Optical and Mechanical Characterization of Spliced Carbon Fibre Composites

This dissertation investigates the optimization of carbon fibre (CF) strands through pneumatic splicing to improve the performance of composite materials. A novel splicing technique was devised and examined using cutting-edge methods. In this process, carbon fibre tows were joined within a splicing chamber under the influence of highly pressurized air, forming a single joint section. These included high-resolution imaging systems, digital image correlation techniques, and thermal and electrical conductivity measurements. A comprehensive image processing workflow was implemented to understand better the spliced composite microstructure, encompassing surface profiling and flow tracking algorithms. Notably, Electrical resistivity measurements and thermal imaging techniques were employed to investigate the physical characteristics of the spliced carbon fibre tows. The Taguchi design of experiment (DoE) was employed to identify tow overlap length as a critical splicing parameter influencing microstructure, mechanical properties, and in-plane permeability.
The effect of the spliced fibres on the transverse in-plane permeability has been examined in this research. The extent of overlap significantly impacted permeability, with longer overlaps resulting in reduced values. This substantial effect led to a 20% difference in ply permeability and a 35% difference in tow permeability values. This is attributed to increased fibre count, whisker formation, cross-sectional variations, and altered microstructure within the spliced region. A dual-scale permeability effect was observed, with the flow front advancing while the spliced fibres remained partially saturated. This phenomenon is attributed to the presence of channels within both intra-fibre bundles and inter-bundle scales within the fabric. Fibre volume fraction exhibited lower values in spliced samples compared to unspliced ones due to the effects of the pneumatic splicing process on the joined CF tows. This resulted in unspliced samples exhibiting the highest permeability due to the absence of twisted fibres. Attempts to model permeability using established models (Gebart and Kozeny-Carman) for spliced samples were unsuccessful compared to experimental results, indicating that the geometry of the channel paths between the fibres was altered during splicing.
Manufacturing studies examined the impact of splice location (centre or edge) and ply spacing on woven composite properties. Significant variations in fibre volume fraction, thickness, and mechanical properties were observed, with edge-spliced samples exhibiting pronounced reductions in strength and modulus compared to centre-spliced counterparts. Multiple spliced plies demonstrated decreased performance relative to single-spliced samples. Failure analysis revealed predominant modes, including fibre breakage, inter-ply cracks, and delamination, with bending test samples exhibiting increased susceptibility to damage.
This comprehensive investigation provides valuable insights into the intricate relationship between splicing parameters, microstructure, and composite performance. The findings contribute to the development of advanced CF composite materials by clarifying the mechanisms underlying the impact of splicing on mechanical properties and structural integrity.