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).

ρσ

σ yS = (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-to-

weight 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]

Graphite Composite

(aerospace grade)

Graphite Composite

(commercial grade)

Fiberglass Composite

Aluminum 6061 T-6

Steel, Mild

Cost $/kg $44-$550+ $10-$45 $3.5-6.5 $6.5 $0.65 Strength (MPa) 620-1400 350-620 140-240 240 410 Stiffness (GPa) 70-350 55-70 7-10 70 210 Density (kg/m3) 1400 1400 1500 2800 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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