ENGLISH for ENGINEERS

Transcription

1 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

2 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 H 2 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

3 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

4 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.

5 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. H 2 containment cylinder The final application of PMCs considered is a H 2 containment cylinder in an advanced fuel vehicle. Such a vessel is designed to contain compressed H 2 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 H 2 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

6 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 H 2 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 H 2 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 H 2 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.

7 References [1] C. Soutis, Carbon fiber reinforced plastics in aircraft construction, Materials Science and Engineering, 2005, pp [2] N. Marks, Polymeric-based composite materials, High Performance Materials in Aerospace, H.M. Flower, ed., 1995, pp [3] Aircraft Structures Summary, Aerospace Students. Available: [4] Case Study of Aircraft Wing Manufacture, Available: [5] F.S. Silva, Fatigue on engine pistons A compendium of case studies, Engineering Failure Analysis, 2006, pp Available: [6] Piston and piston rings, University of Windsor. Available: 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 Available: rbine%20disc%20using.pdf [8] High temperature performance composites. Available: [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 Research. Available: [10] C. Eady, Modes of gas turbine component life consumption. Available: [11] Fundamentals of gas turbine engines. Available: [12] B.D. James et al, Comparison of Onboard Hydrogen Storage for Fuel Cell Vehicles, Ford Motor Company and US Department of Energy, May Available: