Raman Spectroscopic Stress Analysis of Carbon Fibre Composites in Compression

Abstract

Unidirectional carbon fibre reinforced polymer composites (CFRPs) often show compressive strengths around 60% lower than their tensile strengths, making compressive performance a critical design factor. This thesis examines the compressive micromechanics of carbon fibres at the single-fibre scale, utilising in situ Raman spectroscopy and focused ion beam scanning electron microscopy (FIB-SEM).
Raman spectroscopy was validated as an effective, non-destructive, and quantitative tool for analysing compressive behaviour in carbon fibres and single-walled carbon nanotube (SWCNT)-coated fibres. Calibration of G and G+ band shifts enabled the derivation of stress-strain curves for three PAN-based carbon fibres, revealing reversible non-linear behaviour and compressive strength values. Spatially resolved Raman stress maps highlighted improved interfacial shear stress transfer efficiency and reduced transfer lengths with functionalised SWCNT coatings. These coatings doubled interfacial shear stress (IFSS) in some cases but introduced localised stress variations and additional non-linearity effects.
High-resolution Raman mapping revealed fragmentation in high-modulus (HM) fibres, including inter-fragment wedges and shear-type breaks. Stress maps demonstrated that HM fibre fragments retained load-bearing capacity post-failure under compression. Local interfacial shear stress behaviour was analysed, and a mode I debonding mechanism during fragmentation was proposed. SWCNT coatings are hypothesised to improve composite compressive strength by enhancing IFSS and mode I toughness, though further validation is needed.
A novel FIB-SEM approach enabled detailed imaging of compressive fibre and interfacial failure, validating predictions from Raman stress mapping. FIB-SEM revealed that HM fibres exhibited skin-initiated failures with internal kink bands, while AS4 fibres displayed catastrophic failure characterised by significant energy release.
The combination of Raman spectroscopy and FIB-SEM provides a powerful framework for probing fibre micromechanics. These findings offer new insights into compressive failure mechanisms, interfacial behaviour, and the potential of SWCNT coatings to enhance the compressive performance of CFRPs.

Date of Award	17 Jun 2025
Original language	English
Awarding Institution	University of Bristol
Supervisor	Steve Eichhorn (Supervisor) & Richard S Trask (Supervisor)