By Richard Sobieski

A new software package that focuses on automotive steel design helped consultants solve a frame pre-cracking problem in a few days - compared to the weeks that would have been required using conventional hand calculations.

The problem was revealed when a manufacturer experienced excessive warranty expenses on a sport utility vehicle which had been on the market several years. After the vehicle was used a year or two under rugged off-road conditions pre-cracking was seen in several areas on the frame.

In the past, solving this problem would have required extensive manual calculations taking between two to four weeks. Instead, experts from RMS Consulting, St. Albert, Alberta, Canada, used the GAS (Geometric Analysis of Sections) module of AISI/CARS software to solve the problem. The software (from Desktop Engineering Intl. Inc., Woodcliff Lake, NJ, developed for the Auto/Steel Partnership and The American Iron and Steel Institute (AISI), Southfield, MI) is designed specially for analyzing automotive steel structures. The package isolates stress problems and evaluates a wide range of alternatives before selecting the optimum approach.

The vehicle with the pre-cracking problem had been on the road several years when the manufacturer noticed above-normal warranty repairs. The problem, specific to cases of heavy off-road use, was pre-cracking of one of the cross members used to support the main frame. Cracking was occurring both at the ends where it connected to the main frame and also along its length. The cross member where the problem was seen was about 30 in. back from the rear axle. It consists of a C-channel 8 in. high, 3 in. wide at the web and 1/4 in. thick. The part is bent at about a 3 in. radius at either end where it attaches to the main frame.

Investigation initially centered on a stamping operation which had the potential to generate precracks during the manufacturing process. However, discussions with the metallurgical engineers and extensive testing of the components did not turn up any manufacturing problems.

So, RMS Consulting used the GAS module to calculate the bending moment of the member and then applied loading based on information supplied by the vehicle's manufacturer. These calculations showed excessive stresses on the radius where the cross-member was bent for attachment to the frame and also on the radius where it was bent to create the C-channel shape.

Next, consultants used the GAS module and evaluated a number of different geometries for the cross member. GAS contains a C-channel and other common configurations built in so that the user merely needs to enter the required dimensions. The GAS module makes it very easy to change the height, web, thickness and bending radius and quickly determine the effects.

This evaluation showed that increasing the bending radius at the ends of the cross member to 5 in. would avoid the stress concentrations. The next step was addressing the cracking problem in the center of the cross member. The thickness and overall shape of the cross member were fixed by the vehicle's design but the bending radius in the C-channel was allowed to change. The program made it very easy to try a wide range of different radii and determine the stress levels that would result from each. The consultants finally decided to increase the value to 5/8 in.

The manufacturer built prototype vehicle incorporation the modifications from the GAS program. This vehicle was then exposed to severe operational stresses in an extensive off-road durability test for about eight months. Upon inspection of the prototype, engineers found no evidence of pre-cracking. Print changes were ordered and the new frame has since been installed in all production vehicles. The manufacturer reports the warranty problems have been eliminated.

The solution of this problem made use of the AISI/CARS application mode of operation. This mode provides numerical and graphical solutions to over 35 design procedures and 225 equations, based on input provided by the user. It uses sophisticated equation-solving techniques to perform the necessary calculations, thus avoiding mistakes and allowing less experienced users to carry out difficult tasks. CARS also provides a reference mode which accesses over 500 pages of text, charts, tables, equations and figures found in the AISI Automotive Steel Design Manual.

The GAS module is fully integrated into CARS and allows the design engineer to calculate geometric section properties for arbitrary thin-walled sections and it can interface with the CARS application mode to determine member capacities. GAS facilitates analysis of thing-walled open-cell, closed-cell, single-cell, and multiple-cell cross sections; nominal section property calculations; effective section property calculations based on yield, actual or user-specified stress levels; prediction of local buckling effects; trend analyses that permit parametric studies; geometry data input using a keyboard, mouse or digitizing tablet; and geometric databases that can be created, edited, and searched.

RMS Consulting has found the GAS module to save time in solving problems related to on- and off-highway truck frame design. Used in the initial design, it can optimize frames in a manner that prevents such problems in the first place.