ULSAB: An Inside Look at the Details

Image


ULSAB...
A Look Inside the Details

The UltraLight Steel Auto Body (ULSAB) project has concluded and has produced unmatched results: a lightweight steel auto body structure that significantly outperforms benchmarked averages and can also cost less to build.

An ULSAB structure has been assembled, weighed and tested, validating results from the concept phase of a global steel industry study and satisfying the project goals. ULSAB has proven to be lightweight, structurally sound, safe, executable and affordable.

The ULSAB structure weighs merely 203 kg, up to 36 percent less than a range of benchmarked vehicles in the concept phase of the study. Physical tests of the structure reveal similar remarkable results: torsion and bending tests showed improvements over benchmark of 80 percent and 52 percent, respectively, and first body structure mode indicates a 58 percent improvement. Crash simulations also show ULSAB satisfies mandated crash requirements, even at speeds exceeding the requirements. In addition to reduced weight and superior performance, ULSAB costs no more to build than typical auto body structures in its class and can even yield potential cost savings, according to economic analysis.

The ULSAB structure consists of 94 major parts necessary for its structural integrity, plus brackets and additional reinforcements normally included in this type of auto body structure. It does not include doors, hood or decklids, which are the subject of a separate, independent study.

Creation and testing of ULSAB body structures culminated this aggressive $22 million project to demonstrate a lightweight steel auto body structure that meets a wide range of safety and performance criteria. The ULSAB project was sponsored by a consortium of 35 steel companies representing 18 countries around the world, including 11 in North America.

The goal of the consortium was to meet the challenges issued by their automotive industry customers: reduce the weight of steel auto body structures at no additional cost, while maintaining or improving performance. The most prominent sheet steel producers from around the world joined together to design and validate an UltraLight Steel Auto Body.

The ULSAB consortium contracted Porsche Engineering Services, Inc. (PES) to provide engineering and manufacturing management for the ULSAB project and also worked with them to define the project goals. They took a two-stage approach, encompassing a concept phase and a validation phase.

Design

Benchmarking
At the beginning of the design process, PES benchmarked mid-sized four door sedans to determine current standards against which to measure ULSAB. For instance, PES established package constraints through this benchmarking process. To determine an "average base model," 32 different cars representing varying worldwide customer requirements were selected for package benchmarking.

For structural benchmarking (static torsion, static bending, first body structure mode), PES evaluated nine cars that represented current performance standards. Results of that benchmarking study produced the following performance specifications:

Benchmark Performance

11,534 Nm/deg

Static Torsional Rigidity

11,534 Nm/deg

Static Bending Rigidity

11,902 N/mm

First Body structure Mode

38 Hz

Mass

271 kg.

This information was then used to predict a future reference structure with which ULSAB must ultimately compare. Recognizing continuing improvements in body design and engineering, the future reference structure represents improved performance in all areas. Assumptions for that future reference structure are as follows:

Future Reference Structure Performance

11,534 Nm/deg

Static Torsional Rigidity

13,000 Nm/deg

Static Bending Rigidity

12,200 N/mm

First Body structure Mode

40 Hz

Mass

250 kg.

Philosophy
Design considerations for ULSAB were, in large part, governed by mass reduction and improved performance. ULSAB's design team started with a "clean sheet of paper" and used an iterative holistic approach to design, whereby the body structure was treated as an integrated system rather than as an assembly of individual components. The holistic approach emphasized total structure analysis. Sophisticated computer models enabled design engineers to view the body structure as an integrated whole. This perspective enabled them to evaluate how changes in one area affected other areas and where future optimization opportunities existed. Through each iterative step, re-analysis confirmed the effectiveness of the latest optimizations. This approach promoted weight savings and improved structural integrity by enabling engineers to reduce weight in certain areas while strengthening strategic locations. The net effect was the creation of a more efficient structure.

Computer models were also used to analyze all structural aspects and to simulate specific crash events, demonstrating acceptable performance. Sophisticated architecture and refined joint designs ensured continuous load flow, which improved stiffness and strength in the body structure.

Package
ULSAB did not save mass through downsizing. Its package design included a 3-liter V6 engine, transverse front wheel drive, rack and pinion steering, McPherson front suspension, twist beam rear suspension, generous occupant space, exhaust system routing and a 65 liter (17.16 gallon) fuel tank. Its wheelbase is 2700 mm (106 inches); vehicle width is 1819 mm (72 inches); and vehicle length is 4714 mm (186 inches).

Styling
Exterior styling of the ULSAB was necessary to create surfaces for design. Styling also provided the major feature lines for the doors, decklid, hood, fender and front and rear bumpers, which were used in development of mating structural parts. Styling also gave ULSAB a look that is easily recognized while preserving the opportunity to conduct further design studies - for doors or other closures - in the future.

Materials Selection and Processes
Advanced materials and technologies were employed to meet the project goals. The design relies on high-strength steels, steel sandwich materials, tailored blanking, hydroforming and assembly laser welding for reduced weight and structural efficiency.

Materials and Processes

The materials for the body structure were chosen to meet mass, performance and safety goals. They include some grades and thicknesses of steel currently available but not commonly used in auto bodies. Steel is the most recycled material, and all steel chosen for the ULSAB structure is recyclable.

ULSAB uses high-strength steel and ultra high-strength steel for more than 90 percent of the body structure to improve structural performance and save mass. And nearly half of ULSAB's mass consists of parts that use tailored blanks, which enable the design engineer to locate various steels within the part precisely where their attributes are most needed, thereby removing mass that does not contribute to performance.

ULSAB also features a hydroformed side roof rail, which provides an essential load path for structural performance and crash energy management. In addition to tubular hydroforming, ULSAB uses sheet hydroforming for the roof panel. The work hardening effect produced improved dent resistance in the formed part, especially in the center of the panel.

Steel sandwich material was chosen for mass reduction in the spare tire tub and dash panel.

Additionally, laser welding was used in ULSAB assembly for high static and dynamic strength of joints, for areas where access was available on only one side and for good aesthetic appearance at joint areas.

These advanced materials and processes enabled the design engineers to consolidate functions in fewer parts, resulting in mass savings and improved performance.

Manufacturing

Early Involvement
Manufacturability of parts was crucial to the project's success and was contemplated throughout the design phase. Early in the design process the design engineers worked with component fabricators and steel producers to optimize the design. The steel producers were called upon to provide high-strength and ultra high-strength grades of steel for use in tailored blanks, hydroformed tubing and steel sandwich material. The project producers also used forming simulations early in the project to predict manufacturability.

Tooling
To prove manufacturing feasibility of ULSAB the consortium specified production intent standards for ULSAB parts, requiring that all parts be manufactured from tools with no manual forming. All stamping tools in this program were soft tools made of material such as kirksite. Due to pressure requirements, hydroforming was accomplished using hard tools. In all cases, part fabrication tolerances and quality standards were maintained the same as intended for full volume production.

Analysis
To help ensure that the part designs were feasible, the project partners performed forming simulation analysis on the most complex parts. Forming simulation was performed using finite element methods to show locations of strains and material thinning. The project partners then used this information to recommend product design and tooling adjustments accordingly.

Part Validation
Upon completion of tooling, the component fabricators manufactured the parts and evaluated them using circle grid strain analysis to confirm that they were formed to full volume manufacturing standards. Confirmation was also established through measurement of key part dimensions and the use of checking fixtures. Complete information to support manufacturing feasibility was documented and includes material characteristics, tooling parameters and press conditions.

Assembly
ULSAB assembly sequence was quite conventional, deviating from convention only in its two-stage body side framing. The assembly sequence included floor complete assembly, front end assembly, body side inner assembly, underbody complete assembly, framing and final assembly. The assembly sequence, processes and tolerances were established by PES. All tooling holes and locators that would be used for production were used during the build. Porsche in Germany assembled the demonstration hardware using a flexible, modular assembly fixture system.

ULSAB employs about one-third fewer spot welds and significantly more laser welding than a conventional body structure, resulting in improved structural integrity, as well as some cost savings.

Structural Performance

Physical tests show ULSAB to exceed all concept phase averages.

ULSAB Structure
Benchmark Structure
Average

Future Reference
Structure

Performance
Torsion

Bending

First body structure mode (Hz)

20,800

18,100

60


11,531

11,902

38


13,000

12,200

40
Mass (in kg)
203*

271

250
*natural range of variation ± 1 percent

Crash simulations to help predict safety indicated excellent crash behavior of the ULSAB structure in the following crash events: 35 mph frontal NCAP; 55 km/h 50 percent offset AMS frontal impact; 35 mph rear moving barrier; European side impact and roof crush. The 35 mph frontal NCAP and 35 mph rear moving barrier simulations were run at speeds that exceed mandated safety requirements by 17 percent, and the AMS frontal is widely considered one of the most severe offset crashes performed today.

Economic Analysis

Although lightweighting without sacrificing performance was ULSAB's priority, affordability was also important. Early in ULSAB's concept phase the consortium commissioned an economic analysis to establish a reference by which to compare ULSAB. IBIS Associates estimated the cost to manufacture a current, typical body structure in the same class as ULSAB at $1116 each.

In the validation phase, the cost issue was revisited more thoroughly. A Porsche-led team of analysts developed a detailed cost model that included all aspects of fabrication and assembly. The cost model can be used to analyze ULSAB costs in comparison with other options and also to generate costs associated with alternative designs.

This model comprehends United States manufacturing costs, including investments for both plant and tooling, piece fabrication costs and assembly costs, through to the end of the body shop. The analysts used these details to generate a part-specific cost model for ULSAB. They also created assumptions about a future typical four-door sedan (Year 2000) reference structure with which to compare ULSAB.

Basic economic assumptions were identical for both ULSAB and the reference structure; however, inputs about specific parts and assembly steps were not directly comparable because the Year 2000 structure represents an average of typical vehicle structures in the same class as ULSAB. For the Year 2000 structure, the study identified part groupings - instead of individual parts as with ULSAB - and assumed significantly improved fabrication and assembly techniques as compared to the concept stage benchmark.

The cost model with these inputs showed the ULSAB body structure to cost $947 each to manufacture and the Year 2000 structure to cost $979 each, demonstrating that sophisticated design of a steel body structure can achieve lightweight at no cost penalty and with potential cost savings.

Sharing Knowledge

The ULSAB project employed many techniques and processes that were unique and deemed patentable by international attorneys. The consortium chose to make all patentable features along with other project results freely available to its customers and to the public. All intellectual property generated by ULSAB has been placed in the public domain.

Conclusion
The steel industry has demonstrated the viability of a lightweight, structurally superior steel auto body structure that is also affordable. ULSAB weighs less and performs better than benchmarked averages, while also providing potential cost savings.