Reprinted courtesy of BNP Media

Materials are important in every industry. They’re critical to aerospace manufacturers. If companies select the wrong material for a particular application, it can have catastrophic consequences.

Commercial and military aircraft are subjected to a wide variety of harsh conditions, such as atmospheric forces, temperature extremes and ultraviolet rays. Materials become even more complex and challenging when spacecraft are involved.

Aerospace engineers face many unique challenges. For instance, planes experience temperature changes between 20 and -40 C every time they take off and land, with huge differences in pressure and humidity. Materials need to withstand water condensing and freezing inside fuselages. They also need to endure lightning strikes and electromagnetic interferences.

A century ago, first-generation aerospace engineers were busy developing metal airframes and wings to replace flimsy wood fuselages and fabric-covered wings. As aircraft climbed higher and faster, aluminum eventually became the go-to material.

Today, aluminum is still widely used in aerospace applications, but advanced carbon-fiber composites and superalloys now captivate the attention of engineers scrambling to increase efficiency, optimize performance and reduce weight.

Easier Said Than Done

Replacing heavier materials is easier said than done. It requires the development of viable alternatives with lower densities and higher strengths. Many aerospace engineers are scrambling to find new production processes and assembly techniques to cost-effectively join dissimilar materials.

“Materials use in the aerospace industry is extremely complex, because manufacturers must often support legacy programs at the same time that they are developing new aircraft,” says George “Nick” Bullen, technical fellow at Northrop Grumman Aerospace Systems.

“For instance, in the defense sector, companies are typically engaged in aircraft modifications and updates that rely on traditional materials,” explains Bullen. “At the same time, manufacturers are constantly working on new programs that demand cutting-edge products.

“Once materials are qualified, you generally don’t change,” claims Bullen. “Therefore, older military aircraft, such as the F-18, use a mix of aluminum, composites and titanium. But, on the latest aircraft designs, composites are increasingly the dominant material. This recent trend has enabled engineers to produce airframes without the need for numerous fasteners, which is a big cost contributor because of issues related to drilling, filling and countersinking holes.”

When fasteners are used to join traditional materials, there are more holes to drill and fill. By bonding a composite structure together, aerospace engineers eliminate those headaches.

“Fasteners create numerous issues relating to tolerance and destacking,” Bullen points out. “They also contribute to lost-time injuries and defects.

“However, composites have limits,” warns Bullen. “For instance, all hot spots, such as engine-fuselage interfaces or engine exhaust points, must be metal. Wing attach points also still contain a lot of metal, due to the high levels of stress that occur there.”

As the aerospace industry continues to develop new types of aircraft ranging from hypersonic missiles to battery-powered urban air mobility vehicles, lightweight materials will ultimately determine their success or failure.

“Flight systems are becoming more and more high-speed, even going into hypersonic systems, which are five times the speed of sound,” says Richard Liang, director of the High-Performance Materials Institute at Florida State University. “When you have speeds that high, there’s more heat on a surface. Therefore, we need a much better thermal protection system.

“The world of aerospace increasingly relies on carbon-fiber reinforced polymer composites to build the structures of satellites, rockets and jet aircraft,” explains Liang. “But, the life of those materials is limited by how they handle heat.”

Advanced Materials

Liang and his colleagues have been experimenting with using carbon nanotubes to develop a new type of heat shield that protects hypersonic aircraft.

“Existing heat shields are often very thick compared to the base they protect,” Liang points out. “[Our new design] lets engineers build a very thin shield, like a sort of skin that protects the aircraft and helps support its structure.”

“Integrating graphene and related materials into fiber-reinforced composites has great potential to improve weight and strength, and [it can] help overcome bottlenecks limiting the application of these [materials] in planes,” claims Vincenzo Palermo, vice director of the Graphene Flagship, a European organization that is working with manufacturers such as Airbus.

Graphene-integrated composites are constructed by introducing thin graphene sheets, a few billionths of a meter thick, into hierarchical fiber composites as a nano additive that improves the material’s mechanical properties.

“Graphene’s high aspect ratio, high flexibility and mechanical strength enable it to enhance the strength of weak points in these composites, such as at the interface between two different components,” explains Palermo. “Its tunable surface chemistry also means that interactions with the carbon fiber and polymer matrix can be adjusted as needed.”

The fiber, polymer matrix and graphene layers all work together to distribute mechanical stress, resulting in a material with improved strength.

“Whether the goal is to achieve lower weight, increase fatigue life or enable higher heat resistance, material advancements are critical to increasing aircraft performance,” says Michael Eff, applications engineer at EWI. “However, new materials come with a wealth of new challenges.”

According to Eff, the biggest development in aerospace materials during the last five years has been the growing use of powder-based alloys and the joining of carbon-fiber composites to metals such as titanium.

Today, Eff says more aerospace manufacturers are looking at ceramic matrix composites (CMCs) and metal matrix composites (MMCs). “CMCs offer elevated temperature options over conventional alloys,” he explains. “MMCs offer weight savings without sacrificing performance.”

CMCs are ideal for jet engine applications. They are made of coated ceramic fibers surrounded by a ceramic matrix. They are tough, lightweight and capable of withstanding temperatures 300 to 400 F hotter than metal alloys can endure.

“A critical issue for wider use of CMC is the development of cheap, user-friendly joining methods to assemble large components into more complex structures,” says Alber Sadek, technology lead for materials engineering at EWI. “[That requires] joint properties comparable to base materials, including good mechanical strength, oxidation and corrosion resistance, good thermal shock resistance and leak-tight joints.

“In many elevated temperature applications using CMC, there is a requirement to join them to other materials, such as metals,” adds Sadek. “Brazing is a highly effective joining technique for many ceramic-metal joint systems. However, the differences in coefficients of thermal expansion between CMC and metal require specialized approaches to accommodate the mechanical stresses introduced by joining.”

In addition to brazing, Sadek says other CMC joining methods include diffusion bonding, reaction forming, microwave joining, electron beam joining and selective area laser deposition.

“The mismatch in material properties and metallurgical incompatibility makes joining CMCs and MMCs difficult,” warns Eff. “Carbon fibers are difficult to join to any material, as there isn’t much experience with the materials. Joining MMCs to traditional aluminum alloys is difficult, because fusion techniques don’t provide the desired mechanical properties.”

“Interfaces have always been the Achilles’ heel for any system of materials or composite concepts,” adds Timothy Bunning, Ph.D., chief scientist in the materials and manufacturing directorate at the Air Force Research Laboratory. “Novel [manufacturing and design] tools are being explored.

“[We’re also exploring] nondestructive evaluation tools to certify structural performance in dissimilar materials, such as polymer matrix, ceramic matrix and metal-ceramic composites,” says Bunning. ‘The convergence of automation, big data and high efficiency computational tools is allowing for the exploration of systems with many elements via a smart, efficient, calculated manner vs. the historical empirical test methodology.

“The method of materials discovery and design is becoming digital, and will exponentially accelerate as diverse experimental techniques, modeling approaches and databases are integrated into a future materials cloud,” predicts Bunning.

Composite Challenges

During the past two decades, carbon-fiber composite has transformed aerospace manufacturing. The lightweight material is now found in a wide variety of applications, including fuselages, wings, bulkheads, overhead storage bins and other components.

“Composites are here to stay,” claims Bullen. “I don’t see any new materials emerging during the next 15 to 20 years. In fact, we’re just beginning to realize the benefit of fibers.

“For instance, aerospace engineers have been learning how to orient fibers to achieve flexible control surfaces that could eventually replace traditional wing flaps,” explains Bullen. “Instead of mechanical, linear-activated control surfaces, fiber is enabling them to experiment with wing warping.”

However, composites still present many unique challenges to aerospace engineers.

“We can age-test aluminum and titanium, but there’s no way to age-test composites, which tend to have a rapid failure rate,” Bullen points out. “And, although adhesive bonding is widely used to join composites, engineers still default to fasteners when assembling dissimilar materials.

“In addition, bonded wings cannot handle the atmospheric and temperature extremes found at high altitudes,” says Bullen. “Aerospace engineers are always looking for composites that can withstand higher temperatures. Material properties have been improving, but not by leaps and bounds.”

Aerospace manufacturers also face many pesky issues when creating molds to make components.

“When you lay up a composite part, thickness can vary by ±5 percent,” laments Bullen. “That creates big challenges when you try to mate airflow surfaces that don’t have gaps or steps. In fact, that’s the big benefit of using aluminum; you always get smooth surfaces and tolerances.

“But, there have been some recent breakthroughs in the variability of composite materials that hold promise,” says Bullen. “For instance, engineers at Vanderbilt University are working on some interesting ways to maintain consistency.”