Effect of processing dwell time on the microstructure of a fibre-reinforced composite

Carbon-fibre/epoxy-resin unidirectional laminates were prepared in the laboratory by vacuum-bag processing. Three plates were produced using different 'dwell-times', before pressure was applied to consolidate the laminate.Sections from the three plates were cut perpendicular to the fibre direction and were examined with a Quantimet 5 70 Image Analyser. The fibre positions were recorded. The method of microstructural description is based on the size of the Voronoi cell around each fibre; the Voronoi cell around a fibre is defined as the area nearer to that fibre than any other. Thus the dimensions of the cell are a measurement of half the interfibre distance.The measured distributions of Voronoi distances were compared with distributions calculated using the null hypothesis that the fibres are randomly distributed. Comparison showed that fibres from all three sections were clustered, but that the sample from the medium dwell time was most homogeneous. Examination of long Voronoi distances, not predicted by the null hypothesis, showed anisotropy in orthogonal directions arising from the manufacturing process. This method of microstructural analysis revealed a dependence on the plate processing method, and may now be used to evaluate more general processing problems.