, these heat-damaged materials exhibit si gnificant changes in fluorescence intensity and in the wavelength of m aximum fluorescence intensity. These spectroscopic changes can be corr elated with the extent of heat damage. An imaging approach has been de veloped to monitor these fluorescence signatures, allowing rapid nonde structive inspection of large surface areas. This capability provides qualitative and quantitative information about the location and severi ty of damage.