C O V E R S T O R Y CHA SING EFFICIENCY
LIGHTWEIGHT DESIGN FOR MORE ENERGY EFFICIENCY In order to ensure sustainable success, most OEMs are currently focusing their innovation concepts on achieving technology leadership in lightweight design and the use of lightweight materials. For this reason, innovation management with composite materials and lightweight design concepts is also particularly important for engineering partners such as Edag.
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AUTHORS
DR. MARTIN HILLEBRECHT is Head of the Competence Centre for Lightweight Design, Materials and Technology at Edag GmbH & Co KGaA in Fulda (Germany).
JÖRG HÜLSMANN is Head of the CAE Department of the Vehicle Integration Division at Edag GmbH & Co KGaA in Fulda (Germany).
BACKGROUND
The importance of lightweight design in vehicle development continues to increase. Not only the ambitious CO2 targets for 2020, but also the introduction of a whole range of electric vehicles on the market, which require mastery of the substantial changes to weight distribution and the load paths involving lightweight construction, are bringing the focus back to this issue. Consequently, there are all sorts of requirements for components, from which specific design criteria can be used. At its own Competence Centre for Lightweight Construction, Materials and Technologies, Edag is pursuing selected lightweight strategies approaches, from the conception stage through to the evaluation stage.
COMPOSITE LIGHTWEIGHT DESIGN
ANDREAS RITZ is Head of the Sales and Project Management Department of the Tool and Body Systems division at Edag GmbH & Co KGaA in Eisenach (Germany).
PROF. DR. UDO MÜLLER is Professor of Mechanical Engineering at the University for Applied Sciences in WürzburgSchweinfurt (Germany).
In order to make weight reduction and functional improvements with innovative fibre-reinforced plastics (FRP) a reality, the plastics experts of BASF have, in collaboration with Edag, developed a fibrecomposite sandwich demonstrator applied to a convertible roof module [1]. A convertible roof module was selected since this shows promise of a relatively speedy market entry on account of the relatively moderate production volume of around 20,000 units
per year. On the one hand, this offers manageability, while on the other hand, the volume is nonetheless sufficiently large that, without automated production processes, the use of FRP technology would not be possible from an economic viewpoint. In addition, weight reduction above the centre of gravity of a vehicle has a positive effect on drive dynamics and, compared to costlier steel or aluminium lightweight construction, this type of construction can be realised more readily for this vehicle segment, ➊ and ➋. Altogether, the convertible roof module exhibits the following six key features: :: Dry CF mat with fibre orientation based on the load applied, :: Low-density polyurethane foam core with high compressive strength, :: Fast-curing RTM resin systems with good flow properties, :: Unidirectional reinforcements, :: Metal inserts at points where forces are applied, and :: Short glass fibre reinforced plastic inserts. Despite the high potential for weight reduction, there are some obstacles with regard to the application of FRP lightweight construction methods for largeseries vehicle production. These are the relatively high costs in relation to the weight reduction and the current lack of experience of OEMs and suppliers in merging the individual supply-related
Aluminium reference
Wind load
Eigenmodes
Collision load
Polishing work for dents
Central block
Design
Generic concept
Snow load
FRP foam sandwich design
4553 g
2915 g
Dual-shell inner structure with plating and reinforcement plus connecting elements sealers and surface reinforcement for improvement concerning dent stiffness and natural frequency
Sandwich composed of carbon-fibre cover layers, with unidirectional reinforcement and PUR foam core with inserts
❶ Fibre-composite sandwich demonstrator applied to a convertible roof module autotechreview
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C O V E R S T O R Y CHA SING EFFICIENCY
❷ Cross-section of the demonstrator: the characteristic features of the novel fibre-reinforced composite concept, the polyurethane foam core, the impregnated CFRP skin layers and a metallic insert can be seen
production competencies into a coherent value chain. While BASF was able to offer its comprehensive product range and processing know-how for the three material classes of epoxy resin, polyurethane and polyamide, along with the closed-cell PUR
structural foam, Edag applied its FRP development expertise in the project. For the design of fibre-reinforced plastic components, as in the case of isotropic materials, numerical computation is essential for achieving the required component properties with
minimum weight. In order to ensure that the predictions of the computations are reliable, it must be possible to characterise the fibre-reinforced plastics or their material properties, as well as the influencing factors arising from production, ➌. In contrast to typical isotropic materials, additional material testing is required. For the production of material cards for fibre-composite materials, the following procedure has been determined to be effective: :: Experimental characterisation of the material properties via testing, :: Selection of the material modelling software (Abaqus, Pam-Crash, etc.) or implementation of a new method for determining solutions, :: Verification of the material model and calibration for the material card, :: Initial validation of the material card, and :: Validation of the calibrated model via component testing.
❸ CAE material cards can be prepared in the accredited testing laboratories of Edag
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LIGHTWEIGHT STEEL DESIGN
In spite of the lightweight construction potential of FRP, steel will continue to be a prominent lightweight construction material in high volume vehicle production. This fact is underlined by the current FutureSteelVehicle (FSV), which has been developed on behalf of the international steel industry WorldAutoSteel [2]. The FSV utilises CAE methods in combination with an extended portfolio of high-strength and ultra high-strength steels, which will come onto the market between 2015 and 2020. Besides two hybrid drive units and one fuel cell drive unit, the focus was on the electric powertrain. As a battery electric vehicle (BEV), a body weight of just 188 kg could be realised, demonstrating a future potential >20 %. The project not only illustrates that steel is the most cost-effective material for bodies in volume production; it also underlines the importance of a lifecycle assessment, since steel is the most recycled material worldwide, ➍. Lightweight steel-plate construction is always limited owing to the complex joint sections in small spaces. Furthermore, joining processes suitable for series production have technical and geometric limitations. Low-pressure, thin casted steel parts may offer potential for the future. For the Edag Light Car [3] concept vehicle, a space frame joint involving a thin caststeel component was realised for the first time. The purpose of this was to demonstrate the application potential as well as to subject the joining technology and the corrosion properties to close scrutiny. Weight reduction is possible because of the increase in local stiffness, the constructive degrees of freedom in the design (for example, ribs) as well as the reduction in sheet thickness in the area around the components. Optimisation of production costs arises from the use of similar joining technologies rather than bonding methods, and is reinforced if the degree of flexibility is very high or if highly diverse components are used. The additional cost per kilogram saved appears to be extremely positive. Consequently, thin-plated cast steel structures could well be the vision of each body developer if process reliability and technical realisation were autotechreview
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❹ The body concept of the FutureSteelVehicle demonstrates the lightweight construction potential of steel applications of the future
❺ Comparison of sheet and cast joints in a typical steel-intensive body design of today
already guaranteed. Following promising preliminary work in cooperation with the CX Group, it is a declared aim to help make the breakthrough, using new casting methods, bonding technologies, system concepts and design methods, ➎, ➏ and ➐.
LIGHT METALS
For certain component groups of the vehicle concepts of today, in particular
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sliding doors of vans, there is further lightweight construction potential in the application of magnesium, which is already an established design material in body construction. The declared aim here is to compensate for the additional weight of sliding doors (15 % to 40 % higher than conventional swinging doors) by using lightweight construction material, and, if possible, to reduce the weight even further. The example of a sliding door of a car illustrates the lightweight construction potential of magne-
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ing properties are of particular importance for the outer door panel. Regarding the examination of the door structure, a door with a die-cast inner section, a frame involving deeplydrawn components and a side-impact member as an extruded section turns out to be the best solution. Concerning the design of the components, the load conditions include excess door pressure when being opened, a vacuum on the outside, an idealised collision and misuse due to a person being supported by a door. Compared to the benchmark door made of steel, a weight reduction of 44 % is achieved for the magnesium door with the same stress resistance. If an outer magnesium panel is also included in the analysis, the total weight potential rises, reaching up to 50 % [4], ➒. ❻ Comparison between steel-sheet design and thin cast-steel-plate design for the “rear collision” and “axle attachment” load cases
SMALL-SERIES PRODUCTION IN THE PREMIUM SEGMENT STEEL-SHEET DESIGN
THIN CAST-STEEL-PLATE DESIGN
5458
4921*
4
3
5
4
29
26**
BOND SEAM
None
None
COST OF MATERIALS [EURO]
3.28
3.13
COST OF MANUFACTURING [EURO]
8.72
9.24
997,500
765,000
*** DYNAMIC STIFFNESS
100 %
100 %
COLLISION PERFORMANCE
100 %
> 120 %
WEIGHT [g]
NUMBER OF COMPONENTS NUMBER OF JOINING OPERATIONS/DEVICES WELD POINTS
COST OF TOOLS [EURO]
* from which cast part weight = 980 g; ** additional weld points for load path optimisation; *** design criterion
❼ Evaluation of potential based on a generic body structure for a thin cast-steel-plate design compared to a steel sheet design
sium, when applied in sheet form, extruded section form or in cast form, ➑. The theoretical weight reduction potential of substituting steel with magnesium is essentially dependent on the geo-
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metrical shape of the component as well as the stress, and lies in the range from 0 % to 60 %. The stiffness and strength are decisive factors for the door structure. In contrast to this, the local and global dent-
The practical application and realisation of widely differing lightweight construction strategies are currently taking place with all sorts of small-scale premium products. These often involve specific body developments and adjustments for high-performance vehicles. Edag has been making specific developments in the area of body and tool systems for such vehicle structures at its site in Eisenach, Germany, ranging from individual part production processes and joining processes for add-on modules (bonnets, mudguards) through to small-series supplier conditions. In the area of body and tool systems, the multi-material lightweight construction strategy can be clearly recognised from widely-differing products. Alongside aluminium and composite materials (organic sheets), hot-forming and ultra high-strength, cold-forming steels also belong to the single component production range. For this reason, process and method planning concerned with forming occurs directly at the site. From the direct exchange of experience between tool makers and method planners, results can be verified and, if necessary, further developments or corrections can be made. The application of various proven joining processes helps to shape www.autotechreview.com
case too, highly specialised technical knowledge is essential and demands will be made on generalists to be able to identify and assess pioneering technologies. REFERENCES
❽ Overview of components for a sliding door
the character of a functional, multi-material lightweight design structure. Joining processes such as punch riveting, clinching, roller hemming and laser joining techniques are, along with conventional joining methods such as spot welding and MAG welding, continually being applied and the experience of using these methods is beneficial for the functional and production-oriented development of individual parts and structures with lightweight construction properties.
SUMMARY AND OUTLOOK
Only with interdisciplinary teamwork is it
Thickness of magnesium part [mm]
Description
A
Reinforcement (top)
B
Window frames
C
possible for lightweight construction and multi-material concepts to be implemented speedily and successfully. Of central importance in achieving this are the material specialists in specialised companies with experience in automotive development, and in particular those with many years of experience in lightweight design, from the concept stage to series production readiness. Lightweight construction is, and will continue to be, a challenging engineering discipline. Moreover, the initial technical implementation of lightweight construction production technology requires the involvement of all partners of a new value chain that has not yet been established. In this
[1] Dallner, C.; Sandler, J.; Reul, W.; Hillebrecht, M.: Faserverbundkonzept für ein CabrioDachmodul. In: ATZproduktion 5 (2012), No. 3, pp. 178 – 183 [2] Ten Broek, C.; Singh, H.; Hillebrecht, M.: FutureSteelVehicle: Innovativer Stahl-Leichtbau und neue Entwicklungsmethoden. In: ATZ Automobiltechnische Zeitschrift 114 (2012), No. 5, pp. 370 – 377 [3] Hillebrecht, M., Schwarz, W.; Reul, W: Leichtbau durch Multi-Material-Design am Beispiel des Elektrofahrzeugs “Light Car Open Source”. Karosseriebautage Hamburg, 2010 [4] Rathfelder, A.; Müller, U.: Das Leichtbaupotenzial verschiedener Werkstoffkonzepte am Beispiel einer Pkw-Schiebetür. 4. Nano- und Material-Symposium Niedersachsen, 2011
Production method
3.3
Deep drawing
2
Deep drawing
Window shaft reinforcement
1.6
Deep drawing
D
Side impact member
3.3
Extrusion
E
Inner section
2–5
Die casting
THANKS Edag is grateful for the successful and collaborative cooperation with customers and business partners who have given their agreement to publish their contributions with us. These are
B
Dr. Claus Dallner, Dr. Jan Sandler and Dr.
A
Katrin Nienkemper of BASF SE, Cees ten Broek of WorldAutoSteel and Ivo Herzog of the CX Group.
C
E
D
Local thickness increase to 5.0 mm
❾ Components of the door structure with the associated production methods autotechreview
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