Creep of aerospace materials
Creep is a process that involves the gradual plastic deformation of a material over time. The remarkable thing about creep is that plastic deformation occurs at stress levels below the yield strength of the material. In other words, creep causes a material to plastically deform and permanently change shape over time when subjected to an elastic load. This runs counter to the concept that plastic deformation can only occur when the applied stress exceeds the yield strength of the material.
When most engineering materials, including the metal alloys and composites used in aircraft structures, are elastically loaded then the amount of deformation that occurs does not change with time. However, this is only true when the temperature is moderately low and the elastic load is applied to the material for a short time. When the temperature is raised then ‘elastic loads’, which give no permanent deformation at room temperature, can cause the material to plastically deform via creep. (Creep of materials does occur at room temperature, but the rate of plastic deformation is usually extremely slow and any permanent deformation is not noticeable.) Most metals undergo creep at temperatures higher than 30–40% of their absolute melting temperature (in Kelvin). Creep of polymers and polymer composites occurs at lower temperatures than in metals, and in some materials is noticeable at only 50–75 °C. Creep deformation of metals, polymers and composites can continue unabated under elastic loading until eventually fracture occurs via a process called stress rupture.
Creep of aerospace materials can be a serious problem when they are required to withstand high elastic loads and elevated temperatures for long periods of time. Aerospace metals must have high resistance to creep and stress rupture, otherwise the aircraft component may be damaged (Fig. 22.1). For example, without high creep resistance the materials used in aircraft jet engines, such as the turbine blades, discs and compressor parts, distort owing to the high operating stress and temperature. Close tolerances are critical in jet engines, and even a small amount of plastic deformation caused by creep can cause an engine to seize. Excellent creep resistance is also essential for structural materials used in the body skins of supersonic aircraft, rocket nose cones and re-entry spacecraft such as the space shuttle. High temperatures are generated by frictional heating from molecules in the atmosphere, and this can cause the skin materials to permanently deform and warp when they lack sufficient resistance against creep. There are many other examples when high resistance to creep and stress rupture is needed for the materials used in aircraft, such as engine components, and spacecraft, such as rocket nozzles. It is essential to aircraft safety that aerospace engineers understand the creep behaviour of structural materials.
In this chapter we study the creep and stress rupture properties of metal alloys, polymers and polymer composites. We also discuss ways to improve the resistance of aerospace materials against creep and stress rupture.
When a material is held under a constant stress for a period of time, the process of creep can be divided into three stages of development: (I) primary creep when the process begins at a fast rate, (II) secondary creep when the process proceeds at a steady rate, and lastly (III) tertiary creep that occurs quickly and eventually leads to failure (or rupture). These three stages are observed in the creep curve of a material, which is a plot of increasing strain against time under load (Fig. 22.2).
The initial strain represented by εo occurs when load is first applied to the material and is the result of elastic deformation, whereas the higher strains are caused by time-dependent plastic deformation owing to creep. The creep rate is initially very rapid in the earliest period of the primary stage, but slows over time as the material resists the deformation by strain hardening.
The second stage of creep, called ‘steady-state creep’, is a period of nearly constant creep rate defined by the slope dε/dt. The creep rate is constant because a balance exists between the competing processes of plastic deformation and strain hardening. Secondary creep occurs for most of the creep life.
The tertiary creep stage occurs when the creep life is nearly exhausted, and the material specimen begins to neck or develop internal voids which reduce the load capacity. The creep rate accelerates during the tertiary stage as the load capacity drops owing to increased necking or void growth until eventually the specimen fails. By holding the specimen at a constant stress level and temperature until failure, the stress rupture life can be measured.
An important property determined from the creep curve is the steady-state creep rate, , which occurs during the second stage. Because this stage lasts for most of the creep life, the steady-state creep rate is used to calculate the change in shape of a material over most of the operating life. The creep rate is calculated using the Arrhenius relationship:
where T is the absolute temperature, R is the universal gas constant, A and n are creep constants specific to the material, and Qc is the creep activation energy for the material. The creep constants and activation energy are determined from creep testing (which is explained in chapter 5) and, using the results, it is possible to calculate the creep rate of the material for any operating stress and temperature. This equation allows aerospace engineers to calculate the change in shape of a metal component during service and hence specify its design creep life. Alternatively, the equation can be used to determine the maximum operating stress and temperature for an aerospace material without it suffering excessive creep deformation.
The creep life, otherwise known as the stress rupture time, of aerospace materials can be determined from their creep curve, provided of course that the test conditions replicate the in-service operating stress and temperature. The stress rupture time (tr) is calculated using the expression:
where K and m are material constants and Qr is the activation energy for stress rupture, and these are measured by experimental testing. The stress rupture time decreases rapidly with increasing stress and temperature, as shown in Fig. 22.3 for a nickel-based superalloy used in jet engines. Therefore, it is important to limit the operating stress and temperature to avoid stress rupture as well as excessive creep deformation.
Creep failure of aircraft components can be defined in several ways. The failure modes are known as ‘displacement-limited creep’, ‘stress-limited creep’, ‘buckling-limiting creep’, and ‘stress rupture’. These are now explained in this order:
Displacement-limited creep failure occurs when the material changes shape beyond a specified limit, such as 0.1% elongation. This type of failure is important for aircraft components that have precise dimensions or when small clearances must be maintained, such as discs and blades for gas turbine engines.
Stress-limited failure is when the permanent change in shape owing to creep relaxes the initial stress on a material. For example, stress relaxation creep can loosen pretensioned fasteners in aircraft joints.
Buckling-limited creep occurs in beams, panels and other structures that carry compressive loads. This failure mode involves the buckling or collapse of thin sections owing to creep. For example, an upper wing skin could experience creep-induced softening and buckling as a result of frictional heating when flying at supersonic speeds for a long time. For this to occur in practice, however, the skin material would need to have exceptionally low creep resistance.
Creep of metals occurs from the action of two plastic deformation processes: dislocation slip and grain boundary sliding. It is often assumed that dislocations do not move when the stress acting on a metal is below its yield stress. Strictly, this assumption is only true when the temperature is absolute zero (− 273 °C). Above this temperature, the metal atoms have sufficient mobility to cause the dislocations to move. At room temperature, the atomic mobility is low and therefore the movement of dislocations is extremely slow and an extraordinarily long period must pass before plastic deformation is noticeable. For this reason, at room temperature it is assumed that the deformation of a metal is completely elastic when the applied stress is below the elastic limit. Atomic mobility increases with temperature and can, with sufficient time, aid dislocations to move through the crystal structure and thereby cause plastic deformation.
Dislocation movement during creep occurs by two processes: dislocation slip (which is described in chapter 4) and dislocation climb. The former process involves the movement of dislocations along the slip planes of the crystal lattice whereas the latter process is the movement (or climb) of dislocations perpendicular to the slip planes. Dislocation slip occurs at all temperatures above absolute zero when the applied stress is high enough whereas dislocation climb usually occurs only at high temperature. The dislocation climb process is illustrated in Figure 22.4, and requires atoms to move either to or from the dislocation line by diffusion involving a lattice vacancy. This action allows dislocations to ‘climb’ around obstacles impeding plastic flow, such as precipitate particles or clusters of solute atoms (e.g. GP zones), and thereby cause creep deformation even at low stress levels. Dislocation movement by slip or climb increases rapidly with temperature, as observed by an increase to the creep rate. At high temperatures, new slip systems become operative in the crystal lattice of some metals, thus further assisting dislocation motion and thereby increasing the creep rate.
The other important deformation process controlling the creep rate of metals is grain boundaries sliding. At high temperature the grains in polycrystalline metals are able to move relative to each other by plastic flow at the grain boundaries. The sliding that occurs between grains increases with temperature, which thereby raises the creep rate. Grain boundary sliding is the dominant creep process in most metals when the applied stress and temperature are low. Dislocation movement by slip and climb are the more dominant creep mechanisms at high stress and temperature.
Dislocation slip and, in particular, grain boundary sliding promote the formation of small voids at the grain boundaries (Fig. 22.5). Voids initiate at grain boundaries which are oriented transverse to the direction of the applied creep load. The voids develop at the start of the tertiary stage of the creep life, and then increase in number and size during this stage until eventually the metal fails by intergranular fracture. The time taken for the metal to fail under a constant stress and temperature is used to define the stress rupture life.
A potential problem with using polymers in aircraft components is viscoelastic creep, which can cause permanent distortion and damage. Polymers exhibit both elastic (instantaneous) and viscous creep (time-dependent) deformation when under an applied stress that is below the yield strength. This combination of viscous and elastic deformations is called viscoelasticity. When a polymer is under load there is an immediate elastic response. As explained in chapter 13, this is caused by the elastic stretching of bonds along the polymer chains and the partial straightening of twisted and coiled segments of the chains. When the load is removed the chains relax back into their original position, and this is the elastic component of viscoelasticity. However, when a polymer is held under load for a period of time then a second deformation process known as viscous creep occurs which is time-dependent. The chains have time to unfold and slide relative to one another when load is applied for a sufficient time. This viscous or creep flow is a time-dependent process, which decreases with increasing time until a steady-state condition is reached when the initially folded chains reach a new equilibrium configuration. When the polymer is then unloaded, there is an immediate (elastic strain) recovery followed by a time-dependent recovery; however, a permanent deformation remains.
The viscoelastic effect in polymers is dependent on the loading and environmental conditions, as shown in Fig. 22.6. Permanent deformation caused by viscous flow increases when the loading (or strain) rate is reduced. When load is applied rapidly, the polymer chains do not have sufficient time to uncoil and slide and, therefore, the creep effect can be quite low and the polymer behaves in a brittle manner. When load is applied slowly or stress is applied for a long period, there is sufficient time for the chains to slip and straighten and, therefore, the viscoelastic creep behaviour is more pronounced. Creep deformation also increases rapidly with temperature because there is more internal energy available for the chains to slide and uncoil. A polymer eventually fails by stress rupture when the load is applied for a sufficiently long time. The stress rupture time decreases with increasing stress and temperature as shown in Fig. 22.7.
The importance of creep is that permanent deformation to loaded plastic aircraft parts (including bonded connections) increases with the loading rate and operating temperature. Creep can occur in some polymers at room temperature, and these materials must be avoided in aircraft. Polymers should only be used when creep cannot occur, such as in lightly-loaded parts. Although creep occurs in all polymers, the creep rate can be controlled by the molecular structures. Creep is reduced by any process that resists the unfolding and sliding of chains, such as increasing the degree of crystallinity in thermoplastics or the amount of cross-linking in thermosets. Polymers that have large side-groups along the chain also have higher resistance to creep.
In chapter 15 we discussed the importance of aligning the fibres in the load direction for high stiffness, strength and fatigue life of polymer composites, and it is also important for high creep strength and resistance to stress rupture. The creep properties of fibre–polymer composites are anisotropic, and depend on the fibre direction. Carbon and glass fibres used in aerospace composites are resistant to creep and, therefore, when the applied load is parallel with the fibres then composites do not experience creep or stress rupture. However, when the load is not acting on the fibres and is carried by the polymer matrix (such as through-thickness loading) then significant creep can occur.
There are several methods used to improve the creep resistance and prolong the stress rupture life of aerospace materials. The most important method is selecting a material with a high melting (or softening) temperature. As a general rule, creep occurs when metals are required to operate at temperatures above 30 to 40% of their absolute melting point. Rapid creep of polymers occurs at 30–40% of their glass transition temperature, whereas creep of heat-resistant ceramic materials starts above 40–50% of their melting temperature (in Kelvin). Table 22.1 gives the melting or softening temperatures of various aerospace materials. Most polymers and polymer composites have low softening temperatures (typically under 150–180 °C) and so are not suited for high-temperature service. Aluminium and magnesium alloys have relatively low melting temperatures and not suitable for aircraft components required to operate for long periods at temperatures above about 150 °C. Nickel, iron–nickel and cobalt superalloys have high melting temperatures, which makes them suitable for gas turbine engines and other high-temperature components. Ceramic materials have very high softening temperatures that make them useful for extreme temperature applications, such as rocket nose cones and the heat insulation tiles on the space shuttle.
The creep resistance of metals is controlled by their alloy composition and microstructure as well as by their melting temperature. The creep resistance of metal alloy systems increases with the concentration of alloying elements dissolved into solid solution. The presence of alloying elements in interstitial crystal sites increases the lattice strain and thereby resists the processes of dislocation slip and climb that drive creep. This is one reason for the high alloy content of nickel-based and iron–nickel superalloys used in jet engines. Creep resistance is also improved by the presence of finely dispersed intermetallic precipitates, which are stable at high temperature. The precipitates resist the movement of dislocations that causes high temperature creep. It is for this reason that many nickel-based superalloys contain small amounts of aluminium and/or titanium which combine with the matrix to form γ and γ′ intermetallic precipitates Ni3Al, Ni3Ti or Ni3(Al,Ti). Chapter 12 gives more information on the control of creep using the metallurgical properties of superalloys.
Control of the grain size and structure is also an effective method of reducing creep. Increasing the grain size by thermomechanical processes reduces the creep rate and extends the stress rupture life of metals by lowering the amount of grain boundary sliding. Therefore, metals with a coarse grain texture are often used in creep-resistant components. The elimination of transverse grain boundaries along which sliding occurs provides an even greater improvement to the creep resistance and rupture life. High-pressure turbine blades for jet engines are fabricated using the directional solidification process, which involves chilling the metal casting from one end when removed from the furnace (chapter 6). The sharp temperature gradient used in directional solidification forces the grains to grow continuously from one end of the casting to the other. The final casting has a columnar grain structure with few or no transverse grain boundaries, thus providing high creep resistance. The improved creep properties of turbine blades fabricated from directionaly solidified metal allows them to operate at a temperature about 50 °C higher than the material with a coarse polycrystalline structure, thereby providing greater propulsion efficiency. Even better creep properties are achieved by casting metals using the single crystal process. Because there are no grain boundaries in single crystal metals, they have outstanding creep resistance and prolonged stress rupture life.
There are several ways of increasing the creep resistance of polymers, structural adhesives and fibre–polymer composites. The creep resistance of thermoset polymers increases with the amount of cross-linking between the chains. The cross-links increase the glass transition temperature by resisting the sliding and straightening of the chains under applied loading, and this raises the creep-softening temperature. Therefore, thermoset polymers such as epoxy should be fully cured to maximise the amount of cross-linking. The creep resistance of polymers also increases with their molecular weight. Thermoplastics are generally less resistant to creep than thermoset polymers, and their creep properties are controlled by the arrangement of the network structure of the chains; with crystalline and semicrystalline polymers being more creep resistant than amorphous (or glassy) polymers. The most effective method of maximising the creep resistance of composite materials is aligning the fibres in the load direction. Carbon-fibre and glass-fibre composites have good creep resistance when the fibres carry the applied load.
Creep is an important deformation process in aerospace materials required to operate at high temperatures and stresses for long periods of time, and it is especially important for jet engine materials.
Creep involves the plastic deformation of material when an elastic load is applied for a long time. The amount of creep increases with time until eventually the material breaks; this is the stress rupture time.
Polymers are susceptible to plastic deformation when load is applied for a sustained period as a result of viscoelastic creep. The rate of creep deformation increases with the applied stress, temperature or loading rate. The operating load and temperature of polymers used in aircraft components must be sufficiently low to avoid creep deformation which can eventually lead to stress rupture.
Creep resistance of thermoset polymers (including structural adhesives) and fibre–polymer composites is improved by increasing the amount of cross-linking and the molecular weight. Creep resistance of thermoplastics is improved by increasing the molecular weight and the amount of crystalline polymer. Creep resistance of composites is improved by ensuring the fibres carry the applied load, and not the polymer matrix. Carbon-fibre composites have much higher creep resistance in the fibre direction compared with the anti-fibre direction.
Grain boundary sliding: A plastic deformation process that usually occurs at elevated temperature in which grains slide past each other along, or in a zone immediately adjacent to, their common boundary.