CNT fiber-reinforced polymer composites offer increased strength, durability in aerospace applications

A stronger and more durable carbon nanotube polymer composite and laminate material for aerospace applications has been developed by Texas A & M University System. The composites and laminate materials include a fiber component, a polymer matrix component and a quantity of carbon nanotubes coating at least a portion of the fiber component. The fiber component can be a plurality of carbon fibers. The carbon nanotubes coating the fiber component strengthen a fiber-matrix interface between the fiber component and the polymer matrix component. Methods for improving the fatigue durability of a fiber-reinforced polymer composite are also disclosed by the inventors. These carbon nanotube fiber-reinforced polymer composites (CNFRPCs) utilize nanotechnology enhancements to provide advantageous durability and structural stability improvements over conventional fiber-reinforced polymer composites (FRPCs) that do not contain carbon nanotubes. In particular, the CNFRPCs provide increased resistance to tension-tension and tension-compression fatigue failure compared to conventional FRPCs. Inclusion of carbon nanotubes at the fiber-matrix interface in CNFRPCs provides advantageous resistance to polymer matrix cracking, longitudinal cracking along the fiber-matrix interface, and fiber delamination, all of which are dominant failure mechanisms in conventional FRPCs. The CNFRPCs provide a nanotechnology solution to mitigating the evolution of failure mechanisms and extending failure lifetimes under fatigue loading. The polymer composites include a fiber component, a polymer matrix component, and a first quantity of carbon nanotubes. The polymer matrix component and the fiber component form a fiber-matrix interface. The first quantity of carbon nanotubes coats at least a portion of the fiber component. The fiber-matrix interface further includes the first portion of carbon nanotubes. Materials used in aerospace applications are subject to a number of forces, many of which are quasi-static or cyclical in nature. Some of these forces include shear, compression, tension and bending forces which lead to fatigue and ultimate failure of components. Fiber reinforced polymer composites (FRPC), particularly epoxy laminate composites, experience substantial use in aerospace applications due to their light weight and good mechanical strength under tensile loads. For example, it is estimated that the next generation of military and commercial aircraft may include more than 50% by weight of polymer composite materials. Many of these polymer composite materials will be carbon fiber and fiberglass composite materials. Carbon fibers are particularly beneficial in FRPCs due to their high tensile strength and elastic modulus. Epoxy polymers, which are used in many FRPCs, typically have low tensile strength alone but provide bulk to the composite material and aid in load transfer to the fiber component. As a result, FRPCs having good tensile strength are obtained. Poor compression strength is a known weakness of carbon fibers and epoxy polymers, and FRPCs derived from these materials are likewise lacking in compression strength. As a result, FRPCs typically have substantially shorter failure lifetimes under quasi-static compression or cyclical tension-compression loading. A carbon nanotube fiber-matrix interface is more resistant to fiber delamination and fatigue crack growth compared to a reference laminate material not comprising carbon nanotubes. Materials having improved strength and fatigue failure lifetimes are of considerable potential benefit in many applications. Since carbon nanotubes (CNTs) are 5 times less dense than steel and approximately 30 times stronger they could be the ultimate mechanical filler for reinforcing polymers, with very low densities, and Young moduli (the measurement of stiffness of a material) superior to all other carbon fibers, as per nanowerk.com.
Kevlar is a well known high-strength polymer, is not soluble in any common solvent and, having no melting point, decomposes above 400°C. As a result, Kevlar fibers must be produced by wet spinning from sulphuric acid solutions. Researchers in Ireland have found a way to develop a new effective post-processing technique which would allow to incorporate carbon nanotubes into already formed polymer products, such as for example Kevlar yarns. It enables the incorporation of nanotubes into already formed polymer yarns or fibres by swelling them in carbon nanotube suspensions. The core of the findings by Gun'ko's team is the development of new swelling under ultrasound approach for the fabrication of new reinforced composites. In their experiments, commercially sourced Kevlar yarns were placed in stable suspensions of multiwalled CNTs in a selected organic solvent. These Kevlar-nanotube mixtures were processed using an ultrasonic bath for an optimized period of time at ambient temperature. This processing resulted in swelling of Kevlar and in an uptake of nanotubes inside Kevlar fibres. Mechanical testing of these Kevlar-CNT composites showed considerable increases in all mechanical parameters of the nanocomposite material compared to the original Kevlar fibers : Young�s modulus, from 115 to 207 GPa; strength, from 4.7 to 5.9 GPa; strain at break, from 4.0 to 5.4%; toughness, from 63 to 99 J/g. These improvements have been achieved at only 1 - 1.75 wt% of carbon nanotube content. This can be considered a quite significant advancement in the area of nanotube-polymer composites. The CRANN team believes that there will be a number of possible important applications for their new technique. This approach can be used to incorporate various nanoparticles, nanotubes, nanowires etc. inside pre-fabricated polymer fibres, yarns, films, ribbons etc. A promising likely application is the production of conductive CNT-polymer composites (films and wires), which could be potentially used as electrodes in flexible displays, electronic paper, solar cells and in electronic devices. Another very promising area of future research involves the reinforcement of various polymer fibres and films using carbon nanotubes as reinforcing additives. Potential applications of new ultra-strong polymer-nanotube materials would include bullet-proof vests, protective clothing, high-performance composites for aircraft and automotive industries like seat belts, cables, reinforcement of tires, break linings, bumpers, etc.
There are many economic and technical issues currently preventing carbon nanotube-reinforced polymers from being applied to large-scale composite structures. The high cost and relatively short lengths of CNTs, combined with our inability to effectively disperse and align them within a host matrix, prevent us from developing composite structures that could supplement or replace conventional aerospace materials.