Laser induced fluorescence based nondestructive evaluation of heat damaged composite aircraft

This paper provides an overview of a Navy program to develop a portable, non-destructive evaluation (NDE) prototype unit based on the Laser Induced Fluorescence (LIF) technique for the reliable detection and quantification of incipient heat damage in polymer matrix composite (PMC) aircraft components. Fire, engine exhaust impingement, and equipment overheats can cause heat damage and degrade the mechanical properties of PMC's. Incipient heat damage occurs at exposures for which there are no visual indications such as matrix cracking or ply delaminations but nonetheless there is a mechanical property loss in the composite. Currently, there is no depot level technique capable of reliably determining the degree of incipient heat damage in a composite part. Results are presented in development of LIF correlation curves for three composite materials: the un-toughened epoxy composite AS4/3501-6, the toughened epoxy composite IM7/977-3, and the bismaleimide (BMI) composite IM7/5250-4. LIF spectra were recorded of a series of heat-treated test panels followed by interlaminar shear strength (ILS) testing of coupons from the panels. The ILS measurements were compared with the LIF spectral features information and found to give good correlations.