Aerospace engineers have for the past few decades exploited the outstanding in-plane strength and stiffness of fiber-reinforced polymer composites (FRPCs) to create robust, lightweight aircraft structures that typically outperform their metallic counterparts. But FRPCs have a well-known Achilles’ heel: the impressive mechanical properties along the fiber axes are lacking in the direction perpendicular to the plane of the fabric plies.
“Current composite materials have poor mechanical properties in the through-the-thickness direction because there is no fiber reinforcement between plies,” explained Brian L. Wardle, Associate Professor of Aeronautics and Astronautics at Massachusetts Institute of Technology (MIT). “All the strength normal to the fiber plane derives from the polymer matrix. As a result, the thermoset resin can fracture and the plies delaminate.” That means engineers must overdesign composite aircraft structures to accommodate the lower properties in the z-axis, making them thicker and heavier than desired.
Wardle, who is Director of MIT’s Nano-Engineered Composite Aerospace Structures (NECST) Consortium, and his research colleagues in the academic-industry partnership are attempting to overcome that long-standing shortcoming by placing carbon nanotubes (CNTs) in the weaker regions of resin composites to more-securely bind the composite layers together as an integrated whole. The new technique could lead to aircraft structures that are some 10 times tougher, as well as significantly safer and lighter at only a nominal cost increase.
Improved properties in the through-plane direction could, for example, enhance damage tolerance. Use of the electrically conductive CNTs should also enable aircraft designers to tailor electrical and stealth characteristics as well as offer better handling of lightning strikes, electrostatic discharges, and electromagnetic interference.
NECST, which was launched in mid-2007, includes Airbus, Boeing, Composite Systems Technology, Embraer, Lockheed Martin, Saab, Spirit AeroSystems, Textron, and Toho Tenax. The consortium has applied for eight patents, the first of which was granted recently.
Materials scientists have explored various ways to buttress the interfaces between fabric layers by stitching, braiding, weaving, or pinning them together, but these methods are often either prohibitively costly, mechanically counterproductive, or technically difficult. More recently, researchers have tried to introduce CNT reinforcements into the polymer resins that bind together advanced fibers in composites, but “you can’t just mix CNTs into the matrix, because you get a highly viscous goo that you can’t do much with,” Wardle said. The tangled nanotubes tend to agglomerate and refuse to disperse evenly during resin infusion, which leads to only marginal rises in mechanical properties.
Although the NECST team is pursuing several related approaches, they all involve “placing ordered arrays of the strongest fibers we know—carbon nanotubes—where the composite is weakest, and where they’re most needed,” he stated.
In the lab, the researchers grow CNTs using a catalytic chemical vapor deposition process in which hydrocarbon molecules decompose and form the carbon crystal lattice of the nanotubes with the help of a catalyst. The technique yields aligned “forests” of upright CNTs on substrates. During impregnation, the nanotubes readily draw up the resin through capillary action. After curing, adhesion between the CNTs and the plastic creates an effective composite.
One of the group’s novel techniques is called interlaminar nanostitching, a crack-closure scheme whereby nanofibers bridge the gap between prepreg composite plies.
“Since nanotubes are produced at 750°C, you can’t grow the CNTs in situ due to the polymer,” Wardle explained. “We produce a forest of CNTs and then transfer print the array using a roller onto the prepreg.” When processed following standard procedures, they reinforce the interfaces between plies of the laminate, producing nanostitched materials that are two to three times tougher than unmodified carbon fiber-reinforced polymer composites.
Another of the group’s methods is based on the use of “fuzzy fibers” in resin infusion processes: “We grow aligned CNTs in situ on fiber surfaces of a dry form of woven cloth, yielding so-called fuzzy fibers that have nanotubes projecting radially around the fiber circumference,” he said.
In this case, the CNTs boost both interlaminar and intralaminar performance. The hybrid laminate, for instance, showed a 69% rise in interlaminar shear strength compared to unreinforced composites. Although the NECST researchers have demonstrated the methodology using carbon fibers, most of their work to date focuses on inexpensive alumina fibers, which are resistant to the high processing temperatures and are compatible with the CNT growth chemistry they employ.
Wardle said that his group currently makes small quantities of the enhanced FRPCs slowly on benchtop apparatus, but the technique is nearing the application research stage. “It needs to be scaled-up,” he said.
In the fall, the consortium plans to use venture capital funding to establish a commercialization entity for this purpose. He is optimistic of success as NECST members consider these innovative processes to be a platform technology for diverse uses: “We expect this technology to eventually supplant unmodified composites in most structural applications.”
The good news, he noted, is that the new approach adds only a few percent to the cost of the material, concluding, “CNTs just aren’t that expensive.”