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Introduction to aerospace materials
1.1
The importance of aerospace materials
The importance of materials science and technology in aerospace engineering
cannot be overstated. The materials used in airframe structures and in
jet engine components are critical to the successful design, construction,
certification, operation and maintenance of aircraft. Materials have an
impact through the entire life cycle of aircraft, from the initial design phase
through to manufacture and certification of the aircraft, to flight operations
and maintenance and, finally, to disposal at the end-of-life.
Materials affect virtually every aspect of the aircraft, including the:
∑
∑
∑
∑
∑
∑
∑
∑
∑
purchase cost of new aircraft;
cost of structural upgrades to existing aircraft;
design options for the airframe, structural components and engines;
fuel consumption of the aircraft (light-weighting);
operational performance of the aircraft (speed, range and payload);
power and fuel efficiency of the engines;
in-service maintenance (inspection and repair) of the airframe and
engines;
safety, reliability and operational life of the airframe and engines;
and
disposal and recycling of the aircraft at the end-of-life.
Aerospace materials are defined in this book as structural materials that
carry the loads exerted on the airframe during flight operations (including
taxiing, take-off, cruising and landing). Structural materials are used in
safety-critical airframe components such as the wings, fuselage, empennage
and landing gear of aircraft; the fuselage, tail boom and rotor blades of
helicopters; and the airframe, skins and thermal insulation tiles of spacecraft
such as the space shuttle. Aerospace materials are also defined as jet engine
structural materials that carry forces in order to generate thrust to propel the
aircraft. The materials used in the main components of jet engines, such as
the turbine blades, are important to the safety and performance of aircraft
and therefore are considered as structural materials in this book.
An understanding of the science and technology of aerospace materials
is critical to the success of aircraft, helicopters and spacecraft. This book
provides the key information about aerospace materials used in airframe
1
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Introduction to aerospace materials
structures and jet engines needed by engineers working in aircraft design,
aircraft manufacturing and aircraft operations.
1.2
Understanding aerospace materials
Advanced materials have an important role in improving the structural
efficiency of aircraft and the propulsion efficiency of jet engines. The
properties of materials that are important to aircraft include their physical
properties (e.g. density), mechanical properties (e.g. stiffness, strength and
toughness), chemical properties (e.g. corrosion and oxidation), thermal
properties (e.g. heat capacity, thermal conductivity) and electrical properties
(e.g. electrical conductivity). Understanding these properties and why they
are important has been essential for the advancement of aircraft technology
over the past century.
Understanding the properties of materials is reliant on understanding
the relationship between the science and technology of materials, as shown
in Fig. 1.1. Materials science and technology is an interdisciplinary field
that involves chemistry, solid-state physics, metallurgy, polymer science,
fibre technology, mechanical engineering, and other fields of science and
engineering.
Materials science involves understanding the composition and structure of
materials, and how they control the properties. The term composition means
the chemical make-up of the material, such as the types and concentrations
of alloying elements in metals or the chemical composition of polymers.
The structure of materials must be understood from the atomic to final
component levels, which covers a length scale of many orders of magnitude
(more than 1012). The important structural details at the different length
scales from the atomic to macrostructure for metals and fibre-polymer
composites, which are the two most important groups of structural materials
used in aircraft, are shown in Fig. 1.2. At the smallest scale the atomic
and molecular structure of materials, which includes the bonding between
atoms, has a large influence on properties such as stiffness and strength.
The crystal structure and nanoscopic-sized crystal defects in metals and the
molecular structures of the fibres and polymer in composites also affect the
properties. The microstructure of materials typically covers the length scale
Composition
Atomic bonding
Crystal structure
Defect structure
Microstructure
Macrostructure
Materials
science
Materials
properties
Materials
technology
Density
Stiffness
Strength
Fatigue
Toughness
Corrosion
High temperature
1.1 Relationship between materials science and materials technology.
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Introduction to aerospace materials
3
(a)
Macrostructure
Microstructure
Defects
Crystal structure
Atomic
10–12
1 ¥ 10–10
1 ¥ 10–8
1 ¥ 10–6
1 ¥ 10–4
Length (m)
1 ¥ 10–2
1 ¥ 100
1 ¥ 102
(b)
Macrostructure
Microstructure
Defects
Fibre structure
Polymer molecular
structure
Atomic
10–12
1 ¥ 10–10
1 ¥ 10–8
1 ¥ 10–6
1 ¥ 10–4
Length (m)
1 ¥ 10–2
1 ¥ 100
1 ¥ 102
1.2 Structural factors at different sizes affect the properties of (a)
metals and (b) fibre–polymer composites.
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Introduction to aerospace materials
from around 1 to 1000 mm, and microstructural features in metals such as
the grain size, grain structure, precipitates and defects (e.g. voids, brittle
inclusions) affect the properties. Microstructural features such as the fibre
arrangement and defects (e.g. voids, delaminations) affect the properties
of composites. The macrostructural features of materials, such as its shape
and dimensions, may also influence the properties. The aim of materials
science is to understand how the physical, mechanical and other properties
are controlled over the different length scales. From this knowledge it is then
possible to manipulate the composition and structure of materials in order
to improve their properties.
Materials technology (also called materials engineering) involves the
application of the material properties to achieve the service performance
of a component. Put another way, materials technology aims to transform
materials into useful structures or components, such as converting soft
aluminium into a high strength metal alloy for use in an aircraft wing or
making a ceramic composite with high thermal insulation properties needed
for the heat shields of a spacecraft. The properties needed by materials are
dependent on the type of the component, such as its ability to carry stress
without deforming excessively or breaking; to resist corrosion or oxidation;
to operate at high temperature without softening; to provide high structural
performance at low weight or low cost; and so on. Materials technology
involves selecting materials with the properties that best meet the service
requirements of a component as well as maintaining the performance of the
materials over the operating life of the component by resisting corrosion,
fatigue, temperature and other damaging events.
Most aerospace engineering work occurs in the field of materials technology,
but it is essential to understand the science of materials. This book examines
the interplay between materials science and materials technology in the
application of materials for aircraft structures and jet engines.
1.3
Introducing the main types of aerospace
materials
An extraordinarily large number and wide variety of materials are available
to aerospace engineers to construct aircraft. It is estimated that there are more
than 120 000 materials from which an aerospace engineer can choose the
materials for the airframe and engine. This includes many types of metals
(over 65 000), plastics (over 15 000), ceramics (over 10 000), composites,
and natural substances such as wood. The number is growing at a fast pace
as new materials are developed with unique or improved properties.
The great majority of materials, however, lack one or more of the essential
properties required for aerospace structural or engine applications. Most
materials are too expensive, heavy or soft or they lack sufficient corrosion
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Introduction to aerospace materials
5
resistance, fracture toughness or some other important property. Materials
used in aerospace structures and engines must have a combination of essential
properties that few materials possess. Aerospace materials must be light, stiff,
strong, damage tolerant and durable; and most materials lack one or more
of the essential properties needed to meet the demanding requirements of
aircraft. Only a tiny percentage of materials, less than 0.05%, are suitable
to use in the airframe and engine components of aircraft, helicopters and
spacecraft.
It is estimated that less than about one hundred types of metal alloys,
composites, polymers and ceramics have the combination of essential
properties needed for aerospace applications. The demand on materials to
be lightweight, structurally efficient, damage tolerant, and durable while
being cost-effective and easy to manufacture rules out the great majority for
aerospace applications. Other demands on aerospace materials are emerging as
important future issues. These demands include the use of renewable materials
produced with environmentally friendly processes and materials that can be
fully recycled at the end of the aircraft life. Sustainable materials that have
little or no impact on the environment when produced, and also reduce the
environmental impact of the aircraft by lowering fuel burn (usually through
reduced weight), will become more important in the future.
The main groups of materials used in aerospace structures are aluminium
alloys, titanium alloys, steels and composites. In addition to these materials,
nickel-based alloys are important structural materials for jet engines. These
materials are the main focus of this book. Other materials have specific
applications for certain types of aircraft, but are not mainstream materials
used in large quantities. Examples include magnesium alloys, fibre–metal
laminates, metal matrix composites, woods, ceramics for heat insulation tiles
for rockets and spacecraft, and radar absorbing materials for stealth military
aircraft.
Many other materials are also used in aircraft: copper for electrical wiring;
semiconductors for electronic devices; synthetic fabrics for seating and other
furnishing. However, none of these materials are required to carry structural
loads. In this book, the focus is on the materials used in aircraft structures
and jet engines, and not the nonstructural materials which, although important
to aircraft operations, are not required to support loads.
Seldom is a single material able to provide all the properties needed by
an aircraft structure and engine. Instead, combinations of materials are used
to achieve the best balance between cost, performance and safety. Table
1.1 gives an approximate grading of the common aerospace materials for
several key factors and properties for airframes and engines. There are large
differences between the performance properties and cost of materials. For
example, aluminium and steel are the least expensive; composites are the
lightest; steels have the highest stiffness and strength; and nickel alloys have
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Aluminium
Cheap
Light
Low/medium
Medium
Medium
Low/medium
Medium
Low
High
Property
Cost
Weight (density)
Stiffness (elastic modulus)
Strength (yield stress)
Fracture toughness
Fatigue
Corrosion resistance
High-temperature creep strength
Ease of recycling
Expensive
Medium
Medium
Medium/high
High
High
High
Medium
Medium
Titanium
Table 1.1 Grading of aerospace materials on key design factors
Medium
Very light
Low
Low
Low/medium
Low
Low
Low
Medium
Magnesium
Medium
Heavy
Very high
Very high
Low/medium
Medium/high
Low/medium
High
High
High-strength
steel
Expensive
Heavy
Medium
Medium
Medium
Medium
High
Very high
Medium
Expensive
Very light
High
High
Low
High
Very high
Low
Very low
Nickel superalloy Carbon fibre
composite
Introduction to aerospace materials
7
the best mechanical properties at high temperature. As a result, aircraft are
constructed using a variety of materials which are best suited for the specific
structure or engine component.
Figure 1.3 shows the types and amounts of structural materials in various
types of modern civil and military aircraft. A common feature of the different
aircraft types is the use of the same materials: aluminium, titanium, steel
and composites. Although the weight percentages of these materials differ
between aircraft types, the same four materials are common to the different
aircraft and their combined weight is usually more than 80–90% of the
structural mass. The small percentage of ‘other materials’ that are used may
include magnesium, plastics, ceramics or some other material.
Titanium (3%)
Steel (6%)
Composite (3%)
Other materials (7%)
Aluminium
(81%)
(a)
Titanium (4%)
Steel (9%)
Composite (17%)
Other materials (2%)
Aluminium
(68%)
(b)
Titanium and steel (10%)
Composite and
glare (25%)
Aluminium
(61%)
Other materials
(4%)
(c)
1.3 Structural materials and their weight percentage used in the
airframes of civilian and military aircraft. (a) Boeing 737, (b) Airbus
340-330, (c) Airbus A380, (d) Boeing 787, (e) F-18 Hornet (C/D), (f)
F-22 Raptor. Photographs supplied courtesy of (a) K. Boydston, (b) S.
Brimley, (c) F. Olivares, (d) C. Weyer, (e) J. Seppela and (f) J. Amann.
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Introduction to aerospace materials
Composite (50%)
Steel
(10%)
Titanium
(14%)
Other materials
(6%)
Aluminium (20%)
(d)
Titanium (13%)
Steel (16%)
Composite (9%)
Other materials
(11%)
Aluminium
(51%)
(e)
Composite (35%)
Steel (5%)
Other materials (16%)
Titanium (33%)
Aluminium (11%)
(f)
1.3 Continued
1.3.1
Aluminium
Aluminium is the material of choice for most aircraft structures, and has
been since it superseded wood as the common airframe material in the
1920s/1930s. High-strength aluminium alloy is the most used material for the
fuselage, wing and supporting structures of many commercial airliners and
military aircraft, particularly those built before the year 2000. Aluminium
accounts for 70–80% of the structural weight of most airliners and over 50%
of many military aircraft and helicopters, although in recent years the use of
aluminium has fallen owing to the growing use of fibre–polymer composite
materials. The competition between the use of aluminium and composite is
intense, although aluminium will remain an important aerospace structural
material.
Aluminium is used extensively for several reasons, including its moderately
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Introduction to aerospace materials
9
low cost; ease of fabrication which allows it to be shaped and machined into
structural components with complex shapes; light weight; and good stiffness,
strength and fracture toughness. Similarly to any other aerospace material,
there are several problems with using aluminium alloys, and these include
susceptibility to damage by corrosion and fatigue.
There are many types of aluminium used in aircraft whose properties are
controlled by their alloy composition and heat treatment. The properties
of aluminium are tailored for specific structural applications; for example,
high-strength aluminium alloys are used in the upper wing skins to support
high bending loads during flight whereas other types of aluminium are used
on the lower wing skins to provide high fatigue resistance.
1.3.2
Titanium
Titanium alloys are used in both airframe structures and jet engine components
because of their moderate weight, high structural properties (e.g. stiffness,
strength, toughness, fatigue), excellent corrosion resistance, and the ability
to retain their mechanical properties at high temperature. Various types of
titanium alloys with different compositions are used, although the most common
is Ti–6Al–4V which is used in both aircraft structures and engines.
The structural properties of titanium are better than aluminium, although
it is also more expensive and heavier. Titanium is generally used in the
most heavily-loaded structures that must occupy minimum space, such as
the landing gear and wing–fuselage connections. The structural weight of
titanium in most commercial airliners is typically under 10%, with slightly
higher amounts used in modern aircraft such as the Boeing 787 and Airbus
A350. The use of titanium is greater in fighter aircraft owing to their need
for higher strength materials than airliners. For instance, titanium accounts
for 25% of the structural mass of the F-15 Eagle and F-16 Fighting Falcon
and about 35% of the F-35 Lightning II. Titanium alloys account for 25–30%
of the weight of modern jet engines, and are used in components required
to operate to 400–500 °C. Engine components made of titanium include fan
blades, low-pressure compressor parts, and plug and nozzle assemblies in
the exhaust section.
1.3.3
Magnesium
Magnesium is one of the lightest metals, and for this reason was a popular
material for lightweight aircraft structures. Magnesium was used extensively
in aircraft built during the 1940s and 1950s to reduce weight, but since
then the usage has declined as it has been replaced by aluminium alloys
and composites. The use of magnesium in modern aircraft and helicopters
is typically less than 2% of the total structural weight. The demise of
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Introduction to aerospace materials
magnesium as an important structural material has been caused by several
factors, most notably higher cost and lower stiffness and strength compared
with aluminium alloys. Magnesium is highly susceptible to corrosion which
leads to increased requirements for maintenance and repair. The use of
magnesium alloys is now largely confined to non-gas turbine engine parts,
and applications include gearboxes and gearbox housings of piston-engine
aircraft and the main transmission housing of helicopters.
1.3.4
Steel
Steel is the most commonly used metal in structural engineering, however its
use as a structural material in aircraft is small (under 5–10% by weight). The
steels used in aircraft are alloyed and heat-treated for very high strength, and
are about three times stronger than aluminium and twice as strong as titanium.
Steels also have high elastic modulus (three times stiffer than aluminium)
together with good fatigue resistance and fracture toughness. This combination
of properties makes steel a material of choice for safety-critical structural
components that require very high strength and where space is limited, such
as the landing gear and wing box components. However, steel is not used in
large quantities for several reasons, with the most important being its high
density, nearly three times as dense as aluminium and over 50% denser than
titanium. Other problems include the susceptibility of some grades of highstrength steel to corrosion and embrittlement which can cause cracking.
1.3.5
Superalloys
Superalloys are a group of nickel, iron–nickel and cobalt alloys used in jet
engines. These metals have excellent heat resistant properties and retain
their stiffness, strength, toughness and dimensional stability at temperatures
much higher than the other aerospace structural materials. Superalloys also
have good resistance against corrosion and oxidation when used at high
temperatures in jet engines. The most important type of superalloy is the
nickel-based material that contains a high concentration of chromium, iron,
titanium, cobalt and other alloying elements. Nickel superalloys can operate
for long periods of time at temperatures of 800–1000 °C, which makes
them suitable for the hottest sections of gas turbine engines. Superalloys are
used in engine components such as the high-pressure turbine blades, discs,
combustion chamber, afterburners and thrust reversers.
1.3.6
Fibre–polymer composites
Composites are lightweight materials with high stiffness, strength and fatigue
performance that are made of continuous fibres (usually carbon) in a polymer
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Introduction to aerospace materials
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matrix (usually epoxy). Along with aluminium, carbon fibre composite is
the most commonly used structural material for the airframe of aircraft and
helicopters. Composites are lighter and stronger than aluminium alloys, but
they are also more expensive and susceptible to impact damage.
Carbon fibre composites are used in the major structures of aircraft,
including the wings, fuselage, empennage and control surfaces (e.g. rudder,
elevators, ailerons). Composites are also used in the cooler sections of jet
engines, such as the inlet fan blades, to reduce weight. In addition to carbon
fibre composites, composites containing glass fibres are used in radomes
and semistructural components such as fairings and composites containing
aramid fibres are used in components requiring high impact resistance.
1.3.7
Fibre–metal laminates
Fibre–metal laminates (FML) are lightweight structural materials consisting of
thin bonded sheets of metal and fibre–polymer composite. This combination
creates a material which is lighter, higher in strength, and more fatigue
resistant than the monolithic metal and has better impact strength and damage
tolerance than the composite on its own. The most common FML is GLARE®
(a name derived from glass reinforced aluminium) which consists of thin
layers of aluminium alloy bonded to thin layers of fibreglass composite.
FMLs are not widely used structural materials for aircraft; the only aircraft
at present that use GLARE® are the Airbus 380 (in the fuselage) and C17
GlobeMaster III (in the cargo doors).
1.4
What makes for a good aerospace material?
Selecting the best material for an aircraft structure or engine component is
an important task for the aerospace engineer. The success or failure of any
new aircraft is partly dependent on using the most suitable materials. The
cost, flight performance, safety, operating life and environmental impact
from engine emissions of aircraft is dependent on the types of materials
that aerospace engineers choose to use in the airframe and engines. It is
essential that aerospace engineers understand the science and technology
of materials in order to select the best materials. The selection of materials
for aircraft is not guesswork, but is a systematic and quantitative approach
that considers a multitude of diverse (and in some instances conflicting)
requirements. The selection of materials is performed during the early design
phase of aircraft, and has a lasting influence which remains until the aircraft
is retired from service.
The key requirements and factors that aerospace engineers must consider
in the selection of materials are listed below and in Table 1.2.
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Introduction to aerospace materials
Table 1.2 Selection factors for aerospace structural materials
Costs
Purchase cost.
Processing costs, including machining, forming, shaping
and heat treatment costs.
In-service maintenance costs, including inspection and
repair costs.
Recycling and disposal costs.
Availability
Plentiful, consistent and long-term supply of materials.
Manufacturing
Ease of manufacturing.
Low-cost and rapid manufacturing processes.
Density
Low specific gravity for lightweight structures.
Static mechanical
properties
Stiffness (elastic modulus).
Strength (yield and ultimate strength).
Fatigue durability
Resistance against initiation and growth of cracks from
various sources of fatigue (e.g. stress, stress-corrosion,
thermal, acoustic).
Damage tolerance
Fracture toughness and ductility to resist crack growth
and failure under load.
Notch sensitivity owing to cut-outs (e.g. windows),
holes (e.g. fasteners) and changes in structural shape.
Damage resistance against bird strike, maintenance
accidents (e.g. dropped tools on aircraft), impact from
runway debris, hail impact.
Environmental durability Corrosion resistance.
Oxidation resistance.
Moisture absorption resistance.
Wear and erosion resistance.
Space environment (e.g. micrometeoroid impact,
ionizing radiation).
Thermal properties
Thermally stable at high temperatures.
High softening temperatures.
Cryogenic properties.
Low thermal expansion properties.
Non/low flammability.
Low-toxicity smoke.
Electrical and magnetic
properties
High electrical conductivity for lightning strikes.
High radar (electromagnetic) transparency for radar
domes.
Radar absorbing properties for stealth military aircraft.
Cost. The whole-of-life cost of aerospace materials must be acceptable
to the aircraft operator, and obviously should be kept as low as possible.
Whole-of-life costs include the cost of the raw material; cost of processing
and assembling the material into a structural or engine component; cost of
in-service maintenance and repair; and cost of disposal and recycling at the
end of the aircraft life.
Availability. There must be a plentiful, reliable and consistent source of
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Introduction to aerospace materials
13
materials to avoid delays in aircraft production and large fluctuations in
purchase cost.
Manufacturing. It must be possible to process, shape, machine and join the
materials into aircraft components using cost-effective and time-efficient
manufacturing methods.
Weight. Materials must be lightweight for aircraft to have good manoeuvrability,
range and speed together with low fuel consumption.
Mechanical properties. Aerospace materials must have high stiffness, strength
and fracture toughness to ensure that structures can withstand the aircraft
loads without deforming excessively (changing shape) or breaking.
Fatigue durability. Aerospace materials must resist cracking, damage and
failure when subjected to fluctuating (fatigue) loads during flight.
Damage tolerance. Aerospace materials must support the ultimate design
load without breaking after being damaged (cracks, delaminations, corrosion)
from bird strike, lightning strike, hail impact, dropped tools, and the many
other damaging events experienced during routine operations.
Thermal properties. Aerospace materials must have thermal, dimensional and
mechanical stability for high temperature applications, such as jet engines
and heat shields. Materials must also have low flammability in the event
of aircraft fire.
Electrical properties. Aerospace materials must be electrically conductive
to dissipate the charge in the event of lightning strike.
Electromagnetic properties. Aerospace materials must have low electromagnetic
properties to avoid interfering with the electronic devices used to control
and navigate the aircraft.
Radar absorption properties. Materials used in the skin of stealth military
aircraft must have the ability to absorb radar waves to avoid detection.
Environmental durability. Aerospace materials must be durable and resistant
to degradation in the aviation environment. This includes resistance against
corrosion, oxidation, wear, moisture absorption and other types of damage
caused by the environment which can degrade the performance, functionality
and safety of the material.
1.5
Summary
The materials used in aircraft have a major influence on the design, manufacture,
in-service performance and maintainability. Materials impact on virtually
every aspect of the aircraft, including cost, design options, weight, flight
performance, engine power and fuel efficiency, in-service maintenance and
repair, and recycling and disposal at the end-of-life.
Understanding the materials used in aircraft relies on understanding both the
science and technology of materials. Materials science involves studying the
effects of structure and composition on the properties. Materials technology
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Introduction to aerospace materials
involves understanding how the material properties can be used to achieve
the in-service performance requirements of a component.
Although there are over 120 000 materials, less than about 100 different
materials are used in the airframe and engines of aircraft. The four major
types of structural materials are aluminium alloys, fibre–polymer composites
(particularly carbon fibre–epoxy), titanium alloys and high-strength steels;
these materials account for more than 80% of the airframe mass in most
commercial and military aircraft. An important high temperature material
for jet engines is nickel-based superalloy. Other materials are used in the
airframe or engines in small amounts, and include fibre–metal laminates,
ceramic matrix composites, magnesium alloys and, in older and light aircraft,
wood.
Selection of the best material to meet the property requirements of an
aircraft component is critical in aerospace engineering. Many factors are
considered in materials selection, including whole-of-life cost; ease of
manufacturing; weight; structural efficiency; fatigue and damage tolerance;
thermal, electrical, electromagnetic and radar absorption properties; and
durability against corrosion, oxidation and other damaging processes.
1.6
Further reading and research
Askeland, D. R. and Phulé, P. P., The science and engineering of materials, Thomson,
2006.
Ashby, M. F. and Jones, D. R. H., Engineering materials 1: an introduction to their
properties and applications, Butterworth–Heinemann, 1996.
Barrington, N. and Black, M., ‘Aerospace materials and manufacturing processes at the
millennium’, in Aerospace materials, edited B. Cantor, H. Assender and P. Grant,
Institute of Physics Publishing, Bristol, 2001, pp. 3–14.
Peel, C. J. and Gregson, P. J., ‘Design requirements for aerospace structural materials’,
in High performance materials in aerospace, edited H. M. Flower, Chapman and
Hall, London, 1995, pp. 1–48.
© Woodhead Publishing Limited, 2012