Effect of the surface characteristic of the ultra-high-strength carbon fibers on the fracture behavior of the filament wound composites

Abstract

High-pressure resistance performance is the most crucial factor in the design of pressure vessels used to store gaseous and liquid fuels in the aerospace and automotive industries. Accordingly, composite materials have been widely used to fabricate the pressure vessels because of their high specific stiffness and strength. However, verifying the reliability of the composite pressure vessels using conventional material property testing methods poses predominant difficulties owing to the geometrical shape of the vessel and the pattern of filament winding manufacturing process. Therefore, in this research, a study on the inherent property evaluation method for securing the reliability and safety verification of the composite pressure vessel was conducted, and the mechanical properties and fracture behavior were compared according to the analysis of interfacial characteristics at the composite material. Composite pressure vessels should exhibit mechanical properties that allow them to withstand high pressures without being damaged. Therefore, an accurate measurement of their mechanical properties, particularly the burst pressure, is essential prior to their commercial release. A segment-type ring burst test is a facile method to determine the burst pressure

however, the application of uniform pressure to the test specimen remains a crucial issue to be addressed. Therefore, in this study, the conventional segment-type ring burst test device is newly designed to enhance the uniformity of pressure applied to the ring specimen. First, the equation of load transfer ratio from the applied load is derived. To verify the uniformity of pressure in the segment-type ring burst test device, the hoop strain distribution of the ring specimen is compared with that of the hydrostatic pressure test obtained from the theoretical equation and finite element analysis. Based on the result, the number of segments is determined, and a polytetrafluoroethylene sheet is placed between the segments and the ring specimen to reduce the discrepancy of the hoop strain. Furthermore, to verify the reliability of the redesigned test device, the strain distribution and fracture behavior are assessed using strain gage, the digital image correlation method and a high-speed camera. Moreover, the fracture toughness of the ring-shaped specimen derived from the composite pressure vessel was estimated by utilizing segment-type ring burst testing method. The tests were conducted with the ring specimens after machining a single edge notch (SEN) on it. The load-relationship of the testing device and the orthotropic condition of the composite vessel were considered to derive the fracture load. Furthermore, a camera was utilized to investigate the crack initiation and propagation at the crack tip, and then a comparison of fracture toughness with respect to the different initial crack lengths was conducted. Furthermore, the surface characteristics of carbon fibers with different sizing properties were analyzed, and the results were compared with the fracture toughness results. Firstly, the surface roughness, which is one of the physical characteristics of carbon fibers, was measured and analyzed by using atomic force microscopy (AFM) method and compared with the specific surface area measured by using the BET (Brunauer–Emmett–Teller) method. In addition, X-ray photoelectron spectroscopy (XPS) method was used to analyze the distribution of the chemical functional group properties on the carbon fiber surfaces and the results was compared with the surface free energy of the carbon fibers. Finally, the effect of the fiber surface characteristics on the mechanical properties and fracture behavior of the carbon fiber reinforced composite material was analyzed by comparing it with the measured interlaminar shear strengths and fracture toughness results.