Reliable and universally applicable chemometric techniques based on infrared spectroscopy to non-destructively quantify thermal damage of carbon fibre reinforced epoxy matrix composites

Abstract

This work provides multivariate data analysis (chemometric) techniques based on infrared spectroscopy (FTIR) to separately determine temperature and duration of a moderate thermal pre-load on polymer matrix composites as well as their residual strength. The aim is to assess the precision of various techniques such as hand-held and bench top spectrometers, attenuated total reflection (ATR) and diffuse reflectance (DR) spectroscopy, as well as spectra recorded on surfaces and in the bulk of a specimen. A focus is laid on methods which are not only applicable for a single composite for which a calibration is established, but to universally apply them for epoxy based matrix systems with various thermoplastic tougheners. Therefore, data pre-treatments are investigated to focus on spectral changes which are selectively characteristic for the degradation of the thermally less stable epoxy resin. Firstly, difference spectra with pristine material are calculated and secondly, chemometric analysis is limited to spectral ranges characteristic for the epoxy resin. Commercially available composites (HexPly ® M18-1/G939, 8552/IM7, RTM6/G939, and Cycom® 977-3/IM7) are investigated. Multivariate analyses provide reliable values for time and temperature (average deviation < 10 °C) of the thermal pre-load and residual interlaminar shear strength of specific composites (average deviation < 5%). However, average deviations dramatically increase for universally applicable methods, limiting the use of this approach.