Materials, Structures and Manufacturing for Aircraft

Al-Li alloys are the focus of attention in the aviation industry with their high modulus of elasticity, high hardness, high fatigue crack growth resistance, and low density. These specially designed alloys are used in fuselage, fuselage beam, lower wing, and upper wing structures in aircraft. The use of Al–Li alloys is thought to be extended to different sections in aircraft due to the effort of weight reduction. However, various drawbacks are encountered in either the production or material properties of these alloys. In this chapter, the progressive advantages and disadvantages of Al–Li alloys that are used in aviation are explained. Historical development, aviation usage, microstructural properties, processing metallurgy, corrosion, and mechanical properties of Al–Li alloys are expressed.

The most attractive aspect of metal foams is that they can combine the properties of metallic materials (e.g., strength, high melting temperature) with the properties of porous materials (e.g., light-weightiness, sound/heat energy absorption). This makes them favorable in aerospace applications where multifunctional lightweight materials are required since the amount of work done against inertial forces is larger. Within the scope of this chapter, after giving information about the primary and secondary processes of metal foams with open and closed-cells, some applications of these materials in aircraft and spacecraft structures are mentioned.

Recently polymer composites have been in high demand for various types of aircraft applications due to their positive and significant impact. Thirty to forty-five percent of the latest aircraft frames are now made with the help of composite materials and the percentage is increasing day by day due to major technological advances in the sector. Polymer materials are commonly used in aviation because of their mechanical, tribological, and structural properties that reduce weight. Old-fashioned materials are subject to oxidation and fatigue, whereas the composite material is resistant to both. Due to the hardness and high strength of the fiber, the polymer compound gives excellent rigidity and strength to the weight ratio; they have good shear properties and low density. Aircraft designers and engineers are moving toward the next generation of integrated materials to make their aircraft more powerful, more fuel-efficient, and lighter. This review paper gives a brief overview of the polymer composite material, components of an aircraft structure, manufacturing process, its properties, and way ahead for composites in aircraft application.

Fibrous composite materials have been gaining immense interest in many applications such as construction, automotive, medicine, sports, aerospace, and aircraft industries. Their unique properties such as high strength-to-weight ratios in comparison with conventional materials such as metals and ceramics, make them suitable candidates for aircraft and construction applications. These structures exhibit good mechanical properties and high impact and fatigue resistance. Moreover, the ability to shape the structure and fabricating preforms for composite manufacturing allows the researchers to design and produce the specimens with different architectures and configurations. Different textile processes such as weaving, knitting, braiding, stitching, and nonwoven for the fabrication of preforms, and their performance and applications in composite structures are highlighted. This chapter focuses on the design and development of new fibrous composite structures for aircraft applications. Different types of advanced fibrous composite materials as well as their properties, advantages, and disadvantages for modern aircraft construction are discussed. Recent advances using fibrous composite structures on aircraft constructions are also presented. Fiber, matrix, and different fiber architectures and production methods are given. Finally, some directions for new developments and future work are outlined.

Composites are well established as the natural response to the growing interest and demand for high-performance materials. These key engineering materials are employed in a wide range of applications, substituting other engineering materials such as metals in highly demanding applications, which is the case of the aerospace industry. The excellent mechanical properties and low specific weight justify its increasing use in structural applications. However, currently used composites based on synthetic materials such as fiberglass and carbon fibers reinforced composites have negative implications for the environment. The sustainability issues regarding synthetic fibers have led to a growing interest and intensive research on natural materials and their integration in composite structures. Natural fibers, such as flax and sisal, are characterized by good mechanical properties such as their mechanical strength and characteristics such as abundance, biodegradability, and low density and cost. Bio-based matrixes are also one of the current hot topics regarding their importance for full natural composites. Most importantly, natural fiber reinforced composites, depending on the fiber-matrix combination, may present interface issues, regarding the proper adhesion between both components of a composite, resulting in poor performance of a natural composite. These and other challenges, regarding the development of natural composites, are reviewed, as well as their design for aerospace applications. In the first part of this chapter, natural fibers and matrixes are reviewed, from properties to processing. Then, the manufacturing of these composites is reviewed, specifically for aircraft parts fabrication. Finally, a discussion addressing future trends and challenges is performed, from the composite side to the application, regarding the interface dependence on the compatibility between fibers and matrix, and possibilities and trends of natural composites in aircraft.

The expectation of increased air travel in the future decades is met with the issue of a growing environmental effect. The aviation sector is developing potential solutions to reduce the environmental effect. Gas turbine engines are still the most common type of engine used in aircraft, and due to population growth and the number of flights, the emissions produced by these engines have become a major challenge in their future application. The use of lightweight materials is a powerful option to reduce fuel consumption and thus reduce aviation emissions. This chapter gives a brief history of aircraft engines, their classification, principles, common materials used in the main components, and the environmental effects of aircraft engine materials.

In this chapter, first, basic information about the landing gear (definition, purpose, and duties, etc.) is given. Examples of different aircraft are given throughout the chapter. Systems in large aircraft such as extend-retract mechanisms are explained on Boeing 737 aircraft. As mentioned in the chapter introduction, landing gears are the elements that provide contact with the ground during the landing—takeoff phases and movements on the ground (during taxi), the friction resistance is minimized, and they best absorb the loads caused by the horizontal and vertical movements of the aircraft. Afterward, different landing gear layouts and their usage areas, advantages, and disadvantages are mentioned. Landing gear struts, which are one of the basic elements of the landing gear and provide structural strength, are mentioned. By specifying the characteristics of different strut structures, the damping structure of the energies arising from the landing is explained. After the strut structures, wheels, tires, and brake structures, which are the other basic elements of the landing gear, are explained. Since there are retractable landing gears in large aircraft, the extend-retract mechanisms of the main and nose landing gear, their controls, and indicators are mentioned. Finally, the nose wheel steering mechanism that provides guidance during movement on the ground and the tail skid structure that prevents the tail from hitting the ground directly during takeoff are mentioned.

The airplanes’ fuselage and their equipment are specified for challenging conditions. They carry a huge amount of fluids such as fuel, hydraulics, conditioned gases, etc. While these state-of-art machines are flying, the mentioned fluids transfer through the body maintaining flight stability. Besides transferring, the weight of the fuel changes during flight depending on the characteristics of the relevant flight stages. During the takeoff and climb stage, the aircraft consumes more fuel than the cruise and descent stages of a regular flight. Out of question, if the aircraft has vertical takeoff capability the consumed fuel will be more than the similar airplanes that use the runway for departure. As a matter of fact, the weight change of the fuel is a crucial issue that must be monitored by pilots during the flight path. No sudden changes are wanted since they have direct impacts on the Center of Gravity (cg) of the aircraft. To prevent these unwanted changes, many state-of-art techniques have been implemented in aircraft fuel tanks. Another important feature for fuel tanks is crashworthiness. The “fuel-tank crashworthiness” can be defined as “the ability of the fuel-tank structure to protect the fuel inside and therefore retard any possible fire after crash.” Obviously, aircraft parts are generally made of fire-proof material but the fuel system, itself needs more meticulous measurements than the other airborne components since it contains highly flammable substances as a necessity of its task. In this chapter, the types of fuel tanks are examined while focusing on the bladder ones since they have increased attention with the improvement of novel materials such as carbon fibers and retardant chemicals. These materials are used for manufacturing the skin of the bladder fuel tank while the fuel-resistant sponge fills the inside the tank for preventing the slosh and hence the stick-lateness phenomena which is another unwanted issue by pilots. Besides, bladder fuel tanks are far lighter than the other types of fuel tanks with close fuel capacity. With the lightweight feature, they have a direct positive effect for decreasing the direct operational cost (DOC) since even 1 (one) kg of weight reduction is important for airliners because of the fierce competition in the aviation industry. From the follow-on-support side, any fuel-system maintenance staff wants a fuel tank that is foldable and easy to remove and replace. The vacuum ability feature of the bladder fuel tanks provides solutions to maintenance staff for making easy repair and easy maintenance operations. It is even possible to make brush-up repairs from maintenance doors. Eventually, the bladder-type aircraft fuel tanks will be used more widely in the future with their splendid features.

Piezoelectric sensors are attached to the structure to make sense of changes in the structure. Changes in the material properties of the piezoelectric sensor and the attached structure are caused by changing environmental conditions. In this study, electromechanical impedance method (EMI) is proposed for in situ inspection of landing gears in service. Knowing that these structures serve at low-temperature conditions, the tests were conducted between −10 °C and −45 °C. The specimen and sensor were modeled using ANSYS^® finite element program. The experimental and simulation results are very close. The temperature effect was compensated using Effective Frequency Shift (EFS) method and compensated/non-compensated situations were compared using different damage metrics. The genetic algorithm was used to solve the compensation algorithm.

Airplanes are certainly a remarkable success of engineering. They are designed for operating in a formidable environment. Every aircraft is a compound of systems that are necessary to work safely and efficiently. All of these systems have particular features and they cannot function independently. They rely on all the other aircraft systems in order to operate properly while ensuring flight safety. Depending on the functionality, some of the systems carry high sensitivity while others may not. For example, when it is compared with auxiliary power unit (APU), the in-flight entertainment system may have a relatively lower hazardous risk impact. Some aircraft units have crucial impacts on aircraft functionality. They are called major components in general. This chapter is intended to provide information about major components such as the APU, environmental control system (ECS), flight data recorder (FDR), cockpit voice recorder (CVR), and Auto-pilot system. They were meticulously selected by the author’s many years of field experience in the aviation industry initially as a senior engineer, then a pilot, and eventually an academician who concentrates on airworthy part manufacturing. Each of them has crucial importance over ensuring a safe flight while maintaining cost-effectivity. Obviously, in the open literature, it is possible to encounter the resources that provide information about the mentioned major systems. Some resources provide information about one system while others do not. This chapter differs from such open resources as it is solely written for providing information about APU, ECS, FDR, CVR, and auto-pilot system. Hopefully, it will give the reader a great sense for reaching the required information about these crucial aircraft systems.

Recent technologic advancements, especially in cutting-edge sectors like aerospace industries, call for new materials with superior properties. Like advanced engineering alloys, composites, and superalloys, these new materials provide the required specifications; however, to make use of these materials, they are needed to be formed into a final product. Machining is one of the most used manufacturing processes. Since in this process, the chip removal action occurs with direct contact between the cutting tool and workpiece, therefore, cutting materials with superior mechanical properties become a backbreaking process to be carried out. Along with the desired properties of the new advanced engineering materials in the aerospace industry, superior mechanical properties such as high wear resistance and low thermal conductivity of these materials lead to low machinability and difficulties in producing the desired end products by machining. As traditional machining methods are not efficient enough in machining such materials, new machining techniques have been invented to deal with these problems. Nontraditional machining processes are developed to deal with such obstacles that use chemical, electrochemical, thermal, and mechanical energy sources to facilitate the material removal process, reduce cost, and enhance product quality. However, in some cases, these methods’ low production efficiency forced engineers to combine the advantages of multiple machining methods in one hybrid process and improve the process efficiency by expediting the manufacturing process. One of these hybrid manufacturing methods is vibration-assisted machining. The vibration-assisted machining method aims to improve the material removal process by giving high frequency and low amplitude mechanical energy in vibrations to the workpiece or cutting tool. Vibration-assisted machining methods first emerged in the late 1960s and gained popularity in the early 2000s, and nowadays, research stages have gained momentum and are used even in mass production. Vibration-assisted machining has many benefits over traditional machining processes, like reducing costs, cutting forces, required power, secondary operations, cutting tool wear, and increasing the machined surface quality, tool life, and finally, the process performance. In this chapter, a detailed literature survey on the effects of vibration implementation on the performance of various machining processes, including turning, milling, drilling, and cutting advanced aerospace materials, is systematically summarized and discussed. At the end of this chapter, a case study is provided to understand the topic deeply. The detailed review shows that vibration-assisted machining enhances the cutting process in terms of cutting forces, tool wear, and surface roughness compared to traditional methods. Also, case study outcomes support those findings. Likewise, future studies show that vibration-assisted machining process still needs to be investigated deeply and it is a promising research area.

Incremental sheet metal forming (ISF) processes are part of a set of non-classical techniques that allow producing small-batches, customized and/or specific geometries for advanced engineering applications, such as aerospace, automotive, and biomedical parts. Combined or not with other joining processes and additive manufacturing techniques, ISF processes permit rapid prototyping frameworks and can be included in the class of smart manufacturing processes.

This chapter discusses the fundamentals of ISF technology, key attributes, future challenges, and presents a few examples related to the use of incremental forming for the development of complex parts as specifically found in aerospace applications such as airfoils. The use of incremental forming to produce customized designs and to perform quick tryouts of ready-to-use parts contributes to decreasing the time to market, decreasing tooling cost and increasing part design freedom.

An aerospace system is made of metallic or nonmetallic materials. The main goal of this chapter is to obtain for designers, engineers, students, and researchers a big source of the newest information about the topics associated with the joining of different dissimilar materials in the aerospace industry, the newest progress as well as subsequent directions in addressing the topics. Scientists met the fact with the necessity of dissimilar welding of materials since they are looking for creative constructures or parts with outstanding properties. Sometimes a member needs a big temperature resistance in one zone and a good resistance to corrosion in another zone. Systems can need to wear resistance or toughness in one zone integrated with elevated tensile strength in another zone. Enhancing the potential of dissimilar welding with high properties is enabling new processes to light-mass aerospace systems. Most of the dissimilar welding research from 50 years ago has been related to metallic materials. There has been enhancing the research of dissimilar material joining including composites, ceramics, and polymers beginning from the 1980s. Enhanced application of these engineering materials is extending due to the special effecting needs for the resistance to corrosion, big ratio of strength to weight, resistance to erosion, strength at the elevated temperature, and producing light vehicles with decreased fuel consumption. The most important welding techniques used in aerospace systems involve liquid- and solid-state weldings such as GTAW, GMAW, flash welding, big energy density techniques like electron beam welding and laser welding as well as friction welding, diffusion welding, brazing, and ultrasonic welding.

Development of powder bed fusion additive manufacturing technologies enabled the introduction of novel components into various industries such as aerospace, biomedical, mold, and die. These novel components, which can be produced by powder bed fusion additive manufacturing, possess several advantages including internal features, lightweight structures, and integrated functionalities. Components with lattice structures are typical examples that possess most of the listed advantages. Lattice structures can be described as volumes or solids which mostly contain internal voids or spaces arrayed along with one or more directions in an orderly manner. They are categorized under three main groups as strut-based lattice structures, shell lattice structures, and triply periodic minimal surface lattice structures. Although their advantages are compelling for various industries, a sufficient understanding is essential to have the benefits. This chapter broadly presents the types and characteristics of lattice structures together with used analytical techniques. Furthermore, it explains different approaches for the design and analysis of these, considering the topology optimizations and the software used. Additionally, discussions on powder bed fusion additive manufacturing of lattice structures are included in the chapter with different aspects of the technique including but not limited to process parameters and process boundaries. All the provided information is supported with application examples from various industries.

The wire arc additive manufacturing (WAAM) is one of the advanced manufacturing processes to fabricate full-density 3D Inconel 718 (IN718) metal parts in an open freeform environment. Thus, there is no size restriction of the fabricated parts using this process which is suitable for industry-led medium to large production supply chain. So far, the use of WAAM process in the fabrication of IN718 parts is solely focused on the structure–property relationship under heat-treated conditions. Therefore, the present study is attempted to investigate the effects of welding parameters, heat-treatment, and high-oxidation temperature on the processing–microstructure–property relationship of IN718 alloys manufactured via gas tungsten arc welding (GTAW)-based WAAM process. A wrought IN718 alloy was also studied for comparison.

It was observed that increasing the arc current increased the width and reduced the height of the walls as a result of higher surface tension and arc pressure acting upon a constant volume of material under constant wire feed speed and travel speed. A complete opposite trend was seen with increasing wire feed speed under constant arc current and travel speed. Increasing the travel speed adversely affected both the width and height of the walls due to the deposition of lower volume of material. Irrespective of welding conditions, a highly textured and homogeneous microstructure of γ-matrix was developed parallel to the build-up direction. Due to the elemental segregation of heavy elements, the matrix microstructure was mostly composed of Nb-depleted dendritic core region (DCR) along with Nb-enriched interdendritic region (IDR). The mechanical properties in terms of microhardness and tensile strength were found to be similar and independent of the effect of processing parameters. A modified homogenization (1100 °C for 1 h/air cooling)-annealed (720 °C for 8 h/furnace cooling at ~71.2 °C/h to 620 °C for 8 h/air cooling) condition was performed on WAAM IN718 alloys to dissolve laves phase and precipitate out strengthening phase of γ″. The heat-treated WAAM parts showed weakly anisotropic tensile properties at room temperature and exceeded the minimum requirements for cast IN718, but not that of wrought IN718 due to its large columnar grain structure. The high-temperature oxidation study at 1000 °C revealed that the kinetics of oxidation followed the parabolic rate law and were independent on the thermal history, microstructural, and compositional heterogeneities of WAAM parts. Both AF and HA alloys formed oxide scales that were identical in nature. The external oxidation of the protective Cr₂O₃ scale was formed at the air/alloy interface, which was covered by an outermost thin layer of rutile-TiO₂ and spinel-MnCr₂O₄ at air/scale interface. The internal oxidation of Nb-rich rutile-Ti_0.67Nb_1.33O₄ scale at the scale/alloy interface and subscale of Al₂O₃ within the alloy was observed. Based on the thermodynamic data and kinetics abilities of metal cations, a mechanism of oxide layer formation was suggested.