Fibre swelling during laser drilling of carbon fibre composites

Fine holes have been laser-drilled in bundles of carbon fibres, using a pulsed Nd:YAG laser (λ=1.06μm). Examination of the drilled fibres showed that some were significantly swollen, with diameter increases up to ∼60%. Scanning electron microscopy (SEM) and Raman spectroscopy have been used to investigate the effects of fibre type on the extent of laser-induced fibre swelling. It has been established that extensive swelling only occurred with low-modulus, poly acrylo-nitrile (PAN)-based fibres. Based on the Raman spectra obtained from both as-received and laser-drilled fibres, the swelling mechanism is attributed to simultaneous structural rearrangement and rapid, heating-induced volatilisation of impurities. A prior heat treatment was found to reduce the laser-induced swelling in low-modulus fibres. A 2D numerical heat flow model has been used to investigate the thermal fields generated during laser drilling of carbon fibres, and this information is correlated with the observed effects and the proposed swelling mechanism.