Set In Steel: Find Out How One Automotive Supplier Used Innovative Design to Kill Several Birds With One Stone

By using software that calculates section properties to optimize design, engineers at one North American steel company helped a metal stamper save weight, welding cost and material cost on an automobile anti-intrusion door beam.

The stamper producing the door beam was originally using imported steel, but due to the exchange rate, it experienced escalating, material costs. Engineers analyzed the design of the beam using software that quickly calculates effective section properties that are extremely difficult to calculate by hand. By evaluating a wide range of alternate designs, the engineers developed a new approach that made it possible to use a lower strength, lower cost material, and reduced the number of welds from 84 to 8 and reduced weight by six percent.

The original design of the anti-intrusion door beam involved the use of four components:

1. a hat section which runs the full length of the beam with the top of the hat towards the car interior;
2. a flat plate with stiffener lipped edges running the full length of the beam
3. a short length smaller hat section located in the interior of the larger hat; and
4. a short length flat plate located above the smaller hat.

Each component part was roll formed and then assembled with spot welds. The two shorter components were then welded inside the closed section at the center of the beam to provide additional support. The flat plate with lipped edges was then welded to the full length hat section to form a closed section. Then ten holes were punched out of the ends of the beam to reduce the beam weight.

The part was originally manufactured from cold rolled high strength steel with a minimum 140 ksi yield strength. Competition from other automotive parts producers made it clear that manufacturing, costs had to be reduced to remain competitive after the contract expiration date. The manufacturer of the door beam originally considered using aluminum as a substitute for the high strength steel. However, they discovered this would result in a considerable material cost disadvantage. Although aluminum has lower density than steel, it also has a lower yield strength and considerably higher cost per pound. Also, in this case, the use of aluminum requires costly changes to the manufacturing process that can not be passed on to the automotive company that purchases the component.

Going to the source

By sourcing the steel supply from a domestic supplier, it was thought that material cost could be reduced, thereby eliminating material cost volatility due to a fluctuating exchange rate. However, this required changing the design of the part because the exact grade of ultra high-strength steel was not available from a domestic source. A lower cost, high-strength galvanized steel with a minimum yield strength of 120 ksi was chosen as a material substitute. A manufacturer of flat rolled, hot rolled, cold rolled and galvanized steel was asked to improve the beam design so that this lower cost material could be used as the substitute.

The anti-intrusion door beam needs to resist the bending moment resulting from the applied load at the center of the beam. Based on static loading tests conducted by the manufacturer on or the original design, a peak load requirement of 5100 pounds was determined for a simply supported beam with two simple end supports, approximately 35.4 inches apart, loaded at the center of the beam. The window mechanism and other door components restricted the maximum design height and width of the beam to only one and a quarter inches by eight inches.

The crux of this and many other similar problems, is the "effective width concept" that accounts for the post-buckling strength of a thin walled section. The post-buckling strength is the additional load carried by elements of the cross section after they have buckled locally by means of a redistribution of stress. Under the effective width concept, only certain portions of the plate width are considered to be effective in carrying loads after exceeding the local buckling, stress. Accordingly, the effective width of each segment in the cross section is computed. If the results indicate that any element is not fully effective, the calculation becomes iterative. The iterative nature of effective property calculation makes manual calculation very time consuming. So engineers used the Geometric Analysis of Sections (GAS) module of AISI/CARS software developed by Desktop Engineering Int'l, Inc. of Woodcliff Lake, New Jersey, for the Auto/Steel Partnership (A/SP) and The American Iron and Steel Institute (AISI) of Southfield, Michigan to evaluate several different alternative section designs.

GAS uses the following procedure to compute the effective cross section properties:

1. Compute the normal stress distribution, assuming all segments are fully effective,

2. Calculate the effective width of each segment;

3. Calculate the cross section properties of the section based on the effective portion of each segment;

4. Compute the stress distribution using the calculated effective section,

5. Calculate the effective width of each segment based on the revised stress distribution;

6. Calculate the cross section properties of the section based on the effective portion of each segment; and

7. Repeat Steps 4 through 6 until the area, and moment of inertia of the effective section, converge to the assumed trial value.

The first design evaluated by the steel company's engineers was a one piece hat shaped beam suggested by the manufacturer. A one piece beam is desirable because it is inexpensive to produce. After the cross section geometry was entered into the CARS program, calculations were performed showing the effective section modulus to be 0.0747 cubic inches at the proposed steel thickness. The yield load was also calculated, and found to be only 1012 pounds, versus the 5,100 pounds required. Additional iterations with increased metal thickness were then performed until the yield load of the beam exceeded the peak load requirement. However, with this design, the required steel thickness of an acceptable beam would have to be greater than the manufacturing capabilities of the stamper.

The second proposed design was a one piece "W" shaped section. A number of "W" shaped beams with different dimensional characteristics were analyzed using the CARS program. Varying the dimensions made it possible to achieve the required load carrying capacity; however, the beam weight was calculated to be 11.54 pounds, or 1.5 pounds heavier than the original beam. An increase in weight was unacceptable so this approach was abandoned.

The next approach involved the use of two full-length "W" sections designed to interlock when one is placed upside down over the other. Two lengths of the same cross section can be produced with only a slight marginal cost increase over a one piece beam. Once again, however, the resulting beam design proved to be heavier than the existing beam and therefore unacceptable. Engineers then conceived a new design with less steel on the ends of the beam which carry less bending moment. The reinforcing "W" section was shortened so it only covers the central portion of the beam where the bending moment is the highest.

The final design provides yield load of 5977 pounds using the lower cost 120 ksi steel while weighing 9.4 pounds-6 percent lighter than the original design. The calculated peak load is higher than the original beam while the reduced weight of the new component contributes to improved fuel economy. Cost is reduced both by the fact that the new design uses a smaller volume of a lower cost steel, and by the number of part components is reduced from four to two. Only eight spot welds are required to assemble the redesigned anti-intrusion beam, which provides a significant reduction in manufacturing cost.