Background

What do you do after you have slashed the weight of a steel car structural body through the use of state-of-the-art materials, manufacturing technologies and design? Do you sit back and admire the lighter, more environmentally friendly vehicle you have created? No, you apply the same principles to other parts of the car. Following on from the ultra-light steel auto body (ULSAB) project, which was aimed at optimising the use of steel in a car body, an international consortium of steel producers, including British Steel, has embarked upon the ultra-light steel auto closures (ULSAC) project. The project is intended to demonstrate the potential for steel-based car closure panels that offer major weight savings without penalties to structural performance or cost.

Target Areas for Weight Reduction

ULSAC focuses on four closure panels - doors, bonnets (hoods), bootlids (decklids) and tailgates (hatchbacks). The first phase of the project involved generating design concepts for lightweight steel closures that can be made affordably using current vehicle manufacturing technologies. The new closures also had to meet pre-defined targets for their key structural parameters.

The designs produced in the first phase of the project form the basis for building demonstration closure panels in phase two. The ULSAC consortium contracted Porsche Engineering Services (PES) of Troy, Michigan, to provide engineering management for the project and worked with the company in defining the goals of the project.

Project Approach

The ULSAC approach comprised benchmarking, target setting, conceptual design, FEA calculations on the concept designs and cost analysis. Benchmarking was performed on 18 upper-medium size 1997 North American, European and Japanese cars to define current state-of-the-art design concepts. The study established mass, dimensional and structural performance standards for each of the four closures. Structural performance test methods and specifications were defined from a survey of carmakers, so as to represent OEM internal targets.

Following benchmarking, PES developed the targets for each closure. Dimensional targets for doors, bonnets and bootlids were based on the dimensions used in the ULSAB car body. Structural performance targets were set at the midpoint of the range derived from the OEM survey, and mass targets for each closure were set as 10% lower than the best of the benchmarked parts.

Materials and Design Concepts

Steels

The balance between lower weight, structural performance and cost was achieved using state-of-the-art steels and manufacturing techniques combined with an iterative holistic approach to design. In this approach, the structure is treated as an integrated system rather than as an assembly of individual components. For the doors, efforts to optimise design were directed at the sheet metal panels and the components. Weight savings were achieved in both areas, whereas savings in the other less-sophisticated closures came about through attention to design, materials and the manufacturing of the inner and outer panels. High-strength steels were used for all outer panels. Bake-hardenable steel with a minimum yield stress of 210MN.m^-2 has excellent formability in the as-delivered condition and gains additional strength and dent resistance after press forming and paint baking. Higher strength steels were chosen for other components, such as hinge areas of the inner door panels, and a 1200MN.m^-2 yield stress ultra high-strength steel was used for side intrusion bars to give good impact resistance at low weight.

Sandwich Panels

Steel sandwich material can also be used for mass reduction. The ULSAC design study showed that using it for bonnet and bootlid inners can increase weight savings by 3%. Steel sandwich material can withstand bake ovens so that parts can be assembled prior to painting. However, it increases the cost of the closure compared to current designs.

Sheet Metal Hydroforming

Sheet metal hydroforming was used in the manufacturing of the outer panels to increase dent resistance through better strain distribution and added work hardening. All four closures could be produced using a sheet-hydroformed outer panel made from bake-hardenable steel. High-strength hydroformed tube also reduced the weight of the frame-integrated and frameless door and the tailgate designs.

Tailored Blanks

As in the ULSAB project, the use of tailored blanks was crucial to reducing weight and cost. All the door design concepts used tailored blanks for inner and/or outer panels. This allowed reinforcement of the belt area of the roof-integrated door design through the use of thicker material. More sophisticated blank construction involving non-linear welds meant that laser welds could be located for optimum formability, while still giving weight savings.

Assembly Methods

Assembly methods giving continuous joints increase stiffness and provide the opportunity for mass savings compared to spot welded assemblies. Therefore, adhesive bonding and laser welding were specified where possible in the closure designs. Bonnet and bootlid inner-to-outer hemmed joints were formed using adhesive bonding and reinforcements were bonded to the bonnet inner. Laser welding was used extensively in the door and tailgate designs to join the tubular hydroformed parts, and for attaching the hinges and impact beam.

Project Performance

Weight Reduction Techniques

Various techniques were used to reduce mass and/or improve structural stiffness. These included part consolidation, functional integration, incorporating feature lines in outer panels and designing inners to support outer panels. For example, in the frameless door design, a thin wall die casting was used as a structural node to connect the upper and lower frame. This incorporated the mirror patch, upper hinge and joint node into one part. The bonnet and bootlid inners were designed with triangular beams in a ‘V’ pattern supporting the outer panel.

Overall Weight Reduction

The weight savings achieved in the design study are summarised in table 1. This shows that the door designs were between 21% and 27% lighter than the benchmarked average. The bonnet designs were 26% or 32% lighter than the benchmark depending on whether sandwich steel was used. Similarly, with the bootlid, a weight saving of 23% or 29% could be obtained depending on the use of sandwich steel. The tailgate designs were 22-32% lighter than the benchmark average.

Table 1. Summary of weight savings achieved by ULSAC

Part	Benchmark	Target	ULSAC design	Weight Saving (%)
Roof integrated door	19.7	15.5	15.1	23
Frame integrated door	19.7	15.5	15.5	21
Frameless door	19.7	15.5	14.3	27
Bonnet	11.6	8.0	7.9-8.5	26-32
Bootlid	11.2	8.0	8.0-8.6	23-29
Tailgate	13.9	11.3	9.5-10.9	22-32

FEA and Stiffness

Finite element analysis calculations were performed on each part to confirm that the design would give acceptable structural performance. For the doors, frame rigidity, door sag, torsional rigidity, check load and side intrusion load were evaluated. Torsional rigidity and bending stiffness were looked at for the bonnet, bootlid and tailgate designs, while data on side beam stiffnesses were obtained for both the bonnet and bootlid. All designs met the set targets derived from the OEM survey.

Cost Analysis

To conclude the study, PES performed a preliminary cost analysis on each of the ULSAC closure concepts. To create a baseline with which to compare the ULSAC closures, PES developed cost estimates for current closures made from similar materials in similar sizes and geometries. The costs were estimated based on manufacturing experience and knowledge of business economics.

This economic analysis showed no discernible difference between the costs of the ‘concept’ and ‘baseline’ in two of the three designs of doors. The frame-integrated door was estimated to cost about 7% more than the baseline. For bonnets, there was no additional cost for the sheet steel solution and an increase of about 10% for the steel sandwich design. Similarly, for bootlids the study revealed no additional cost compared with baseline for the steel sheet solution and a 16% increase for the steel sandwich design. Costs for the concept tailgates were estimated to be between 12% and 24% above the baseline.