Table of Contents
Related Titles
Title Page
Copyright
Foreword
Preface
List of Contributors
Part I: Metals
References
1: Steel and Iron Based Alloys
1.1 Introduction
1.2 Sheet Steels
1.3 Forging Steels
1.4 Casting Steel
References
2: Aluminum and Aluminum Alloys
2.1 Introduction
2.2 Wrought Alloys and Associated Processes
2.3 Casting Alloys and Associated Processes
2.4 Secondary Processes
2.5 Case Studies
2.6 Summary and Outlook
2.7 Further Reading
Acknowledgment
References
3: Magnesium and Magnesium Alloys
3.1 Introduction
3.2 Wrought Alloys and Associated Processes
3.3 Cast Alloys and Associated Processes
3.4 Other Aspects
3.5 Case Studies
3.6 Summary and Outlook
3.7 Further Reading
References
4: Titanium and Titanium Alloys
4.1 Introduction
4.2 Fundamental Aspects
4.3 Applications in Automobiles, Aerospace, and ShipBuilding
4.4 Future Trends
4.5 Further Reading
References
Part II: Polymers
5: Thermoplastics
5.1 Introduction
5.2 Fundamentals and Recent Advancements in Thermoplastics
5.3 Processing and Evolution of Structure – Basics and Recent Developments
5.4 Properties
5.5 Summary
Acknowledgment
References
6: Thermosets
6.1 Introduction
6.2 Advanced Thermosets and Associated Processes
6.3 Thermosets for Coatings and Adhesives
6.4 Case Studies – Thermoset Composites
6.5 Summary and Outlook
References
7: Elastomers
7.1 Introduction
7.2 Classification of Elastomers
7.3 Natural Rubber
7.4 Synthetic Rubbers
7.5 Thermoplastic Elastomers
7.6 Fluorine-Containing TPEs
7.7 Bio-Based TPEs
7.8 Conclusions
References
Part III: Composites
References
8: Polymer Matrix Composites
8.1 Introduction
8.2 Further Reading
References
9: Metal Matrix Composites
9.1 Introduction
9.2 Relevant MMC Systems
9.3 Case Studies
9.4 Summary and Outlook
9.5 Further Reading
Acknowledgments
References
10: Polymer Nanocomposites
10.1 Introduction
10.2 Fiber-Reinforced Nanocomposites
10.3 Sandwich Structures
10.4 High-Temperature Fiber-Reinforced Nanocomposites
10.5 Age and Durability Performance
10.6 Concluding Remarks
References
Part IV: Cellular Materials
References
11: Polymeric Foams
11.1 Introduction
11.2 Blowing Agents for Polymer Foams
11.3 Thermoplastic Foams: Conventional Processing Technologies
11.4 Thermoplastic Foams: New Trends, Materials and Technologies
11.5 Thermosets Foams: Conventional Processing Technologies
11.6 Thermosets Foams: New Trends, Materials and Technologies
11.7 Nanocomposite Foams
11.8 Case Studies
11.9 Summary and Outlook
11.10 Further Reading
Acknowledgments
References
12: Metal Foams
12.1 Introduction
12.2 Foams Produced by Means of Melt Technologies
12.3 Foams Produced by Means of Powder Metallurgy (P/M)
12.4 Porous Structures for Structural Applications Produced from Wires and Other Half-Finished Parts
12.5 Case Studies
12.6 Summary and Outlook
12.7 Further Reading
Acknowledgments
References
Part V: Modeling and Simulation
13: Advanced Simulation and Optimization Techniques for Composites
13.1 Introduction
13.2 Multiphysics Homogenization Analysis
13.3 Probabilistic Homogenization Approaches
13.4 Optimization
13.5 Summary and Conclusions
References
13:An Artificial-Intelligence-Based Approach for Generalized Material Modeling
14.1 Introduction
14.2 Strain Measures
14.3 Stress Measures
14.4 Example
References
15: Ab Initio Guided Design of Materials
15.1 Introduction
15.2 Top-Down and Bottom-Up Multiscale Modeling Strategies
15.3 Ab Initio Based Multiscale Modeling of Materials
15.4 Modeling of Ultralightweight Mg–Li Alloys
15.5 Ternary bcc MgLi–X Alloys
15.6 Summary and Outlook
15.7 Further Reading
Acknowledgments
References
Part VI: Higher Level Trends
16: Hybrid Design Approaches
16.1 Introduction
16.2 Motivation
16.3 From Monomaterial to Hybrid Multi-Material Design Approach in the Automotive Sector
16.4 ULSAB AVC Project/FSV Future Steel Vehicle Project
16.5 S-in Motion – Steel BiW Project
16.6 Multi-Material Hybrid Design Approach
16.7 Optimum Multi-Material Solutions: The Reason for Hybrid Design Approach
16.8 SuperLIGHT-Car Project
16.9 Hybrid Solutions: Overview of Current Automotive Production
16.10 Trends in Automotive Materials and Structural Design
16.11 Hybrid Solutions in Aircraft, Rail, and Ship Market
16.12 General Aspects on Joining Technologies for Multi-Material Mix
16.13 Conclusion
References
17: Sensorial Materials
17.1 Introduction
17.2 Components
17.3 Case Study
17.4 Further Reading
Books:
Conferences:
Acknowledgment
References
18: Additive Manufacturing Approaches
18.1 Introduction
18.2 Metal Materials
18.3 Nonmetal Materials
18.4 Secondary Processes
18.5 Case Studies
18.6 Summary and Outlook
18.7 Further Reading
References
Index
Related Titles
Schmitz, G. J., Prahl, U. (eds.)
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2012
ISBN: 978-3-527-33081-2
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The Editor
Dr.-Ing. Dirk Lehmhus
University of Bremen
ISIS Sensorial Materials
Scientific Centre
Wiener Stra{\ss}e 12
28359 Bremen
Germany
Prof. Dr.-Ing. Matthias Busse
University of Bremen
ISIS Sensorial Materials Scientific
Centre and Fraunhofer IFAM
Wiener Stra{\ss}e 12
28359 Bremen
Germany
Prof. Dr.-Ing. Axel S. Herrmann
Faserinstitut Bremen e.V.
Am Biologischen Garten 2
28359 Bremen
Germany
Prof. Dr. Kambiz Kayvantash
Cranfield University
School of Applied Sciences
Building 61, Cranfield campus
Cranfield, MK43 0AL
UK\hb
and\hb
CADLM Sarl
43 rue du Saule Trapu
91300 Massy
France
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Foreword
Transportation has at all times given engineers, craftsmen, and builders a challenge: making it easier to move. This means: make it lighter, stronger, more reliable, more comfortable, easier to build, less expensive…all with available materials.
Although means of transportation have been designed and built over millennia, truly engineered products have arisen recently. The first engineering book, La Science des Ingénieurs, by Belidor, was published in 1729. A. Wöhler developed the theory of mechanical fatigue and used it to improve the rolling stock of Northern German Railways in the mid-nineteenth century. A.N. Krylov applied modern stress analysis methods to ship design, based on his initial publication in 1906. A.A. Griffith published his theory of the strength of glass fiber, underlying structural composite performance, in 1920. The commercial production of glass fibers, by Owens Corning, dates back to no further than 1935 Ref. [1]. Also the field of polymer chemistry for technical plastics – the molecular design of new materials – emerged in the second half of the twentieth century. This paved the way for the widespread replacement of natural, renewable materials for strong lightweight structures.
We are only a century away from the major roots of our current design and materials science for transportation! But the new scientific tools developed since then have allowed engineers to tackle some big challenges during the twentieth century.
Big strides were made, for example, in protecting metal structures from corrosion using organic coatings, and where relevant, using noncorroding materials such as stainless steel (patented in 1912) and plastics.
Sometimes problems with fabrication processes led to improved design rules. During World War II, the massive failure of a number of Liberty Ships practically gave birth to the discipline of fracture mechanics. More than 2700 Liberty Ships were built between 1941 and 1945 at unprecedented war-time rates. “One shipyard built a Liberty ship in five days. The massive increase in production was possible in large part because of a change from riveted to electric-arc welded construction… saving a thousand tons of weight in the hull” Ref. [2]. The better understanding of crack propagation led to durable progress in ship – and other – production thereafter.
Air transport, with its literally sky-high requirements, stimulated a race for energy efficiency and mass reduction, and thus prompted great strides in the development of new grades of aluminum, titanium, and carbon-fiber composites. A significant step in modern aircraft design was the De Havilland Mosquito, whose airframe largely consisted of sandwich structures built up of thin skins of laminated plywood over a balsa core. Its lightweight and aerodynamics gave it a speed superior to any other airplane of the day. Sandwich panels are now widespread in all kinds of vehicles, including trains.
The emergence of powerful and affordable computers quickened the pace of development by providing a means to create and “test” designs numerically. It thus provided a new way to meet the challenge of improving the robustness of material models and predictive simulation. This led to significant progress in reliability and safety. A second effect was the capacity to imagine and realize structures with much more complex geometries. Ensuring, in particular, the safety of ever more sophisticated structural systems became intimately linked to finding the compromise between strength and formability in new-generation materials, be they steel alloys, nonferrous alloys, or structural plastics.
Airplanes, boats, trains, trucks, and cars today are thus built of very different materials than they were even 50 years ago.
In this twenty-first century, armed as we are with numerical tools and a rich knowledge of materials and process science – to which this book is contributing – the new challenge is applying our technological base to make our prosperity last and share it more widely. We have become obsessed with finding the optimal materials to push the performance envelope, improve safety, reduce costs, and preserve our resources – in a competitive global market environment.
From the point of view of materials and mechanical engineering, a number of standards have gained preeminence for various transportation applications such as plastic bumpers, magnesium steering wheels, composite train cabins, aluminum bus frames, titanium ship propeller shafts, and carbon-fiber composite structural components for aircraft. We have mastered the most pressing trade-offs so far, but the uncomfortable realization remains, that our best solutions are local, rather than global optima, increasingly subject to extrinsic factors.
Mass reduction for the improvement of energy consumption and emissions has become one of the principal drivers in material selection for transportation. The debate has evolved from the primitive level of asking which material – polymer composites or metals – future vehicles will be made of, to an understanding that significant progress can only be made by thinking in terms of pragmatic, fully optimized multimaterial designs used in appropriate vehicle architectures.
In the automotive sector, performance improvement has finally become synonymous with downsizing, for higher efficiency in use. The newest generations of internal combustion engines, with outputs upward of 90 kW l−1 and 200 Nm l−1, rely on increasingly sophisticated materials with high temperature and dynamic wear resistance. A typical engine consists of more than 60 grades of metallic alloys and 20 types of organic materials, plus a variety of coatings and surface treatments. Electric vehicle (EV) motors, running at 20 000 rpm, will require yet different structural materials with improved creep and magnetic resistances.
Although very promising, however, advanced materials such as carbon-fiber composites are not suited for some of the applications with the strongest projected growth. For both individual and shared means of transportation, the access to mobility in new markets is opened under different economic constraints. Here the conditions for success are low initial and operation costs, local availability of materials, damage tolerance under loads that are more severe than in countries with mature infrastructures, good repairability, and adaptability to a widely varying, sometimes minimal industrial system.
Durable development requires not only robustness and durability of the means of transportation itself but also a design that favors repair, reuse, and recycling. The recycling pathways for many material classes are not yet reliable or even inexistent in many regions. Designs that favor easy separation for maximal recovery rates will become an essential element of a durable mobility for all.
Finally, for a global economy that is growing well above 3% per year according to the World Bank, using recycled materials will not suffice to cover our industries' needs – no matter how intensely we recycle. At such growth rates, we will have to continue injecting primary materials into the production streams. That means that we must pay increased attention to our resources, with the aim of reducing the footprints in minerals and metals, energy, and water. Given the expected long-term upward trend in energy costs, getting a grip on our material consumption will soon be synonymous with protecting ourselves against cost impacts, and thus finding a better path toward durable business.
Several trends are outlining the way ahead for structural materials:
We are indeed shifting our paradigm from one of pure technical efficiency to one of a broader sustainable material management (SMM). As a recent European report remarked, “the EU is the world region that outsources the biggest part of resource extraction required to produce goods for final demand” Ref. [3]. SMM is thus not just a catchphrase for durability – be it environmental or business – but in fact one of raw material security. In several ways, beyond the pure technical challenge, mass reduction is becoming a strategic society issue.
The general term of ecodesign is gaining a strong, concrete footing as a credible practice to make “green” good for business. Michael Ashby has dedicated a recent book to the issue, extending his popular material selection charts to cover environmental criteria for broad classes of materials and applications. Ref. [4]. A metric of particular relevance, in relation with the mainstream considerations of CO2 emissions, is the energy trade-off between manufacturing and in-use consumption. Life cycle analysis (LCA) is thus finally becoming established as an economic tool, with strong bearing on not only corporate social responsibility, but also on mid-to-long-term competitiveness. It helps us understand where, in the supply chain, we can reduce our energy impacts, and therefore costs. Such approaches aim at guiding economical material selection in a broader perspective. Beyond the traditional steel versus aluminum versus composite choices, they allow us to construct optimized material–process–localization combinations or aim for the right pace of introduction of recycled materials.
The conditions of success are multiple. Starting with reliable and comparable data, we need to define new value equations that integrate extended design drivers such as sustainability, and allow arbitration between them. In some cases, we will need to make hard choices between adapting the requirements to available technologies that may be more economical or durable and developing new technologies when more sophisticated materials are considered readily accessible over the longer term. Third, management needs to provoke and actively support more interdisciplinary research and improved cross-functional interactions within and between companies. Underlying all this is an integrated approach that gives appropriate simultaneous consideration to design, materials, and process: a truly production- and life-cycle-oriented engineering. And finally, we may simply need to have more courage and willingness to make decisions that could pay off only in the midterm – to truly build a vision.
The future of structural materials lies in optimized hybrid structural systems that conform to a larger set of requirements and constraints. A commonly heard expression is “the right material at the right place, at the right time.” In order to develop durable solutions to the technical challenges of safe and efficient transportation, we will need to solve equations that integrate macro-economic and policy factors, trade considerations and externalities, and customer appeal and reassurance.
To do this successfully, the necessary starting point is a comprehensive knowledge of the options, of the potentials, and limitations of the different classes of structural materials. This is precisely what this book aims to give you. Beyond the basic yet extensive design and process reference data, it aims to bring you new insights based on recent, first-hand information from some of the top research centers in the EU.
As we look toward the future, we will want to keep our scope wide: state-of-the-art research is continuously extending the horizon for novel, sometimes surprising applications of classical materials. This is the spirit behind concrete canoes, carbon-fiber-reinforced bridges, plastic engines, and cars made of castor-oil-based polymers. Having ready access to comparable data will, it is hoped, promote synergies between fields of application.
We are confident that this book will be useful to you to orient your current material selection and also as a starting point to imagine the means of transportation of tomorrow.
Dr Patrick Kim
VP R&D Benteler Automotive
formerly VP Materials Engineering
Renault
References
1. Timoshenko, S.P. (1953) History of Strength of Materials, McGraw-Hill.
2. matdl.org/failurecases/Other_Failure_Cases/Liberty_Ship.
3. Wuppertal Institute et al. (2010) Sustainable Materials Management for Europe – from Efficiency to Effectiveness. Report for the Belgian Government, March 2010, http://www.euractiv.com/sites/all/euractiv/files/SMMfor%20EuropeStudy_0.pdf.
4. Ashby, M. (2009) Materials and the Environment, Butterworth-Heinemann.
Preface
This book is meant to provide an introduction to current developments in the field of structural materials for the transportation industry. This includes rail, maritime, automotive, and aerospace industries, with a focus on the last two. Deliberately excluded from the scope are purely functional materials.
Quite literally, structural characteristics of materials are the backbone of any engineering design. They provide self-supporting capabilities to components where mechanical stability is a secondary concern. Whenever the bearing of mechanical loads becomes the primary role, and materials are optimized in view of this demand, we speak of structural materials. Solutions that address this challenge are what this book revolves around. The perspective chosen to deal with the topic is that of materials science and engineering. We have structured our work accordingly by dedicating the central parts to the main material classes.
However, any structural material we see in a specific application is in fact a combination of material and process. Its properties are defined by both, and thus, treating one aspect while neglecting the other is not a viable option. Besides, it is development tools which are built on similarly advanced modeling and simulation techniques that finally enable usage of emerging materials by allowing their evaluation in diverse application environments. With this in mind, we have included these aspects in our book. In all of them, the perspective is forward-facing: We do not intend to comprehensively cover the fundamentals of the various fields. Instead, we have attempted to identify major trends and highlight those that we see at the threshold to practical application.
Materials are evolving. So are the processes associated with them, as well as the tools and methodologies that allow their development and application. The rate of change in material development is dictated by external pressure. We have defined the major periods in the development of early mankind by the structural materials that dominated them. When bronze technology evolved, stone had to yield. The same occurred to bronze once iron became available on a larger scale. In transportation, we have seen change from wood and other natural materials to metal, and nowadays to composites. The rate of change steps up once pressure rises. The period of time that one material prevails appears to become shorter and shorter. On the other hand, since the shift from natural to technical materials, we mostly observe additions to the spectrum of materials rather than complete replacement. This may not be true for individual exponents of a material class, but definitely so for the classes themselves. Transportation, in all its width, is currently under considerable pressure to increase resource efficiency. One major handle to achieve this is lightweight design. This affords either new structural concepts or new materials offering improved performance. Very often, both go hand in hand. While such general pressure strengthens the motivation to search for entirely new approaches, it will also fuel inter-area competition. The past has shown that this may significantly speed up developmental processes within one class of materials. A good example in this respect is the recent evolution of high-strength steels, which took place at least partly in response to aluminum-centered automotive body designs entering volume production. A comparable situation can be observed in the commercial aircraft industry, where large-scale introduction of carbon-fiber-reinforced composites challenges the established status of aluminum alloys. New production processes support such tendencies are enablers of cross-fertilization between modes of transport: considering their properties, automotive design could profit greatly from application of carbon-fiber-based composites, too, but the sheer cost of state-of-the-art aerospace materials and processes forbids immediate takeover. Adaptation of processes to match another industry's needs, like transition from single part to large-scale series production, can help diminish such barriers. As a result, we currently see an extremely high rate of change in the range of available materials for load-bearing structures in the transport industry. With this in mind, it is the conviction of editors and authors of this book that a work is needed that familiarizes materials scientists, design engineers, and innovation managers in industry with developments in structural materials science and engineering that are likely to find their way into high-technology products within the next 5–10 years. We do give some background on the various materials and technologies, but the major focus is on what is currently on the verge of application.
Besides our primary target audience, we are confident that students and graduates in mechanical engineering, as well as academic researchers in the field, will find this compilation helpful to first get and then adjust their bearings through a highly dynamic field of research. In this sense, we intend our book to serve as a guideline for both groups. As such, it is meant to give them the first idea of the respective material class as well as a clear vision of where the present focus of developmental work will lead it within the near to mid-term future. This knowledge base shall allow them to decide which material classes and subclasses to study in more detail in view of their specific interest. Suggestions on where to search for in-depth fundamental information and keep track of future advances shall complete the picture.
The book is structured along the major material classes relevant for transport industry structural applications. All of these are treated in separate parts, starting with metals (Part I) and proceeding via polymers (Part II) and composites (Part III) to cellular materials (Part IV). Each part covers associated processes on the level of its individual chapters, that is, for the exemplary case of metals separately for iron-, aluminum-, magnesium-, and titanium-based materials (Chapters 1–4). In a further section (Part V), selected aspects of modeling and simulation techniques are being treated. Highlights have been set here in terms of modeling approaches covering multiple scales of material description (Chapter 13) and adaptation of artificial intelligence (AI) techniques to material modeling (Chapter 14). The use of fundamental ab initio techniques in designing new metallic material compositions and states is treated in yet another subsection (Chapter 15). Finally, specific trends that go beyond an individual class of materials are discussed in Part VI. An example are hybrid design approaches, which attempt to locate the optimum material for a purpose at the place where best use can be made of its properties, thus leading to complex, multimaterial structures (Chapter 16). In extrapolating trends already discussed in terms of structural health monitoring for composite materials (Chapter 8), material-integrated sensing and intelligence, summarized under the descriptive term of sensorial materials, are covered in Chapter 17. Additive manufacturing as an approach with promise for highly versatile production and structural complexity that in some respects cannot be reached by other processes is presented in the final chapter (Chapter 18).
We have attempted to organize each of the main chapters in Parts I–IV and Part VI in a similar way. In these predominantly material-related chapters, we start with some fundamentals and go on to detail new developments. In this, we do not separate material and process because of the close link between both. However, we do subdivide the chapters according to distinctions that are already established for the respective class of materials. An example is the distinction between wrought and cast alloys realized in the chapters on aluminum and magnesium. A similar approach, though adapted to the specifics of such composites, is reflected in the separation between processes involving thermoplastic versus those employing thermoset matrices in the chapter on polymer matrix composites. The major chapters are concluded with a section on further reading for intensified study and a hint at major organizations, conferences, or other events dedicated to the respective topic.
We are extremely grateful to the many authors who have shouldered the task of providing the content to this work. We are indebted to Dr. Martin Preuss (Wiley-VCH) who encouraged us to venture this endeavor, which was originally based on two symposia organized in the course of the Euromat 2009 Conference held in Glasgow (UK) from 7 September 2009 to 10 September 2009 (www.euromat2009.fems.eu). Finally, our thanks go to Lesley Belfit, again of Wiley-VCH, who helped us steer our course through all the hindrances of the editorial process with grace and patience.
Dirk Lehmhus
Matthias Busse
Axel S. Herrmann
Kambiz Kayvantash
List of Contributors
Part I
Metals
Metals belong to the eldest engineering materials of the humankind. The history can be traced back to the Copper Age when metallic products were firstly made of native metals. However, in contrast to natural materials, such as wood or stone, native metals were very rare. The rising needs of metallic products implied the invention and control of metallurgical processes, such as ore smelting and casting. In the period of the Bronze Age (2200–800 BC), when copper was firstly alloyed with tin, followed by the Iron Age (1100–450 BC), when advanced smelting and working techniques were introduced, the cornerstones for the industrial production were put in place. Since the beginning of the industrial production in the late eighteenth century, the demand for metals has been increased steadily (see Figure P1.1) [1]. The rapid technological development has led simultaneously to today's alloys and production processes. Nowadays, metals are spread in all technical applications of human life. The present means of transportation would have been inconceivable without metals.
Figure P1.1 World's annual production during the last century (following [1]).
On microscale, metals are characterized by a crystalline structure. Electrons of the outer shell being freely movable between the atomic cores keep the crystal together. The electron's free mobility is responsible for both the high electrical and thermal conductivity of metals. Furthermore, the formation of close atomic packing within the crystal [2] enables a ductile atomic sliding under mechanical shear load. The siding proceeds on the densely packed planes along the close packed directions without changing the bonding conditions. The spatial arrangement of these sliding planes is shown in Figure P1.2. The number of sliding systems is the product of the number of sliding planes and the number of close packed sliding directions. While body centered cubic (BCC) and face centered cubic (FCC) structures, which can be found in ferritic steel and aluminum alloys exhibit respectively 12 and more systems for atomic sliding, the number of sliding systems in hexagonal crystal structures, which are typical for magnesium and α-titanium alloys, is dependent on the c/a ratio of the unit cell. Besides three basal sliding systems, alloys with c/a below 1.63 exhibit up to nine additional pyramid and prism sliding systems. In polycrystals, a number of more than five enables any deformation [3]. This fact explains the outstanding plasticity of most metals and, consequently, their excellent workability.
Figure P1.2 Sliding planes of (a) body centered cubic (BCC), (b) face centered cubic (FCC), and (c) hexagonal crystal structure.
The mass and the diameter of atoms as well as how close atoms are packed determine the density of a material [2]. Metals with a density of below 5 g cm−3 belong to the class of light metals [4]. In this class, magnesium is the lightest metal with a density of about 1.74 g cm−3 followed by aluminum with 2.70 g cm−3, and titanium with 4.50 g cm−3. The density difference between the pure metal and its alloys are usually very low. With a density of 7.86 g cm−3, iron, as the base metal of steel, belongs to the class of heavy metals. Because of its very high Young's modulus, and the variety of technical and metallurgical means to reach highest strength properties, nevertheless, this metal is still among the materials being attractive for light-weight designs.
Weight reduction is an important subject for the transportation industry to improve their products' energy efficiency, particularly during the operation time. Low energy consumption benefits the reduction of CO2 emission and helps the electric mobility by increasing the transportation range. The main lightweight drivers are illustrated in Figure P1.3 [5].
Figure P1.3 Drivers of weight reduction and multimaterial design as a strategy for weight reduction (following [5, 7]).
Generally, the potential of materials for weight savings can be evaluated by regarding their density-specific properties [6]. Compared to fiber-reinforced polymers (FRPs), metallic materials, such as steel, aluminum, and magnesium alloys as well as titanium alloys, provide an excellent impact toughness, a good wear, and thermal resistance as well as a high life cycle fatigue along with moderate materials and processing costs and, furthermore, an outstanding recyclability.
Within this material class, steel is distinguished by its high specific tensile stiffness combined with a good fracture toughness and low material costs. Light alloys such as aluminum and magnesium alloys are characterized by high specific bending stiffness and strength values along with excellent compression stability. Titanium alloys are more expensive, but advantageous when, for example, a very high specific tensile strength and a good wet corrosion resistance is required. Figure P1.4 gives an overview of the mechanical properties for different loading conditions relative to those of steel as the reference material.
Figure P1.4 Density and relative mechanical properties of metals (a) for tension/compression load and (b) for bending/buckling load (reference: steel) (values according to [6]).)
However, owing to the ongoing need of weight reduction, alternative structural materials, such as FRP, penetrate more and more into transportation applications (see Figure P1.3). Compared to metallic materials, for example, carbon-fiber-reinforced polymers (CFRPs) are still very expensive, but on the other hand they exhibit specific strength and stiffness values, which are significantly higher than those of metallic materials. Consequently, effective weight reduction calls for advanced multimaterial designs (Figure P1.3) [7]. Thus, in order to ensure a cost efficient weight reduction, a strong position of metals is required. This necessitates a multidisciplinary approach that combines materials science and production technology with designing and dimensioning.
The following sections shall give an overview of recent advances in the development of steel, aluminum, and magnesium alloys as well as titanium alloys, which are intended for application in the transportation sector. Basically, the alloy development is closely related to process development, comprising a variety of production techniques associated with the development of advanced metallic products. Therefore, descriptions of selected process developments are also part of these sections.
References
1. Degischer, H.P. and Lüftl, S. (eds) (2009) Leichtbau–Prinzipien, Werkstoffauswahl und Fertigungsvarianten, Wiley-VCH Verlag GmbH, Weinheim, ISBN: 978-3-527-32372-2, p. 80.
2. Ashby, M.F. and Jones, D.R.H. (2005) Engineering Materials 1, 3rd edn, Butterworth–Heinemann as an imprint of Elsevier, Oxford, ISBN: 978-0750663809.
3. Gottstein, G. (2010) Physical Foundations of Materials Science, 1st edn, Springer-Verlag, Berlin, Heidelberg, ISBN: 978-3-642-07271-0.
4. Roos, E. and Maile, K. (2004) Werkstoffkunde für Ingenieure: Grundlagen, Anwendung, Prüfung, Springer-Verlag, ISBN: 978-3540220343.
5. Kasai, J. (2000) Int. J. Life Cycle Assess., 5 5316.
6. Ashby, M.F. (1999) Materials Selection in Mechanical Design. Butterworth-Heinemann Verlag, Oxford, ISBN: 978-0750643573.
7. Goede, M., Dröder, K., Laue, T. (2010) Recent and future lightweight design concepts–The key to sustainable vehicle developments. Proceedings CTI International Conference–Automotive Leightweight Design, Duisburg, Germany, November 9–10, 2010.
1
Steel and Iron Based Alloys
As a consequence of global warming the demand for transport vehicles with lower emissions and higher fuel efficiency is increasing [1]. Reduction of travel time and CO2 emissions per passenger is also noticed in transport applications. Different methods have been utilized to improve the fuel economy, which is also demanded by the governments. For instance, the Obama administration in the United States has recently proposed a law requiring a 5% increase in fuel economy during 2012–2016 and the development of the Corporate Average Fuel Economy (CAFE) to obtain an average automotive fuel efficiency of 35.5 miles per gallon by 2016 [1–3]. Since the development of new power generation system in hybrid-electric vehicles is expensive, new technologies aim to improve fuel economy through improving aerodynamics, advanced transmission technologies, engine aspiration, tires with lower rolling resistance, and reduction of vehicle weight [4]. Usage of new kinds of lightweight steels in vehicle design for automotive and transport applications is of great importance nowadays for enhancing safety, improving fuel economy, and reducing lifetime greenhouse gas emissions.
Sheet and forged materials for transport applications require both high strength and formability. High strength allows for the use of thinner gauge material for structural components, which reduces weight and increases fuel efficiency [5]. High strength also improves the dent resistance of the material, which is esthetically important. Another advantage of higher strength material is improved passenger safety due to the higher crash resistance of the material. Therefore, the high strength material must be formable to allow economical and efficient mass-produced automotive parts.
New tools and technologies such as modeling and simulations are being innovated for the design of steels for future ecologically friendly vehicles. There are also several different organizations that work on the evaluation of new kinds of lightweight steels. One of them is the World Auto Steel organization that has developed the latest version of the so-called advanced high strength steels (AHSSs) with the newest technology to evolve body structures with higher strength, lower weight, and consequently lesser CO2 emissions compared to the other body materials. Another program, new steel body (NSB), of ThyssenKrupp made a point of weight-saving potential of steel; it provides the benefits of tubular components with conventional stamped parts using advanced multiphase steels. Also, the future steel vehicle (FSV) program worked on the steel body structure designs. Over 35% mass reduction in the benchmark vehicle and approximately 70% reduction in the whole life cycle emissions are achievable in this program [6–10]. Finally, the most extensive research is however conducted by the ultralight steel auto body-advanced vehicle concepts (ULSAB-AVC) Consortium. This program was established about a decade ago by the collaborative efforts of 35 steel companies in the world to find steel solutions to the problems faced by automotive companies all around the world. The ULSAB-AVC program follows the ultralight steel auto body (ULSAB) program (results announced worldwide in 1998). Here, two important drivers for the ULSAB-AVC were the US Partnership for a New Generation of Vehicles (PNGV) and the EUCAR (The European CO2 reduction program) projects, which provided references for setting ULSAB-AVC targets [8–11]. This program focused on the development of steel applications for future vehicles, considering the increasing demands shown in Figure 1.1. Weight reduction, crash safety, compatibility with the environment, and economic features can be addressed as the main goals of this program [6–13].
Figure 1.1 Increasing demands on the transportation [13].
The ULSAB program deals with both high strength steels (HSSs) and ultrahigh strength steels (UHSSs), which were mostly conventional microalloyed sheet grades. Within the ULSAB-AVC program, the application of newer types of high-strength sheet steels, the so-called AHSSs, has been considered. Therefore, as the first task, a consistent classification of the various grades of steels has been identified in order to manage the goals of the program [4, 6–11]. In this terminology, steels are identified as “XX aaa/bbb,” where the first digits (XX; these are not constricted to be two digits) represent the type of the steel, the next digits (aaa) are responsible for the minimum yield strength (YS) of the material in MPa, and the last digits (bbb) depict the minimum achieved ultimate tensile strength (UTS) of the investigated steel. The world wide steel classification developed by the ULSAB-AVC program is reported in Table 1.1 [11]. The identified mechanical properties of the steels chosen for the ULSAB-AVC body structure, closures, ancillary parts, suspension, and wheels are reported in Table 1.2.
Table 1.1 The Universal Classification Introduced by the ULSAB-AVC Consortium [11].
Table 1.2 Mechanical Properties of Steels Chosen for the ULSAB-AVC Components [11].
There are various mechanisms available to strengthen steels. HSSs such as BH (bake hardening), HSLA (high strength low alloy), CMn (carbon manganese), or ISs (isotropic steels) have been optimized by the application of classical concepts such as solid solution strengthening, precipitation strengthening, and grain refinement. In addition, AHSS concepts such as DP (dual phase), TRIP (Transformation-Induced Plasticity), and CP (complex phase), or Mart (martensitic) use the multiphase character of the microstructure, which may also include phase transformation and additional deformation and strengthening concepts [1]. The special class of MnB (manganese boron) steels in some sense stands outside of this logic as it is based on martensitic structure (to achieve maximum strength, while accepting poor deformation values), which is reasonable only by application of the very specific processing concept of press hardening. Figure 1.2 shows a comparison between the tensile strength and total elongation of HSS and AHSS steels in comparison to HSS. Classical press forming steels such as mild steels and IF (interstitial free) are included, as well as austenitic steels for comparison, but will not be discussed in detail.
Figure 1.2 Strength-formability relationships for mild, conventional high strength steels (HSS), and three generations of advanced high strength steels (AHSS).
Source: After Ref. [9].
While HSS typically shows 10 GPa% ecoindex, first generation AHSS shows 15–20 GPa% and second generation AHSSs, namely TWIP