NEW MATERIALS IN AEROSPACE INTRODUCTION Aircrafts design and manufacture is a complex process, designers consider of several factors such as engine efficiency and weight of the aircraft. In success-full meeting of the factors mentioned above fuel efficiency is met which on its turn permits longer distances to travel and larger quantity of cargo. The improvement of engine efficiency is achieved by permitting the engine to operate at higher temperatures. Thus designers aim to use new materials to withstand higher operating temperatures. Wood and fabric materials on an old aircraft evolved to advanced metal alloys and advanced composite materials in order to meet
Transcript
NEW MATERIALS IN AEROSPACE
INTRODUCTION
Aircrafts design and manufacture is a complex process, designers consider
of several factors such as engine efficiency and weight of the aircraft. In
success-full meeting of the factors mentioned above fuel efficiency is met
which on its turn permits longer distances to travel and larger quantity of
cargo. The improvement of engine efficiency is achieved by permitting the
engine to operate at higher temperatures. Thus designers aim to use new
materials to withstand higher operating temperatures. Wood and fabric
materials on an old aircraft evolved to advanced metal alloys and advanced
composite materials in order to meet weight reduction and engine
efficiency. The above picture shows the Boeing 787, this super-jumbo has
an all composite materials fuselage.
METAL ALLOYS
The needs of jet engines in old aircrafts were met only by using metal
alloys. New aircrafts are designed to incorporate engines with greater
horsepower and advanced metal alloys. The advancement of metal alloys is
the super-alloys. Super-alloys are divided into three categories: nickel
based, cobalt based and iron based. Their properties as resistive materials
to high temperatures, of high strength, very great mechanical stress and
finally the surface stability lead to their selection for use in jet engines,
space aircrafts and general aviation.
ALUMINIUM ALLOYS
FUSELAGE FRAME
Aluminium alloys is widely used in aerospace. Aluminium is used as a
primary material to form alloys with other metals such as magnesium,
silicon or zinc. This is an effective way to improve the mechanical properties
of metals.
MAGNESIUM ALLOYS
GEAR BOX FRAME
Magnesium is a light material weighting only two –thirds of aluminium or
one-fifth of steel. However the level of magnesium structural strength is low
and as a result alloys have been produced. Aluminium magnesium alloys
with magnesium lower than 5% have higher strength lower density and are
better corrosion resistive materials than aluminium. Aluminium magnesium
alloys with 5 % magnesium are easier to weld, cast or machine.
MOLYBDENUM ALLOYS
Molybdenum has a very high melting point and providing strength at
temperatures where other metals or alloys are passed their melting point.
Molybdenum alloys are very important materials for the turbine engines
due to their high melting point and their corrosion resistance.
NICKEL ALLOYS
DRIVE CONE
Nickel alloys are alloys using nickel as the primary element. Nickel
aluminide is considered to be in the nickel alloys category. It is composed of
nickel joined with a number of metals including aluminium, chromium,
molybdenum, zirconium and boron. This particular material is considered to
be an inermetallic material; that is the material properties are between
ceramic and metallic properties. The resistance to heat and corrosion of
this material is at very high levels and can be used as a coating material on
the blades of gas turbine or jet turbine engines.
TITANIUM ALLOYS
HIGH PRESSURE DISC
Titanium is a ductile material with low density, low electrical and thermal
conductivity and it is weakly attracted by the poles of a magnet with the
level of magnetism falling to zero when the magnetic field is removed.
Titanium is considered to be a vital material when used as an alloying agent
with metals. Titanium alloys exhibit tensile strength higher than titanium
material. It is used for many parts in aerospace due to its high strength low
density, the extremely good corrosion resistance and excellent performance
at very high temperatures. Titanium alloys are high cost materials.
TUNGSTEN ALLOYS
HIGH DENSITY AVIATION PART
Tungsten is a steel grey metal that exhibits the highest melting point of all
non-metal alloys. It has very high density and thus can be used as balanced
weight requiring minimum space. Tungsten is used in super-alloys used in
rocket nozzles, turbine blades, wear resistant parts and coatings.
COMPOSITE MATERIALS
The tremendous progress made in alloys for aerospace enables the
designers to build more efficient engines. However metal alloys are not the
best choice for aerospace structures. Composite materials are much
lighter than metal alloys, thus material scientists are very interested in
developing composite materials that exhibit properties for aerospace
structures applications. The use of composites in aerospace structures
resulted in weight savings and as a result better performance of the
aircraft. The illustration on the bottom of the page shows that fibre
reinforced polymers is primary materials for aerospace structure
applications.
Composite materials development is based on the need of strong and stiff
materials that can be used in aerospace application. The composed
material will be of two or more different materials aiming to develop a new
composite material that exhibits enhanced properties compare to the old
materials. The composite materials used in aerospace refer to fibre
reinforced metal, fibre reinforced polymer and fibre reinforced ceramic. The
strong and stiff properties mentioned at the start of this paragraph refer to
the fibres properties which can be glass, graphite, silicon carbide or aramid
fibres. Properties like electrical conductivity, thermal conductivity and
thermal stability are of the matrix of a polymer or metal or ceramic
material. The effect of a crack that propagates until the component fails can
be avoided with composite materials and this is due to the fibre
reinforcement; if a fibre fails the remaining fibres will not be affected.
Human body if not the most complex body that exists, is considered to be
one of the most complex body’s. The bone structure of the human body is a
source of information for engineers that are trying to develop the light
composite materials. Every bone on the human body is carrying a load
different from the other bones. The bone it self is not monolithic but
constructed of fibres that grow in many different directions as to support
the carrying load.
CARBON FIBRE REINFORCED PLASTIC (CFRP)
Carbon fibres mixed with plastic resin creates the CFRP. The carbon fibres
are used to reinforce the composed material resulting in a very strong and
light material. CFRP can be constructed strong enough to support loads
applied on a component in one direction but weak in the direction where no
load is carried. Manufactures developed carbon fibre pieces that can carry
loads in all directions. The strength to weight ratio of the material enables
the designer to build on efficiency of the aircraft and minimize the impact
on the environment. What is more the longevity of a CFRP component along
with the correct choice of manufacturing process allows the designer to
choose this high cost material.
The white knight two shown in the above picture is the largest all carbon fiber aircraft ever made. White knight two is built to carry a smaller spacecraft up to 48,000 feet from where the spacecraft will blast off into suborbital space.
GLASS FIBRE REINFORCED PLASTIC
Plastic resin mixed with glass fibres to create GFRP. This composite material
provides very good compressive properties due to the plastic material.
Plastic is not famous for its tensile behaviour and this is where the glass
fibres stand to support the composite material due to their high tensile
strength. Thus the material can carry compressive and tensile loads. GFRP
can be used in microelectronics due to its nonconductive properties.
The picture below shows the S-2 GFRP, an enhanced type of GFRP. This
particular product weights less than the typical GFRP and costs less the
AFRP and CFRP. The enhanced properties of GFRP made it ideal for
applications like lightweight airframe structural parts or helicopter blades.
S -2 GFRP
ARAMID FIBRE REINFORCED POLYMER
KEVLAR
The exceptional thing about aramids is the structure of their polymer chain
molecules. The structure is such that produces fibre reinforced composite
materials with exceptional fibre strength and thermal stability. Kevlar is
well-known aramid that combines fibres of ultrahigh strength and ultrahigh
stiffness. Kevlar is five times stronger per kilogram than steel enabling
designers to reduce weight and thus built efficient aircrafts. Kevlar or
equivalent aramids have many applications in aerospace such as landing
gear doors, aircraft cabin and jet engines.
ENGINEERING CERAMICS
This category of materials is about the improvement of some traditional
ceramics such us Alumina and also of some more recent ceramics such as
silicon carbide and silicon nitride. Ceramics are well known for their low
electrical conductivity, brittleness, and excellent resistant to heat. Silicon
carbide is a crystalline composed of carbon and silicon. It is an extremely
hard material with high thermal conductivity high temperature strength,
low thermal expansion and resistant to chemical reaction. In order to
reinforce ceramics or other metals, silicon carbide fibres can be developed.
Silicon carbide can be used in wear resistant parts in rocket engines.
GAS TURBINE ENGINE
The reinforcement of titanium with silicon carbide fiber results in a material
with high strength to weight ratio, elevated temperature strength and
stiffness. This material is ideal for use in gas turbine engines for static and
rotating components in medium temperatures. Silicon carbide fiber
titanium density is 15% less of titanium alloy.
ENGINEERING POLYMERS
EPOXIES (THERMOSETS)
Epoxies can be used as a structural material reinforced with carbon fibres,
glass and Kevlar. For these applications epoxies show high strength when
reinforced with fibers of glass, aramid, or carbon. In aerospace applications
epoxies can be used as adhesives for the structure. Epoxy adhesives can be
developed according to the need of application. Epoxy resins are also made
into structural parts such as laminated boards, laminates and composites
for aerospace applications. The above picture shows the application of a
very strong and flame retardant epoxy (magnobond 92-1) used to reinforce
floor panels and wall panels on the Boeing 777.
POLYMETHYLMETHACRYLATE (THERMOPLASTIC POLYMER)
PMMA is a transparent unable to bend plastic that continues to have its
properties when exposed to ultraviolet radiation and weather. It is used for
aircraft canopies and aircraft windows. It has lower density and greater
impact resistance than glass. In the case of impact this material will brake
into large pieces compare to glass that brake into very small pieces. This
material offers an economical solution to designers.
POLYCARBONATE
The impact-temperature resistant and optical properties make this material
a very interesting choice for designers. Polycarbonate comes in different
types and prices. The polycarbonate material used for the canopy of the US
fighter F22 Raptor that uses the latest technology is a high cost material
made especially for the F22. This particular component incorporate a thin
surface layer used to prevent heat from escaping the canopy and thus
reduce thermal radiation of the fighter. This is one of the many stealth
technologies incorporated on the F22 raptor.
FOAM POLYMER
The foamed polymers exhibit lower thermal conductivity and are less
flexible than a polymer in solid state. Polyurethane, a synthetic resin of a
polymer and urethane that has very good behavior with metallic surfaces
and as a result it can be used in feeling certain aircrafts components.
CONCLUSION
Modern aircrafts materials selection is based on two very important factors,
weight reduction and engine efficiency. The use of metal alloys improved
the overall picture over the last years but was not the best solution. The
super-alloys performance on aircraft applications is very effective due to
their superb mechanical properties and high temperature resistance.
However on structural applications the use of composite materials such as
CFRP is growing as a result of the reduced weigh of the components and
also their very good mechanical properties. As mentioned in the beginning
of this section the two most important factors for materials selection is
weight savings and engine efficiency. New materials such as ceramic with
reinforced fibres resulted into new materials with excellent properties for
use in jet engines. The use of these materials permits higher operating
temperatures and as a result greater engine efficiency. Greater engine
efficiency means better fuel consumption and less impact on the
environment. Ceramic material weights less than metals and has much
better thermal resistance which makes ceramic reinforced with fibres or
whiskers one of the best options for aircraft designers.
