Historically, polymer matrix composites (PMCs) have provided insufficient toughness for some aerospace structures. Rubber toughening agents and thermoplastics are often employed to increase material toughness but such additives increase viscosity of polymer resins and increase processing difficulty and cost. Other reinforcements like aramids exist but also have inherent issues (e.g., Kevlar absorbs moisture, which adds weight and is susceptible to degradation upon UV exposure).
Innovators at NASA Glenn developed a toughened hybrid reinforcement material made from carbon fiber and carbon nanotube (CNT) yarn for use in PMCs. The new material improves toughness and damping properties of PMCs, enhancing impact resistance, fatigue life, and structural longevity.
The new carbon fiber/CNT yarn hybrid material displays excellent toughness and strength for reinforcing PMC materials while also mitigating issues associated with current hybrid reinforcements like rubber, thermoplastics, and aramids. The material is a toughened triaxial braid made from ductile CNT yarn hybridized with carbon fiber, which is ultimately used as reinforcement material to make toughened PMCs. The CNT yarn component of the reinforcement is solely responsible for adding toughness, while the processes used to optimize the fiber braiding parameters and tensile properties of the carbon fiber/CNT yarn hybrid tow material determine the overall improvement in tensile strength for resin-impregnated fiber tows. Bundles of continuous carbon nanotube yarns are combined with a similar format of carbon fiber, yielding an easily scalable process.
Advantages of the material include reduced cost by eliminating use of toughening agents, increased ability to conform to highly complex geometries, greater environmental stability compared to aramid fiber reinforcements such as Kevlar, and possibly decreased density. Many hybrid reinforcements exhibit interfacial compatibility issues, which could lead to premature failure via crack propagation at the polymer matrix interface. In contrast, chemical similarities between the CNT yarn and carbon fiber constituents impart the hybrid reinforcement material with excellent interfacial compatibility.