Engineers at a North American steel company helped a metal stamper save weight, welding cost and material cost on an automobile anti-intrusion door beam by using software that calculates section properties to optimize the design. The stamper producing the door beam was originally using imported steel and experienced escalating material costs due to recent increases in the exchange rate. Engineers analyzed the design of the beam using software that quickly calculates effective section properties which 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, reduced the number of welds from 84 to 8 and reduced weight by 6%.

The original design of the anti-intrusion door beam shown in figure 1 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 4) a short length flat plate located above the smaller hat. Each component was roll formed and then assembled with spot welds. The two shorter components were 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. 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 high strength steel. But, they discovered that this would result in a considerable material cost disadvantage. Aluminum has lower density than steel, but it also has a lower yield strength and considerably higher cost per pound. Also, in this case, the use of aluminum would require costly changes to the manufacturing process that could not be passed on to automotive company that purchases this component.

It was felt that by sourcing the steel supply from a domestic supplier, the material cost could be reduced and the material cost volatility due to a fluctuating exchange rate could be eliminated. This required changing the design of the part, however, 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 a 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 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, 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 the effective property calculation makes the manual calculation very time consuming. So, engineers used the GAS (Geometric Analysis of Sections) module of AISI/CARS software developed by Desktop Engineering Int'l, Inc., Woodcliff Lake, New Jersey, for the Auto/Steel Partnership (A/SP) and The American Iron and Steel Institute (AISI), 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 the 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. 7) Repeat Steps 4 through 6 until the area and moment of inertia of the effective section each converge to the assumed trial value.

The first design that was evaluated by the steel company's engineers was a one piece hat shape beam suggested by the manufacturer. A one piece beam is desirable because it is very 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 calculated and found to be only 1012 pounds versus the 5,100 pounds required. Additional iterations with increased metal thickness were performed until the yield load of the beam exceeded the peak load requirement. However, 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 (shown in figure 2 Design C)designed to interlock when one is placed upside down over the other. Two lengths of the same cross section could be produced with only a slight marginal cost increase over a one piece beam. Again, the resulting beam design proved to be heavier than the existing beam and therefore unacceptable. Then, engineers conceived of a new design with less steel on the ends of the beam which carry less bending moment. The reinforcing 'W' section (shown in Figure 2 Design D) was shortened so that it only covers the central portion of the beam where the bending moment is the highest.

The final design provided yield load of 5977 pounds using the lower cost 120 ksi steel while weighing 9.4 pounds, 6% 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 fact that the number of part components is reduced from four to two. Only 8 spot welds were required to assemble the redesigned anti-intrusion beam, providing a significant reduction in manufacturing cost.