Residual stress distribution in carbon fibre/thermoplastic matrix pre-impregnated composite tapes

The distribution of residual strain in carbon fibre (P75) reinforced thermo-plastic (polyetheretherketone, PEEK) composites has been studied by monitoring the Raman frequencies of the embedded carbon fibres at the microscopic level. The average longitudinal strain in the fibre measured in a 100 mm² section of a P75/PEEK prepreg was found to be compressive and comparable in magnitude to residual strain values computed analytically. The combined effects of differential thermal expansion of the two materials, differences in fibre size and in the thickness of resin layer, are offered as likely explanations for the incurred variations in the Raman frequency values and the considerable spread of the fibre Raman frequency distribution in the prepreg tape. Finally, it is demonstrated that the previously reported high values of apparent compressive strain in similar composite tapes by means of Raman spectroscopy were artefacts of the experimental procedure.