FUNDAMENTAL REPRODUCIBILITY OF RAMAN BAND POSITIONS AND STRAIN-MEASUREMENTS OF HIGH-MODULUS CARBON-FIBER - THE EFFECT OF LASER-INDUCED HEATING

We have shown that the E2g, A1g and second-order (approximately 2700 cm-1) Raman band positions of Hercules HMS4 carbon fibres shift as a function of incident laser power. These shifts arise as a result of changes in the local fibre temperature. The sensitivity of band position to varying laser power is different from fibre to fibre within the same tow - thus equally strained fibres subjected to the same laser power can show widely different band positions and so different apparent strains. The same effect can be observed from point to point on individual fibres. If laser power is not carefully controlled (both in magnitude and stability), errors in the accuracy of the measured strain can be greater than the measured stran itself, and have been shown to approach 70% of the breaking strain. These results mean that strain measurements obtained from composite materials containing this fibre must be interpreted with caution unless the laser beam intensity at the fibre is precisely controlled. This effect will also be important with other types of carbon fibre, since in previous work we have observed that laser-induced sample heating occurs with a wide range of materials from cokes to pitch-based fibres.