Four High-Strength Steel Tubes Provide Structure and Crash Energy Management Functionality

DETROIT, MI, May 24, 2000 – The UltraLight Steel Auto Closure (ULSAC) Validation Phase has succeeded in advancing the state-of-the-art in lightweight, cost effective, safe and environmentally benign auto closures, and close examination of the details of the design of the frameless door hardware demonstrates the point.

A consortium of 33 international steel producers undertook the ULSAC study in 1997, beginning with a Concept Phase, which comprised benchmarking and conceptual design. Following the successful Concept Phase, the ULSAC program proceeded in November 1998 to the validation of a frameless door concept design. The Consortium selected the frameless door to build and test because it is representative of all the closure designs developed in the Concept Phase in that it embodies much of the materials and manufacturing technologies of all the designs. Its successful manufacture demonstrates the value and structural efficiency of combining innovative architecture, advanced manufacturing technology and advanced grades of steel.

With use of high- and ultra high-strength steels and technologies such as tailor-welded blanks, stamping and hydroforming, the ULSAC door achieved 33 percent mass savings over the average Concept Phase benchmark from a wide range of door structures. It is 42 percent lighter than the Validation Phase benchmarked average of frameless doors only, and 22 percent lighter than the lightest benchmarked unit, a framed door structure.

The validated door weighs 23.15 pounds (10.5 kg) and would cost $66.50 to manufacture in typical high-volume production (greater than 225,000 units). With no compromise to safety, the door meets or exceeds a range of performance requirements and would cost no more to build than doors in the benchmark group.

The ULSAC consortium commissioned Porsche Engineering Services, Inc. (PES), Troy, Mich., to provide design and engineering management for both the Concept and Validation Phases of the program.

Scope of the Validation Phase

The ULSAC Validation Phase included further optimization of the frameless door design from the Concept Phase, with emphasis on mass reduction and high-volume production. Following design optimization and using detailed design data, PES built tooling and fixtures, manufactured and assembled actual doors to validate the concept. Once the door was complete, PES conducted a series of confirming analyses and tests, including structural and forming analyses, dent resistance and oil canning testing as well as structural testing for door sag, torsional performance and side intrusion. An economic analysis detailed ULSAC’s costs and compared the results to a generic, state-of-the-art door structure cost analysis. The Validation Phase also included documentation of material properties, part manufacture and door assembly.

ULSAC’s Comparison to State-of-the-Art Benchmarks

In the Concept Phase, benchmarking defined current state-of-the-art design concepts; target setting provided specific objectives; and conceptual design demonstrated ideas that would meet the targets and produce data to support the concepts. The Concept Phase benchmarked a selection of 18 doors taken from 1997 model vehicles, the lightest of which, or best in class, was a framed door.

Because it selected the frameless door design for representative validation, PES benchmarked three additional frameless doors during the Validation Phase to which ULSAC’s door could be more precisely related. These units are Doors A, B and C in the table below.

To accurately compare ULSAC to all benchmarks, PES normalized the mass of all doors by dividing the door structure mass by the true outer surface area, taking surface curvature into account (kg/m²), a procedure carried over from the Concept Phase.

Compared to benchmarks, ULSAC gained significantly in closure mass reduction:

This mass reduction is due to efficient design combined with the best use of steels and manufacturing technology.

Design and Technology Combine to Create Efficient Structure

The frameless door validation uses mild, high-strength and ultra high-strength steels taken from normal steel mill production. However, to demonstrate optimal mass, safety and performance results at affordable cost, ULSAC’s door pushes the envelope with selective use of thinner, high yield strength materials that have not been common in closure panels.

These materials, along with advanced processes, enabled design engineers to consolidate functions into fewer parts, resulting in mass savings while maintaining performance, safety and affordability.

Designers also used a holistic approach to design, which views the structure as an integrated whole and enables evaluation of how changes in one area affect other areas, and where further optimization opportunities exist. This approach resulted in the creation of an efficient, optimized door structure. Consequently, the ULSAC frameless door sets a new standard for efficient material usage and the design and functionality of several of its parts.

The design eliminates the need for a structural full door inner panel. This is partly a result of the four tubular parts that make up the basic door structure, which are high-strength workhorses for achieving necessary crash safety and structural performance at significantly reduced weight. The door structure provides an excellent example of both part and functional consolidation. The two vertical hydroformed tubular parts eliminate the need for several reinforcements, particularly at the hinge and mirror flag attachments points. The upper and lower tubes provide stiffness and work together as side intrusion beams, simultaneously meeting both basic structural and crash energy management responsibilities.

The structure comprises high-strength steel tube hydroformed latch and hinge parts and two straight ultra high-strength steel tubes. These separate components allow for selection of precise diameter, material grade and thickness combination for each part — independently of one another and based entirely on functional requirements. The tube used to hydroform the latch component is 1.0 mm, 280 MPa yield strength steel and the tube for the hinge component is 1.2 mm, 280 MPa yield strength steel. These two parts provide structural attachment points for the hinges and latch and join to the lower tube and outer belt reinforcement to form the inner door structure.

The lower tube and outer belt reinforcement use Dual Phase ultra high-strength steel. In a frontal collision, these two parts provide excellent load carrying capabilities between the A- and B-pillars. In side collisions, they provide strength and absorption capabilities to effectively manage impact energy forces. The lower tube features a larger section with a relatively light wall thickness of 1.6 mm, providing the necessary attributes for excellent crash performance, but at a low mass.

The frameless door also features a stamped tailor welded blank inner front, which incorporates the mirror flag inner. The upper portion of the blank is 1.0 mm, 140 MPa yield strength mild steel, and the lower portion is 1.2 mm of the same material.

The material in the upper portion adds stiffness to the mirror flag for support of the rear view mirror and outer panel attachment, but at lower mass. The thicker lower portion of the tailored blank is necessary to achieve acceptable structural performance in the hinge area. The inner front and mirror outer stampings form the glass drop channel and capture the outer belt reinforcement. This creates a strong structural node, which transfers loads to and from the hinge tube. It also consolidates as many functions as possible for attachment of the mirror and window components, with fewer parts.

Choosing the Best Steel for Outer Door Panel Performance

ULSAC’s door outer panel uses Bake Hardenable (BH) 260 MPa high-strength steel in 0.7 mm thickness. To select this material for the outer panel, PES considered six different materials in both 0.6 mm and 0.7 mm thicknesses:

All six of these high-strength steel grades were successfully manufactured into quality door outers, using stamping. This is an important achievement in stamping using these particular grades and steel thicknesses and advances the state-of-the-art in such applications.

After evaluation, PES selected three materials for comparative testing: BH 210, BH 260 and DP 600, all in both 0.6 and 0.7 mm thicknesses. The engineers selected these three grades because they represent a good range of steel grades for comparison purposes, and they are at the leading edge of steel material use in closure panels. After completion of dent resistance and oil canning testing of stamped parts made from these three materials, Consortium material experts selected BH 260 at 0.7 mm thickness because it proved to provide the best dent resistance and oil canning behavior for this particular door design.

The ULSAC Consortium member companies provided all necessary material-specific data to design, develop and construct the ULSAC frameless door demonstration hardware and provided all materials used in manufacture. These included sheet steel and tailor welded blanks, as well as raw tubes for manufacturing of the straight tubular and hydroformed tubular parts. In addition, member companies supported the program with data and expertise related to material selection and tailor welded blank development, as well as forming simulation on selected parts.

ULSAC Includes a Complete Door Package

In developing ULSAC’s frameless door, PES took the design beyond the structural components, the main focus of the program, to selecting the complete door component package to ensure the door’s total functionality. This approach resulted in an example of a complete lightweight, functioning door and an investigation of the impact of selected components on the final door assembly and assembly sequence.

Style, along with driver safety, comfort and convenience, were all factors in choosing the component package. The complete door features a trim panel-integrated energy absorbing foam block (rather than a separate foam inner as is current practice), electronic door latch and outer handle, power window and lock, and heated, electronic rear view mirrors.

Manufacturing and Assembly

PES selected suppliers for part manufacture according to a range of criteria, the most important of which was the supplier’s experience in producing "production intent" prototypes. Another was the supplier’s proximity to Porsche AG’s Weissach, Germany, facility where the doors were finally assembled.

The ULSAC program incorporated simultaneous engineering -- involving PES, parts and materials suppliers and assembly specialists -- to design and manufacture the parts for the frameless door. Simultaneous engineering drives an efficient process of implementing changes to tool designs prior to their release for manufacture and provides feedback regarding feasibility and cost. It also ensures successful and timely manufacture of all parts.

ULSAC’s door structure assembly occurred in three sub-assembly stages: The first sub-assembly joins the four tubular parts; the second sub-assembly joins the door inner parts to the first tubular parts; and the third bonds and hem-flanges the door outer panel to the rest of the door structure. Adhesive bonding and laser, spot and MIG welding are used in the assembly process.

The door components package is incorporated in the three sub-assemblies to form an inner panel module. The inner panel module then fits into the door structure after painting.

Test Results

PES physically tested the ULSAC door structure to confirm performance and to select the best-suited door outer panel material for the ULSAC demonstration hardware. Two types of testing occurred: Dent resistance/oil canning testing and structural performance testing.

ULSAC’s performance meets or exceeds requirements for dent resistance, oil canning, upper and lower lateral stiffness, and quasi-static intrusion. Rather than perform a physical test for longitudinal door crush, PES simulated this test using computer-aided engineering (CAE) non-linear analysis. Results of this analysis suggest that the door structure would make a considerable contribution to crash load management when tested in a full vehicle front impact event. Benchmarking of frameless doors shows that the ULSAC door, in respect to vertical door sag, performs similarly to doors currently in production – yet ULSAC is 42 percent lighter than the average frameless door benchmark.

Cost Results

Results of the economic analysis, using an interactive spreadsheet with a large degree of detailed inputs including parts fabrication and assembly, show that a lightweight door structure with comparable performance to state-of-the-art generic doors would cost no more to build in production volume. According to the detailed economic analysis, an ULSAC door would cost $66.50 to manufacture in annual production volumes of 225,000, compared to the generic door at $138 for a pair.

Future Developments

PES is conducting additional development of the same frameless door design, using a sheet hydroforming process for the door outer panel. This work focuses on attaining the maximum possible benefit from steel’s excellent strain hardening characteristics. The Consortium will release additional information concerning this work when the work is complete.

Like the UltraLight Steel Auto Body (ULSAB) study, released in 1998, the UltraLight Steel Auto Suspensions (ULSAS), released in May 2000, and ULSAB-AVC (Advanced Vehicle Concepts), which will be complete in 2001, the UltraLight Steel Auto Closure (ULSAC) program is a study undertaken by the global steel industry to demonstrate the effective use of steel in producing lightweight, structurally sound steel automotive closure panels that are manufacturable in production volume and affordable. The ULSAC Consortium consists of the following leading steel companies from around the world:

ACERALIA Corporación Siderúrgica, S.A. - Spain
AK Steel Corporation - USA
Bethlehem Steel Corporation - USA
BHP Steel - Australia
China Steel Corporation - Taiwan, ROC
USINOR/Cockerill - Belgium
Corus Group - The Netherlands
Corus Group - United Kingdom
Dofasco Inc. - Canada
Ispat Inland, Inc. - USA
Kawasaki Steel Corporation - Japan
Kobe Steel, Ltd. - Japan
LTV Steel Company, Inc. - USA
National Steel Corporation - USA
Nippon Steel Corporation - Japan
NKK Corporation - Japan
Pohang Iron and Steel Co., Ltd (POSCO) - Republic of Korea
Rautaruukki/RAGAL Feinblech GmbH - Finland
Rouge Steel Company - USA
Salzgitter AG - Germany
SIDERAR S.A.I.C. - Argentina
Sidmar NV - Belgium
Stelco Inc. - Canada

The Automotive Applications Committee (AAC) is a subcommittee of the Market Development Committee of AISI and focuses on advancing the use of steel in the highly competitive automotive market. With offices and staff located in Detroit, cooperation between the automobile and steel industries has been significant to its success. This industry cooperation resulted in the formation of the Auto/Steel Partnership, a consortium of DaimlerChrysler, Ford and General Motors and the member companies of the AAC.

American Iron and Steel Institute/
Automotive Applications Committee:
AK Steel Corporation
Bethlehem Steel Corporation
Dofasco Inc.
Ispat Inland Inc.
National Steel Corporation
Rouge Steel Company
Stelco Inc.
United States Steel Corporation
WCI Steel, Inc.
Weirton Steel Corporation