Nondestructive inspection of heat damage to graphite-epoxy laminates using diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS)

Diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) has been used as a nondestructive inspection (NDI) technique to spatially map heat damage to AS4/3501-6 graphite-epoxy laminate specimens. Specimens were furnished in the heat damaged condition by the Great Lakes Composites Consortium as part of a program to evaluate nondestructive methods for detecting oxidative embrittlement (i.e., the loss of resin strength without delamination due to heat in air). In this study, 3846 individual spectra, all of high quality, were obtained from 64 AS4/3501-6 panels (12-in.×12-in., 16-ply) having their as-prepared central region damaged by radiant heat or by contact with a heated aluminum disc over a wide range of times (0 to 120 min.) and temperatures (178-302 °C). Facilities and techniques for mapping surfaces using infrared spectra are described. The thrust of these experiments was to obtain spectra at 6.25-mm intervals across these panels along a line through the center of the panels at right angles to the long axis of short beam shear specimens to be fabricated after NDI measurements were made. Other data collected included data lines at right angles to the main line, data lines at spatial resolutions as high as 2.5 mm., and area maps. In all cases, the effect of heating was detected by DRIFTS and reported in a spatially accurate format. Panels having quasiisometric layups designed to fail in the resin when tested by short-beam shear, good correlation between failure load and DRIFTS measurements were obtained. Algorithms for converting DRIFTS spectra to simple numbers to evaluate heat damage and deduce mechanical property loss are described.