Steel Closures Study Follows ULSAB Model - New Steels and Technologies Help Reduce Weight of Doors, Hoods, Decklids, Hatches

Holistic Design is Key to Low-Cost, High-Performance Solutions

DETROIT, MI, September 29, 1998 - Fresh from the success of its UltraLight Steel Auto Body (ULSAB) project, the steel industry has turned its attention to doors, hoods, decklids and hatchbacks, the "closures" on a vehicle. The UltraLight Steel Auto Closures (ULSAC) study yielded designs that could save more than 50 pounds of weight in an automobile while meeting stringent targets for safety and structural performance at little or no cost increase compared to conventional closures and those made of other materials.

Accounting for about six percent of a typical vehicle's mass, closures offer further opportunities for the steel industry to deliver low-cost, high-performance solutions to its automotive customers' goal to develop and market environmentally benign vehicles in the next decade.

Conducted by Porsche Engineering Services, Inc., (PES) Troy, Mich., the ULSAC study followed a similar approach to that of ULSAB, the comprehensive study of auto bodies, whose results appeared in March. The project comprised benchmarking, target setting, conceptual design, finite element analysis and cost analysis.

Benchmarking

PES began its benchmarking by selecting eighteen 1997 models, which used the most common closure design approaches. These included:

Doors:

  • Roof-integrated
  • Frame-integrated
  • Frameless

Hoods:

  • Conventional
  • Grille-integrated

Decklids:

  • Conventional with tail

Hatches

  • Lift-gate

The goal for benchmarking was to define current state-of-the-art design concepts and provide the basis for target setting. The benchmark study established mass (without glass), dimensions and structural performance standards for doors, hoods, decklids and hatchbacks.
PES normalized the data to make accurate comparisons among closures of different sizes and then evaluated designs and components of the benchmarked closures. To normalize mass, PES used the formula: Mass (in kg) divided by outer surface area (in square meters). PES also assessed costs associated with manufacturing each of the closures. From the benchmarking results, PES developed mass and performance targets for each of the specific closure designs.

Target Setting

Dimensional targets for doors, hoods and decklids were based on existing ULSAB styling surface dimensions because those dimensions were similar to ULSAC benchmarked averages and they provided PES essential outer surface data. For hatch dimensional targets, PES used the measurements from a lift-gate type hatch, which was the lightest and smallest one in the benchmark group.

PES set structural performance targets at the midpoint in a range derived from a survey of the manufacturers of the benchmark cars. Because of the project emphasis on weight reduction, PES set mass targets at 10 percent better than best in class of the benchmarked closures.

Conceptual Design

Using the basic closure approaches identified in the benchmarking phase as a point of departure, PES started with a "clean sheet" and applied a holistic design strategy. Holistic design, used so successfully in ULSAB, treats the structure as an integrated system, rather than as an assembly of individual components. PES specified materials, processes and joining technologies that would meet the mass and performance targets. To guard against oil canning and provide dent resistance, PES employed several conventional techniques:

Incorporate feature lines in outer panels to stiffen unsupported areas:

  • Design inner panel structures to provide adequate support to outer panels
  • Use sheet metal hydroforming to increase outer panel dent resistance through work hardening
  • Use high-strength steel for outer panels.

Doors

In each of the door design concepts (roof-integrated, frame-integrated and frameless), PES employed a 0.7 mm sheet hydroformed outer panel with feature lines to improve dent resistance and counter oil canning. In the roof-integrated design, PES saved mass and improved formability in the inner panel by specifying non-linear weld lines in the tailored blank. Additional mass savings in this part became available by using a double cable window regulator to eliminate the lower glass drop channels. The tailored blank outer panel reinforces the belt area, increasing stiffness there. High-strength steel in the hinge area helps the structure withstand sag and check load stresses.

In the frame-integrated and frameless design concepts, a high-strength steel hydroformed tube that forms the lower door frame withstands sag and check load stresses and incorporates the side intrusion beam to save mass. The hydroformed tube structure also aids assembly accessibility and facilitates service of the window regulator, latch system and wiring harness. Continuous laser welding used in both designs to attach parts to the hydroformed frame enhances rigidity.

In the frameless design, a thin wall die casting used as a structural node to connect the upper and lower frame saves mass by incorporating several features in one part, including the mirror patch, upper hinge and joint node.

The roof-integrated design achieved a normalized mass of 15.1 kg/m2 compared with the target of 15.5 kg/m2. It is 23 percent lighter than the benchmarked average. The frame- integrated design results in a structure with a normalized mass of 15.4 kg/m2 compared with the target of 15.5 kg/m2. It is 22 percent lighter than the benchmarked average. The frameless design has a normalized mass of 14.3 kg/m2 compared with the target of 15.5 kg/m2. It is 27 percent lighter than the benchmarked average.

Hoods

Both hood designs (conventional and grille-integrated) employ a 0.6 mm sheet hydroformed outer panel and feature lines to improve dent resistance in the thin material. Adhesive bonding in the hem flanges enhances structural performance. To improve stiffness, a 'V' pattern inner panel connects the hinges to the latch area. A hole - not the typical depression - is the crush initiator in the outer most ribs of the hood. Triangular beams designed within the 'V' pattern support the outer panel.

Steel sandwich material in the inner panel contributes to mass reduction. This material comprises a 0.8 mm engineered polypropylene core sandwiched between two 0.2 mm sheets of steel. This material can withstand bake ovens and can be assembled prior to painting.

The conventional hood design results in a structure with a normalized mass of 7.9 kg/m2 compared with the target of 8 kg/m2. It is 32 percent lighter than the benchmarked average.
Given that steel sandwich material is not yet readily available in large production quantities, PES recommended an alternative 0.6 mm steel sheet inner panel. This alternative demonstrates performance results that are quite similar to the sandwich material concept but has a normalized mass of 8.24 kg/m2 compared with the target of 8 kg/m2. Nonetheless, it is 29 percent lighter than the benchmarked average.

The grille-integrated design and materials are similar to the conventional hood design with the addition of a grille formed by extending the inner and outer panels.

Normalized mass of this design was virtually identical to that of the conventional hood. As with the conventional hood design, PES offered a steel sheet hood as an alternative to the sandwich material. Again, the results were equivalent.

Conventional Decklid With Tail

Like the hood design, the decklid employs a sheet hydroformed outer panel to improve dent resistance; adhesive bonding in the hem flanges for structural performance; a 'V' pattern inner panel to connect the hinges to the latch area; and steel sandwich material in the inner panel for mass reduction. In addition, PES specified down standing flanges on both edges for greater rigidity.

The decklid design resulted in normalized mass of 7.9 kg/m2 compared with the target of 8 kg/m2. It is 29 percent lighter than the benchmarked average. PES also offered a steel sheet alternative to the sandwich material.

Hatch

For the lift gate hatch design, PES again relied on sheet hydroforming, thin gauge, stamped steel sheet and adhesive bonded hem flanges. In addition, PES created stiffness at lower mass by specifying a tubular hydroformed frame to maximize section size and reduce part count. Urethane bonding of glass to the frame contributes to torsional stiffness.

The normalized mass of this design is 10.3 kg/m2 compared with the target of 11.3 kg/m2. It is 26 percent lighter than the benchmarked average.

FEA Calculations

Throughout the design process, PES used very detailed shell models to perform finite element analysis (FEA) calculations on each part. This step confirmed that local closure vibration modes would not coincide with body structure modes. The calculations also showed that all designs met or exceeded structural requirements and validated the use of steel to reduce weight and achieve effective structural performance.

Cost Estimation

PES performed a cost analysis of each of the closure concepts it developed for the ULSAC project. To create a baseline with which to compare ULSAC closures, PES developed cost estimates for current closures similar in material, size and geometry. Then PES estimated the cost of the concept based on manufacturing experience and knowledge of business economics.

This economic analysis found no discernable difference in the costs of the door concepts and the baseline for doors. For hoods, it found no additional cost over baseline for the sheet steel solution and an increase of about 10 percent for the steel sandwich design. Likewise, for decklids, the study revealed no additional cost compared with baseline for the sheet steel solution and about an 18 percent increase for the steel sandwich design. Costs for the concept hatch came in at approximately 24 percent above baseline. In all cases, however, ULSAC solutions remain highly cost competitive, both with heavier steel designs as well as using alternative materials.

The American Iron and Steel Institute (AISI) is a non-profit association of North American companies engaged in the iron and steel industry. The Institute comprises 48 member companies, including integrated and electric furnace steelmakers, and 173 associate/affiliate members who are suppliers to or customers of the steel industry. For more news about steel and its applications, view American Iron and Steel Institute’s website athttp://www.steel.org.

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
Acme Steel Company
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