Proposal for a long-fibre microbuckling scenario during the cure of a thermosetting matrix

Recently, real-time video images of the appearance of undulations on a long T300 single carbon fibre during the curing of a LY556 epoxy matrix were proposed by Jochum et al. [Jochum Ch, Grandidier JC, Smaali M. Experimental study of long T300 carbon fibre undulations during the curing of LY556 epoxy resin. Compos Sci Technol 2007;67:2633–42]. The correlation of the fibre undulation with cure kinetics has established that undulations appear prior to glass transition. The present study presents a description of the evolution of the elastic and loss modulus during curing, which is correlated with cure shrinkage estimations and previous video results. The comparisons between these results lead to a clear proposal for a single long-fibre microbuckling scenario during the curing of an epoxy matrix. Novel results are that fibre instability is triggered at local heating rates higher than 3 °C/min and coincides with the maximal chemical shrinkage observed prior to glass transition. Similarly, the observation of the relaxation of partial fibre undulation relaxation is coherent with the viscoelasticity of the matrix during formation. The instability appears some time after exposure to a rubbery state. The analysis of single fibre undulation is an excellent indicator of cure quality and an appropriate testing structure for further chemical, thermal and mechanical models of the co-operative buckling of many fibres.