Structural Connections for Lightweight Metallic Structures

Multiple Site Damage (MSD) and Widespread Fatigue Damage (WFD) became a concern after the Aloha incident in 1988. Both, Multiple Site Damage and Widespread Fatigue Damage are typical effects which may occur in structures of aging aircraft. They consist in the development of scenarios of sets of small cracks, which interact at a certain stage and may suddenly burst into one large crack. The chapter consists of five topics, namely the description of the relevance of such an assessment method from an industrial point of view, the fast calculation method, which is used to account for the interaction of cracks in riveted joints, a Monte Carlo Simulation (MCS) method based on the model mentioned above, typical results of such a MCS procedure and its validation and in the end some considerations of methods which may be used to improve the calculation effort needed for a MCS.

This chapter starts with an overview of the fusion welding processes used in aluminium welding and further progresses by analysing in detail the characteristics of laser welding of aluminium. Laser sources for welding are available for a few decades but new concepts are coming to the market. The chapter addresses the most commonly used lasers for materials processing, CO₂ and Nd-YAG (neodymium–yttrim aluminium garnet) and their interaction with aluminium alloys in welding applications. More recent laser types are also included, namely fibre lasers and disc lasers as, though only more recently available in the market, their potential is foreseen as being interesting for welding of aluminium. Hybrid laser MAG (Metal Active Gas) welding has proven to lead to good results in welding aluminium plates namely for long seam welding.

Since extensive literature is published on the topic of laser welding in different bibliographic resources, this chapter is intended to present only brief discussions on important aspects of the laser welding process. Taking into account that the automotive industry is the main industrial sector where laser welding process is widely used, the impact of the laser welding process on this industry is analyzed compiling the fundamental applications of this welding process into car manufacturing process. That includes one of the most significant improvements in lightweight structures manufacturing for the automotive industry, the tailor welded blanks. This chapter, besides introducing the main features of the laser welding process, shows the breakthrough that this welding process involved in the automotive industry.

The friction stir welding (FSW) process was invented in 1991 by Wayne Thomas et al., one of the authors of this chapter. This machine tool based process is currently considered an important development in welding technology, saving costs and weight for a steadily expanding range of applications of lightweight metallic structures. Evidences of the disruptive character of the FSW process are the prompt adoption by world-wide industry of the significant advantages of FSW and the numerous technic-scientific papers and patents published. The FSW technology has been subjected to the most demanding quality standard requirements and used in challenging industrial applications over a wide range of structural and non-structural components. In this chapter, some of the basic fundamentals underpinning the invention of FSW technology are presented with emphasis for the concept of the third-body region. The state-of-the-art concerning tooling in FSW for conventional and bobbin stir welding approaches are introduced. The non-destructive testing assessment of the most relevant imperfections in FSW is also discussed for butt and lap joints. In summary, the FSW is a key joining technology for lightweight metallic structures. The international organization for standardization standard for welding aluminium alloys by FSW is available and the most recent European standards for design of structures—Eurocodes, already include guidelines for the application FSW process.

Even if in the last years several researches have studied the Friction Stir Welding (FSW) process, it should be observed that most of these studies are concerned with the butt joint and just a few of them extend to more complex geometries. It is worthy to notice that the acquired knowledge on FSW process of butt joints is not immediately extendable to lap and T-joints. The first observation is that in butt joints the surface to be welded is vertical, while in lap and T-joints it is horizontal and placed at the bottom of the top blank to be welded; in this way a major vertical component of the material flow is required to obtain sound joints. In the FSW of lap-joints four different geometrical configurations are possible—actually reducible to two—on the basis of the combination of the mutual position of the sheets to be welded and of the tool rotation direction, strongly affecting the process mechanics and the effectiveness of the final part. Furthermore, in the FSW of T-parts a proper clamping fixture is needed in order to fix the stringer during the process; such fixture is characterized by two radii, one for each side of the joints, corresponding to the radii between skin and stringer in the final welded part (corner fillets). Actually during the FSW process such radii must be filled by the flowing material. Consequently, an actual forging operation is required to force the sheet and the stringer material in fulfilling the radii of the clamping fixture, resulting in the radii of the T-joint. In other words, the material flow induced by the tool in the FSW process must be effective enough to get both the bonding of the two blanks and the fulfillment of the fixture radii. On the basis of the above observations, once the material of the blanks to be welded is selected, the most effective set of operating and geometrical parameters that optimize the FSW of butt joints will not, in all probability, work for the lap or T-joints. In particular, the tool geometries together with the tool feed rate and rotating speed must be redetermined in order to get an effective material flow and bonding conditions during the FSW process since the plastomechanics of the two processes are completely different. The specific peculiarities of the two processes must be properly investigated and the correlations between the characteristics of the materials to be welded and the mechanics of the welding configurations must be highlighted.

The present chapter seeks to summarize the ideas of damage tolerance of aircraft panels, keeping in mind the effect of residual stresses. First concepts and techniques are briefly reviewed, and afterwards their application to experimental work is discussed.

Aston Martin uses lightweight materials in combination with adhesive bonding technology to manufacture its vehicles Body-in-White and this Chapter gives an overview of the motivations, advantages and challenges behind this choice.

A detailed material breakdown and associated joining processes is presented, showing how Aston Martin succeeded in manufacturing a leading performance multi-material automotive body structure. It is also highlighted how joint design and materials selection were carried out for structural and semi-structural applications.

It is demonstrated how adhesive bonding technology and processes were developed and optimised to overcome some of the challenges associated with low volume manufacturing of lightweight, multi-material automotive structures.

Product development is limited by engineering design capabilities. Engineering design is one of the most important phases during the development of a new product, particularly in the case of complex and safety-critical systems, in order to consider all safety concerns, e.g. in aircraft and nuclear power plants. In these cases, the introduction of new design concepts and solutions is tightly tapered by existing materials and manufacturing processes. In this chapter, a breakthrough joining process—friction stir welding (FSW)—is discussed from the point of view of manufacturing costs. Friction stir welding is a solid state welding process, widely considered one of the most relevant advances in welding technologies in the last decades. The final cost of the manufacturing processes has a fundamental role in the success of its infusion and massification since this is one of the most important drivers in almost all industries. The application of FSW in new product development for aerospace components allows large weight and cost savings.

Structural weight reduction is a major driver to improve the transportation efficiency particulary in aeronautics. However the lightweight structural designs can be too costly. Indeed, minimum-weight designs are frequently too costly to manufacture, whereas less expensive and easy to fabricate and assemble designs are often much heavier. The most efficient design on the basis of both cost and weight often lies between these two extremes. The current trend in structural materials selection, by the principal commercial aircraft producers, consists of the extensive use of composite materials in the airframe, as in the last generation of twin aisle aircrafts. Composite materials have high specific strength, are less prone to fatigue crack initiation and provide enhanced flexibility for structural optimization compared to the aluminum alloys. On the other hand, aluminum alloys display higher toughness and better damage tolerance in the presence of defects. In order to improve the material selection and the comparison of airframe materials, this chapter presents an weight assessment based on the specific weight for different damage scenarios taking into account their damage tolerant properties.