StructuralDurability: Methodsand Concepts

Ruediger Heim

Structural Integrity 17Series Editors: José A. F. O. Correia · Abílio M. P. De Jesus

Enabling Cost and Mass Efficient Products

Structural Integrity

Volume 17

Series Editors

José A. F. O. Correia, Faculty of Engineering, University of Porto, Porto, PortugalAbílio M. P. De Jesus, Faculty of Engineering, University of Porto, Porto, Portugal

Advisory Editors

Majid Reza Ayatollahi, School of Mechanical Engineering, Iran University ofScience and Technology, Tehran, IranFilippo Berto, Department of Mechanical and Industrial Engineering, Faculty ofEngineering, Norwegian University of Science and Technology, Trondheim,NorwayAlfonso Fernández-Canteli, Faculty of Engineering, University of Oviedo, Gijón,SpainMatthew Hebdon, Virginia State University, Virginia Tech, Blacksburg, VA, USAAndrei Kotousov, School of Mechanical Engineering, University of Adelaide,Adelaide, SA, AustraliaGrzegorz Lesiuk, Faculty of Mechanical Engineering, Wrocław University ofScience and Technology, Wrocław, PolandYukitaka Murakami, Faculty of Engineering, Kyushu University, Higashiku,Fukuoka, JapanHermes Carvalho, Department of Structural Engineering, Federal University ofMinas Gerais, Belo Horizonte, Minas Gerais, BrazilShun-Peng Zhu, School of Mechatronics Engineering, University of ElectronicScience and Technology of China, Chengdu, Sichuan, ChinaStéphane Bordas, University of Luxembourg, ESCH-SUR-ALZETTE,LuxembourgNicholas Fantuzzi , DICAM Department, University of Bologna, BOLOGNA,Bologna, ItalyLuca Susmel, Civil Engineering, University of Sheffield, Sheffield, UK

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Topics of interested include but are not limited to:

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Ruediger Heim

Structural Durability:Methods and ConceptsEnabling Cost and Mass Efficient Products

123

Ruediger HeimResearch Division Structural DurabilityFraunhofer-Institute for Structural Durabilityand System Reliability | LBFDarmstadt, Germany

ISSN 2522-560X ISSN 2522-5618 (electronic)Structural IntegrityISBN 978-3-030-48172-8 ISBN 978-3-030-48173-5 (eBook)https://doi.org/10.1007/978-3-030-48173-5

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Preface

Often, the term metal fatigue is introduced together with the high visibility ofspectacular mechanical failures which have been publicized from bridges, aircraftstructures, power turbines, or other structures. In 1982, an extensive study from theBattelle Columbus Laboratories was published in which the annual costs of materialfracture to the US economy were estimated in a range of 119 billion USD that isequivalent to 300 billion USD and about 1.5% of the GDP today. More than 53% ofthose costs were considered reducible, to a larger extent through the adoption ofbest practices, technology transfer, and education. Materials and structures whichare subjected to fluctuating loads in sufficient numbers are liable to fail by fatigue.In that study, hundreds of components were examined with regard to their indi-vidual failure cause and about 44% failed due to improper maintenance. Half ofthose incidents showed fatigue-related fractures. For other failure causes—such asmaterial defects or manufacturing issues—fatigue was the dominant mechanismthat finally led to the loss of structural integrity. Fatigue is the primary originator forunintentional material fractures of parts which are dynamically loaded. At thecomponent level, about 80% of mechanical failures are somehow related to fatigue,alongside other mechanisms such as wear, corrosion, or creep.

Many people still recall that in 1979 a McDonnell Douglas DC-10 crashed intothe ground shortly after taking off from the runway at O’Hare International Airportin Chicago. All passengers and the complete crew were killed as well as two morepeople on the ground because the left engine separated from its wing due tomaintenance-induced fatigue damage.

Another dramatic accident was the Eschede derailment in 1998 when 101 peoplewere killed because an ICE-type high-speed train from Deutsche Bahn derailed andcrashed into a concrete road bridge in the North of Germany. Few wheel sets of thatICE were equipped with resilient wheels which had a rubber damping ring betweenthe wheel body and the rail-contacting steel tyre. The latter became thinner due towear which limited its load-carrying capabilities and resulted in crack initiation andgrowth until it fractured.

v

The word fatigue is derived from the Latin ‘fatigo’ which means ‘get tired’; in atechnical context, it was used for the very first time in 1839 in a book on mechanicsby Jean-Victor Poncelet, who gained experience about mechanical failures of castiron axles and supposed that the axles became tired, or fatigued, after a certainperiod of operational use before they finally break.

Preliminary assumptions about the nature of metal fatigue were developedduring the industrial revolution in Europe when rail vehicles failed under cyclicloads. Later, this topic became even more important because of serious accidentswhich took place around World War II and the years after. At that time, simple andlow-cost liberty class naval cargo ships were built in a large number using extensivewelding instead of time-consuming riveting. Soon subzero temperatures led tobrittle fractures of the welds and the parent metals.

We now understand that fatigue failures can result in injuries, downtime andreduced availability, repair and rework, scrap, recalls, lost output and liabilityclaims. Fatigue-related incidents simply constitute a significant part of costs for amanufacturer. It sounds like fatigue is synonymic to disaster—a micro-mechanicalmechanism initiating cracks which start to grow and enter a macroscopic scaleviolating structural integrity.

While the fatigue of structural materials is virtually inevitable with certain typesof loading, the use of advanced design methods is concerned with avoiding or atleast mitigating the often very negative effects of material fatigue. Structuraldurability is one such method, which leads to very useful, engineer-scientificfindings on suitability for use and operating life. But structural durability is muchmore than just a risk assessment method, and here, we want to look at it in a slightlydifferent way: Structural durability is a genuine design method for particularlysustainable products and for reliable lightweight construction.

We basically understand that fatigue is a progressive structural damage ofmaterials and components under cyclic loads. Hence, fatigue life is an importantcharacteristic of an engineering component and is measured by the number ofcycles before a failure occurs. Since safety, reliability, and profitability are corecriteria for the development and successful commercialization of products,managing fatigue life according to customer expectations and product requirementsis an enabler for sustainability: Lifetime-oriented design makes it possible to use theminimum quantity of material, or the most cost-efficient material without com-promising fatigue life. Once we understand how to design a structure for a specifiedoperational life, overengineered features and un-efficient material utilization can beavoided. In this way, structural durability becomes a powerful method enablingsustainable design.

So, our mission is to analyze the in-service conditions carefully and to get thecomponent’s strength characteristic aligned to the requirements from the opera-tional loading. Having a well-balanced product which recognizes both stress andstrength in a proper way is much more than a safety pointer—it is a fundamentalapproach for a minimal effort design. That is the path toward mass and cost-efficientproducts which serve profitability while not violating future requirements ofenvironment and societies.

vi Preface

The structure of this book is based on my lecture at the University of AppliedScience in Kaiserslautern. For many years, I have been teaching the basics ofstructural durability there in an international master’s course of study, in whichparticular importance is attached to a consistent treatment of both load andstressability. In structural durability, material and component strength properties arecombined with typical operational load sequences to make a conclusion regardingthe expected service life.

A consistent and coherent presentation in this respect for students and engineersin mechanical engineering and mechatronics has been missing until now. Thepresent book is intended to close this gap.

It is a rather brief introduction, even in terms of its scope, which actually aims tobe read straight from the first to the last page. This book is a quick introduction tostructural durability and is intended to provide basic concepts and methods, i.e., anunderstanding of how structural durability works.

It is therefore not a specialist book for specialists—there are other excellentworks by well-known authors for this purpose.

I would like to thank Springer Nature for its courage to get involved in such anexperiment.

I thank the Fraunhofer LBF and the University of Applied Science inKaiserslautern for the opportunity to work, research, and teach in the field ofstructural durability.

And, finally, I would like to thank my wife Sandra for her patience with awriting spouse who, for more than a year, put his thoughts down on paper everyweekend at half past five in the morning. Of course, this is not right: Even I alreadyworked with a computer. Nevertheless, my wife was really patient with me.

Darmstadt, Germany Ruediger Heim

Preface vii

Contents

1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Environment and Emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Business Sustainability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2 Operating Life Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.1 Stress–Strength Interference . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.2 Load Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2.2.1 Counting Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372.2.2 Spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442.2.3 Finite Element Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 45

2.3 Strength Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 462.3.1 Fatigue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

2.4 Damage Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

3 Influencing Factors for Fatigue Strength . . . . . . . . . . . . . . . . . . . . . 673.1 Repeating Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 673.2 Material- and Process-Related Effects . . . . . . . . . . . . . . . . . . . . . 72

3.2.1 Material Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 733.2.2 Size Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 753.2.3 Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 773.2.4 Surface Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 863.2.5 Applied Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 883.2.6 Mean Stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 933.2.7 Process-Related Conditions . . . . . . . . . . . . . . . . . . . . . . . 973.2.8 Environmental Conditions . . . . . . . . . . . . . . . . . . . . . . . . 103

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

4 Fatigue Strength Under Spectrum Loading . . . . . . . . . . . . . . . . . . . 1094.1 Blocked-Program Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1094.2 Standardized Random-Type Testing . . . . . . . . . . . . . . . . . . . . . . . 111

ix

4.3 Limitations of the Miner’s Rule . . . . . . . . . . . . . . . . . . . . . . . . . 1164.4 Microplastic Straining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

4.4.1 Local Stress Approaches by Math Modeling . . . . . . . . . . . 1204.5 Weldings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126

4.5.1 Standards and Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . 1264.5.2 Structural Hot Spot Stress Concept . . . . . . . . . . . . . . . . . . 1364.5.3 Effective Notch Stress Approach . . . . . . . . . . . . . . . . . . . . 1394.5.4 Fracture Mechanics Approach . . . . . . . . . . . . . . . . . . . . . 142

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144

5 Structural Durability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1475.1 Working with Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1475.2 Experimental Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151

5.2.1 Severity Level of Operational Loads . . . . . . . . . . . . . . . . . 1595.2.2 Sufficient Amount of Test Results . . . . . . . . . . . . . . . . . . 1625.2.3 Accelerated Life Testing . . . . . . . . . . . . . . . . . . . . . . . . . 1645.2.4 Probability of Failure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170

5.3 An Application: Biaxial Wheel Fatigue . . . . . . . . . . . . . . . . . . . . 1725.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1725.3.2 Wheel Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1745.3.3 ZWARP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1815.3.4 Math Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185

5.4 Beyond ‘Safe Life’ and Traditional Durability Performance . . . . . 1905.4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1905.4.2 Fracture Mechanics Approach for a Multitiered Integrity

Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1965.4.3 Standards for Axle Design . . . . . . . . . . . . . . . . . . . . . . . . 1995.4.4 Metro Rail Vehicle Integrity Concept . . . . . . . . . . . . . . . . 200

5.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209

x Contents

Abbreviations

AC Alternating currentASD Acceleration spectral densityASIP Aircraft structural integrity programASME American society of mechanical engineersASTM American society for testing and materialsBCC Body-centered cubicBDC Bottom dead centerBRICS Brazil, Russia, India, China, and South AfricaBS British standardC Complexity of objectCAD Computer aided designCARLOS Car loading standardCoG Center of gravityCPSC Consumer product safety commissionCS Coordinate systemDAQ Data acquisitionDC Direct currentDIN Deutsche Industrie NormDOF Degree of freedomEAC Environmentally assisted crackingEN European NormEoL End of lifeEPS Equivalent pre-crack sizeEV Electric vehicleFAT Fatigue classFAW Front axle weightFCC Face-centered cubicFEA Finite element analysisFEM Finite element methodGDP Gross domestic product

xi

GFC Guide function for corneringGM General motorsHAZ Heat-affected zoneHCF High cycle fatigueHCP Hexagonal closest packedHFP High frequency peeningICE Internal combustion engineIIW International Institute of WeldingLCF Low cycle fatigueLEFM Linear elastic fracture mechanicsM Aesthetic measureMEMS Microelectromechanical systemsMIC Microbiologically influenced corrosionMPI Magnetic particle inspectionMY Model yearNDI Non-destructive inspectionNVH Noise, vibration, and harshnessO Order/symmetryOECD Organization for Economic Co-operation and DevelopmentOEM Original equipment manufacturerPDE Partial differential equationspdf Probability density functionPG Proving groundPoD Probability of detectionPSD Power spectral densityPWT Post-weld treatmentQ&T Quenched and temperedRAW Rear axle weightRFS Required fatigue strengthRMS Root-mean-squareSAE Society of Automotive EngineersSIF Stress intensity factorS-N Stress vs. lifeSUV Sports utility vehiclesTDC Top dead centerTIG Tungsten inert-gasUPT Ultrasonic peening treatmentUSAF US Air ForceUTS Ultimate tensile strengthVHCF Very high cycle fatigueZ standard scoreZWARP Zweiaxiale Räderprüfung

xii Abbreviations

Symbols

A Cross section after fractureA0 Initial cross sectiona Crack sizea0 Initial crack growing from material discontinuitiesa* Notch-related lengtha Constant in Wöhler equationb Coefficient in Wöhler equationC ConfidenceD Wheel press fit diameterD DamageDrel Relative damaged Adjacent cross section to wheel press fitda/dN Fatigue crack growth rateE Elastic (Young’s) modulusFv,stat Static force in vertical directionFx Force in x-directionFy Force in y-directionFz Force in z-directionH Cumulative number of cyclesH0 Reference number of cyclesi Number of repetitionsK Stress intensity factorKC Critical stress intensity factorKf Fatigue notch factorKt Theoretical stress concentration factork Slope of S-N curvek* Slope of S-N curve after kneepointk' Slope of S-N curve after kneepoint for damage accumulationl Number of lifetimesM Mean stress sensitivity

xiii

Mx Moment around x-axisMy Moment around y-axisMz Moment around z-axism Actual number of load cyclesN Number of cycles to failureNk Number of cycles to failure at kneepointNcalc Calculated number of cycles to failureNtest Number of cycles to failure from testsn Fatigue life cyclesntotal Total numberP ProbabilityPe Probability of exceedancePs Probability of survivalp IntervalQ Constant crack growth termR Load ratio/stress ratioR2 Correlation coefficientRa Average roughnessRz Mean roughness depthr Crack growth exponentr Effective notch root radiusSa Stress amplitudeSa,e Stress amplitude at endurance limitSa,k Stress amplitude at knee pointSa,max Maximum stress amplitudeSmean Mean stress of cyclic loadingSD Standard deviations Growth rate adjustment parameter of Walker equationTN Scatter related to number of cyclesTSa Scatter related to stress amplitudestflight Operational flight hourst ThicknessV Highly stressed volumeX Stress as random variableX Normally distributed stressY Strength as random variableY Normally distributed strengthC Parameter of Palmgren equationDKth Threshold stress intensityU Interference random variableea Strain amplituded Test time factorη Length of success run/sample sizeµ Expected value/mean valuem Safety margin

xiv Symbols

r Sigma unitsrallow Allowable stressrhs Hot-spot stressryield Yield stressrUTS Ultimate tensile stressr/ Far-field stress valueu Load severity factor/ Ductility parameterv Spectrum shape parameterw Geometry parameterx Geometry and load dependent parameter

Symbols xv

Chapter 1Motivation

Abstract Sustainability is a widely used, yet new degree subject that attempts tobridge social science with future technologies. Often, we think about reducing car-bon emissions and protecting the environment, while driving innovation and notcompromising the way of life we are used to live. For transport and mobility, wehave to look for systems and solutions which provide a much better eco-friendlinessthan everything what we did in the past. But it is not only about electric vehiclesand renewable fuel sources: Transportation is much about lightweighting and energyefficiency too. Hence, there is a strong need for methods and concepts to designready to manufacture, lightweight, yet durable enough to withstand the rigors ofchallenging use. In the first chapter of this book, emphasis is placed on the impact oflightweight design for energy savings in transportation that is the hidden backgroundof structural durability.

1.1 Environment and Emissions

The Sustainable Development Goals of the United Nations are a blueprint to achievea better and more sustainable future for all. These goals address the challenges weface globally, including those related to climate and environmental degradation.

The concept of sustainability was initially defined by Lester Brown in 1981 andexpressed as a society

that is able to satisfy its needs without diminishing the chance of future generations.

In these societies, a number of indicators related to social, environment and econ-omy point to a high level of wellbeing. Since 2006 a Sustainable Society Index ispresented which today is available for 154 countries, comprising 99% of the worldpopulation.

While the scores of human wellbeing and economic wellbeing increased on aver-age since then, the situation for the environment becomes unhealthier because ofnegative effects related to renewable water resources, energy use and greenhouse gasemissions. Energy is the dominant contributor to climate change and accounts for

© Springer Nature Switzerland AG 2020R. Heim, Structural Durability: Methods and Concepts, Structural Integrity 17,https://doi.org/10.1007/978-3-030-48173-5_1

1

2 1 Motivation

60% of the global greenhouse gas emissions. But it is not only about carbon dioxide:Pollution is a severe issue especially for the huge cities in India—half of the 20 mostpolluted cities in the world are in India. Two-third of all households in India eventoday rely on wood, coal, charcoal or animal waste for cooking and heating—glob-ally that concerns 3 billion people. The population of India will grow to 1.6 billionin 2040 and the electricity demand will be four times higher than today.

What do we have to expect for the future when talking about environmentalissues due to energy use? While the world population today is 7.8 billion people,the population is currently growing at a rate of around 1.05% per year, and by 2040,the total population is expected to be more than 9 billion. As an effect of social andeconomic sustainability, the per capita gross domestic product (GDP) is projectedto rise significantly, particularly in the countries outside of the Organization forEconomic Co-operation and Development (OECD): A huge number of people willjoin the global middle class. By 2030, the global middle class will likely expandfrom 3 billion to more than 5 billion people.

For this expanding population, the rising living standards come with high needfor energy which raise the global energy demand by about 25% until 2040. Nearly,all growth will be in non-OECD countries, where the energy demand will increaseabout 40% as well as the electricity demand almost will double.

That ismore than understandablewhenwe look at thefigures of the energy demandper person and the GDP per capita for different countries (Fig. 1.1).

For India, Japan and the USA, we almost find the same value when the energydemand is divided by the GDP per capita, though the individual economic poweris pretty different. The same is found for China, Germany and Singapore, but thoseare way more energy efficient. The simple message from such a chart is that a

Fig. 1.1 Energy demand versus GDP per capita for different countries

1.1 Environment and Emissions 3

significant expansion of the middle class in the emerging countries and their greateraccess to modern energy in homes as well as the significant increases in personal andcommercial transportation needs will rise the energy demand per person radically.Hence, we have to look for substantial energy efficiency improvements to curb thegrowth in the global demand for energy.

The growth of economic activity and personal income inevitably leads to anincrease of commercial transportation as well as personal mobility demands. Theglobal fleet will go up from today about 1 billion cars to likely reach the twobillion mark in 2040—not including commercial vehicles which today make upapproximately 26% of the total motor vehicle annual production in the world.

While in Europe the number of vehicles per 1000 people will slightly increase by10% in 2040, we will see a much bigger increase in some of the BRICS1 countries.In Brazil, the access to four-wheeled vehicles for personal mobility will double andin China that will triple. A huge increase is expected for India too, but remaining ona comparatively lower number because that market will see an extraordinary hugenumber of two- and three-wheelers. Motorcycles offer a lower-cost entry point topersonal mobility, with ownership particularly high in India and other countries inAsia Pacific.

Though we will see significant changes in the future fleet mix and a huge numberof hybrids and electric vehicles (EV), the liquid fuel demand for light-duty vehiclesis still expected to be relatively flat to 2040. Currently, there are slightly more than2 million electric vehicles in the global fleet, or about 0.2% of the total, but for 2040it is expected that 55% of all new car sales and one-third of the global fleet will beelectric [1].

All those numbers and figures immediatelymake it clear that we have to anticipatean increasing global population and number of middle-class people which make theglobal fleet much, much bigger than today. Consequently, we have to look into everysingle aspect where efficiency gains could come from.

Among other aspects, our special attention is aimed to vehicle lightweight designimmediately. The fundamental relation between the mass of a vehicle and its energyconsumption is generally known by the public: Once people ride a heavy bicycle andmay have to transport additional luggage, they understand the value of lightweight.All vehicles need energy to operate because of the driving resistances which theyhave to conquer. Most of those resistances depend on the mass of the vehicle. Onlythe aerodynamic drag of the vehicle is not influenced by the mass. Different to that,the rolling resistance, the inclination resistance and the acceleration resistance showa linear relation to the vehicle’s weight.

For amix of routes—including city, country road andmotorway—the fuel savingsof classical car models range from 0.15 to 0.5 l per 100 km for a 100 kg weightreduction. The higher fuel efficiency is achievedwhen lightweighting comes togetherwith adjustments in the transmission to benefit from the different power-to-weight

1BRICS is an international political organization of leading emerging market countries consistingof Brazil, Russia, India, China and South Africa.

4 1 Motivation

ratio [2]. As a rule of thumb, a value of 0.3 l per 100 km is often used to explain theeffect of 100 kg weight reduction for a passenger car under normal operation.

Now, let us think about the potential savings throughout the whole operatinglifecycle of such a car—assuming it would have 100 kg less weight and travels amileage of 200,000 km. The energy savings would be in a range of 25 GJ for a car.Much higher savings would be achieved for such cars when used for taxi operation:Here, the operating lifecycle comeswithmuch highermileage andmass-related stop-and-go of urban driving which then would account for lifecycle savings in the rangeof almost 120 GJ [3].

Substantial, weight-related savings are expected for classical car models but forelectric vehicles too. Though sometimes lightweighting for EV’s is thought not to beof vital importance because those cars get energy back from braking, the vehicle’sweight is still a major parameter for its energy consumption. Based on my ownresearch work with an EV fleet, the energy consumption can be determined as 0.1–0.11 Wh km−1 kg−1. That fleet included a range of vehicles from 1345 to 2125 kg.Certainly, brake energy recovery helps the heavy electric car models to improvetheir efficiency, but first thing one have to consider is the energy that is needed toaccelerate a heavy mass: You may get some energy back when braking, but you havespent so much more energy to get such a car up to speed—and you love to do thatas often as you can. Hence, you do not find a serious reason to neglect the aspect oflightweighting an electric car. Due to the mass and size of the Li-ion energy storagesystems that mission is not an easy one.

Automakers therefore like the idea of producing sports utility vehicles (SUV)which have become one of the most popular vehicle segments in the past years: TheSUVs are bigger and taller and have more packaging space for the battery systemunder the floor. Premium electric SUV then has an engine power of 300 kW and amass of more than 2500 kg. Though they are zero emission vehicles, they are far offfrom being energy efficient.

Consequently, the European Union wants to see for future urban mobility a totallydifferent kind of a city car that is a small, light quadric-mobilewith amaximumpowerof 6 kW and an enclosed driving and passenger compartment accessible through amaximum of three sides. Those vehicles are four-wheeled and have a top speed ≤45 kph, while the operational mass is ≤425 kg which does not include the battery.Compared to the premium electric SUV that bespoke city car needs only half of thespace and its mass is reduced to the fifth part of the SUV. The SUV itself has 50times more power than the light quadric-mobile which makes it clear that electricmobility does not mean it is automatically a fundamental part of sustainable futuretransport.

For heavy commercial vehicles—such as a truck or an express route bus—theoperationalmileage is typicallymore than 1million km. For 100 kgweight reduction,the lifecycle energy savings then turn out to be about 30 GJ, which is even more thanfor average passenger cars. And it does not sound too difficult to reduce the weightof those vehicles by 100 kg, doesn’t it? A European 4 × 2 tractor unit with 500 HPweighs 7500 kg.

1.1 Environment and Emissions 5

Trains and trams can save even more energy when getting lighter, due to frequentstops at train stations and again because of their huge operational mileage—whichcan be 1 million km annually for a high-speed train. A 100 kg weight reduction oftrains and trams then counts for 65–125 GJ savings through their lifecycle.

The most effective lightweighting starts when doing the mass optimization foraircrafts. Does that mean any surprise to us? Isn’t the aircraft industry the majorplayerwhen it comes to the use of high-end compositematerials or titanium? Indeed, a100 kg weight reduction would give lifecycle energy savings in the range of 10,000–20,000 GJ for short-distance aircrafts and even higher savings for long-distanceplanes. Thus, the lifecycle-related energy savings for an aircraft are expected to bemore than 100 times higher than for rail vehicles and almost a 1000 times higherthan for passenger cars.

Where do we go from here? Does it make any sense to look for lightweight designin automotive engineering? Isn’t it right to focus on ride comfort and NVH (noise,vibration and harshness) and built heavy SUV, because there is a comparativelysmall impact of automotive lightweighting? Shouldn’t we spend all the effort formass optimization on trains and planes only?

The answer is: Certainly not, because there is such a huge number of cars and dutyvehicles in the world. Though the individual impact of a 100 kg weight reduction ismuch smaller for road vehicles, the extraordinary large number of vehicles makesthe difference: Assuming a 100 kg weight reduction for all road vehicles, trains andaircrafts in the world, the annual energy savings today would be more than 2600Peta-Joule, a nicely shaped number in the range of 1018, and the same energy than420 million barrels of oil equivalent.

Almost 90% of these savings would come from lightweighting of the roadvehicles, while the contribution of the trains would be negligible.

And there is another interesting aspect we have to look at: For a sustainablefuture, we have primarily to get the urban environment managed, since even todaythe cities contain more than half of the world’s population and create almost 80% ofthe greenhouse gas emissions, while they cover just 2% of the globe’s surface.

1.2 Business Sustainability

Often sustainable business simply means to manage a business’ financial, environ-mental and social opportunities, obligations and risks. Managing that triple bot-tom line makes the business aligned to healthy economic, environmental and socialsystems and helps companies to ensure resiliency over time.

Under a range of best practices to foster business sustainability, we often find theterm life cycle analysis which means the systematic analysis of the environmentaland social impact of the products a company use and produce. Tech firms thereforemay introduce a paperless office environment, and cell phone manufacturers maylook for conflict-free supply chains for the minerals and metals they need.

6 1 Motivation

But business is not working without profitability which often is a short-term goalfor stock corporations looking to the next quarterly report.

As an example, wemay look back to the early 1990s at which a Spanish-born autoexecutive helped GeneralMotors (GM) to slash costs and to stop the carmaker’s hugelosses both in Europe and the USA by ripping up long-standing contracts with GMsuppliers and demanding ever-lower prices and faster deliveries. The annual savingsstarted at US$ 1.1 billion in 1991 and were more than doubled in the followingyear. For a while, GM got a steroid-type boost in profitability: End of April 1992the corporation announced its first quarterly profit since the second quarter of 1990,and a year later, it reported a first-quarter net income of more than US$ 500 millionbecause revenue went up and many costs down.

When looking at the average retail price of cars, GM’s procurement strategy seemscomprehensible because buying material and parts contribute to the pro-rate costsmore than 40%. Engineering, assembly and labor costs are 15% of the retail price,while overhead costs, depreciation and distribution add up to slightly more than 35%(Fig. 1.2).

But on long term, the damage to GM from beating up on suppliers for lower costswas immense: GM went from being the most preferred customer of the automotivesupply base to becoming the least preferred one. That happened in a time whentechnology leadership started to shift from the original equipment manufacturer(OEM) to the vendors, and consequently, GM became the last customer to receiveinnovative products and technologies. In a supply industry survey GM ranked laston supplier relations among all carmakers having production sites in the USA [4].GM won immediate savings, but ignored total cost: Too often parts from lower-costalternatives flunked quality tests. GM’s quality hurt for years after that Spanish-bornauto executive left the company.

Fig. 1.2 Material, warranty and profit as part of the retail price in the car industry

1.2 Business Sustainability 7

Typically, warranty-related costs are somewhat more than 4% of the retail price.With an average price of a new car in Germany of more than 31,000 Euro in 2018that means a price tag of almost 1400 Euro for repair parts and services within the24 months warranty period. Often the company’s profit from car production andsales is smaller than those pro-rate costs for warranty claims. And that is a point asustainable business strategy has to consider: Quality and durability improvementslead to better profit margins by lowering the expenses for unwanted repair parts andservices.

When talking about the term durability, we do mean the ability that an equipment,machine or material exist for long without significant deterioration. Hence, durableproducts resist the effects of heavy use, drying, wetting, heating, freezing, corrosion,oxidation and many more. Avoiding wear and tear just means for the manufacturera much smaller effort for product fixes, or even retrieving and replacing defectivegoods for its customers.

In January 2017, a leading smart phone manufacturer from South Korea sum-marized that battery fires and explosions—quite literally—sparked two recalls fora specific model due to irregularly sized batteries causing the first round of batteryfires in August 2016 and a number of different manufacturing issues which createdtrouble a second time.

The part itself was a sizable 3500 mAh (milliamp hour) lithium battery that waspackaged in a 7.9 mm thin phone. Those batteries were prone to exploding becauseof missing insulation tape and sharp edge protrusions which caused severe issues innearly 100 cases in theUSA alone. Themanufacturer together with theUSConsumerProduct Safety Commission (CPSC) officially recalled the complete product line inthe USA. Finally, the OEM got back 96% of the 3 million devices which were sold,and started to extract more than 150 t of valuable rare earth metals, gold, silver,copper and cobalt to reuse them for new products. The ill-fated smartphone createdtotal costs of more than $5 billion to the company [5].

Errors both in design and manufacturing which affected batteries from two dif-ferent suppliers were the root cause for such a huge recall—and can happen any timeagain, not only in the smartphone business.

Electric cars heavily rely on the technology of lithium–ion batteries too. Andwhile EV battery fire or explosions do not happen with unusually high frequencyyet, the battery itself shows gradual energy or power loss with time and use. Again,durability could be an issue—specifically when the battery has to be replaced beforethe end of vehicle life. Since the lithium–ion battery system of an EV is the mostexpensive single part, its useful life should be aligned to the vehicle’s life whichis—as we saw before—12–15 years or a mileage of at least 200,000 km.

Does a customer accept a costly replacement much before that time? That iscertainly not a genuine question, but a rhetorical one: An EV customer is typicallynot even interested in size, mass or layout of the battery pack, but simply wants tobe sure that it operates properly over a longer period.

Talking about what a customer normally wants brings us to the importance ofdetermining the target customers’ needs and wants. Having product features which

8 1 Motivation

help to achieve customer satisfaction and to develop a minimum viable product iskey for product development.

That can be shown by the Kano model that was developed by Noriaki Kanoin the 1980s to classify product features depending on the value they provide tocustomers (Fig. 1.3). Understanding the Kano model helps to keep the focus onoptimizing important features as well as to fade out features which are not necessaryor superfluous.

According to the Kano model, a feature can be categorized by its level of func-tionality and satisfaction provided to the user. Depending on the parameter of theKano model, a feature is placed into one of the following categories:

• must-be,• one-dimensional,• attractive.

Features falling in the must-be category are those which users deem essentialfor the product to operate as expected. Those features have to be present, becauseotherwise the product would not hold any value for the user.

But, while features of the pure must-be category introduce a certain level offunctionality, that does not mean a positive level of customer satisfaction. Much

Fig. 1.3 Positioning basic durability performance in the Kano diagram