1.1 The design challenge: Low weight, low cost, low carbon footprint and high performance ......... 2 1.2 Introduction to the design process described in this manual ..................................................... 4
1.2.1 Overview of design process ........................................................................................... 4 1.2.2 Design Phases and Objectives ........................................................................................ 5 1.2.3 Design Tools ................................................................................................................ 9 1.2.4 .......................................................................................................................................
1.1 The design challenge: Low weight, low cost, low carbon footprint and high performance
The principal reason for introducing aluminium into automobiles, i.e. the application of aluminium alloys for vehicle components, sub-assemblies and full vehicle structures is to achieve a significant weight reduction compared to traditional car designs based mainly on steel and cast iron. At the same time, the aim must be to develop designs that are easy to manufacture and cost effective whilst meeting the required performance criteria. Today, the design and manufacture of cars must not only satisfy engineering criteria, but also fulfil environmental, social and political aims, such as minimising waste and deposition, satisfying recycling targets, respecting health and safety regulations as well as reducing emissions and saving resources. High emphasis is now given to the environmental performance of an automobile over its total life cycle. The aim is to minimise the overall environmental impact of the vehicle production stage (including materials production), service life and end-of-life phase (including recovery and recycling of the materials). A particular challenge is the treatment of end-of-life vehicles. In the future, therefore, it is envisaged that increased emphasis will be put on the disassembly of selected components for optimum recovery of the material value. This will enhance the traditional recycling route of end-of-life vehicles (shredder and subsequent metal sorting). Design for disassembly is thus an important development theme, requiring careful consideration of material combinations and manufacturing methods during early design phases in order to meet both ecological and cost targets. The finally realised weight of a system or a sub-system, however, may not be the lowest weight achievable since cost is generally an overriding issue. Nevertheless, with proper application, lightweighting with aluminium almost always delivers an improved performance of the vehicle in addition to the lower fuel consumption (which translates into lower CO2 emissions). Lightweighting with aluminium enables better acceleration, shorter braking distance, easier handling, increased car body stiffness and a lower centre of gravity at a modest cost increase, factors which also contribute in particular to an enhanced safety performance. A good example of for such a performance enhancement is body structure of the Audi TT roadster. Different aluminium product forms, but also steel are used at specific locations in the body structure with the following results:
Weight reduction of 100 kilograms compared to an equivalent all steel construction Significantly reduced body-in-white mass for coupé and roadster versions (weight
reduction of 100 kg leading to a total weight of 206kg) Balanced front / rear axle weight distribution Improved torsional stiffness (coupé +50%, roadster +100%).
An interesting part is the lower A post, a high pressure die cast multi-functional aluminium component that links the side member, the sill, the A post and the wind-shield cross-member. The specific benefits include a very precise part geometry and the maximum use of the available space to ensure high stiffness and structural integrity in crash. For the car owner, the extensive use of aluminium translates to:
Easier handling during cornering Shorter braking distance 7.5 to 12.5 grams lower CO2 emissions per kilometre Lower fuel consumption More uniform tyre wear.
1.2 Introduction to the design process described in this manual
Automobile manufacturers have at their disposal a wealth of expertise in designing and manufacturing vehicles with a wide range of materials. The body structure and the skin panels have traditionally been manufactured from various steel grades. However, with the current focus on the reduction of fuel consumption and overall greenhouse gas emissions, other materials, and in particular aluminium, are used to achieve a significant reduction in the weight of the car body. While most of the general design principles that have been developed for steel are also valid for aluminium structures, best results are always obtained if there is enough design freedom to apply an aluminium-oriented design, rather than just to substitute aluminium for steel in existing part designs. This section will describe a design process that is later illustrated by case studies. Some vehicle manufacturers and first tier suppliers are already expert with aluminium as the example of the Audi TT structure shows. They have developed sophisticated procedures based on their accrued knowledge, in order to manage the design process with aluminium. We hope, however, that the following will be a useful introduction for both young and experienced engineers who wish to explore more fully the differences between designing with steel and aluminium.
1.2.1 Overview of design process A design process can be used for further developing existing objects or for producing completely new ones. The detailed content of each design project is likely to be different since it will depend on its specific objectives, the starting point and the available resources. Certain logical steps are often taken in order to be able to make qualified decisions selecting the best of the options available at specific points along the time-line called gates.
Design Pre-Prototype Design Development IndustrialisationNew technology scanning / Concept studies
Prototype Studies Detailed Design Product and Process Design / Validation
Industrialisation and Trials
Pilot runs
SOP ramp
GATE Technology
Selection
GATE Design Freeze
GATE Tooling
Simulation Acceptance
Tooling tryoutmanufacturing
plant
GATE Implementation
Readiness
GATE Prototype Selection
GATE Design & Process Sign-Off
Tooling tryout at tooling
manufacturer
SOP
Design Brief (Cahier de Charge)
Time-line for complete Concept to Start of Production (SOP) projects
Source: Roger Hall
The single common element in all design projects is the design brief that describes the overall objectives of the project, such as, for example, “an electric niche vehicle with a very short wheelbase, whose target customer is young families living in the city”, etc. The design brief is then translated into specific deliverables for each department working on the project. Typical deliverables might include weight reduction, improved safety for occupants and pedestrians, cost reduction, etc.
The scope of a design project is further defined by detailed product requirements and the imposed constraints which form the specific boundary conditions of the project.
Some common project boundary conditions
Source: Roger Hall
1.2.2 Design Phases and Objectives In the automotive industry, “clean sheet” design projects are relatively rare because of their long duration, high cost and potentially higher risk than the development of proven designs, but also because of the possibilities ti integrate carry-over parts. Some examples of recent “clean sheet designs” are the Mercedes SLS, BMW 1 series, Fiat 500 and the GM electric vehicle. The advantages of a clean sheet design include a full freedom in the location and choice of power unit, power train and suspension. Thus they are most interesting for vehicles using new, alternative power systems. Better packaging can improve safety and access to hydrogen storage units or batteries and other rigid components that may be required by fuel cell and electric motor powered vehicles. Improved packaging of the main functional units directly influences the potential to maximise the ratio of passenger compartment volume to the vehicle length (e.g. important for the attractiveness of city cars for families). When designing with aluminium, a clean sheet approach will also deliver more efficient structures. Larger beam sections, large thin-walled castings and straight or bent extrusions may be used more effectively and also packaged to deliver stiffness and strength targets as efficiently as possible.
Design and development projects can start at different points along the above time-line according to their nature and the degree of experience that has been accumulated with all of the relevant technologies. Until the mid-1980’s, aluminium applications in the car body and in particular all-aluminium vehicles were very rare and almost all car manufacturers envisaging the use of aluminium for structural applications worked intensively with their materials suppliers, universities and engineering partners to validate aluminium-oriented design concepts. Nowadays, aluminium concept studies tend to concentrate on the development of new technologies delivering greater design possibilities, improvements in car body stiffness and crash energy absorption capability, final quality and cost reduction. Aluminium structures are becoming more cost and weight efficient owing to the development of stronger, more formable alloys, better joining systems and the availability of improved numerical simulation methods for aluminium. Simultaneous engineering enables the integration of new technologies at all phases of the design and development project.
1.2.3 Design Tools New materials and processes can lead to step-change improvements in product performance and attractiveness. The drawback is that, until they are fully known, there is always an associated risk that needs to be managed. In reality, even known products can suddenly develop a new failure mode. There are four main levels at which physical uncertainty or scatter is seen: loads, boundary / initial conditions, material properties, and geometry. Numerical simulation assumptions and simplifications add another equally important form of uncertainty that may not be associated with the physics of the problem. The automotive industry is no stranger to these problems and has adopted and developed many tools to enable robust and efficient production of vehicles. This section intends to highlight the major differences between aluminium and steel that influence the physics that forms the basis for design decisions from concept to product sign-off. These differences are highlighted by a brief discussion of aluminium in a multi-material structure using basic tools that are already familiar to most automotive design engineers. DFMEA – Design Failure Modes and Effects Analysis This is an iterative process that becomes more refined as the design and development process matures. Its aim is to identify potential failure modes, identify corrective actions and to assess remaining risk. The designer can also use this technique to identify useful physical properties of the material that might otherwise go unnoticed. Consider the four main uncertainties identified by Dr Marczyk in the context of a stamped aluminium panel in a steel structure.
Under each major heading is a list of the parameters which have an affinity with each other, and which might be important for the design of the envisaged component. Potential manufacturing and in-service conditions may be thus investigated and the high-risk items can be identified for solution very early in the design or development stages. The selected parameters that are relevant for the design concept may then be laid out as a herring-bone diagram in order to add further details for each parameter. In the simplified example shown below, the product form is assumed to be a stamped sheet panel.
Once all of the options have been considered, one or more potential solutions can be outlined specifying the aluminium product form, alloy and temper, surface conditions, joining methods, etc. These then form the boundary conditions for the design development process. A herring-bone diagram may be omitted for the further development of an existing, proven design unless one or more of the major parameters is significantly changed. Aluminium components can be shaped by a wide range of technologies including various casting techniques, hot forging, cold forging, room temperature stamping, warm forming, super-plastic forming, spinning, extruding, roll forming, etc. Each of these forming processes has some specific limitations regarding the applicable alloy compositions, but it has also an effect on the final material properties that have to withstand the loading conditions. Some major differences between aluminium and steel with respect to the most commonly used automotive component forming operations:
Casting
◦ Lower energy requirement (casting temperatures lower than steel and iron)
◦ Wide range of available casting methods
▪ Steel dies may be used because of low melting point of aluminium
▪ Advanced high pressure die casting processes produce thin-walled high precision parts with very low porosity delivering almost wrought properties
Extrusion
◦ Only available for aluminium (and magnesium) as commercial products
◦ Aluminium may require more rolling stands than steel of the same strength
◦ See Stamping comments for E, galling, n and r values Stamping
◦ Elastic Modulus
▪ More spring back compensation may be required than for comparable strength steel grade (Elastic modulus of aluminium 1/3 of steel)
◦ Yield surface / Locus shape
▪ Aluminium is closer to TRESCA than HILL 48 (commonly used for steel)
▪ Use Cazacu-Barlat, Vegter or Hill 90 for aluminium
◦ Friction
▪ Tends to be slightly higher than with galvanised steel sheet
▪ Appropriate surface topographies and modern hot melt lubricants minimise this difference
◦ Galling
▪ Lower surface hardness and melting point than steel
▪ Use suitable tooling materials and lubricants
◦ Low temperature ductility
▪ Ductility of aluminium improves at low temperature
▪ Ductile brittle transition temperature (DBTT) is not an issue for aluminium
◦ Strain rate sensitivity
▪ Very low sensitivity at room temperature (compared to steel)
▪ Flow stress and plastic strain tends to increase with increasing strain rate
◦ Work hardening (strain hardening exponent n)
▪ Higher than equivalent strength steel grades
◦ Plastic Anisotropy (Lankford coefficient r)
▪ Inferior to steel at room temperature
▪ result of a deep-drawing operation is, however, for aluminium sheets less dependent on the r-value than for steel sheets
In sheet stamping, these differences between aluminium and steel produce correspondingly different post forming thickness and strain distributions in the final part. Additionally, local design and process modifications, for example to compensate for the low r value, may further influence the local material characteristics in the formed panel and thus the behaviour of the final part under critical loads such as a crash.
Experimental and theoretical yield loci and r-value - AA 3103-O
Source: Proceedings of the Romanian Academy Series A, Volume 3, Number 3/2002. Recent Anisotropic Yield Criteria for Sheet Metals
In the example outlined below in more details, the idea of inserting an aluminium panel into a steel structure is considered simultaneously from product, operating environment and manufacturing perspectives. Additional details on the function of the panel need to be added now in order to complete the FMEA specification. We will define its function as a non-visible panel defining the central floor of the vehicle. Software tools are commercially available for engineers to construct and track an FMEA process. Most vehicle manufacturers have developed their own systems that enable a high degree of integration with their other product development management activities. This section aims to demonstrate the main principles of risk and benefit assessment that automotive design engineers use in order to make informed and reasoned decisions when a new application of aluminium may be required.
The concept of failure mode potential is simple, but like any prediction, it relies heavily on experience. It forms an important part of risk and benefit analyses that are normally carried out and updated periodically for review at each design gate. In this context, failure means that the component, assembly or system does not meet the requirements of the design intent. As the concept matures, the potential risks should all have some corrective action in order to pass the decision gate allowing the concept to proceed through to the next phases. If perceived benefits outweigh the resulting risks or cost of investigation and correction, then the concept may continue. A greater level of confidence is required for each subsequent decision gate, and it is usual for a panel of experts to contribute to this process in order to arrive at implementation readiness for design integration, and later implementation readiness for production. The FMEA sheet in its simplest form is made up of three parts:
1. List of potential risks 2. Proposed corrective actions 3. Results of actions
Corrosion Strength 4 Dissimilar materials 5 Test 8 160
Clamping Force 5 Build tolerances 4 Manufacturing
Engineering
4 80
8 Incompatible Material
strength
3 Product
development
5 120
10 Incompatible
thicknesses
4 Product
development
5 200
Joint design 5 Gun Clearance 5 Manufacturing
Engineering
3 75
Joint design 10 Only single side
access
5 Product
development
3 150
Cost of Rivet Business case 3 Number needed 5 Product
development
5 75
Labour Cost Business case 7 Fully automated 1 Manufacturing
Engineering
4 28
Component Cost Business case 0 No special
preparation required
0 Manufacturing
Engineering
0
Cycle time Business case 5 Weight of Gun 1 Manufacturing
Engineering
4 20
Cost of Gun Business case 8 Number of Guns 8 Manufacturing
Engineering
1 64
Description / Purpose
Layer failure
Blind Rivet: floor/sill
Self piercing Rivet: floor/sill
and seat runner
Access
Assembly
Shear strength
T Peel /tension
Fatigue
Shear strength
T Peel /tension
Fatigue
Potential risks: Joining aluminium to steel with the selected joining systems
Source: R Hall / M Shergold
Severity of failure (S) is a subjective measure unless numerical simulations or experience can quantify the consequences of a failure with respect to the function of the component or the end user. Occurrence (O) is an estimate of the probability of failure. Anticipated loading, environment and mechanical properties of the system elements can help to define this rating in the early design stages. Detection (D) is an estimate of the capabilities of the controls to detect the cause of failure. S, O and D values of 10 indicate maximum risk, RPN of 125 or greater indicates a high priority risk.
This example would not be considered as the end of the FMEA, since some high ratings remain to be investigated. To complete the assessment of the joining system, we need to compare the specific features and capabilities of each system. The features that have been identified as benefits for our specific case can then be highlighted enabling the best choice to be identified easily.
Responsibility
Prepared by RWH / MS
Date 14/04/2010
Revision 1.0
Features
Single Side Access
Dissimilar materials in layers
Different layer thicknesses
Accepts high thicknesses Ratio
Misalignment tolerance
Corrosion resistance galvanic
Piece price
Component (drilling) cost
Gun Price
Cycle Time
Shear strength
Peel Strength
Fatigue strength
Single Side Access
Dissimilar materials in layers
Different layer thicknesses
Accepts high thicknesses Ratio
Misalignment tolerance
Corrosion resistance galvanic
Piece price
Component (preparation) cost
Gun Price
Cycle Time
Shear strength
Peel Strength
Fatigue strength
Check particular case
Advantage
Disadvantage
Blind Rivet: floor/sill
Self piercing Rivet: floor/sill
Part / Process Name / N° Aluminium Floor Panel in steel structure
Advanced Design / Manufacturing
Description / Purpose
Low
High
High
High
Check with manufacturer & test
Check with manufacturer & test
Check with manufacturer & test
No
Select appropriate coating
High
High
High
Select appropriate coating
Low
High
Medium
High
Medium
No
Low
Low
Rating
Yes
Yes
Yes
Yes
Yes
Comparison of features of two joining systems
Such an evaluation can also lead to the identification of specific areas where the unique advantages of aluminium can be exploited. In this example, an extruded sill section with a shear flange could be used to accommodate cross car slip tolerances.
Many definitions and techniques come under this heading, but all have one thing in common, the refinement of an existing design. Design optimisation is an iterative procedure usually employed in the design development and industrialisation phases. Design optimisation should not be confused with striving to obtain the highest performance since this may only be obtained in most cases at the cost of the robustness of the solution. An affinity matrix approach can be useful to help to identify the parameters that need to be included into the optimisation boundary conditions, the variables and the object function we wish to minimise in order to obtain an “optimal” solution. Considering our example:
Selection of the parameters for an optimisation analysis
As the number of variables and object functions increase, so does the complexity of the optimisation process. Non-feasible results can be obtained if boundary conditions and variables are defined incorrectly. In this case the variables can be further defined as:
All commercial finite element optimisation software packages require that the optimisation process is controlled by boundary conditions that define the practical spatial, geometrical, material-related and loading limits of the problem. The drawback of any optimisation analysis is that it is very difficult to know if the identified solution is robust. This issue has lead to the development of statistical methods for finite element predictions (see brief discussion of Stochastic analysis). Furthermore, it is difficult to include the effect of tolerance on each of the parameters used. Another danger is that some important parameters may not be selected, although, affinity matrix methods can help to avoid this problem. Aluminium extrusions and castings lend themselves particularly well to geometrical and limited topological optimisation tools. Aluminium sheet parts can benefit from the very tight tolerances on rolled thickness. Aluminium has a density of one third of that of steel, with similar if not tighter rolling tolerances. This combination enables a higher degree of mass optimisation than can be obtained with commercially available sheet steel. It is usual to try to minimise mass which automatically reduces material cost and carbon foot print, although many other object functions may be chosen.
b) Topological optimisation
Topology optimisation relies on algorithms that manipulate two- or three-dimensional spatial relationships with given materials properties to satisfy all functional requirements of a system within a given package. This technique is most often used in early design phases when packaging is defined. Most linear finite element analysis software offers this capability. In the most general cases, a somewhat organic structure is identified as the optimum solution, satisfying all of the defined load cases and boundary conditions.
Design space for topological optimisation
Source: Aachen Body Engineering Days 2009, Realization of an Uncompromising Sportscar Concept, The Body of the Mercedes SLS AMG
While careful definition of the design volume can steer the optimisation process, it is usually necessary to further develop the shapes to produce feasible solutions for rolled and/or extruded products.
Source: Aachen Body Engineering Days 2009, Realization of an Uncompromising Sportscar Concept, The Body of the Mercedes SLS AMG
Topological optimisation is a particularly suitable technique for aluminium solutions. The low density of aluminium and the possibility of a consequent wall thickness adaptation often make it possible for cast and extruded aluminium products to achieve close approximations to the output from these optimisation tools while delivering significant weight savings compared to other technologies.
c) Optimisation and stochastic methods
Finite element (FE) simulation is increasingly used for the prediction and validation of the performance of automotive components and sub-systems. Unfortunately, this approach cannot in itself address uncertainties in tolerances, loading and boundary conditions. Optimisation techniques are most useful in the early design phases where the part geometry is dimensioned according to the nominal loads and packaging constraints. It may then be necessary to judge the robustness of the “optimised” solution using stochastic methods. If used correctly to further modify the design parameters in order to avoid an undesired behaviour, then the final result will be efficient and robust. Monte Carlo Simulation techniques are used to enable for uncertainties to be modelled and run on the computer thousands of times. In this way, FE analysis and test results can be compared to verify that the model represents the functional reality. It enables the identification of many failure modes that might otherwise not be known. It is also possible to identify which variables are to be controlled most stringently in order to avoid random behaviour in highly non-linear systems. These studies are very expensive, but their results can be very valuable in explaining what might influence bifurcation in behaviour. Ant-hill plots are used to identify chaotic and deterministic behaviours.
