Design Bonnet Study

Design – Case study: Bonnet and boot lid Table of contents 6 Case study: Bonnet and boot lid .................................................................................................. 2  6.1 Introduction ........................................................................................................................ 2  6.1.1 Purpose of a Bonnet system ........................................................................................... 2  6.1.2 Purpose of a Boot lid ..................................................................................................... 3  6.2 Design Aspects ................................................................................................................... 3  6.2.1 Design Boundary Conditions for Bonnet and Boot Lid ..................................................... 3  6.2.2 Requirements for Bonnet components ............................................................................ 8  6.2.3 Prioritisation Matrix for Outer Skin Panel ..................................................................... 10  6.2.4 Conclusions:............................................................................................................... 17  6.2.5 Overall conclusions of concept evaluation..................................................................... 20  6.2.6 Comparison of “state of the art” systems ....................................................................... 21  6.3 Conclusions from bonnet and boot lid case study ................................................................. 25 

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6 Case study: Bonnet and boot lid 6.1 Introduction Bonnet and Boot lid form a sub-segment of vehicle “closures”, which also contains doors and tail gate. Bonnet and boot lid usually do not directly open onto the passenger cell which greatly reduces the importance (weighting) of certain of the functional requirements normally associated with closures such as passenger safety, air tightness, low cycle fatigue strength etc. Legislation for new vehicle registrations in Europe, the United States of America and Japan all include requirements for pedestrian safety. EuroNCAP (European New Car Assessment Program) and other independent vehicle assessment bodies have been instrumental in increasing public awareness of the effectiveness of design for pedestrian safety. Additionally, their test results are factored into the insurance ratings that are delivered for new vehicles. The objective of these measures is to reduce the number of road accident fatalities and the severity of injuries sustained by pedestrians involved in a collision with a vehicle in urban traffic. Impact frequency and seriousness of injury has been studied for many years, resulting in rating systems and improved design. One such study based on 246 passenger car / pedestrian collisions (Bosch Automotive Handbook 4th Edition 1998) clearly shows that the bonnet zone accounts for a substantial proportion of the risk associated with pedestrian safety.

This is the major difference between bonnet and boot lid safety functional requirements. Bonnet and boot lid systems influence the following performance measures:  Overall vehicle mass  Fore / aft weight distribution  Height of vehicle centre of gravity (the bonnet and the boot lid are usually located above the C of G of the vehicle, hence weight reduction is beneficial)  Vehicle drive-by noise intensity.

6.1.1 Purpose of a Bonnet system The bonnet system is an access panel to the engine compartment to enable maintenance of power train, drive belts, battery, fluid levels and lamp units. It is fundamentally a reinforced skin panel with many safety and quality requirements.

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6.1.2 Purpose of a Boot lid The boot lid system is an access panel to a rear storage compartment often enabling access to auxiliary systems such as spare wheel, tool box, jack and rear light units. It is fundamentally an opening reinforced skin panel.

6.2 Design Aspects 6.2.1 Design Boundary Conditions for Bonnet and Boot Lid The boundary conditions for commencing a new design process using aluminium can be identified and analysed by grouping together requirements using affinity matrix methods. Bonnet and boot lid are treated separately and are then compared to highlight major differences. Bonnet (Hood) Pedestrian Safety

Vehicle quality

Occupant Safety

Styling

Manufacturing

Child Head Impact Criterion

Torsional Stiffness

Frontal Crash Collapse

Tight radii on hemmed panel edges

Paint drain features

Adult Head Impact Criterion

Flutter Resistance (Bending stiffness)

Frontal Crash Maintain integrity with hinges etc.

Excellent painted surface quality

Outer panel Dent resistance during assembly

Bonnet Leading Edge geometry

Dent Resistance (Palm Print)

Frontal Crash Retention of loosed components

Stretch Flanges

Outer panel Clean sheared edges

Good Shape: Low strain doubly curved surfaces

Outer panel No Lüder lines or other visual defects resulting from stretch forming

Dent Resistance (Hail Stone and Stone Chip) Corrosion Resistance Low mass (Ease of opening)

Inner panel Gauling resistance

Low mass Weight distribution Low mass Vehicle Centre of Gravity

Noise Attenuation

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Boot (Trunk lid) Auxilliary

Vehicle quality

Occupant Safety

Styling

Manufacturing

Electrical resistance Lighting earth return

Torsional Stiffness

RearCrash Collapse

Tight radii on hemmed panel edges

Paint drain features

Dent Resistance (Palm Print)

Excellent surface quality

Outer panel Dent resistance during assembly

Corrosion Resistance General

Stretch Flanges

Outer panel Clean sheared edges

Corrosion resistance Seal Surface

Good Shape: Low strain doubly curved surfaces

Outer panel No Lüder lines or other visual defects resulting from stretch forming

Corrosion resistance Cut edges & holes

Inner panel Gauling resistance

Low mass (Ease of opening)

Low mass Vehicle Centre of Gravity

Pedestrian Safety

Vehicle quality

Occupant Safety

Styling

Manufacturing

Child Head Impact Criterion

Torsional Stiffness

Frontal Crash Collapse

Tight radii on hemmed panel edges

Paint drain features

Adult Head Impact Criterion

Flutter Resistance (Bending stiffness)

Frontal Crash Maintain integrity with hinges etc.

Excellent painted surface quality

Outer panel Dent resistance during assembly

Bonnet Leading Edge geometry

Dent Resistance (Palm Print)

Frontal Crash Retention of loosed components

Stretch Flanges

Outer panel Clean sheared edges

Dent Resistance (Hail Stone and Stone Chip) Corrosion Resistance Low mass (Ease of opening) Low mass Weight distribution Low mass Vehicle Centre of Gravity Noise Attenuation

Good Shape: Low strain doubly curved surfaces

Bonnet (Hood)

Outer panel No Lüder lines or other visual defects resulting from stretch forming Inner panel Gauling resistance

Boot (Trunk lid) Auxilliary

Vehicle quality

Occupant Safety

Styling

Manufacturing

Electrical resistance Lighting earth return

Torsional Stiffness

RearCrash Collapse

Tight radii on hemmed panel edges

Paint drain features

Dent Resistance (Palm Print)

Excellent surface quality

Outer panel Dent resistance during assembly

Corrosion Resistance General

Stretch Flanges

Outer panel Clean sheared edges

Corrosion resistance Seal Surface

Good Shape: Low strain doubly curved surfaces

Outer panel No Lüder lines or other visual defects resulting from stretch forming

Corrosion resistance Cut edges & holes Low mass (Ease of opening)

Inner panel Gauling resistance

Low mass Vehicle Centre of Gravity

Main functional differences between Bonnet and Boot Lid

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Main conclusions from affinity matrix analysis:  The bonnet has more safety (pedestrian and passenger) and vehicle quality (manufacturing and in-service performance) requirements than the boot lid.  An aluminium bonnet may improve fore / aft weight distribution  Both seem to have similar manufacturing boundary conditions From this point onwards we will just focus on the bonnet system. Design options assessment Aluminium enjoys the advantage of being available in a wide range of product forms that may closely represent the output from topological optimisation tools. Each product form possibility may be combined with the intrinsic properties of aluminium in order to identify the most suitable design space for this application. Herring-bone diagrams are a good way to start to develop a Design Failure Modes and Effects Analysis (DFMEA). They are also a good starting point for identification of the key design requirements, wishes and constraints in order to assist the selection of candidate materials, product forms and assembly methods, etc. Bonnet (Hood) Material Characteristics

Shape Characteristics

Elastic limit

Manufacturing Tooling cost

Swages

Corrosion resistance Section collapse Initiation features « Bird Beak »

N-value R-value

Stretched Skin

Total elongation

Blanking & punching Face Spot Welds Self Pierce Rivet

Drawn inner panel

Surf ace hardness

Adhesive bonding

Stretcher Strain defects

Anti-f lutter mastic

Surf ace quality post forming Lubricated and non-lubricated f riction co-ef ficients Conversion coating Surf ace Roughness

Potential solutions

Stamping

Finishing

Flanged holes

Flat hem flanged front edges skin panel

Electro Coating

Rope hem f langed upper edge skin panel

Painting

Paint Bake Response Hem f lange capabilities Fatigue resistance Low density

Flanged panel edges

Outer panel 6xxx Laminated 6/7XXX Sheet Steel Inner panel 5xxx 6xxx Laminated 5/5/7XXX High performance Plastic / Composites Thin-walled Casting Sheet Steel

Cycle Time

Desired characteristics and manufacturing techniques arranged in a herring-bone diagram. Whole vehicle system considerations Pedestrian impact energy is absorbed by a sequence of different mechanisms. In most cases, the leg of the pedestrian is first impacted by the bumper system (lower leg impact is a safety critical load case for the pedestrian that is entirely managed by the bumper system). Initial contact with the pedestrian is therefore at a point below the centre of gravity of the head and torso causing rotation. At relatively low velocity impacts ( 40 km/h

Skin Panel