Design Methodology

Design – Design Methodology

Table of contents

1. Design methodology ...... 2 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 ......

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1. Design methodology

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.

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Body structure of the AUDI TT roadster Source: http://www.automobilesreview.com/tag/tt/

The body structure consists of 140 kg Aluminium (68%):  63 kg of sheet,  45 kg of castings,  32 kg of extruded sections, as well as of 66 kg steel (32%).

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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.

GATE Design Brief GATE Tooling tryout GATE Tooling (Cahier de Technology manufacturing Design Freeze Simulation Charge) Selection plant Acceptance

Design Pre-Prototype Design Development Industrialisation New technology scanning / Prototype Studies Detailed Design Product and Process Industrialisation Pilot SOP Concept studies Design / Validation and Trials runs ramp

GATE Tooling GATE GATE Design & tryout at Implementation Prototype SOP Process tooling Readiness Selection Sign-Off manufacturer

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.

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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.

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Cost ReductionExercise Face-Lift Mid-life New Design Cycle on Existing Platform Full Freedom of Design "Clean Sheet" INTRODUCTION timing Examples of TECHNOLOGY e Design New Fuel Design Design Brief Implementation Concept studies technologyNew scanning / (Cahier de de (Cahier Charge) Readiness

Cell GATE GATE ‐ GM

MB /

Electric

einPePooyeDsg eeomn Industrialisation Development Design Pre-Prototype Design

Architecture

EV1/2 SLS

Vehicle ouein Module Technology Selection Audi ‐ GATE GATE ‐ BMW

Prototype Studies Detailed Design Product and Process Product andProcess Design Detailed Studies Prototype

new

Mark

Series

material

Prototype Selection GATE GATE Weight, Styling Functional ‐

ride Bonnet,

and Laguna

handling,

Bumper, Enhancement Design Freeze

II GATE GATE

Design / Validation wings, power

door train Design & Design Sign-Off Sign-Off Process GATE GATE Process n grade and Material and Trials Industrialisation Acceptance Simulation Tooling

GATE GATE fine

gauge tuning

tuning manufacturer

tryout at at tryout Tooling Tooling tooling manufacturing Tooling tryout Tooling runs Pilot plant SOP ramp SOP

Design and Development Projects Source: Roger Hall

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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.

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Supplier production Department Supplier Engineering Department Supplier research department Profesional research organisations Universities Auto ManufacturerPurchasing Auto ManufacturerProduction Engineering Auto ManufacturerEngineering Auto Manufacturer Engineering Advanced Auto ManufacturerResearchers PHASES ACTIVITY PROJECT Design Design Brief Implementation Concept studies technologyNew scanning / (Cahier de de (Cahier Charge) Readiness GATE GATE einPePooyeDsg eeomn Industrialisation Development Design Pre-Prototype Design Technology Selection GATE GATE Prototype Studies Detailed Design Product and Process Product andProcess Design Detailed Studies Prototype Prototype Selection GATE GATE Design Freeze GATE GATE Design / Validation Design & Design Sign-Off Sign-Off Process GATE GATE and Trials Industrialisation Acceptance Simulation Tooling GATE GATE manufacturer rotat tryout Tooling tooling manufacturing Tooling tryout Tooling runs Pilot plant SOP ramp SOP

Simultaneous activities for a typical project Source: Roger Hall

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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.

Aluminium Panel in Steel Structure

Load Boundary / Material Geometry initial Properties Conditions

Thermal Joining Mechanical Shape E, , Rp, Rm, Ag,..

Internal stress Stiffness of Thermal Assembly attached structures DBTT, Conductivity, Emissivity, Expansion Coefficient, ...

Impact Environment Shaping Tolerances n-value, r-value, ...

Gravity Resistance Electrical, Corrosion, Thermal, Radiation, Magnetic, ...

Air pressure

Structure Loads

Vibration

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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.

Material Properties Joining / Finishing

Steel Conductivity Cold Metal Transfer

Aluminium Expansion coefficient Rivet Bonding

Creep Spot Welding Zinc Tensile Strength Friction Stir Spot welding Potential Copper Yield Strength Laser Brazing Solutions Elastic Modulus Flame Brazing Al2O3 Linishing / Polishing Material

Carbon Adhesive Bonding Properties

Joining / Temperature Cycling Finishing

Paint system OEM Environment Degrease Re-work Line Robot Control Competence Salt / Acid Repair Shop Hand Machine Scratches Methods Do It Yourself Emery Cloth Rain / Hail

Environment Competence Methods

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

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 Roll forming ◦ Suitable for aluminium and steel ◦ 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.

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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.

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Part / Process Name / N° Aluminium Floor Panel in steel structure Prepared by RWH / MS Design / Manufacturing Responsibility Advanced Design / Manufacturing Date 14/04/2010 Revision 1.0

POTENTIAL ASSESSMENT Description / Purpose Failure Mode Effects of Failure Severity Causes Occurrence Controls Detection Risk Priority Proposed Corrective Actions Responsible Completion Results Number Date (S) (O) (D) (S*O*D) Actions SOD RPN taken Blind Rivet: floor/sill Shear strength Water ingress 2 Gap in joint 0 FEA & Test 5 Noise / Vibration 2 Relative movement 0 " 3

Passenger protection 10 Modified load paths 1 " 4 40

T Peel /tension Water ingress 2 Gap in joint 2 " 4 16 Noise / Vibration 2 Relative movement 1 " 3 6

Passenger protection 10 Modified load paths 3 " 5 150 Analyse loads / add rivets / add adhesive Analysis /Materials 7/5/10 Y 5 1 5 25

Fatigue Water ingress 2 Gap in joint 2 " 4 16 Noise / Vibration 2 Low clamping force 2 Quality 10 40

Passenger protection 10 Modified load paths 3 FEA & Test 10 300 Analyse loads / add rivets / add adhesive Analysis /Materials 7/5/10 Y 1 1 5 5

Corrosion Strength 4 Dissimilar materials 5 Test 8 160 Rivet Coating/ paint & sealing Materials / Test 7/5/10 Y 1 1 5 5

Assembly Hole misalignment 7 Build tolerances 4 Manufacturing 4 112 Larger clearance hole on insertion side CAD / Test 7/5/10 Y 3 2 4 24 feasibility Engineering Access Joint design 5 Gun Clearance 5 Manufacturing 4 100 Simulation of Robot path and fouling Manufacturing 7/5/10 Y 5 1 1 5 Engineering Engineering Cost of Rivet Business case 5 Number needed 5 Product 5 125 Identify areas where pitch can be relaxed Product development 7/5/10 Y 5 2 5 50 development Labour Cost Business case 7 Manual operation 4 Manufacturing 4 112 Only use on niche vehicle volumes Manufacturing N744112 Engineering Engineering Component Cost Business case 6 Hole Drilling 4 Manufacturing 3 72 Engineering Cycle time Business case 5 Location of holes 5 Manufacturing 4 100 Larger clearance hole on insertion side Manufacturing 7/5/10 Y 3 2 4 24 Engineering Engineering Cost of Gun Business case 3 Number needed 1 Manufacturing 1 3 Manufacturing Engineering Engineering Self piercing Rivet: floor/sill Shear strength Water ingress 2 Gap in joint 0 FEA & Test 5 and seat runner Noise / Vibration 2 Relative movement 0 " 3

Passenger protection 10 Modified load paths 0 " 4

T Peel /tension Water ingress 2 Gap in joint 1 " 4 8 Noise / Vibration 2 Relative movement 0 " 3

Passenger protection 10 Modified load paths 1 " 5 50

Fatigue Water ingress 2 Gap in joint 1 " 4 8 Noise / Vibration 2 Relative movement 1 Quality 10 20

Passenger protection 10 Modified load paths 1 FEA & Test 10 100 Analyse loads / add rivets / add adhesive Analysis /Materials 7/5/10 N 10 1 10 100

Corrosion Strength 4 Dissimilar materials 5 Test 8 160 Rivet Coating Materials / Test 7/5/10 Y 1 1 5 5

Assembly Clamping Force 5 Build tolerances 4 Manufacturing 4 80 Engineering Layer failure 8 Incompatible Material 3Product 5 120 Test extreme combinations Product development 7/5/10 Y 3 3 5 45 strength development 10 Incompatible 4Product 5 200 Test extreme combinations after alloy Product development 7/5/10 Y 3 4 5 60 thicknesses development combinations window identified Access Joint design 5 Gun Clearance 5 Manufacturing 3 75 Engineering Joint design 10 Only single side 5Product 3 150 Assembly sequence / put flange on extrusion Analysis / CAD / 7/5/10 Y 10 1 1 10 access development Production Cost of Rivet Business case 3 Number needed 5 Product 5 75 development Labour Cost Business case 7 Fully automated 1 Manufacturing 4 28 Engineering Component Cost Business case 0 No special 0 Manufacturing 0 preparation required Engineering

Cycle time Business case 5 Weight of Gun 1 Manufacturing 4 20 Engineering Cost of Gun Business case 8 Number of Guns 8 Manufacturing 1 64 Engineering

Typical FMEA tracking sheet for joining

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

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Description / Purpose Failure Mode Effects of Failure Severity Causes Occurrence Controls Detection Risk Priority Number (S) (O) (D) (S*O*D)

Blind Rivet: floor/sill Shear strength Water ingress 2 Gap in joint 0 FEA & Test 5 Noise / Vibration 2 Relative movement 0 " 3

Passenger protection 10 Modified load paths 1 " 4 40

T Peel /tension Water ingress 2 Gap in joint 2 " 4 16 Noise / Vibration 2 Relative movement 1 " 3 6

Passenger protection 10 Modified load paths 3 " 5 150

Fatigue Water ingress 2 Gap in joint 2 " 4 16 Noise / Vibration 2 Low clamping force 2 Quality 10 40

Passenger protection 10 Modified load paths 3 FEA & Test 10 300

Corrosion Strength 4 Dissimilar materials 5 Test 8 160

Assembly Hole misalignment 7 Build tolerances 4 Manufacturing 4 112 feasibility Engineering Access Joint design 5 Gun Clearance 5 Manufacturing 4 100 Engineering Cost of Rivet Business case 5 Number needed 5 Product 5 125 development Labour Cost Business case 7 Manual operation 4 Manufacturing 4 112 Engineering Component Cost Business case 6 Hole Drilling 4 Manufacturing 3 72 Engineering Cycle time Business case 5 Location of holes 5 Manufacturing 4 100 Engineering Cost of Gun Business case 3 Number needed 1 Manufacturing 1 3 Engineering Self piercing Rivet: floor/sill Shear strength Water ingress 2 Gap in joint 0 FEA & Test 5 and seat runner Noise / Vibration 2 Relative movement 0 " 3

Passenger protection 10 Modified load paths 0 " 4

T Peel /tension Water ingress 2 Gap in joint 1 " 4 8 Noise / Vibration 2 Relative movement 0 " 3

Passenger protection 10 Modified load paths 1 " 5 50

Fatigue Water ingress 2 Gap in joint 1 " 4 8 Noise / Vibration 2 Relative movement 1 Quality 10 20

Passenger protection 10 Modified load paths 1 FEA & Test 10 100

Corrosion Strength 4 Dissimilar materials 5 Test 8 160

Assembly Clamping Force 5 Build tolerances 4 Manufacturing 4 80 Engineering Layer failure 8 Incompatible Material 3 Product 5 120 strength development 10 Incompatible 4 Product 5 200 thicknesses development Access Joint design 5 Gun Clearance 5 Manufacturing 3 75 Engineering Joint design 10 Only single side 5 Product 3 150 access development Cost of Rivet Business case 3 Number needed 5 Product 5 75 development Labour Cost Business case 7 Fully automated 1 Manufacturing 4 28 Engineering Component Cost Business case 0 No special 0 Manufacturing 0 preparation required Engineering

Cycle time Business case 5 Weight of Gun 1 Manufacturing 4 20 Engineering Cost of Gun Business case 8 Number of Guns 8 Manufacturing 1 64 Engineering 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.

Proposed Corrective Actions Responsible Completion Results Date Actions SOD RPN taken

Analyse loads / add rivets / add adhesive Analysis /Materials 7/5/10 Y 515 25

Analyse loads / add rivets / add adhesive Analysis /Materials 7/5/10 Y 115 5

Rivet Coating/ paint & sealing Materials / Test 7/5/10 Y 115 5

Larger clearance hole on insertion side CAD / Test 7/5/10 Y 324 24

Simulation of Robot path and fouling Manufacturing 7/5/10 Y 511 5 Engineering Identify areas where pitch can be relaxed Product development 7/5/10 Y 525 50

Only use on niche vehicle volumes Manufacturing N 744 112 Engineering

Larger clearance hole on insertion side Manufacturing 7/5/10 Y 324 24 Engineering Manufacturing Engineering

Analyse loads / add rivets / add adhesive Analysis /Materials 7/5/10 N10110100

Rivet Coating Materials / Test 7/5/10 Y 115 5

Test extreme combinations Product development 7/5/10 Y 335 45

Test extreme combinations after alloy Product development 7/5/10 Y 345 60 combinations window identified

Assembly sequence / put flange on extrusion Analysis / CAD / 7/5/10 Y101110 Production

Proposed corrective actions and results of actions Source: R Hall / M Shergold

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.

Part / Process Name / N° Aluminium Floor Panel in steel structure Responsibility Advanced Design / Manufacturing Prepared by RWH / MS Date 14/04/2010 Revision 1.0

Description / Purpose Features Rating Blind Rivet: floor/sill Single Side Access Yes Dissimilar materials in layers Yes Different layer thicknesses Yes Accepts high thicknesses Ratio Yes Misalignment tolerance No Corrosion resistance galvanic Select appropriate coating Piece price High Component (drilling) cost High Gun Price Low Cycle Time Medium Shear strength High Peel Strength High Fatigue strength Medium Self piercing Rivet: floor/sill Single Side Access No Dissimilar materials in layers Check with manufacturer & test Different layer thicknesses Check with manufacturer & test Accepts high thicknesses Ratio Check with manufacturer & test Misalignment tolerance Yes Corrosion resistance galvanic Select appropriate coating Piece price Low Component (preparation) cost Low Gun Price High Cycle Time Low Shear strength High Peel Strength High Fatigue strength High

Check particular case Advantage Disadvantage 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.

a) Design Optimisation 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:

Aluminium Panel in Steel Structure

Load Boundary / Material Geometry initial Properties Conditions

Boundary conditions Thermal Joining Mechanical Shape and Loading E, , Rp, Rm, Ag,..

Function to Minimise Internal stress Stiffness of Thermal Assembly attached Conductivity, structures Emissivity, Expansion Optimisation Coefficient, ... Variables Impact Environment Tolerances Shaping n-value, r-value, ...

Gravity Resistance Electrical, Corrosion, Thermal, Radiation, Magnetic, ... Air pressure

Structure Loads

Vibration

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:  Joining ◦ Structural adhesive ◦ Rivet ◦ Rivet + mastic ◦ Rivet pitch  Shape ◦ Flange length ◦ Flange root radius ◦ Material thickness.

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.

Structure proposed by topological optimisation 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.

Ant-hill plot showing system response to a variation of barrier angle Source: Uncertainty Management in Automotive Crash: From Analysis To Simulation, J. Marczyk Ph.D