Chapter 25: Materials selection for aerospace – Introduction to Aerospace Materials

25

Materials selection for aerospace

25.1 Introduction

The process of selecting materials to be used in the airframe and engine is an important event in the design of aircraft. The material used can be just as important as the design itself; there is little point creating a well-designed structural or engine component if the material is unsuitable. The key objective of materials selection is to identify the material that is best suited to meet the design requirements of an aircraft component. Selecting the most suitable material involves seeking the best match between the design requirements of a component and the properties of the materials that are used in the component. There are an extraordinarily large number of requirements for the materials used in aircraft. For example, Fig. 25.1 shows just a few of the key requirements for different sections of an airliner. Each component must be carefully analysed for its main property requirements in order to select the best material.

25.1 Examples of design requirements for the fuselage and empennage of the Airbus 380.

Materials selection in aerospace involves one of two situations: the selection of either so-called revolutionary or evolutionary materials (as described in chapter 2). Revolutionary materials selection involves selecting a material that has not been used previously in aircraft, such as the first-time application of GLARE to the Airbus A380. The selection of revolutionary material may also involve choosing an existing material for a new application, such as the first-time use of carbon fibre–epoxy composite in the fuselage of the Boeing 787 airliner. Materials selection of an evolutionary material involves selecting an existing material for an application where it has been used before. It can also involve using a material that has been improved slightly from an existing type, such as a new aluminium alloy with a slightly modified alloy content or heat treatment compared with aluminium alloy used previously. The selection of a revolutionary material is usually done to achieve a large improvement in one or more aspects of the aircraft, such as a large reduction in cost or weight or a substantial improvement in fatigue life or damage resistance. Selecting an evolutionary material often results in a smaller improvement in performance, but using the material results in less risk. Regardless of whether new or existing materials are chosen, the process by which the best material is selected is the same.

The process of selecting a material that best meets the design requirements involves considering many factors such as cost, ease of manufacture, structural performance, and operating life. Other considerations can include the space and volume available for the component, the operating environment and temperature, and the number of parts to be produced. The most efficient design for an aircraft structure or engine is achieved by identifying early in the design phase those properties of the material that are most essential to achieving the design requirements. Materials are then selected based on their ability to meet these requirements.

The factors considered in materials selection can be roughly divided into the following categories:

• structural properties,

• economic and business factors,

• manufacturing issues,

• durability in the aviation environment,

• environmental impact, and

• specialist properties.

Table 25.1 lists the material properties most often considered in the design of aircraft structures and engines.

Table 25.1

Design-limiting properties used in materials selection

Class Property
Structural Density
Elastic modulus (tension, shear, etc.)
Strength (yield, ultimate, fracture)
Impact damage resistance
Hardness
Fracture toughness (damage tolerance)
Fatigue (life, strength)
Creep resistance (creep rate, stress rupture life)
Economic and business Cost (raw material, processing, maintenance)
Availability
Service life
Regulatory issues
Manufacturing Fabrication and casting (formability, machinability, welding)
Dimensional (shape, surface finish, tolerances, flatness)
Number of items
Nondestructive inspection for quality assurance
Environmental durability Corrosion rate
Oxidation rate
Moisture absorption rate
Environmental impact Sustainability
Greenhouse and other emissions during manufacture
Recycling
Waste disposal (hazardous)
Health hazards (carcinogens, flammable)
Specialist Thermal conductivity
Electrical conductivity
Thermal expansion
Thermal shock resistance
Stealth (electromagnetic absorbance/infrared)

Seldom is a single material able to provide all of the properties required by an aircraft structural or engine component. The selection of materials is a complex process involving many considerations and it frequently requires compromises by accepting some disadvantageous properties (such as increased cost) in order to attain beneficial properties (such as reduced weight or increased strength).

In this chapter, we discuss the main factors and properties considered in the selection of materials for aircraft structures and jet engines. We examine the structural, business and economic, manufacturing, durability, environmental impact, and specialist properties that are considered in materials selection. In addition, there is an introduction to the process and methodology that aerospace engineers use to select the most appropriate materials for new designs or the modification of existing designs.

25.2 Materials selection in design

Selecting the right material for an aircraft component can seem an overwhelming task because there are over 100 000 materials available. There may appear to be too many choices from which to find the one material that is best suited to meet the design requirements of the component. However, materials selection is not a random, chaotic process in which the engineer is expected to ‘stumble across’ the best material among an immense variety of choices with little or no guidance. Materials selection is an ordered process by which engineers can systematically and rapidly eliminate unsuitable materials and identify the one or a small number of materials which are the most suitable.

The process of selecting materials and how they are then manufactured into an aerospace component is intimately linked to the design process. The design process involves several major stages performed in the sequence shown in Fig. 25.2. The process begins with a detailed assessment of the market need for a new aircraft or the modification of an existing aircraft type. Important issues are assessed in this stage to establish the economic viability for commercial aircraft and the war-fighting requirements for military aircraft. Answers are sought to key questions that must be addressed in the market assessment. What are the required range, speed, passenger capacity and payload for the commercial airliner? What is the required fuel economy of the aircraft, and are there environmental impact limits for greenhouse gas emissions and noise? What are the roles and functions of the new military aircraft? Does the aircraft require stealth and other covert capabilities? How many aircraft are required, and what is their anticipated operating life? Analysis of the market need is not performed by the engineer alone; a team is involved that includes aerospace design engineers, manufacturing engineers, materials engineers, financial analysts and possibly customer representatives. The team then proceeds onto the design of the aircraft using the information obtained in the market analysis.

25.2 Major stages of design and their relationship to materials selection. (adapted from G. E. Dieter ‘Overview of the materials selection process’, ASM handbook volume 20: materials selection and design. American Society of Materials International, Ohio, 1997)

Following the assessment of market need, the design process proceeds in three main stages: concept design, embodiment design, and (lastly) detailed design. Materials selection occurs in all three stages, but is fluid at the concept design stage where a large number of materials might be considered as candidates, and as the design progresses towards completion the selection process becomes more focused towards a single material. As the materials selection process advances through the design stages it does not always lead to a ‘correct’ solution, although the choice of some materials is clearly better than others. Rarely does a single material meet all the design requirements, and often two or more materials are closely matched and would be equally suitable. In this respect, materials selection is different to other problems in aerospace engineering such as aerodynamics and structural analysis, which generally involve a single, correct answer.

Although there is no universally accepted definition of concept design, for the purpose of materials selection, it involves describing how a new product will be configured to meet its design objectives and performance requirements. Another useful definition of concept engineering is the process of translating customer needs (based on market research) to design features and measurable performance parameters. For complex designs, the basic configuration is determined at the concept stage, but all the minor design details may still remain unknown. The functional requirements of the design are determined at the concept stage, but the properties of the materials are not. The design requirements for a component specify what it should do but not what properties its materials should have nor how it will be made. Consider the wing design for a new type of stealth fighter; the functions would include the provision of lift and low radar visibility. These and other functions are determined at the concept design stage, but the materials and manufacturing process used to construct the wing are still not known. However, important questions related to the properties of the wing material are identified:

• Does the wing material require a radar absorption value above a specified limit?

• Will the wing experience heating during supersonic flight?

• Are any special properties required, such as ballistic protection or impact strength?

At the concept design stage, the options for materials are wide and essentially all types are considered. As the concept design develops, the choice is often made about the general class of material, such as whether it is to be metal alloy or composite, but the exact type of metal or composite is not known. If an innovative choice of material is to be made, such as constructing the wing using a new type of composite material with improved radar absorption properties, then it must occur during the concept design stage. Choosing an innovative material later in the design process is often too late because too many other decisions about the design have been made to allow for a radical change.

The embodiment stage of design involves determining the shape and approximate size of the product. The loads exerted on the component and environmental operating conditions are assessed in greater detail than during concept design. During embodiment design, the material properties important to the design are identified, such as cost, weight, strength and corrosion resistance. These properties are then ranked in order of importance. Once the design properties have been identified and ranked then a specific class of material is chosen; for example a variety of titanium alloys or steels. The properties of the candidate materials must be known to a high level of precision at this stage.

Once this is complete, the design process proceeds to detailed design which involves completing all the design details and then converting the design to specifications (e.g. dimensions, tolerances, materials, surface finish) and accompanying documentation. At this level, the decision is narrowed to a single type of material whose properties best match the design requirements.

25.3 Stages of materials selection

The design process from concept design to detailed design involves the progressive culling from a large number of material choices towards a single material, but it does not explain how the selection is made. The process of materials selection involves four main steps in the order: translation, screening, ranking and (finally) supporting information (Fig. 25.3).

25.3 The four steps of materials selection: translation, screening, ranking, and supporting information. (adapted from M. Ashby. Materials selection in mechanical design, Butterworth–Heinemann, Massachusetts, 1999)

25.3.1 Translation

Materials selection begins with translation, which involves examining the functions and objectives of the design. The functions define what the component is designed to do. For instance, any aerospace component used in the airframe or engine has one or more functions: to support a given stress level; to support a load at a given temperature; and so on. The objectives define what aspects of the design need to be maximised or minimised; such as maximum strength for minimum weight or greatest corrosion resistance for minimum cost.

The objectives are subject to a set of constraints, which are the conditions of the design that must be met and cannot be adjusted. As examples, the constraint may be that the component has to be within a certain size (e.g. aircraft landing gear); that the component must operate at high temperature (such as 800 °C) without softening and plastically deforming (e.g. turbine blade); that the component must withstand a specified number of loading (fatigue) cycles without cracking (e.g. helicopter rotor blade); and that the component must survive a bird impact of a defined weight and speed without causing damage (e.g. inlet blade to turbine engine). Examples of common objectives and constraints are given in Table 25.2.

Table 25.2

Common objectives and constraints applied in the translation stage of materials selection for aircraft

The approach for selecting materials is shown by two examples in Fig. 25.4: an aircraft undercarriage and a gas turbine engine. The list of requirements that the material must meet are expressed as objectives and constraints. The objectives for the landing gear include minimum weight and volume and being structurally reliable. The objectives for the engine include a high thrust-to-weight ratio and the ability to produce low levels of greenhouse gases and operate using conventional jet fuel. The outcome of the translation step is a list of constraints expressed as design-limiting properties that must be met by the material. For example, the Young’s modulus and strength must exceed limiting values whereas the rates of corrosion and creep must be under specified limits. Based on the objectives and constraints, the materials selection process moves to the next stage of screening.

25.4 Selecting materials for the (a) undercarriage of a commercial airliner and (b) gas turbine engine. The objectives and constraints are listed on the left side and the properties used to screen and rank the materials are given on the right side.

25.3.2 Materials screening

Screening involves eliminating those materials whose properties do not meet the design constraints. The constraint defines an absolute upper or lower limit on property values, and materials that do not meet the limiting value are screened out. No trade-off beyond this limit is allowed. For example, in Fig. 25.4, one constraint applied to the landing gear material is that the yield strength must be above 1000 MPa, and any material with lower strength is eliminated from the selection process. In addition to constraints on the mechanical properties, other constraints may be applied related to economic/business, manufacturing and environmental factors as well as specialist properties. These can also be used to screen out materials. A description of the major factors and properties considered in materials selection is provided later in this chapter.

25.3.3 Material indices

Once the screening process is complete, the materials that pass are then ranked in the order that they surpass the design constraint limits. How well a material exceeds the constraint limit is quantified using a material index. In other words, a material index measures how well a candidate material that has passed the screening step can do the job required by the component. There are many material indices, each associated with maximising or minimising some property value, such as maximum strength per unit weight or minimum manufacturing cost per unit product.

Equations for calculating the index values for stiffness, strength and cost for different design shapes are given in Table 25.3. Other equations are used for calculating index values for properties such as thermal shock resistance, vibration damping and so on; some of which are given in Table 25.4. Equations such as these are used to calculate the index value for each candidate material that passes the screening stage, and then the materials are ranked in order of excellence. It is also important to consider at this stage whether the materials can be fabricated into the component. There is no point ranking a material if it cannot be cost-effectively processed into the final product.

Table 25.3

Indices for stiffness and strength-limited design

E: Young’s modulus; G: shear modulus; σy: yield strength; τy: yield shear strength; ρ: density; and Cm: cost per unit weight.

Table 25.4

Selected material indices

Design objective Material index
Maximise thermal insulation 1/k
Minimise thermal distortion k/α
Thermal shock resistance σf/Eα
Maximise damage tolerance (beam, plate, etc.) KIc and σf
Maximum pressure vessel strength KIc/σf

E: Young’s modulus; k: thermal conductivity; α: thermal expansion coefficient; σf: failure strength; and KIc: fracture toughness.

25.3.4 Supporting information and final selection

Once the shortlist of candidate materials are ranked in order of excellence using the index values, the final stage of the selection process is performed, and this involves the use of supporting information for a detailed profile of each material. Supporting information involves important factors other than the material properties that are relevant to the design, such as previous uses of the material in similar applications; the availability of the material; whether the company has prior manufacturing experience with the materials; certification issues associated with the material (i.e. has it been previously certified by aviation regulators); whether the material has any special handling requirements or poses occupational health and safety problems during manufacturing; whether the material can be recycled; and so on. Many sources, including databases and case histories, are used to collect as much information as possible about each material. The supporting information is analysed for each candidate material, and from this the final material is selected.

25.4 Materials property charts

The process of screening and ranking materials on their properties can be exhaustive when an extremely large number of materials are under consideration. With over 120 000 materials being available, the task of individually assessing each material against the objectives and constraints of the design is not practical. Material property charts are used to rapidly screen out large numbers of materials and to identify those materials that meet the property constraint. Material property charts plot two properties, as shown for example in Fig. 25.5 for Young’s modulus against density and for strength against fracture toughness. These charts condense a large body of property data into a compact yet accessible and easy to understand form. The charts also reveal correlations between material properties, such as the relationship between strength and toughness or between elastic modulus and density capacity.

25.5 Examples of material property charts: (a) Young’s modulus–density and (b) strength–toughness. (provided courtesy of the Department of Engineering, University of Cambridge)

The properties of materials have a characteristic range of values, although for some properties this range can be large (covering up to five or more orders of magnitude). For instance, the Young’s modulus for all metals is in the range of about 10 to 300 GPa and the density is between around 1300 and 20 000 kg cm−3. With polymers, however, their modulus spans 0.08 to 10 GPa and density covers 800 to 2000 kg m−3. When the property values for the various groups of materials are plotted they cluster together with, for example, the modulus–density combination of metal alloys forming a distinct grouping from ceramics, glasses, polymers and composites. A similar situation is found for strength–toughness and many other property combinations. A boundary is drawn around each group of materials and, for this reason, the graphs are also called bubble charts. These charts provide a global view of the relative performance of different classes of materials.

Material property charts are used to screen materials. The design constraint limits are plotted on the chart, and those materials that do not meet the limits can be eliminated from the selection process. For instance, Fig. 25.6 shows a material property chart whereby the limits of specific stiffness (Young’s modulus/density) of 300 MN m kg−1 or specific strength (strength/density) of 100 kN m kg−1 have been imposed. All materials in the window defined by the limit labelled ‘passed region’ meet both these constraints, and can be considered further. Materials that fall outside this region can be eliminated from the selection process.

25.6 Example of the use of a materials selection chart for screening. Only materials that exceed the constraint limits of specific modulus > 300 MNm kg−1 or specific strength > 100 kNm kg−1 are considered, and all other materials (within the shaded region) are eliminated. (modified from chart provided courtesy of the Department of Engineering, University of Cambridge)

25.5 Structural properties in materials selection

25.5.1 Density

The use of lightweight materials together with optimised design has always been the most effective way of reducing the structural mass of aircraft. The use of low-density materials on their own does not necessarily provide a large weight saving; it must be combined with design methods for reducing mass to be fully effective. Similarly, lightweight design on its own does not reduce the mass significantly unless it includes the use of low-density materials.

The structural mass of most aircraft is within the range of 20 to 40% of the take-off gross weight (Fig. 3.1). Therefore, using low-density materials in the airframe translates to a large saving in the overall aircraft weight. For example, the structural mass of a B737-NG is about 22 500 kg (depending on the exact aircraft type), and substituting all of the aluminium alloy with a density of 2.7 g cm−3 with slightly lighter carbon fibre–epoxy composite with a density of 2.0 g cm−3 in principle provides a weight saving of around 3500 kg (or about 15% of the total mass). On a smaller scale, the use of composite material in the front fan case and fan blades of a gas turbine engine for a mid-sized airliner can reduce the mass by 100–200 kg.

As an approximation, reducing the structural mass by 1 kg on a mid-sized airliner provides about another 1 kg reduction in aircraft weight through the use of smaller engines to maintain the same airspeed as well as smaller wings to keep the same wing loading. A strong incentive exists to achieve even a modest reduction in the weight using light materials. It is estimated that, for every 1 kg saved on an averaged-sized airliner, the fuel consumption can be reduced by about 800 l year−1. Therefore, a large reduction in structural weight translates to a substantial reduction in fuel burn with corresponding reductions in fuel cost, greenhouse and other gas emissions. Figure 25.7 shows the projected reductions in fuel consumption and gas emissions by lowering the percentage structural mass of a typical mid-sized airliner through the use of light materials.

25.7 Approximate reductions in annual fuel consumption and carbon dioxide (greenhouse) gas emission with percentage reduction to the weight of a mid-sized passenger aircraft.

Reducing the density of structural materials is recognised as the most efficient way of reducing airframe weight and improving performance. It has been estimated that reducing the material density is anywhere from 3 to 5 times more effective than increasing the tensile strength, modulus and fracture toughness of the material. Figure 25.8 shows the effect of improvements to various properties of a material on the structural weight change. A small reduction in the density of the material is far more effective in reducing the structural weight than increasing the mechanical properties, and this is the main reason why light metal alloys and composites are used extensively in aircraft.

25.8 Effect of property improvement on the structural weight. (reproduced from F. H. Froes et al., Proceedings of the International Conference on Light Materials for Transportation Systems, Korea, 1993)

25.5.2 Structural efficiency

Many mechanical properties are considered when selecting materials for their structural efficiency, which means the mechanical performance of a material per unit weight. Properties such as elastic modulus, strength, fatigue life and fracture toughness are important in the selection of materials for both airframe structures and engines.

Stiffness is an important design constraint for many aerospace structures because of the need to avoid excessive deformation and buckling. Table 25.5 provides the elastic modulus and stiffness efficiency properties of various aerospace materials. It is usually the structural efficiency (also called specific property), expressed as the mechanical property normalised by the density of the material that is considered in aircraft materials selection rather than the absolute property of the material. This is because some lightweight materials may have relatively low stiffness and strength, but when these properties are normalised by the density they are superior to heavier materials with higher mechanical properties. For example, the elastic modulus of carbon fibre–epoxy composite used in stiffness-critical structures is about 120 GPa, which is less than the modulus of steel at 210 GPa. However, the composite material is around 3.5 times lighter than steel and, therefore, when the specific stiffness (E/ρ) of these materials is compared then the carbon–epoxy is nearly three times greater. The relative improvement in stiffness efficiency is even higher for beams (E/ρ2)and plates (E/ρ3) under bending loads.

Table 25.5

Elastic modulus and structural efficiency of aerospace materials used in stiffness-critical applications Structural efficiency

Quasi-isotropic carbon-epoxy with 60% by volume of high modulus carbon fibres.

Quasi-isotropic glass-epoxy with 60% by volume of E-glass fibres.

The specific static strength is a key factor in materials selection for aerospace structures. Aircraft components are designed to withstand the maximum operating stress plus a safety factor, which is typically 1.5. The yield strength and strength efficiency of various aerospace materials are given in Table 25.6. The material with the highest strength is high-strength (maraging) steel, which is used in safety-critical components requiring high yield strength such as the undercarriage landing gear and the wing carry-through structure, although it does not have the highest strength efficiency. Titanium alloys and, in particular, carbon–fibre composites have high strength and structural efficiency and are also used in heavily-loaded aircraft components. Fibreglass composites also have high-strength efficiency, but their stiffness efficiency is relatively low.

Table 25.6

Yield strength and structural efficiency of aerospace materials used in strength-critical applications

Quasi-isotropic carbon-epoxy with 60% by volume of high strength carbon fibres;

Quasi-isotropic glass-epoxy with 60% by volume of E-glass fibres.

Damage tolerance is another important property in materials selection, and this defines the ability of a load-bearing structure to retain strength and resist crack growth when a defect or damage is present. Aerospace materials can contain small flaws such as processing defects (e.g. porosity, brittle inclusion particles) or in-service damage (e.g. impact, corrosion), and it is essential that these do not grow rapidly during aircraft operations otherwise it can lead to structural failure. Materials with the greatest damage tolerance generally possess high fracture toughness combined with excellent fatigue resistance defined by a slow rate of fatigue crack growth. Other mechanical properties considered in materials selection can include the structural efficiency (specific stiffness and specific strength) at high temperature, creep resistance, and fatigue performance (crack growth rate, life and residual strength).

25.6 Economic and business considerations in materials selection

An important consideration in the selection of materials is their whole-of-life cost. This cost includes all expenses associated with the material from initial manufacturing to final retirement of the aircraft, and consists of the costs of the raw material, processing and manufacturing, in-service maintenance, repair, and recycling and disposal. The decision on materials selection often comes down to a trade-off between performance and cost. Figure 25.9 shows the typical breakdown of the purchase and operating costs for a fighter aircraft, and the materials account for a small percentage of the total lifecycle cost. Therefore, using relatively expensive materials such as carbon-fibre composite or titanium instead of a cheaper material such as aluminium has little impact on the total lifecycle cost. Costs associated with the maintenance of materials over the operating life of the aircraft are often at least two or three times greater than their initial purchase cost. Therefore, using materials that require less maintenance from in-service damage such as impact, fatigue or corrosion provides significant cost saving. The fuel cost is also a large operating expense, and a small reduction in aircraft weight by using lighter materials can also provide a substantial cost saving, as mentioned.

25.9 Relative distribution of costs for military aircraft. Material costs are a small fraction of the total cost.

In the materials selection process, the cost is not considered in isolation from the other properties required from the material. The cost of the material is assessed against other important properties such as stiffness, strength and corrosion resistance, and a more expensive material may be selected because it has superior properties to a less expensive material. Sometimes, materials that are expensive are justified because they offer a unique property advantage or because they are cheaper to use than other lower-cost materials; for example, a design might be simplified and, thus, made at lower cost.

Figure 25.10 presents a materials selection chart of cost–strength for different groups of engineering materials. The cost of most materials, including the metal alloys and composites used in aircraft structures and engines, vary over a wide range depending on their composition and processing. Expensive metals such as titanium and nickel alloys are preferred over cheaper materials when high specific strength and creep resistance at elevated temperature are required. Similarly, carbon fibre composites are used in aircraft structures rather than the cheaper glass–fibre composites because of their superior stiffness and fatigue strength.

25.10 Materials selection chart for cost–strength. (Provided courtesy of the Department of Engineering, University of Cambridge.)

Another economic consideration in materials selection is that the purchase and maintenance costs of materials can change over time. The prices of raw materials are rarely stable and usually fluctuate up and down in response to supply and demand. Many aircraft types are sold by the aerospace manufacturers over a period of many years and, during this time, the price of raw materials can change considerably. As examples, the Boeing 747 has been in continuous production since 1969 and the production of the Airbus 300 commenced in 1974 and ceased 33 years later. Over these long periods, the price of the raw materials can rise and fall, which means that selecting a material because of its low cost does not always mean it remains cost competitive for the entire production period. Figure 25.11 shows the fluctuations to the price of titanium over a sixty-year period, during which the cost has varied more than 400%. Such large fluctuations in cost are difficult to predict, but must be recognised when selecting materials to be used in large quantities in aircraft construction.

25.11 Average price of sponge titanium per tonne. The price is normalised to the cost of titanium in 1998.

The maintenance cost of the materials used in the airframe and engine can also change with time owing to deterioration from corrosion, fatigue and other factors. The cost of maintaining some types of materials (such as titanium and carbon fibre–epoxy) is usually less than other materials (such as aluminium or magnesium) owing to fewer problems with fatigue or corrosion. The potential to reduce maintenance cost is a consideration in materials selection.

25.7 Manufacturing considerations in materials selection

Selecting the process by which raw materials are manufactured into airframe and engine components is an essential part of materials selection. Manufacturing includes the primary forming processes (e.g. casting and forging of metals and autoclave curing of composites), heat treatment (e.g. thermal ageing, stress-relief annealing), material removal processes (e.g. machining, drilling, trimming), finishing processes (e.g. surface coatings, anodising), joining processes (e.g. welding, fastening, adhesive bonding), and nondestructive inspection (e.g. ultrasonics, radiography).

Materials and manufacturing are closely linked, and it is impossible to select a material without considering how the material is to be manufactured into the final component. The choice of material is dependent on the choice of process by which it is formed, joined, finished and otherwise treated. The material properties affect the choice of process: ductile materials can be forged and rolled, whereas brittle materials such as composites must be processed in other ways. Conversely, the choice of process affects the material properties: forging and rolling change the grain structure, strength and toughness of metals whereas different manufacturing processes for composites result in different fibre contents and therefore different mechanical properties. The manufacturing process can change the material properties (beneficially or adversely), and thus affect the performance of the component in service. Some processes improve the properties, such as heat treatment raising the strength of metals or removing residual stresses, whereas other processes can degrade the properties, such as casting defects in metals or voids in composites. Selecting the best material for an airframe or engine component involves more than selecting a material that has the desired properties, it is also connected with the manufacturing of the material into the finished product and ensuring it meets the quality assurance requirements set by aviation safety regulators.

Selecting the manufacturing process is not an easy task for there are many processes to choose from, each with benefits and limitations. Figure 25.12 shows the classes of processes used for manufacturing with metals and composites, and there are many to choose from at each stage of the production process. The goal is to select the process that maximises the properties and quality of the component and minimises the cost. It is important to select the manufacturing process at an early stage in the materials selection process, otherwise the cost of changing the manufacturing route later can be costly.

25.12 Classes of manufacturing process and process flowcharts for (a) metals and (b) composites. (from M. F. Ashby. Materials selection in mechanical design, Butterworth–Heinemann, Massachusetts, 1999)

The method of selecting the manufacturing process is shown in Fig. 25.13, and is similar in principle to the materials selection process. The starting point is that all processes are considered as possible candidates until proven otherwise. The sequential steps of translation, screening, ranking and search for supporting information are followed to eliminate unsuitable processes and to identify the best process. The translation step involves transforming the design requirements into constraint limits used in selecting the process. Constraints may include the size, shape, material type and processing temperature of the product. Limits are applied to the constraints, such as the process must be capable of making integrated products larger than 2 m or the process must heat treat the product in an inert atmosphere. Screening involves eliminating the processes which do not meet the constraint limits, and the shortlisted processes are then ranked in order of their ability to manufacture the product in terms of cost, batch size and so on. Supporting information is used to help identify the best process, such as the availability of the capital equipment or the technical complexity of the process operation.

25.13 The four steps of manufacturing process selection: translation, screening, ranking, and supporting information. (adapted from M. F. Ashby. Materials selection in mechanical design, Butterworth–Heinemann, Massachusetts, 1999)

An important consideration in the design and manufacture of an aerospace component that is certified by the safety regulators such as the Federal Aviation Administration is the quality assurance of the finished part, often involving NDI. Safety-critical aircraft components for the airframe and engine must be inspected to ensure they are free from manufacturing defects that may cause damage or failure in-service, such as voids and large intermetallic inclusions in metals and porosity and delamination cracks in composites. Parts that are found to contain defects above a certain size or volume fraction must be repaired or scrapped. Most defects are small, and can only be reliably detected using NDI methods such as ultrasonics, radiography and thermography. Some components are easily inspected using certain NDI methods but not others; for instance thick metal components can be inspected for casting voids using ultrasonics but not thermography. Also, certain NDI methods are more suited to some materials than others; such as the application of eddy–current inspection to conducting materials, but it cannot be used on fibre–polymer composites. The shape, dimensions and material used in the component determines the type of NDI method that can be used to inspect for manufacturing quality, and this must be considered as part of the design process.

25.8 Durability considerations in materials selection

The durability of materials in the operating environment of the aircraft is an important consideration in minimising maintenance and extending the service life. The environment may be hot, humid, corrosive, abrasive or some other potentially damaging condition. Both metals and fibre–polymer composites are susceptible to environmental degradation during service, and selecting a material with the best durability is an important consideration. Figure 25.9 shows that maintenance of the airframe and engine accounts for a large percentage (~ 26%) of the whole-of-life cost, and a significant amount of this cost is the expense of inspecting materials for environmental damage and, when detected, repairing or replacing the component.

25.8.1 Corrosion in materials selection

Corrosion is the most common and expensive type of environmental damage to aircraft metals. Composites, polymers and ceramics are not susceptible to corrosion; the problem is mostly with metals. It is estimated the aviation industry spends around $2 billion per year on maintaining metal components damaged by corrosion. Water is the most common cause of corrosion to aircraft metals, although some acidic or alkaline solvents (e.g. paint strippers, cleaning agents) may also cause corrosion. Metals can be damaged in various ways by corrosion, including uniform attack over the entire surface of the material or localised attack causing surface pitting or cracking. Corrosion is an important consideration in materials selection because not only does it weaken aircraft components by removing material, but under stress it can also cause cracking.

The corrosion resistance of metals is dependent on many factors, including the type and concentration of the corrosive agent, temperature, external load, and material parameters such as the type of base metal, types and concentrations of alloying elements, microstructure (grain structure, precipitates), and residual stresses. Figure 25.14 provides an approximate ranking of the corrosion resistance of several materials in salt water, although such comparisons must be used with caution in materials selection. It is difficult to compare the corrosion resistance of candidate materials unless data is available from corrosion tests that are performed under conditions that closely replicate the in-service environment. There are two approaches used to select aerospace materials for corrosion resistance: choose a material which has a high resistance to corrosion or protect the material using a corrosion resistant coating (such as anodised coating for aluminium or cladding for steel).

25.14 Ranking of the resistance of materials to attack by seawater.

25.8.2 Oxidation in materials selection

Oxidation is another type of environmental damage that must be considered in materials selection, particularly materials for high-temperature applications, such as jet and rocket engines. Oxidation is a reaction process between the material and oxidising agents in the atmosphere, such as oxygen in air or sulfur dioxide in the combustion gas of jet fuel. The reaction damages the metal by forming a thick surface layer of brittle metal oxide. Oxidation is not usually a problem for aircraft materials unless they are used for high-temperature applications, when it is essential to select a material with high resistance to oxidation or a material that can be thermally insulated using an oxidation-resistant coating.

25.8.3 Moisture absorption in materials selection

Unlike metals, fibre–polymer composite materials are not susceptible to corrosion and are not used in hot environments where oxidation is a problem. However, composites are not immune to the environment and may be damaged by other ways. Composites are susceptible to environmental damage by absorbing water in the atmosphere. Water molecules are absorbed into the polymer matrix of composites where they cause softening and lower the glass transition temperature. With some types of composites, the absorption of water can cause delamination cracking, fibre/matrix debonding, and damage to the core material (with sandwich materials). Water can also be absorbed by organic fibres used in composites, such as aramid, which further weakens the material. The deterioration of composites by moisture absorption is a consideration in materials selection, particularly when the aircraft is required to operate in tropical regions where the atmosphere is hot and wet. The polymer matrix and organic fibres used in composites may also be degraded by long-term exposure to ultraviolet radiation in sunlight. Consideration of the environmental stability of composites is essential in materials selection, and numerous types of composites are available which are resistant to degradation by moisture and ultraviolet radiation.

25.8.4 Wear and erosion in materials selection

Damage by wear and erosion may be another consideration in assessing the durability properties of aerospace materials. Wear is not usually a serious problem except when materials are used in engines and other moving parts, when selecting a material with high wear resistance is important. Figure 25.15 presents a materials selection chart for the wear rate constant versus hardness. The wear rate constant ka is a measure of sliding wear resistance: low ka means high wear resistance at a given bearing pressure. With a single group of materials, such as metals or polymers, it is generally found that the wear rate constant decreases with increasing hardness. Therefore, selecting a material for high wear resistance is often based on the hardness and yield strength properties. Erosion is a specific type of wear involving the removal of material under impact from abrasive particles such as sand or dirt. Erosion is a consideration for materials used at the external surface of the main rotor blades for helicopters and propeller blades for aircraft. The erosion resistance of materials, like their sliding wear resistance, increases with the surface hardness and strength.

25.15 Materials selection chart for wear rate constant versus hardness. Reproduced from M. F. Ashby. Materials selection in mechanical design, Butterworth–Heinemann, Massachusetts, 1999.

25.9 Environmental considerations in materials selection

Consideration of the environmental impact of materials is fast becoming a key factor in materials selection for aircraft. Whenever possible, materials obtained from sustainable resources and which have low impact on the environment during their production, usage and disposal should be considered. The aerospace industry is keen to use sustainable materials to minimise the environmental impact of aircraft. Sustainability is defined as the ability of a material to be used infinitely, which involves recycling the material at the end-of-life for reuse in new aircraft or some other application. Recycling avoids the need to extract new material from a non-renewable resource such as ore for metals or petroleum products for carbon fibres and polymers. If some material is deemed ‘sustainable’ and cheap to recycle, this is a favourable selection factor, particularly for materials used in high tonnage such as aluminium. The ability to recycle varies considerably among the various aerospace materials, with aluminium and steel being relatively easy to recycle, titanium and magnesium being more difficult to recycle, and composite materials being extremely difficult (if not impossible) to fully recycle.

The aerospace industry is also keen to minimise the so-called ‘carbon footprint’ of materials, which is another consideration in materials selection. In the USA, Europe and many other places, emissions from manufacturing and recycling processes are significant factors to be dealt with. When selecting a material, consideration should be given to whether its use causes the emission of greenhouse gases and other pollutants to the atmosphere. For example, the production of 1 kg of aluminium, which includes the mining, refinement and smelting, requires about 284 MJ of energy and generates about 35 kg of carbon dioxide when the power source is grid electricity. In comparison, the production of 1 kg carbon–epoxy composite, which includes producing the fibres, polymer and manufacturing the material, requires about 40 MJ of energy resulting in about 5 kg of CO2. Other waste by-products from manufacturing processes, such as effluent, should be considered.

25.10 Specialist properties in materials selection

Specialist properties are considered for aircraft materials used in components for a unique application. The specialist property may be the most important consideration in materials selection, and other properties such as the cost, ease of manufacture or mechanical performance could be of lesser importance. For example, resistance against cracking and spalling owing to rapid heating, known as thermal shock resistance, is an essential property for materials used in the exhaust casing of rocket engines. Listed below are several specialist properties which may be considered in materials selection:

• Electrical conductivity is an important property for materials used in the outer skin of aircraft. The material must have the ability to conduct an electrical charge in the event of lightning strike.

• Thermal conductivity is a consideration for materials used in high-temperature applications such as heat shields and engine components. Heat-shield materials require low thermal conductivity to protect the airframe structure from excessive heating.

• Thermal expansion is also a consideration for high-temperature materials. Materials with a low thermal expansion coefficient are often required to avoid excessive expansion and contraction during heating and cooling.

• Flammability is a consideration for materials where there is the risk of fire, such as aircraft cabins and jet engines. Flammability properties such as ignition temperature, flame spread rate and smoke may need to be considered.

• Stealth is an important property for materials used in the external surface of covert military aircraft. Materials with the capability to absorb radar waves and/or reduce the infrared visibility are important for stealth aircraft.

25.11 Summary

Materials selection is an important process in the design of aircraft. The objective of materials selection is to identify the material that is best suited to meet the design requirements. Selecting the material involves seeking the best match between the design requirements of the aircraft component and the properties of the materials.

Materials selection can be revolutionary or evolutionary. Revolutionary materials selection involves selecting a new material or a material that has not been used previously in an aircraft component. Evolutionary materials selection involves using an ‘old’ material or slightly modified version of an old material in a component where it has been used previously. Most choices for materials in aircraft follow the evolutionary path because it is less risky than using a revolutionary type of material.

Many factors must be considered in materials selection, which are classified as structural, economic/business, manufacturing, durability, environmental impact, and specialist properties for unique applications.

Following the assessment of market need, the design of aircraft involves several major stages in the order: concept design, embodiment design and detail design; followed by the final design solution. At the concept design stage, all materials are considered; then this is reduced to a shortlist of candidate materials in the embodiment design stage; and, finally, one material is chosen in the detailed design stage.

The process for selecting materials and the process for selecting the manufacturing process involves four main steps in the order: translation, screening, ranking and supporting information.

Materials selection charts are used to screen out materials which do not meet the design constraints, and materials indices are used for ranking the shortlisted materials in order of excellence.

The most effective way of reducing the structural mass of aircraft is using light-weight materials (together with optimised design). A reduction in material density is often more effective at reducing aircraft weight than using stiffer or stronger materials of higher weight.

The durability of materials in the aviation environment (e.g. heat, rain, humidity, erosive particles) is a key consideration in materials selection. Materials must be resistant to deterioration when used in service: metals must resist corrosion and oxidation; composites must be unaffected by moisture; and metals and composites must resist wear and erosion.

The environmental impact of using material is becoming an increasingly important consideration in materials selection. Sustainable materials obtained from renewal resources and which have minimal impact on the environment during their production and recycling are considered favourably.

25.12 Terminology

Concept design: Basic design of a product to meet the main functional objectives and performance requirements determined from market research. A large number of materials are considered at the concept design stage.

Constraint (design): Performance requirements that must be met by the product (and its materials).

Detailed design: inal stage of the design process involving all the detailed design work to complete the product. Also involves converting the design into specifications and documentation so the product can be produced. The material to be used in the product is selected in the detailed design stage.

Embodiment design: Determination of shape, size and other major design features of the product. A shortlist of candidate materials for the product are considered in the embodiment design stage.

Function (design): The purpose of the designed product.

Material indices: Quantitative measure of how well a material property (e.g. stiffness, strength, maximum operating temperature) exceeds the design constraint. Index values are used to rank shortlisted materials in order of excellence to exceed the design constraint limit.

Objective (design): The aims of the design, such as to minimise cost or maximise fatigue life.

Ranking: Listing shortlisted materials in their order of excellence to exceed the design constraint limit based on the index value.

Screening: Process of eliminating materials based on their inability to meet one or more of the design constraints.

Supporting information: Various resources (e.g. case histories, past experiences) used in the final stage of the materials selection process.

Translation: Process of translating the design into functions and objectives which can used to define the constraints for materials selection.

25.13 Further reading and research

Ashby, M.F. Materials selection in mechanical design, 3rd edition. Oxford: Butterworth–Heinemann; 2005.

Ashby, M.F., Cebon, D. Cambridge materials selector. Cambridge MA: Granta Design Ltd; 1996.

Budinski, K.G., Budinski, M.K. Engineering materials: properties and selection. Upper Saddle River, NJ: Pearson Education Inc; 2010.

Dieter G.E., ed. ASM handbook volume 20: materials selection and design. Ohio: ASM International, 1997.

Hinrichsen, J. The material down-selection process for A3XX. In: Vermeeren C., ed. Around Glare. Kluwer Academic Publishers; 2002:127–144.