qAndrew Devine
ME 3150.01
Prof. Gan
4-25-23
Aerospace Materials
Summary:
The most common materials for aerospace structures include Al 2024, Al7068, Ti-6Al4V, 300M steel, CFRH and twill weave carbon fiber laminate.
Introduction:
Aerospace is a constantly evolving field of engineering that demands constant
improvements in every aspect of airplanes and spacecraft. “aircraft engineers are increasingly
searching for structural materials that have low densities; are strong, stiff, and abrasion and
impact resistant; and do not easily corrode” [1]. The Aerospace industry has had an enormous
impact on virtually all the primary fields of engineering, especially but not limited to mechanical
engineering, materials engineering, and computer engineering. The space race caused computer
and computer components (especially transistors) to be more compact, redundant and use less
power. Airplanes caused several new materials to be synthesized, materials to reduce structure
weight, increase heat resistance and increase fuel efficiency for rockets and jet engines. Because
of the extreme environments that aircraft and space vehicles operate in, the variety of materials
that are used in aerospace is very broad. The most prevalent materials in today’s aerospace
environment include; aluminum alloys, titanium alloys, composite materials, superalloys,
ceramics and advanced polymers. This materials research paper will explain where these
materials are used and why they are used.
Discussion:
Aeronautics is the study of the science of flight [2]. It is the branch of aerospace
engineering that concerns airplanes and other vehicles that use the atmosphere as their primary
operating environment. Airplanes and other airborne vehicles must constantly fight gravity in
order to stay in the air. In addition to this, aircraft carrying large payloads or traveling at high
speeds must withstand high stresses endured when turning or landing. These criteria force
aircraft to use materials with high strength to weight ratios. The most prominent materials for
aircraft frames currently are aluminum alloys and carbon fiber. Additionally, aircraft traveling at
high speeds experience high levels of skin friction drag which generates lots of heat. Engines and
skin for high-speed aircraft must endure high temperatures and still maintain structural integrity.
Aluminum is a great choice for aircraft frames and many other applications because of its great
strength to weight ratio. In the past materials like wood were used to construct aircraft but are
now phased out because of the large advantages that aluminum possesses. As can be seen from
the strength vs. density graph (figure 1.), aluminum has one of the lowest densities of any
Figure 1. Density Vs. Strength Graph
of the metals in the chart and has a very comparable tensile strength to steel. The most
common aluminum alloys used in fuselage construction are wrought aluminum alloys including
2024, 7068, and 7050, 2024 being the most prominent. The first digit indicates the major
alloying elements (2 series is copper, 7 series is zinc). The last two digits identify the specific
alloy in the series. Aluminum 2024 is composed of 93.5%Al, 4.4%Cu, 0.6%Mn, and 1.5%Mg
(percentages are by weight). Most 2000 series aluminums are precipitation hardened with
copper, making them much harder than unalloyed aluminum and having strengths comparable to
steels. The precipitation hardening allows for small copper crystals to appear on grain
boundaries, increasing the strength of the aluminum. One of the primary downsides of aluminum
is that it has poor fatigue resistance. The reason Aluminum 2024 is so popular is because it has
an excellent fatigue life unlike most aluminum alloys [2]. It is most commonly used in sheet
form for the wings and fuselage. The 2000 series, however, is prone to stress cracking and is
increasingly being replaced by the 7000 series alloys. Aluminum 7068 has a composition of
87.6%Al, 7.8%Zn, 2,5%Mg, 2.0%Cu and 0.12%Zr. In addition to having a higher resistance to
stress cracking it also has a much higher tensile strength of 710Mpa compared to the 420Mpa of
Aluminum 2024. 7050 Aluminum is another 7000 series known for its high strength. Its
composition by weight is 89%Al, 6.2%Zn, 2.3%Mg, 2.3%Cu and 0.1%Zr. An advantage it
possesses over the other alloys is its superior corrosion resistance. For this reason it is used in
exterior components like skins and fuselages. In addition to its corrosion resistance, its high
fracture resistance makes it a popular alloy for military aircraft. One of the things aluminum is
not used for however is landing gear. Today steel and titanium are used for the landing gear of
most passenger planes because steel and titanium alloys generally have higher strengths than
aluminum. The most common steel used for landing gear is 300M carbon steel. Its range of
carbon is between 0.4-0.45% along with silicon and nickel and other trace elements. This
composition gives 300M an ultimate tensile strength of 1.9GPa [4] which is necessary for
landing aircraft upwards of 1 million pounds such as an airbus A380. One of the downsides of
this extreme strength is that 300M is vulnerable to corrosion and must be coated with protective
coats. Most often these coats are comprised of zinc and epoxy. Though these aluminum alloys
have great strength to weight ratios and corrosion resistances, composites can exceed aluminum
and its alloys.
Composites play an increasingly important role in the aerospace industry. Their superior strength
to weight ratio is a desirable property not just in aerospace, but also in the automotive industry.
The first passenger jet that utilized composites was the Boeing 707. At this time fiberglass
comprised about two percent of the structure, and each successive generation had larger portions
of the airframe made of composites. Today, the Boeing 787 is nearly 50% (by weight) comprised
of advanced composites (Figure 2.)
Figure 2. Material Composition and distribution of a Boeing 787 Dreamliner
When fuel efficiency is the goal in passenger transportation, a 20% total fuel efficiency increase
compared to aluminum is a monumental improvement. Other airliners are following suit, and the
Airbus A380 also makes use of a composite fuselage. These composites used to manufacture
wings are largely carbon fiber laminate. For wing sections, carbon fiber fabric is combined with
epoxy to make carbon fiber laminate sheets. Although the type of carbon fiber used on
commercial airliners is proprietary information, they most likely use twill carbon fiber fabric.
This twill weave pattern is somewhat similar to how garments are manufactured. The twill weave
pattern provides filaments woven between each other to provide the same amount of strength in
perpendicular axes. The tensile strengths of individual threads of carbon fiber can be in the range
of 4-10GPa. Since just one sheet of fabric will only provide significant strength in two
directions, multiple layers are stacked on top of each other in differing directions to provide
stiffness in all directions. Omni directional strength is especially important on the wings since
they can experience a wide range of loads and loading conditions for any given flight. Most often
fabric layers will be rotated in increments of 15 degrees. For large scale applications it is most
common that these laminates are manufactured with pre-saturated fabric and then put in an
autoclave to cure the epoxy. Pre-preg fabrics and autoclaves are very expensive to implement so
smaller scale applications may use the resin infusion process. General Atomics is one of the
companies that uses resin infusion to create skins and wings for their aircraft. For a wingspan of
66’ like the MQ-9 Reaper (figure 3.)
Figure 3. MQ-9 Reaper composite surfaces
resin infusion is the optimal manufacturing lightweight and high strength wings. For
sections such as fuselages, a different type of carbon fiber is used. Since fuselages are essentially
pressure vessels that go through cyclic pressurization and de-pressurization, a carbon fiber tow
fabric is used. Carbon fiber tow is a type of carbon fiber that is a collection of carbon fiber
filaments. Whereas twill fabric is multiple bundles of filaments that are combined into threads
then woven together, Carbon fiber tow is one single thread that is made of many filaments. This
carbon fiber tow is then wound around a positive mold and then coated in epoxy resin. This is
essentially the same process that filament wound composite pressure vessels use. [5]
Titanium is an extremely important metal in the aerospace industry and has allowed for
innovations that other materials do not allow for. It can be 30% stronger than steels and half the
weight. Naturally titanium is essentially corrosion proof since it develops a layer of titanium
oxide on its surface which helps dramatically in harsh conditions airplanes experience. Titanium
also has excellent strength retention up to 1,000F. Another significant advantage titanium has is
its coefficient of thermal expansion. This is especially important when considering plane
operating environments. In normal operating environments for commercial aircraft, the aircraft
can experience temperature differences of up to 200 degrees Fahrenheit. CTE is a very important
factor in bulkhead and fastener design since bearing stresses between components can be
amplified by temperature change. Titanium at room temperature and pressure has a close packed
hexagonal phase. Titanium alloys are generally sorted into four categories; alpha, near alpha,
alpha and beta, beta and near beta as seen in Figure 4.
Figure 4. Titanium Phase diagram
Beta titanium alloys exhibit BCC allotropic form of titanium. These alloys contain varying
amounts of molybdenum, vanadium, niobium, tantalum, zirconium, manganese, iron, chromium,
cobalt, nickel and copper. Alpha phase titanium is also a hexagonal close packed crystal structure
and is stable under 882 C. These phases of titanium provide it with an excellent strength to
weight ratio. With all these benefits it is hard to find places where titanium shouldn’t be used. It
is used for structural components, engine components, fasteners, heat exchangers and electrical
components.
The SR-71 Blackbird is perhaps the most well-engineered plane to date, first being used
in 1966. Lockheed-Martin’s engineering team created a plane that outperforms some of the fifthgeneration planes today. This plane would not be possible if not for the use of titanium in
virtually every portion of the plane. The airframe was made from titanium alloys including the
fuselage, wings and empennage. This plane regularly exceeded Mach 2 and was only able to do
this because of titanium’s strength to weight ratio and excellent heat resistance. All the skin
panels on the surfaces of the SR-71 allowed it to withstand the extreme temperatures of
supersonic flight, up to 600 degrees Fahrenheit. Several engine components of the SR-71 such as
the turbine and compressor blades were manufactured from titanium. In all, 93% of the entire
plane was constructed from titanium.
Concluding remarks:
A variety of cutting-edge materials have allowed mankind to continually push the limits
of aerial vehicles. Lifting millions of pounds of cargo and travelling at supersonic speeds has
only been allowed due to the extensive analysis and implementation of materials like titanium,
aluminum and carbon fiber. They each possess incredible strength to weight ratios and have
advantages that designate each material to their respective uses. Carbon fiber makes great wings
because of its unmatchable strength to weight ratio. Titanium makes great skin, fasteners and
structural components because of its corrosion resistance and low coefficient of thermal
expansion. Aluminum makes a great frame material because of its cost effectiveness and great
strength to weight ratio.
References:
[1] Callister, William. Materials Science and Engineering: An Introduction, 10th Edition
[2] Shaw, Robert. What is Aeronautics? 13 May 2021 https://www.grc.nasa.gov/www/k12/UEET/StudentSite/aeronautics.html
[3] Giurgiutiu, Victor, Boeing 787 Dreamliner, May 2022
https://www.sciencedirect.com/topics/engineering/boeing-787-dreamliner
[4] Vartanov, G., "High Strength Corrosion Resistant Steel for Aircraft Landing Gears and
Structures," SAE Int. J. Adv. & Curr. Prac. in Mobility 3(3):1240-1243,
2021, https://doi.org/10.4271/2021-01-0028.
[5] Vicaro, A.A, “Polymer matrix Composites: applications” 2018.
https://www.sciencedirect.com/topics/engineering/filament-wound-composite-pressure-vessel
[6] Engineering ToolBox, (2005). Metals - Temperature Expansion Coefficients. [online]
Available at: https://www.engineeringtoolbox.com/thermal-expansion-metals-d_859.html