Measuring Tensile Stresses in CNF/Polymer composites using Raman Spectroscopy

This research focuses on developing relationships between the mechanical properties and the optical spectra of carbon nanofiber (CNF) reinforced polymer composites. Stress distribution in the sample at discrete force increments is obtained and the effects on the G band of the Raman spectra of CNF are observed. A linear shift of carbon nanofiber Raman frequencies is created by strain applied to their molecular structure. Results of loading experiments combined with optical spectroscopy shows the stress dependency of these bands which can be used in the future for design, analysis and non-destructive structural stress testing of CNF/polymer composites. I. Introduction HE potential of polymer resins enhanced with carbon nanofibers for aerospace applications, where weight, stiffness and strength are critical, presents a need for developing a test methodology to assess the effect of CNF reinforcements with conventional laminates such as glass fiber and carbon fiber reinforced polymer composites. 1 Nanocomposites are a novel class of composite materials that have received special attention because of their improved properties at very low loading levels compared with conventional filler composites. 2 Conventional polymer composite materials reinforced by continuous fibers have excellent in-plane strength, but are usually weak against matrix-dominated failures. Single wall carbon nanotubes (SWNT) , multi-wall carbon nanotubes (MWNT), as well as carbon nanofibers (CNF) are being used for reinforcing polymer matrices for improved mechanical, thermal, and electrical properties. Carbon nanofibers, though not as perfect in structure compared to carbon nanotubes, are demonstrated to have positive impacts on properties of polymer composites and show potential to be reasonably used in metal matrix composites as well because of their mechanical and physical properties. The typical diameters of SWNTs are in the range of 0.7–1.5 nm, MWNTs in the 10–50 nm range, and that for CNF in the 60–200 nm range. In SWNT and MWNT, the graphitic planes are parallel to the tube axis, while in carbon nano fibers, they make a small angle to the CNF axis. 1 CNFs offer chemically facile sites that can be functionalized with additives thereby resulting in a strong interfacial bond with the matrix. Thermoplastics such as polypropylene, polyester, polycarbonate, nylon, poly (ether sulfone), poly (phenylene sulfide), acrylonitrile-butadiene-styrene, thermosets such as epoxy as well as thermoplastic elastomers such as butadiene-styrene diblock copolymer have been reinforced with carbon nanofibers. CNF composites have been studied basically for their potential to improve composite strength. However, unless the nanofiber-matrix interface has been modified, the poor load transfer through this interface, will lead to an interfacial debonding and subsequent premature failure. The structure and properties of the fiber-matrix interface play a huge role in the physical and mechanical properties of the composite. Particularly, for mechanical applications, the nature of the interface determines the load transfer efficiency from the matrix to the fibers, which in turn determines the stress resistance of the composite. The focus of CNF-reinforced composites has been the engineering applications that require superior strength, stiffness, electrical and thermal conductivities.