The tremendous growth and development of the global socioeconomic systems in the past decades has put intense pressure on the existing infrastructure. In particular, it has resulted in a sharp increase in power consumption that has significantly strained the existing power infrastructure. This has necessitated the development of new conductors for high current capacity overhead power transmission with the ability to operate under high temperatures. The high-temperature low-slag (HTLS) conductors, as they are commonly known, have proved to be a promising solution for reducing the strain on the burdened power grid. Generally, the HTLS conductors such as ACCC® conductors are made of hybrid polymer matrix composites (PMC) cores comprising unidirectional reinforcement of E-glass and carbon fibers in an epoxy matrix. This has enabled the conductors to achieve the desirable diameters, strengths and load capacity to transmit power under high temperatures.
The performance of carbon fiber reinforced PMCs is often improved through the fiber hybridization method. However, this process involves excessive bending without axial tensions that can significantly damage the composite core. Previous findings established that the lower comprehensive strengths associated with carbon fiber reinforced polymers (CFRPs) are due to the formation of kink bands. Consequently, an improvement in the conductor resistance to bending-induced damage can be realized by increasing the axial tension. Additionally, the fiber bundle waviness and misalignments, as well as the nonlinear characteristics of the carbon fibers, have also been associated with a significant reduction in the comprehensive axial strengths of unidirectional composites. However, the fiber misalignment-related effects on the composite cores remain a largely underexplored area despite its practical implications.
On this account, Mr. D.H. Waters, Dr. Joe Hoffman and Dr. Maciej Kumosa from the University of Denver carried out a combined numerical, analytical and experimental study to investigate how fiber misalignment affects the bending strength of pultruded hybrid polymer matrix composites under tension and bending loading. They tested a series of ACCC® conductors cores produced by CTC Global to simulate the transportation, installation and in-service conditions of the cores. The tests were carried out in three-point and four-point bending-based experiments. Additionally, analytical and numerical predictions of the axial stresses were carried out to establish the failure initiation locations. The failure characteristics on the three-point and four-point loading and the impact of the fiber alignment on the failure of the rods were discussed in detail. The work is currently published in the journal, Composites Part A: Applied Science and Manufacturing.
The research team showed that the composite rods failed terribly under pure bending due to the effects of the comprehensive stress found in the carbon composite region. Although the axial tension reduced the critical bend radius, an increase in the applied tension shifted from micro-buckling in the carbon region to the glass region. Thus, the initiating damage mechanisms changed from comprehensive to tensile mods with a further increase in the axial tension. Furthermore, it was worth noting that natural fiber misalignment influenced the damage initiation locations apart from reducing the bending strength. Thus, it was necessary to consider its effects in the failure analysis.
In summary, the study presented a thorough and elegant experimental, analytical and numerical investigation of the effects of fiber misalignments on the damages and bending strength of hybrid polymer matrix composites subjected to tension and bending. Based on the results, the bending strength of the composite rods was significantly influenced by the fiber misalignment effects. Even though the reduction in the comprehensive strength was more pronounced in the pure bending phase, it could be mitigated by increasing the applied axial tension. In a statement to Advances in Engineering, the authors explained the new study would lead to the design of robust and high-performance conductors and improve their transportation and installation to increase operational efficiency.
Waters, D., Hoffman, J., & Kumosa, M. (2021). Effect of fiber misalignment on bending strength of pultruded hybrid polymer matrix composite rods subjected to bending and tension. Composites Part A: Applied Science and Manufacturing, 143, 106287.