Microstructural Analysis of Wound Composites with Considerations on the Fiber Winding Force

Abstract

Abstract. The paper concerns issues related to the fabrication and microstructural analysis of carbon and glass fiber-reinforced epoxy composites. For the purposes of the tests, ring samples of carbon fiber and glass fiber were fabricated, each sample being made of the same amount of material. The rings were produced by circumferentially wrapping fibers onto a rigid core. During winding, the fiber tension was changed for each sample to investigate whether the amount and distribution of fibers in the composite depending on the force applied. The applied tension forces ranged from 18 to 138 N. In the next stage of tests, small fragments were cut out of the rings, cutting perpendicularly to the arrangement of fiber bundles. The cut elements were then embedded in epoxy resin. The microsections prepared in this were ground using a grinding disc with a gradation of 80 to 4000 and then polished, washed and dried. After these treatments, microscopic photos of the tested surfaces were taken. Observations were made using a NEOPHOT 32 microscope with an integrated camera and a KEYENCE VHX microscope. The volume fraction of voids and discontinuities in the matrix, as well as the volume fraction of fibers, were determined by measuring the percentage of the surface areas occupied by the appropriate components of the composite. On each cross-section of individual samples, photographs were taken in three planes parallel to the composite layers and three planes perpendicular to the fibers  in the matrix system. The image analysis method was used for verification, in which it is assumed that the assessment of fiber distribution in a two-dimensional section is representative of its volumetric distribution. This method is mainly used to analyze the distribution of fibers with a constant cross-section. The most important conclusions from the conducted analyses allow the authors to state that in both composites the fibers in the matrix are highly packed and that there was no significant and noticeable effect of fiber tension during winding on the "packing" density of fibers in the composites.