COMPOSITE MATERIALS IN AEROSPACE APPLICATIONS

Presentation by : Himanshu Chauhan

Chemical EngineeringI.E.T. Lucknow

CONTENT • Introduction to composite material• Classification of composite

Composite Materials?• A composite material is a material made from two or more constituent

materials with significantly different physical or chemical properties that, when combined, produce a material with characteristics different from the individual components.

• Composite materials generally have two phases:A. Continous phase (metal matrix, ceramic matrix, polymer matrix)B. Dispersed phase (glass fiber, organic fiber)• Composite materials should have the following characteristics: 1. Microscopically it is non-homogeneous material and has a distinct

interface.2. There are big differences in the performance of component materials.3. The formed composite materials should have a great improvement in

performance.4. The volume fraction of component materials is larger than 10%.

Classification of composites

• The classification of is usually based on the matrix, the reinforcing phase , the arrangement (geometry) of reinforcement and architecture.

• Based on matrix, composites are classified into:• METAL MATRIX COMPOSITES• CERAMIC MATRIX COMPOSITES • POLYMER MATRIX COMPOSITES

• Based on reinforcement, composites are classified into:• PARTICULATE COMPOSITES • FIBROUS COMPOSITES • LAMINATE COMPOSITES

Use Of Composite In Aerospace Structure

• The use of composites has been motivated largely by such considerations. The composites offer several of these features as given below: • Light-weight due to high specific strength and stiffness.• Fatigue-resistance and corrosion resistance. • Capability of high degree of optimization: tailoring the directional strength

and stiffness. • Capability to mould large complex shapes in small cycle time reducing part

count and assembly times: Good for thin-walled or generously curved construction.

• Capability to maintain dimensional and alignment stability in space environment.

• Possibility of low dielectric loss in radar transparency. • Possibility of achieving low radar cross-section.

• These composites also have some inherent weaknesses:• Laminated structure with weak interfaces: poor resistance to out-of-plane

tensile loads • Susceptibility to impact-damage and strong possibility of internal damage

going unnoticed • Moisture absorption and consequent degradation of high temperature

performance • Multiplicity of possible manufacturing defects and variability in material

properties.

Even after accepting these weaknesses, the projected benefits are significant and almost all aerospace programmes use significant amount of composites as highlighted in the figure.

Boeing 757-200

Composite materials used in Boeing 787 body

Aerospace Composites• The use of composites in the aerospace industry has increased dramatically

since the 1970s. Traditional materials for aircraft construction include aluminium, steel and titanium. The primary benefits that composite components can offer are reduced weight and assembly simplification. The performance advantages associated with reducing the weight of aircraft structural elements has been the major impetus for military aviation composites development. New aircraft utilize carbon, boron and aramid fibers combined with epoxy resins. Such materials have replaced fiberglass reinforcements.

• It is important to note that the three most common types of composites that exist are composite materials that are reinforced with fiberglass, carbon fiber and aramid fiber. It is also interesting that each of these types has subtypes which provides for a wide variety of composites that exist in the world.

Fibre Density (g/cc)

Modulus(GPa)

Strength(GPa)

Application areas

Glass E-Glass

S-Glass

2.55

2.47

65-75

85-95

2.2-2.6

4.4-4.8

Small passenger aircraft parts, radomes, rocket motor casingsHighly loaded parts in small passenger a/c.

Aramidlow modulus Intermediate modulusHigh modulus

1.441.44

1.48

80-85120-128

160-170

2.7-2.82.7-2.8

2.3-2.4

Fairings, non-load bearing partsRadomes, some structural parts, motor casings.Highly loaded parts

CarbonStandard modulusIntermediate modulusHigh modulus

1.77-1.801.77-1.81

1.77-1.80

220-240270-300

390-450

3-3.55.4-5.7

4.0-4.52

Widely used for almost all type of parts in satellites antenna missiles etc..Space structures, control surface in a/c.

Fabrication Process

• Autoclave MouldingThis method employs an autoclave to provide heat and pressure to the composite product during curing. In this method, prepregs are stacked in a mold in a definite sequence and then spot welded to avoid any relative movement in between the prepreg sheets. After stacking the prepregs, the whole assembly is vacuum bagged to remove any air entrapped in between the layers. The schematic of autoclave molding process is shown in figure. After a definite period of time when it is ensured that all air is removed, the entire assembly is transferred to autoclave. Here, heat and pressure is applied for a definite interval of time. In this process, matrix is uniformly distributed and intimate contact is achieved through proper bonding between fibers and matrix. After the processing, the assembly is cooled to a definite rate and then vacuum bag is removed. The composite part is taken out from the mold. Initially, a release gel is applied onto the mold surface to avoid sticking of polymer to the mold surface. The raw materials used in these techniques are given in the table.

Raw material used in Autoclave moulding

matrix Epoxy, polyester, polyvinyl ester, phenolic resin, thermoplastic resin

Reinforcement Glass fiber, carbon fiber, aramid fiber.

Filament winding

• To begin with, a large number of fibre rovings is pulled from series of creels into bath containing liquid resin, catalyst and other ingredients such as pigments and UV retardants. Fibre tension is controlled by the guides or scissor bars located between each creel and resin bath. Just before entering the resin bath, the rovings are usually gathered into a band by passing them through a textile thread board or stainless steel comb.

• At the end of the resin tank, the resin-impregnated rovings are pulled through a wiping device that removes the excess resin from the rovings and controls the resin coating thickness around each roving.

• The most commonly used wiping device is a set of squeeze rollers in which the position of the top roller is adjusted to control the resin content as well as the tension in fibre rovings. Another technique for wiping the resin-impregnated rovings is to pull each roving separately through an orifice.

• The latter method results in better control of resin content. Once the rovings have been thoroughly impregnated and wiped, they are gathered together in a flat band and positioned on the mandrel.

• Band formation can be achieved by passing through a stainless steel comb and later through the collecting eye.The transverse speed of the carriage and the winding speed of the mandrel are controlled to create the desired winding angle patterns.

• After winding, the filament wound mandrel is subjected to curing and post curing operations during which the mandrel is continuously rotated to maintain uniformity of resin content around the circumference. After curing, product is removed from the mandrel, either by hydraulic or mechanical extractor.

Aerospace Applications• Business and CommercialLear Fan 2100 As one of the first aircraft conceived and engineered as a “composites” craft, the Lear Fan uses approximately 1880 pounds of carbon, glass and aramid fiber material. In addition to composite elements that are common to other aircraft, such as doors, control surfaces, fairings and wing boxes, the Lear Fan has an all-composite body and propeller blades..Beech Starship The Starship is the first all-composite airplane to receive FAA certification. Approximately 3000 pounds of composites are used on each aircraft.Boeing The Boeing 757 and 767 employ about 3000 pounds each of composites for doors and control surfaces. The 767 rudder at 36 feet is the largest commercial component in service. The 737- 300 uses approximately 1500 pounds of composites, which represents about 3% of the overall structural weight. Composites are widely used in aircraft interiors to create luggage compartments, sidewalls, floors, ceilings, galleys, cargo liners and bulkheads. Fiberglass with epoxy or phenolic resin utilizing honeycomb sandwich construction gives the designer freedom to create aesthetically pleasing structures while meeting flammability and impact resistance requirements.

• Airbus In 1979, a pilot project was started to manufacture carbon fiber fin box assemblies for the A300/A310 aircraft. A highly mechanized production process was established to determine if high material cost could be offset by increased manufacturing efficiency. Although material costs were 35% greater than a comparable aluminum structure, total manufacturing costs were lowered 65 to 85%. Robotic assemblies were developed to handle and process materials in an optimal and repeatable fashion.

• Military

• Advanced Tactical Fighter (ATF) Advanced composites enable the ATF to meet improved performance requirements such as reduced drag, low radar observability and increased resistance to temperatures generated at high speeds. The ATF will be approximately 50% composites by weight using DuPont's Avimid K polyamide for the first prototype. Figure 1-51 depicts a proposed wing composition as developed by McDonnell Aircraft through their Composite Flight Wing Program.

• Advanced Technology Bomber (B-2) The B-2 derives much of its stealth qualities from the material properties of composites and their ability to be molded into complex shapes. Each B-2 contains an estimated 40,000 to 50,000 pounds of advanced composite materials. According to Northrop, nearly 900 new materials and processes were developed for the plane. • Second Generation British Harrier “Jump Jet” (AV-8B) This vertical take-off and landing (VTOL) aircraft is very sensitive to overall weight. As a result, 26% of the vehicle is fabricated of composite material. Much of the substructure is composite, including the entire wing. Bismaleimides (BMI's) are used on the aircraft's underside and wing trailing edges to withstand the high temperatures generated during take-off and landing. • Navy Fighter Aircraft (F-18A) The wing skins of the F-18A represented the first widespread use of graphite/epoxy in a production aircraft. The skins vary in thickness up to one inch, serving as primary as well as secondary load carrying members. It is interesting to note that the graphite skins are separated from the aluminium framing with a fiberglass barrier to prevent galvanic corrosion. The carrier- based environment that Navy aircraft are subjected to has presented unique problems to the aerospace designer. Corrosion from salt water surroundings is exacerbated by the sulphur emission from the ship's exhaust stacks.

• Osprey Tilt-Rotor (V-22) The tilt-rotor V-22 is also a weight sensitive craft that is currently being developed by Boeing and Bell Helicopter. Up to 40% of the airframe consists of composites, mostly AS-4 and IM-6 graphite fibres in 3501-6 epoxy (both from Hercules). New uses of composites are being exploited on this vehicle, such as shafting and thick, heavily loaded components. Consequently, higher design strain values are being utilized.

Helicopters

• Rotors Composite materials have been used for helicopter rotors for some time now and have gained virtually 100% acceptance as the material of choice. The use of fibrous composites offers improvements in helicopter rotors due to improved aerodynamic geometry, improved aerodynamic tuning, good damage tolerance and potential low cost. Anisotrophic strength properties are very desirable for the long, narrow foils. Additionally, a cored structure has the provision to incorporate the required balance weight at the leading edge. The favorable structural properties of the mostly fiberglass foils allow for increased lift and speed. Fatigue characteristics of the composite blade are considerably better than their aluminum counterparts with the aluminum failing near 40,000 cycles and the composite blade exceeding 500,000 cycles without failure. Vibratory strain in this same testing program was 510 µ inch inch for aluminum and 2400 µ inch inch for the composite. Sikorsky Aircraft of United Aircraft Corporation has proposed a Cross Beam Rotor (XBR)TM, which is a simplified, lightweight system that makes extensive use of composites. The low torsional stiffness of a unidirectional composite spar allows pitch change motion to be accommodated by elastic deformation, whereas sufficient bending stiffness prevents areoelastic instability.

Advantages of composite in aerospace

• Weight reduction - savings in the range 20%-50% are often quoted.• It is easy to assemble complex components using automated layup machinery and rotational

molding processes.• Monocoque ('single-shell') molded structures deliver higher strength at much lower weight.• Mechanical properties can be tailored by 'lay-up' design, with tapering thicknesses of

reinforcing cloth and cloth orientation.• Thermal stability of composites means they don't expand/contract excessively with change in

temperature (for example a 90°F runway to -67°F at 35,000 feet in a matter of minutes).• High impact resistance - Kevlar (aramid) armor shields planes, too - for example, reducing

accidental damage to the engine pylons which carry engine controls and fuel lines.• High damage tolerance improves accident survivability.• 'Galvanic' - electrical - corrosion problems which would occur when two dissimilar metals are

in contact (particularly in humid marine environments) are avoided. (Here non-conductive fiberglass plays a roll.)

• Combination fatigue/corrosion problems are virtually eliminated.

The Future of Composites in Aerospace

• With ever increasing fuel costs and environmental lobbying, commercial flying is under sustained pressure to improve performance, and weight reduction is a key factor in the equation.

• Beyond the day-to-day operating costs, the aircraft maintenance programs can be simplified by component count reduction and corrosion reduction. The competitive nature of the aircraft construction business ensures that any opportunity to reduce operating costs is explored and exploited wherever possible.

• Competition exists in the military too, with continuous pressure to increase payload and range, flight performance characteristics and 'survivability', not only of airplanes, but of missiles, too.

• Composite technology continues to advance, and the advent of new types such as basalt and carbon nanotube forms is certain to accelerate and extend composite usage.

• When it comes to aerospace, composite materials are here to stay.

• The following are some of the military and commercial aircraft that use significant amounts of composites in the airframe.

• Figher aircraft• U.S. ' AV-8B, F16, F14, F18, YF23, F22, JSF, UCAV• Europe ' Harrier GR7, Gripen JAS39, Mirage 2000, Rafael, Eurofighter,

Lavi, EADS Mako• Russia ' MIG 29, Su series• Bomber ' B2

Transport U.S. ' KC135, C17, 777, 767, MD11• Europe ' A320, A340, A380, Tu204, ATR42, Falcon 900, A300-600• General Aviation ' Piaggio, Starship,

Premier 1, Cirrus SR 20 & SR 22 Rotary Aircraft ' V22, Eurocopter, Comanche, RAH66, BA609, EH101, Super Lynx 300, S92

Conclusion

• Composite materials offer high fatigue and corrosion resistance. Composite materials have high strength to weight ratio. So they are best suited for various aerospace applications.