Improvements in mechanical properties of a carbon fiber epoxy composite using nanotube science and technology
Introduction
The newest commercial aircraft designs propose a reduction in weight by having over 50% of the primary structural components fabricated with epoxy based carbon fibers or carbon-glass fiber hybrid reinforced composites [1], [2]. Using advanced light weight and high strength composites is necessary in order to achieve the reduced fuel consumption and improved passenger comfort goals of these future commercial aircraft design innovations. Fiber reinforced epoxy composite laminates are known to have high in-plane stiffness, strength and fatigue resistance under tensile loadings. The fibers have the primary role to carry the load imposed on the composite laminate. Carbon fibers have an exceptional tensile strength, but they have essentially no compressive load carrying capability [3]. The role of the matrix is to provide bulk to the composite laminate and transfer load between fibers. However, the epoxy matrix can be brittle with poor strength and toughness. There is a third constituent in composite laminates, the fiber/fabric-matrix interfaces. A weak fiber/fabric-matrix interfacial strength in composite laminates could be a principal reason for delamination failures and their subsequent failures under fatigue or cycling loadings [4]. A challenge for research on fiber reinforced composite laminates is to achieve improvements in mechanical properties, such as the compressive strength, which may be as low as 50% of the ultimate tensile strength [5], [6], the in-plane tension – compression fatigue resistance, and through thickness interlaminar shear strength [7].
In this paper carbon nanotube science and technology is used to directly reinforce the laminate fiber/fabric-matrix interfaces of the composite cross-section. The aim is to hinder the onset of axial or longitudinal direction fiber/fabric-matrix interfacial cracking and delamination, a principal component in the evolution of damage in composites under both monotonic and cyclic loadings [8], [9]. As a consequence the matrix rich regions in the laminate cross-section can also become reinforced with nanotubes, which could hinder matrix cracking [10]. Recent research shows well dispersed functionalized carbon nanotubes in an epoxy matrix would enhance the strength [11] and fracture toughness [12] of epoxy nanocomposite materials. This current research illustrates using nanoscale science and technology to further improve mechanical properties of tensile strength and stiffness, and resistance to damage and failure due to cyclic loadings in a carbon fiber reinforced epoxy composite.
Section snippets
Materials
The primary materials used in this study are a high strength carbon fiber, an epoxy resin and fluorine functionalized carbon nanotubes (f-CNTs). The carbon fabric, a four-harness satin weave having identical warp and fill yarns of 6000 filament count, was manufactured by Hexcel Corporation using fiber type IM7. The Hexel IM7 is an aerospace grade carbon fiber with a reported tensile strength up 5.5 MPa and an elastic modulus near 276 GPa. The fibers used in this study are typically surface
Results and discussions
The loading conditions and results from the monotonic tensile and cyclic loading tests are given in Table 1, Table 2. Given for each carbon fiber epoxy laminate material at the prescribed weight percentage (wt.%) CNT are the average measured ultimate tensile strength (UTS) and stiffness (E). For the cyclic testing the maximum stress σmax of both loading types (R = σmin/σmax = + 0.1 and R = σmin/σmax = −0.1) and number of cycles to failure (N) are tabulated. Using these test results, the primary
Summary and conclusions
Carbon fiber reinforced epoxy composite laminates, with 0.2, 0.3 or 0.5 weight percent (wt.%) fluorine functionalized “XD” carbon nanotubes (f-XD-CNTs) strategically incorporated at the fiber/fabric–matrix interfaces, were tested for tensile strength, stiffness and resistance to failure under both tension–tension (R = +0.1) and tension–compression (R = −0.1) cyclic or fatigue like loadings and compared to the neat (0.0 wt.% CNT) case. A spraying technology was used to deposited the f-XD-CNTs at the
Acknowledgments
This research was sponsored in part by the NASA Contract No. NCC-1-02038, University Research Engineering and Technology Institutes (URETIs), Air Force Office of Scientific Research, US. Air Force, Department of Defense ASSURE Program, National Science Foundation NSF Grant No. 0453578 and the Air Force Research Laboratory Clarkson Aerospace Corporation Minority Leaders Program Contract No. FA8650-05-D-1912. For individuals, the authors wish to thank Mr. Thom Stephens for his expertise and
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