The Use of Lightweight Composites in Satisfying the Unique Structural
Requirements of Aircraft Design
by
Brad Peirson
School of Engineering
Grand Valley State University
Term Paper
EGR 250 – Materials Science and Engineering
Section 1
Instructor: Dr. P.N. Anyalebechi
July 14, 2005
Abstract
The design and manufacture aircraft wings require attention to several unique
structural demands. Among other traits all of the materials used must have be
lightweight and have a high strength. Since the first airplane flight over 100 years ago
the materials and processes used in the manufacture of wings have evolved greatly. As
the technology employed in providing the thrust for flight has advanced so too has the
physical requirements of the material. As aircraft become larger and faster the stresses
applied to their wings increase. Wood frames and doped cloth construction eventually
gave way to all metal airframes. The current trend in the aerospace industry is the use of
lightweight, high strength composites.
Introduction
More than a century ago two men forever changed the face of history. In the
winter of 1903 Orville and Wilbur Wright piloted the first powered heavier than air
vehicle. This maiden flight lasted only moments but it ushered in an era of constant
technological advancement in the field of aeronautics. While the techniques have been
perfected over the year, the basic concepts developed by the Wrights are still in practice.
The wings of the Wright flyer consisted of a hardwood truss covered in fabric [1]. The
truss allowed for maximum strength and minimum weight in the aircraft. This basic
combination remained the staple of aircraft design until the twenties. In this decade Jack
Northrop pioneered the shift to stressed skin designs.
Fabric is not strong enough for a stressed skin design so Northrop perfected a
method of forming plywood sheets to the required shape of the airframe [1]. This
advancement increased the overall rigidity of the plane. It allowed for faster planes and
increased cargo capacities. This stressed skin design is the same essential method used in
modern aircraft. The ultimate difference is in the advancement of the materials used. In
the decades after Northrop’s innovation, technology had advanced to the point of metal
framed and skinned aircraft.
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Figure 1: Examples of Jack Northrop’s stressed skin designs [1]
The evolution of metals to the point of being useable in aircraft construction
marked the beginning of the jet age. In the middle part of the twentieth century metals
allowed planes to be built larger and faster. A metal airframe allows the plane an
excellent rigidity. This added rigidity in the wings of an aircraft allow for greater lift
forces and thus greater payloads. Taking aircraft technology into the 21st century are
lightweight, high strength composite materials.
Composite materials are made by combining two different materials in order to
gain properties greater than either of the components. Composites typically consist of
particulate or fibrous material suspended in a matrix of the different material [2]. This
composition can give excellent properties to a composite material such as high stiffness,
fatigue resistance and thermal shock resistance. The properties exhibited by a composite
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material are directly determined by the properties of the constituent materials [2]. Such
materials are continuously being applied to aerospace technologies.
Figure 2: Applications of composites in the Boeing 777 aircraft [2]
Functional Requirements of Airplane Wing Material
There are two primary functional requirements that must be considered when
considering materials for use in an airplane wing. The first is high strength. As aircraft
become larger they naturally become heavier. The heavier aircraft requires a more lift
force to obtain flight. Greater lift directly translates to greater stress on the wings. This
effect on the wing can be illustrated through the use of finite element analysis software.
If a given material was not strong enough it would fail under the high stresses generated
by large passenger aircraft such as the Boeing 777 or the Airbus A380, which has the
largest wings ever produced on an aircraft.
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The second required property of a wing material would be light weight. Again, as
aircraft become larger they become heavier. If the materials used were not of sufficient
light weight the payload of the aircraft would be decreased. When the structure of the
aircraft is made as light as possible the weight savings can be used to carry extra cargo or
passengers. Lightweight is especially important in the Airbus, which will carry 35%
more passengers (555 people) than the current largest airliner. Because of unique weight
saving materials and processes the Airbus will use 20% less fuel per passenger than
current airliners [3].
Figure 3: FEA of aircraft wing under maximum loading (units in psi) [4]
`
Additionally, the material must be able to resist extreme temperature changes.
Within the troposphere the atmospheric temperature can have an extreme amount of
variation. Aircraft such as the Airbus A380 have an operational ceiling of around
thirteen kilometers [5]. At this altitude the temperature is considerably lower than that at
sea level. There are two possible modes of failure for a wing material at such altitudes.
The first is that the temperature drop could make the material brittle. This would lead to
cracking and general failure as the wing would still be subjected to the lift forces.
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Figure 4: Temperature vs. Altitude in the Earth’s atmosphere [5]
The other possibility is that the material would experience creep, or shrinkage,
due to the extreme temperature change. This could cause a void in the skin of the wing.
The resulting airflow disruption could cause a resonance in the wing’s structure that
could tear it from the plane. A similar failure was catalogued by the Transportation
Safety Board of Canada in May of 1998. A small Skyhopper aircraft scraped its wing on
the runway prior to takeoff. The impact left no visible evidence on the runway. The
initial damage to the wing is shown in Figure 5.
Figure 5: Damage to the wing tip of a Shyhopper aircraft on May 1, 1998 [6]
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The photograph in Figure 5 was taken of the recovered wing. The photograph of the
entire wing is shown in Figure 6, after a resonance caused by the scrape tore it from the
fuselage.
Figure 6: The result of resonant frequency on a Skyhopper wing [6]
Material Properties Required to Satisfy Functional Requirements
In the case of an aircraft wing the strength of a material would best be classified
by both its yield strength and elastic modulus. In the wing of an aircraft there are stresses
cause by the lift force along the entire length. The stress applied to the wing increases
closer to the fuselage of the plane. A material with poor yield strength will be prone to
permanent deformation in this area. In the air any variation from tolerance could prove
catastrophic. The material used in the wing would also require a relatively high elastic
modulus. This would mean that he material would resist flexing when lift is applied. Not
only that, it will also prove less susceptible to resonance caused by normal airflow around
the wing [7].
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Figure 7: FEA of simple wing design showing increased stress at joints [8]
The lightweight physical requirement translates almost directly to the density of
the candidate material. Regardless of the material used the same volume of it will
typically be required to produce a given product. This means that given equal volumes
the less dense material will weigh less. This is a crucial factor in any portion of the
aircraft. Light weight is secondary to sufficient strength in this particular application.
The best property to use as a material selection criterion would be the strength-to-weigh
ratio of the material. The strength-to-weight ratio is found by equation (1).
σS =
σy
ρ
(1)
where σS = the strength-to-weight ratio of the material, σy = yield strength of the material
and ρ = density of the material.
Of several candidate materials, the material with the highest strength-to-weight
ratio will have the best combination of light weight and high strength.
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Composite Materials used in the Manufacture of Aircraft Wings
The focus for cutting edge materials in aircraft construction is shifting toward
high strength composites [9]. The materials typically used in the construction of aircraft
wings are graphite composites. Graphite composites offer an exceptional amount of
strength. Because of their structure these composites also have a relatively low density.
These two factors combined give a graphite composite material an excellent strength-toweight ratio.
Figure 8: Effect of temperature on the coefficient of thermal expansion for reinforced
composite aluminum alloys [10]
Table 1 shows that graphite composites outperform common metal alloys in the
areas of concern for the construction of a wing. The aerospace grade composite’s
strength is nearly 3 times that of the AA6061-T6 alloy. This composite even has a
strength that is greater than steel. Also the composite has a stiffness range whose lowest
point is equal that of the AA6061-T6. This means that the composite will most likely
have a higher stiffness than the aluminum. There is even the possibility of the composite
being stiffer than mild steel as the mild steel value falls within the composite range. The
graphite composite also has a much smaller density than either metal. The density of the
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composite is half that of AA6061-T6 and one sixth that of steel. This means that a given
volume of the composite will be lighter than the same volume of either metal alloy.
Table 1: Comparison of physical properties of composite materials and common metal
alloys [11]
Cost $/kg
Strength (MPa)
Stiffness (GPa)
Density (kg/m3)
Graphite
Composite
(aerospace grade)
$44-$550+
620-1400
70-350
1400
Graphite
Composite
(commercial
grade)
$10-$45
350-620
55-70
1400
Fiberglass
Composite
$3.5-6.5
140-240
7-10
1500
Aluminum
6061 T-6
$6.5
240
70
2800
Steel,
Mild
$0.65
410
210
8300
Figure 8: Use of Composites (blackened areas) in construction of the Airbus A380 [12]
Manufacturing Graphite Composite
Composite materials all have a relatively similar composition. The first portion of
the composition is the reinforcing phase [2]. This material is present as either particles,
whiskers or fibers. In the case of graphite composite the graphite is the reinforcing
phase. It is a fibrous state. Because of this graphite composites are known simply as
carbon fiber. The reinforcing phase is suspended in a matrix composed of a different
material. Graphite composite is suspended in a matrix of epoxy polymer. Composites
gain their high strength from the way in which the reinforcing phase and the matrix work
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together. The reinforcing phase is the primary load bearing portion of the composite.
The matrix absorbs most of the laoding on the composite and transfers it to the
reinforcing phase fibers [2].
The composite itself is formed in thin sheets. Within a single sheet all of the
fibers are arranged in a single orientation. In order to form a product from the material it
must be layered [13]. This is accomplished by molding a sheet to the desired shape,
affixing additional layers to it via a resin. The physical properties can be manipulated by
varying the orientation of the fibers between layers [13].
At this stage in the manufacture of composite materials many manufacturing
facilities encounter difficulties. This is because composite technology is a fairly new
science. A majority of manufacturing locations have vast experience cutting metal, but
almost no experience cutting composites [14]. The primary technology used to cut hard
composites is the abrasive waterjet. In this process the composite, or other similarly hard
material, is placed on a CNC type machine. In place of an end mill is a waterjet nozzle.
This nozzle flows a high pressure stream of water that is extremely thin. The force
applied at the water’s point of contact with the material is great enough to allow the jet to
cut nearly any hard material.
Limitations of Composite Materials
Composite materials can possess far greater properties for a given application
than any traditional engineering material. With all of the positive aspects of composite
materials there are two primary factors that continue to limit their widespread application.
The first is the economic drawback. Composite materials are extremely expensive to
produce [15]. Table 1 shows the relationship between the cost of composite materials
and some metals with comparable properties. The price increase for the increased
properties of the composite is the prohibiting factor in many applications. This is also the
reason the aircraft industry has made the greatest advancement in the use of composites.
Because of the harsh operating environment the aircraft industry typically tends toward
performance over cost based design [15].
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The cost of composites is directly related to the difficulty in their production.
Composite materials are essentially a fledgling technology [15]. While composite
materials have been in existence for more than fifty years the technology required to mass
produce them has not. For example, the high strength of most composite materials makes
them extremely difficult to cut to the proper size for an application [14]. The technology
required to produce composites is continually evolving and many professionals maintain
that they remain a viable material for future high performance engineering applications
[15].
References
1.
S.J. Mraz: “A Century of Progress in Aircraft Materials,” Machine Design, 2003,
vol. 75, no. 21, pp. 72-73.
2.
P.N. Anyalebechi: “Essentials of Materials Science and Engineering,” School of
Engineering, Padnos College of Engineering and Computing, Grand Valley State
University, 2005, pp. 137-156.
3.
“Materials & Minerals Processing:” Materials World News, Feb. 2004.
4.
V. Tran, B. Rothrock: “Structural Design,” ch. 7, pp. 45-54.
5.
“Mach vs. Altitude Tables,” www.aerospaceweb.org, June 24, 2005.
6.
Transportation Safety Board of Canada: “Aviation Occurrence Report: In-flight
wing separation,” Report A98O0104, 1999.
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7.
M. Backstrom, et. al.: “Fatigue Assessment of an Aging Aircraft’s Wings Under
Complex Multiaxial Spectrum Loading,” Proc. of the 7th International Conference
on Biaxial/Multiaxial Fatigue and Fracture, 2004.
8.
B. Dreibelbis, J. Barth: “Structural Analysis of Joint Wings,” America Institute of
Aeronautics and Astronautics, pp. 1-10, 2000.
9.
“Sophisticated Aircraft Driving New Market for Composite Materials,” Wireless
News, 2005, pp. 1.
10.
Y.D. Huang, et. al.: “Thermal Expansion and Dimensional Stability of Short Fiber
Reinforced AlSi12CuMgNi Piston Alloys,” SME Technical paper, 2004.
11.
“Graphite Composite Design Guide,” www.performancecomposites.com, June
24, 2005.
12.
A.G. Bratukhin: “Nonmetallic Materials in the Aircraft Industry,” Chemical and
Petroleum Engineering, 2000, vol. 36, nos. 9-10, pp. 521-523.
13.
N. Olsen: “Advanced Pressure Molding (Autocomp) and Fiber Form
Manufacturing Technology for Composite Aircraft/Aerospace Components,”
Composites Group of SME Technical Paper, 1986.
14.
D. Ginburg: “Abrasive Waterjet Cutting of Aerospace Materials,” SME Technical
Paper, 1989.
15.
G.E. Balthes: “Natural Fibers, Thinking Out of the Box,” SME Technical Paper,
2004.
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