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ENGLISH for ENGINEERS
This is an example of a case study written by an MSc student.
Students had been asked to choose one of six given materials to assess in terms of their structural
performance in relation to:
(A) The lower wing of an aircraft
(B) A high performance automotive piston
(C) The turbine disc of a land-based turbine engine
(D) A high pressure H2 gas containment cylinder for advanced fuel vehicles
Case Study: Polymer Matrix Composites
I.
Introduction
Polymer matrix composites (PMCs) are a class of materials consisting of polymer reinforcements
(fibres) embedded in a surrounding matrix. The material properties of PMCs are highly dependent
on the materials chosen for the fibre reinforcement and matrix, the bonding between them, the
orientation of the fibres within each layer, and the manufacturing method and quality. Although this
makes their design and usage more complex, it is also a major advantage in that PMCs can be
tailored for use in specific applications. The most common fibres used in PMCs today are glass,
carbon, aramid, and boron [2]. Glass fibres are the cheapest but also the weakest while carbon and
boron fibres have high strength and stiffness but come at a higher cost [2]. The matrix materials
used in PMCs can be classified as either thermosets or thermoplastics, each with their own unique
properties. Thermosets are resins that are formed into composite components through irreversible
polymerisation (or curing) and are generally cheaper than thermoplastics. Thermoplastics are
already fully polymerized and exist as solids at room temperature but can be melted and reshaped
which eases the manufacture process [2].
The fibres in a PMC provide properties like in-plane tensile strength and flexural strength while the
matrix provides support when the material is under compressive stress, out-of-plane tensile stress,
or shear stress [1]. The interface between the fibre and matrix also plays an important part in the
overall properties of the material by transferring load between the fibres and the matrix. PMCs are
desirable materials due to their high strength and low density as well as their resistance to fatigue.
However, they also have weaknesses including degradation of their properties when performing in a
hot or wet environment and a manual manufacturing process resulting in low quality control over
output.
In this report, the use of PMCs in four different engineering applications will be examined:
· An aircraft wing
· A high performance automotive piston
· A disc in a turbine engine
· An H2 containment vessel in an advanced fuel car
As this paper focuses on polymer matrix composites, any future use of the word composite will refer
specifically to PMCs.
II.
Aircraft wing
Modern aircraft wings consist of a load carrying spar and a skin around this that provides the
required aerodynamic shape [3]. Aerodynamic lift generated during flight causes shear forces and
bending moments on the wing as well as a torsional force as the wing twists [3]. These loads are
initiated in the skin of the wing, but transferred to the spar which is the main load bearing
component [3]. Wings must be able to withstand the maximum possible load experienced during
flight (i.e. due to a gust of wind or flutter) in addition to the fatigue loading from the cycle of take-off
and landing. Loading is highest at the connection between the wing and fuselage [3] making this
connection equally as important as the wings themselves when it comes to design. Heavy aircrafts
have low fuel efficiency so the ideal material for a wing is lightweight and low cost while still being
able to safely withstand loading. Wings are safety critical components on an airplane and therefore
failure should be non-catastrophic or avoidable by damage monitoring and maintenance.
The high specific strength and stiffness of PMCs make them very desirable materials for aircraft
which is why composite materials are actually used in many modern day planes [4]. The most
common variant of PMC used in the aerospace industry today is carbon fibre/epoxy [2] which can
result in a 20% weight reduction when compared to traditional aluminium alloys [1]. Carbon fibres
can either be polyacrylonitrile (PAN) based or pitch-based. Pitch-based fibres have higher stiffness
and strength but require higher temperatures during processing which makes them more expensive
[2]. The majority of carbon fibre used in aircraft is PAN-based due to the lower cost.
Another advantage of using PMCs in aircraft wings is their ability to withstand fatigue loading. When
PMCs experience cyclic loading, a variety of damage mechanisms occur including matrix cracking,
delamination, and fibre fracture. These mechanisms slowly degrade the material’s stiffness and
strength until failure occurs [1]. Sudden and catastrophic failure of critical airplane components is
unacceptable as this could result in the loss of human life. This makes the slow degradation of
composites to failure under fatigue loading a desirable characteristic for airplane wings. The
toughness (resistance to crack growth) of a composite material is highly dependent on the strength
of the bond between the fibre and matrix. When this bond is strong, cracks will propagate through
both the matrix and fibres quickly resulting in rapid failure. However, this material will have a higher
strength and stiffness as the superior properties of the fibres are transferred into the matrix through
the bond. Alternatively, a weak matrix/fibre bond will result in a much tougher material as crack
growth within the matrix is deflected at the fibres. In this case ultimate failure of the material is a
result of multiple slower damage mechanisms as opposed to a single crack; however, the material
will have lower strength and stiffness as the fibre properties are not transferred to the matrix as
effectively. It is therefore necessary to strike a balance between strength and toughness when it
comes to the matrix/fibre interface.
A final advantage of composites in wing design is the ability to form complex shapes that aren’t
possible with existing metal manufacturing techniques. Using specially shaped moulds, complex
optimised aerodynamic shapes for the wings can be created [1]. It should be noted that when
creating complex geometries, automated lay up is not yet possible. Additionally, the use of woven
fibres makes manufacture easier; however woven fibres are not as strong as unidirectional fibres [2].
Weaknesses of PMCs in an aircraft wing application include lack of visible damage as well as
sensitivity to notches and holes required for connecting the various components. Maintenance and
inspection account for a significant proportion of the cost of owning and operating an airplane [3].
In traditional metal wings, fatigue cracks can be identified and their growth monitored by visual
inspection. In PMCs, damage often consists of delaminations, matrix cracking, and broken fibres
within the material that are not visible on the surface. Advanced techniques like radiography,
ultrasonics, acoustics, and thermography [2] are required to detect and monitor such damage which
increases maintenance costs for the plane. In addition, drilling holes in PMC components for bolts
and rivets provides sites for damage initiation and can cause premature failure. In an airplane wing,
the connection between the wing and the fuselage is heavily loaded in shear [3] and must not fail
under stress. The necessity of creating notches in a composite wing to attach it to the fuselage can
weaken the material and potentially cause failure.
III.
Automotive piston
High performance automotive pistons are an essential part of a combustion engine. They convert
energy from fuel combustion into mechanical energy by sliding back and forth within the engine
cylinders [6]. Modern engines can reach temperatures up to 400°C so it is essential that the pistons
can operate at these temperatures as well as having good thermal conductivity to transfer the heat
generated within the cylinders to the cooling system [5]. In addition, the pistons must be able to
withstand pressures up to 20MPa and high frequency cyclic loading [5]. Due to the speed at which
the pistons move (up to 25m/s [5]) it is also a benefit to minimise their weight and therefore their
inertial forces.
The use of PMCs in automotive pistons is limited by the high operating temperature within the
engine cylinders. The stiffness and strength of a composite matrix are degraded above the glass
transition temperature which is less than 200°C for most matrix materials [1]. This leaves the PMC
unable to withstand the same loads it could at a lower temperature. Thermoplastic matrices are less
effective than thermosets for high temperature performance, as the matrix will soften and melt as
the temperature is raised. Thermosets, on the other hand, can usually operate safely up to their
curing temperature under dry conditions. Many thermoset epoxies cure between 120 °C and 180°C
but special matrices for high temperature applications can cure at temperatures up to 350°C [1].
Unfortunately, with modern technology good performance of the matrix cannot be expected at
400°C. For high temperature applications, metals and ceramics perform much better than polymer
matrix composites [8]. Another important aspect of PMCs that make them unsuitable for this
application, is their poor thermal conductivity [2]. Pistons must absorb the heat generated from
combustion within the cylinders and transfer it to the cooling system in order to manage the overall
temperature within the engine. PMCs thermal conductivity is too low to successfully transfer the
required heat.
In terms of withstanding the high pressures within the cylinders and minimising the inertia of the
pistons as they move through the cylinder, PMCs are a good material choice. As discussed for the
wing application, PMCs have high specific strength and stiffness so PMC pistons would have low
mass as well as the load bearing capabilities necessary in the engine. In the future, the discovery of
new matrix materials that can operate at higher temperatures and conduct heat more effectively
could make PMCs a viable choice for engine pistons.
IV.
Turbine engine disc
In land-based turbine engines, air enters through an inlet at low temperature and pressure, is
compressed to a high temperature and pressure in the centre where combustion takes place, and
then returns to low temperature and pressure as it exits the engine [11]. The fuel ignition at the
centre of the engine powers a series of bladed discs that operate under extreme conditions of
temperatures and pressure and rotate at high angular velocity [11]. Design of a turbine disc must
consider high and low cycle fatigue, thermo-mechanical stresses, corrosion, and wear as well as
external factors like a strike by a foreign object [10]. Additionally, safety in the event of a failure is
an important aspect of turbine discs and the engine as a whole.
Operating temperatures for the turbine discs can exceed 1000°C [11] which essentially rules out the
use of PMCs for this application. Matrix materials that can operate at these temperatures without
seeing a serious degradation in their mechanical properties have not yet been discovered. However,
assuming that a suitable high temperature matrix is discovered in the future, the suitability of PMCs
for the other requirements of a turbine disc can be assessed. It should be noted that if PMCs can be
used at high temperatures in the future, special attention must be paid to the coefficient of thermal
expansion of the fibres and matrix to avoid damage or weakening of the composite due to internal
stresses.
One of the most important qualities of a turbine disc is the ability to withstand both low and high
cycle fatigue. PMCs have higher fatigue endurance than metals due to their complex failure
mechanisms [2]. In metals, a crack forms and propagates leading to eventual failure of the specimen.
In a properly designed composite cracks that form in the matrix are redirected when they hit a fibre
leading to a number of different damage mechanisms that slowly weaken the specimen eventually
leading to failure. The energy required to generate the damage in the composite is greater than that
required for the propagation of a single crack in a metal which makes composites more tolerant to
fatigue than metals. The gradual failure of composites is also an advantage in terms of safety as
catastrophic failure during operation is less likely to occur. A final advantage of composites used as
turbine discs is that they are not affected by corrosion [2].
One potential disadvantage of PMCs in turbine engine discs is the attachment of blades to the disc.
Traditional metal discs have notches where the blades are attached so that damaged or broken
blades can be replaced individually at a lower cost than the entire disk. Notches in PMCs create
damage initiation sites where matrix and fibres are exposed which can significantly decrease the
fatigue life of the material [1]. The sensitivity of composites to notches also increases with
temperature. If turbine discs were to be manufactured from composites, the design would most
likely need to be adapted to use bonding to join the disc and the blades or to manufacture the entire
part as a single component.
V.
H2 containment cylinder
The final application of PMCs considered is a H2 containment cylinder in an advanced fuel vehicle.
Such a vessel is designed to contain compressed H2 at room temperature that is then used to power
the vehicle [12]. Important design considerations are the ability of the cylinder to withstand hoop
stresses due to the internal pressure and the safety of the system in extreme or fatigue failure [12].
Additionally, the size and weight of the cylinder should be minimised for optimal fuel efficiency.
According to a study funded by the Department of Energy in the US, a storage cylinder for a midsized vehicle should be able to hold 6.8kg of H2 while keeping the system weight under 47.6kg [12].
PMCs are an ideal material to meet these requirements as high stiffness and strength can be
achieved at low weights compared to other materials. As with airplane wings, carbon fibre with an
epoxy matrix is the preferred material due to its balance of strength and stiffness with cost [12].
Additionally, in the case of pressure vessels, glass and aramid fibres are subject to failure due to
creep while carbon is not [12]. The stress in a gas containment cylinder is almost entirely tensile
stress in the hoop direction. PMCs can be tailored for cases where loading is highest in a single
direction by orienting the fibres in the direction of the stress (which in this case means “wrapping”
them around the cylinder).
Safety is a critical issue with H2 vessels in cars as they could be subjected to impact loading in the
event of a crash. Hydrogen is highly flammable so it should always be safely contained within the
cylinder. Carbon fibre/epoxy composites will be able to withstand higher impacts than the car itself
[12] which is mostly made from metal and plastic so the vessel is likely to remain intact in the case of
an accident. However, PMCs are susceptible to barely visible impact damage in which extensive
damage within the material is not visible on the surface. After an accident, it would be important to
inspect the damage in the cylinder using radiography, ultrasonics, acoustics, or thermography [2].
Manufacturing is often a weakness when it comes to PMCs due to the high cost in terms of time and
labour and the inconsistency due to the manual nature of the process. However, in the case of the
H2 cylinder this is not the case. Filament winding can be used to manufacture the vessels, which is a
cheaper, faster, and more automated process than the traditional lay up process [2]. This is an
additional advantage to using composite materials in this application.
VI.
Conclusions
Polymer matrix composites are a versatile class of materials that can function successfully in a
variety of applications. They are particularly well suited for applications where high strength is
required with low mass, where loading of a component is known to be primarily in one direction, or
where high fatigue resistance and non-catastrophic failure are required. Such applications include
aircraft wings and H2 containment vessels where carbon fibre/epoxy materials are already in use.
Polymer matrix composites are not suitable for high temperature applications as current matrix
materials lose their reinforcing properties at higher temperatures resulting in an overall weakening
of the material. The manufacturing process for PMCs is currently cost and labour intensive as well as
lacking automation which results in poor quality control over the components produced. However,
as manufacturing processes progress and new matrix materials are created PMCs are likely to
become an essential material in many more engineering applications.
References
[1] C. Soutis, “Carbon fiber reinforced plastics in aircraft construction,” Materials Science and
Engineering, 2005, pp. 171-176.
[2] N. Marks, “Polymeric-based composite materials,” High Performance Materials in Aerospace,
H.M. Flower, ed., 1995, pp. 202-226.
[3] “Aircraft Structures Summary,” Aerospace Students. Available:
http://aerostudents.com/files/aircraftStructures/aircraftStructuresFullVersion.pdf
[4] “Case Study of Aircraft Wing Manufacture,” 2003. Available:
http://www.oup.com/us/static/companion.websites/9780195157826/Chapter_19.pdf
[5] F.S. Silva, “Fatigue on engine pistons – A compendium of case studies,” Engineering Failure
Analysis, 2006, pp. 480-492. Available: http://mea.pucminas.br/palma/fca-art2.pdf
[6] “Piston and piston rings”, University of Windsor. Available:
http://courses.washington.edu/engr100/Section_Wei/engine/UofWindsorManual/Piston%20and%2
0Piston%20Rings.htm
[7] R.A. Claudio, C.M. Branco, E.C. Gomes, and J. Byrne, “Life prediction of a gas turbine disc using
the finite elemento method,” Joranadas de Fractura, 2002, pp. 131-144. Available:
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rbine%20Disc%20using.pdf
[8] “High temperature performance composites.“ Available:
http://www.unitedcomposites.net/usapages/hightempperformancecomposites2.htm
[9] K.C. Sheth, R. R. Gallucci, and G. Haralur, “Thermoplastic polyimide – polyetheretherketone
blends with exceptional high temperature properties,” Savic Innovative Plastics and GE Global
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[10] C. Eady, “Modes of gas turbine component life consumption.“ Available:
http://ftp.rta.nato.int/public//PubFulltext/RTO/TR/RTO-TR-028///TR-028-04.pdf
[11] “Fundamentals of gas turbine engines. “ Available: http://www.castsafety.org/pdf/3_engine_fundamentals.pdf
[12] B.D. James et al, “Comparison of Onboard Hydrogen Storage for Fuel Cell Vehicles,” Ford Motor
Company and US Department of Energy, May 1996. Available:
http://www.directedtechnologies.com/publications/storage/H2Storage.pdf