Because lithium is the least dense elemental metal, materials scientists and engineers have been working for decades to develop a commercially viable aluminum-lithium (Al-Li) alloy that would be even lighter and stiffer than other aluminum alloys. The first two generations of Al-Li alloys tended to suffer from several problems, including poor ductility and fracture toughness; unreliable properties, fatigue and fracture resistance; and unreliable corrosion resistance. Now, new third generation Al-Li alloys with significantly reduced lithium content and other improvements are promising a revival for Al-Li applications in modern aircraft and aerospace vehicles. Over the last few years, these newer Al-Li alloys have attracted increasing global interest for widespread applications in the aerospace industry largely because of soaring fuel costs and the development of a new generation of civil and military aircraft. This contributed book, featuring many of the top researchers in the field, is the first up-to-date international reference for Al-Li material research, alloy development, structural design and aerospace systems engineering. Provides a complete treatment of the new generation of low-density AL-Li alloys, including microstructure, mechanical behavoir, processing and applications Covers the history of earlier generation AL-Li alloys, their basic problems, why they were never widely used, and why the new third generation Al-Li alloys could eventually replace not only traditional aluminum alloys but more expensive composite materials Contains two full chapters devoted to applications in the aircraft and aerospace fields, where the lighter, stronger Al-Li alloys mean better performing, more fuel-efficient aircraft
The material and manufacturing property requirements for selection and application of 3rd generation aluminium-lithium (Al–Li) alloys in aircraft and spacecraft are discussed. Modern structural concepts using Laser Beam Welding (LBW), Friction Stir Welding (FSW), SuperPlastic Forming (SPF) and selective reinforcement by Fibre Metal Laminates (FMLs) are also considered. Al–Li alloys have to compete with conventional aluminium alloys, Carbon Fibre Reinforced Plastics (CFRPs) and GLAss REinforced FMLs (GLARE), particularly for transport aircraft structures. Thus all these materials are compared before discussing their selection for aircraft. This is followed by a review of the aluminium alloy selection process for spacecraft. Actual and potential applications of 3rd generation Al–Li alloys are presented. For aircraft it is concluded that the competition between different material classes (aluminium alloys, CFRPs and FMLs) has reached a development stage where hybrid structures, using different types of materials, may become the rule rather than the exception. However, aluminium alloys are still the main contenders for spacecraft liquid propellant launchers.
Aluminium-Lithium (Al–Li) alloys have been of interest since the 1950s when they were first used on a military aircraft. Having lithium as the main alloying element in Al alloys is attractive since (i) each 1 wt% Li reduces the density by ~3% and increases modulus by ~5%, and (ii) high strengths can be achieved by precipitation-hardening. During the 1980s, extensive research and development was carried out on alloys with high lithium contents (>2 wt%≡~8 at%) such as AA 8090 (Al 2.4 Li 1.2 Cu 0.7 Mg 0.12 Zr) (wt%). The mechanical properties of these ‘second-generation’ Al–Li alloys, however, did not match those of conventional Al (-Zn)-Mg-Cu alloys, and the lower fracture toughness of these alloys (for equivalent strengths was a particular problem. Thus, 2nd generation Al–Li alloys did not see widespread use. The experience with 2nd generation Al–Li alloys led to the development of ‘3rd generation’ alloys with lower Li contents (0.75–1.7 wt%), and some of these alloys have a better overall balance of properties, including fracture toughness, than the best available conventional Al alloys. These 3rd generation Al–Li alloys should therefore see extensive use in future civil and military aircraft. This chapter on fracture toughness and fracture modes of aerospace Al–Li alloys outlines why fracture toughness is important for aerospace structures and components, and summarises testing procedures and terminologies in regard to plane-strain and plane-stress fracture toughness. The relationships between fracture toughness/fracture modes and microstructural features such as grain morphology, constituent particles, impurity phases, matrix precipitates, grain-boundary precipitates, and grain boundary segregation, are then discussed. Proposed explanations for the low fracture toughness of 2nd generation Al–Li alloys, associated with low-energy intergranular and transgranular shear fractures, are discussed in some depth, followed by a summary of the alloy-design principles behind the development of 3rd generation Al–Li alloys with a much improved resistance to low-energy fracture modes. Quantitative data for fracture toughness of 2nd and 3rd generation Al–Li alloys in comparison with conventional Al alloys are provided, showing that 3rd generation Al–Li alloys have outstanding combinations of toughness and strength combined with reduced densities. The superior toughness of 3rd generation Al–Li alloys compared with 2nd generation alloys is reflected in the differences in fracture-surface topography and fracture path. The chapter concludes with a summary of the current and proposed uses of 3rd generation Al–Li alloys in aircraft structures and components
The low cycle fatigue (LCF) and high cycle fatigue (HCF) properties of Al–Li alloys are influenced by alloy composition, microstructural characteristics, tensile stretching prior to artificial aging, and crystallographic texture. In general the fatigue properties, notably the notched HCF resistances, of Al–Li alloys are similar to those of conventional aerospace aluminium alloys. Alloy development programs on newer Al–Li alloys aim to study further the effects of minor alloying additions (rare earths, beryllium, silver and TiB); various thermomechanical treatments; alloy microstructure, notably crystallographic texture and grain size; and the fatigue load history and environment on the mechanical behavior, including the fatigue properties. It is important to note that the occurrence of bilinearity in LCF life-dependence on strain amplitude in most Al–Li alloys engenders the overestimation of the LCF lives in both the hypo-transition (lower strain amplitudes; longer fatigue lives) and hyper-transition (higher strain amplitudes; shorter fatigue lives) regions if the lives are estimated by extrapolation from either of these regions. Further, in cases such as in Al-Li alloys where there are large differences in strength-based (Basquin-like) and plastic strain – based (Coffin-Manson) power-law relationships, it is appropriate to develop an alloy design philosophy based on either plastic strain energy per cycle (Halford-Morrow) or fatigue toughness (total plastic strain energy to fracture). All of these aspects are discussed in detail in this chapter.
The structural and engineering property requirements for widespread deployment of aluminium-lithium (Al-Li) alloys in aircraft are discussed, particularly with respect to commercial transport aircraft. The development of Al-Li alloys has been driven mainly by the fact that additions of lithium to aluminium alloys lowers the density and increases the elastic modulus, thereby offering the potential of significant weight savings with respect to conventional (non-lithium containing) alloys. The first use of Al-Li alloys in aircraft goes back to the late 1950s (alloy AA 2020) and mid-1960s (alloys 1420 and 1421). These materials are referred to as the 1st generation Al-Li alloys. Subsequently there have been two major development programmes resulting in the 2nd and 3rd generation alloys. Development of the 2nd generation alloys began in the 1970s and continued through the 1980s. Attempts were made to develop families of Al-Li alloys for widespread replacement of conventional alloys. Ultimately this was unsuccessful except for ‘niche’ applications. The failure to find widespread application was associated largely with the too-high lithium contents of the alloys (typically more than 2 wt%). This resulted in serious disadvantages, including mechanical property anisotropy, low short-transverse ductility and fracture toughness, and thermal instability. Development of the 3rd generation Al-Li alloys began in the late 1980s and is ongoing. These alloys have significantly reduced lithium contents (0.75 – 1.8 wt%) and there are other important compositional changes. Silver and zinc have been added for strength, and zinc improves the corrosion resistance; and manganese is added besides zirconium, which was already present in 2nd generation alloys, to control recrystallization and texture. These differences and improved knowledge about thermomechanical processing and heat-treatment have resulted in a family of alloys with significant property advantages covering all major structural areas and applications for transport aircraft.
Airworthiness regulatory bodies are authorised and responsible for verifying and ensuring the safety and reliability of aircraft. There are many civil and military aviation organisations and regulatory bodies. The functions and responsibilities of several of these organizations are summarised in this chapter. Owing to the importance of aircraft structural fatigue, a survey of fatigue design philosophies is also given. This is followed by (i) a discussion of the airworthiness certification methodology for materials and structures, starting with the initial mill products and proceeding via incremental levels to the finished aircraft; and (ii) an example of material certification for an aluminium–lithium (Al-Li) alloy that is a candidate for use in the airframes of light combat aircraft (LCA).
Most aluminium-lithium (Al–Li) alloy fatigue crack growth (FCG) data have been obtained for 2nd generation alloys, specifically under constant amplitude (CA) and constant stress ratio (CR) loading, and for long/large cracks. These data show the alloys in a favourable light, but this FCG ‘advantage’ essentially disappears under realistic flight simulation loading, and is also absent for short/small cracks. Furthermore, the FCG advantage is due to inhomogeneous plastic deformation, which has undesirable consequences for other important properties. These consequences have greatly restricted the use of 2nd generation alloys in aerospace structures. FCG data for 3rd generation Al–Li alloys are becoming more available. Many of the issues associated with 2nd generation alloys have been eliminated or greatly alleviated as a result of several changes, including reduced Li contents and innovative thermomechanical processing. Consequently, the FCG behaviour of 3rd generation alloys is more similar to that of conventional alloys. Nevertheless, the 3rd generation alloys tend to have better FCG properties than equivalent conventional alloys; and these and other improvements have already led to many aircraft applications.
Mechanical working of Al–Li alloys is primarily concerned with aerospace alloy rolled products (sheet and plate), extrusions, and to a lesser extent forgings. These products are fabricated by hot working with intermittent and final heat treatments. This thermomechanical processing (TMP) can be rather complex for the modern 3rd generation Al-Li alloys, but is necessary to obtain optimum combinations of properties. This Chapter is in two parts. Part 1 discusses the ‘workability’ of metals and alloys and the hot deformation characteristics of Al–Li alloys, leading to the concept of Process Maps. A comprehensive Process Map for a binary Al–Li alloy illustrates the usefulness of these Maps for defining temperature–strain rate regions for safe and unsafe hot working, recrystallization and recovery, and superplastic behaviour Part 2 provides some general considerations about processing Al–Li alloy products, followed by a review and discussion of the currently available information for 3rd generation alloys. It is concluded that their complex TMP schedules may make it difficult to obtain optimum combinations of properties for thicker products.
This publication reviews most of the available literature on the fatigue properties of β annealed Ti-6Al-4V and titanium alloys with similar microstructures. The focus is on β processed and β heat-treated alloys because β annealed Ti-6Al-4V has been selected for highly loaded and fatigue-critical structures, including the main wing-carry-through bulkheads and vertical tail stubs, of advanced high-performance military aircraft. An important aspect of the review is a concise survey of fatigue life assessment methods and the required types of fatigue data. This survey provides the background to recommendations for further research, especially on the fatigue behaviour of β annealed Ti-6Al-4V under realistic fatigue load histories, including the essential topic of short/small fatigue crack growth. Such research is required for independent fatigue life assessments that conform to the aircraft manufacturer’s design requirements, and also for life reassessments that most probably will have to be made during the service life of the aircraft.
Since the first edition published in 1991, this has been one of the top-selling books in the field. The first and second editions have been used as a required text in over 100 universities worldwide and have become indispensable reference for thousands of practising engineers as well. The third edition reflects recent advances in the field, although it still retains the characteristics that made it a best-selling title. Providing thorough coverage of a wide range of topics, this book covers both theoretical and practical aspects of fracture mechanics and integrates materials science with solid mechanics. This edition includes expanded coverage of weight functions and a new chapter on environmental cracking.
Thank you for visiting our website. Would you like to provide feedback on how we could improve your experience?
This site does not use any third party cookies with one exception — it uses cookies from Google to deliver its services and to analyze traffic.Learn More.