Raman stress measurement in opaque industrial Cf/epoxy composites submitted to tensile strain

Low crystallinity IM7 carbon fibres were analysed by Raman micro-spectrometry in two ‘industrial’ epoxy matrix composites. The matrix had been polymerised either by thermal treatment or by γ-irradiation. The very limited translucency range of the matrices prevented in-depth laser focusing and ‘semi-infinite’ or long fibres were, therefore, analysed on the surface of the composites. The ‘extensometry’ law had been measured on longitudinally strained non-embedded fibres:
• Δν∼−4.6±0.3 cm−1/% for Csp2−Csp3 stretching mode;
• Δν∼−5.1±0.3 cm−1/% for Csp2−Csp2 stretching mode.These results were used for in situ assessment of the fibre strain in composites tested from 0% (residual strain) up to 2%. Optical control showed fibre straining was limited to ∼1% in the cured epoxy matrix. This was due to fibre/matrix or coupon/support creep. The loading profile and ineffective length were determined in the γ-polymerised matrix.