AM GENERAL SAVES LIVES WITH COMPUTER SIMULATION SOFTWARE WHILE REDUCING COSTLY, TIME-CONSUMING PROTOTYPING TESTING
In the field since the 1980s, the M939/A1/A2 series, one of the Army's 5-ton military vehicles, experienced approximately 230 rollover accidents in a six-year period.

September 4, 1998, Pittsburgh, Pennsylvania - The U.S. military operates in some of the most dangerous situations and conditions on earth. It demands transportation vehicles that can safely negotiate intense, rough terrain. As with civilian transit, a military's vehicle manufacturer's top priority is to respond to field experience in order to improve safety. So when rollover accidents involving military transport trucks in the M939/A1/A2 series repeatedly caused injuries and fatalities, the U.S. Army's Tank and Automotive Command (TACOM) unit turned to AM General Corp. (AM General) in Livonia, Michigan to improve the truck's safety. TACOM suggested adding a rollover protection structure to the truck's existing frame. Using Accupak/VE Mechanical Event Simulation with Linear and Nonlinear Stress Analysis software by Pittsburgh-based Algor, Inc. to test the modified truck frame design, AM General was able to create a safer vehicle, reduce the number of prototypes needed and shorten the design cycle's length.

AM General Enlisted for Safety Improvement

The M939/A1/A2 series is one of the Army's 5-ton military vehicles in the field since the 1980s. These truck models are generally used to transport troops and supplies. A recent accident study conducted by the U.S. Army indicated that these truck models experienced a relatively high number of accidents due in part to brake lock-up or driver errors such as driving too fast for the road conditions.

In response to the study, TACOM formulated a number of safety measures for the M939/A1/A2 series including the addition of a rollover protection structure that would protect the occupants in the cab in the event of a rollover accident, installation of ABS brakes and three-point seat belts, speed control enforcement and improved tires. While most of TACOM's safety measures could be implemented using existing components, the rollover protection structure needed to be designed, tested and manufactured. For that kind of work, TACOM needed the help of an experienced military vehicle manufacturer.

TACOM brought the truck's frame design to AM General with recommendations for a rollover protection structure. AM General is the original manufacturer of approximately 50 types of passenger and equipment transportation vehicles. Many of these vehicles are produced for the U.S. Army and Navy and AM General's Hummer, the civilian sibling of the military's HMMWV (High Mobility Multipurpose Wheeled Vehicle), is sold to the public.

The AM General design team created plans for a T-shaped configuration to be added to the original truck frame that would protect occupants during a rollover accident. Anticipating that the proposed design would be sufficient, AM General produced a truck frame prototype without first testing a model of the design with any type of computer simulation.

AM General developed a laboratory prototype test using information about the forces and impact energy that the frame and rollover protection structure must withstand, which was provided by TACOM and based on government standards and field experience. During the laboratory test procedure, they applied a series of loads to the prototype's fixed frame to simulate the impact loads experienced in a rollover scenario. The frame would be a success if it absorbed the energy of impact and did not deform into the driver and passenger area. However, the first prototype failed the laboratory test because the design team had built a structure that was too flexible. Rather than continuing to test designs with prototypes, AM General turned to finite element analysis testing to save time and money.

Computer Simulation Overcomes Excessive Prototype Testing

AM General stress analyst Michael Yan performed the finite element analysis with Algor's Mechanical Event Simulation with Linear and Nonlinear Stress Analysis software. Mr. Yan knew that Accupak/VE would enable him to apply loads sequentially and see resulting displacements and maximum principal stresses. Accupak/VE also enabled him to analyze nonlinear effects such as large deflections and plastic deformations.

Using the design team's 3-D solid AutoCAD model, Mr. Yan created a beam/truss element model of the truck's cab frame with rollover protection structure. The frame was made of stainless steel tubes welded together. "Beams were appropriate for this design because the geometry of the structure consisted of relatively slender members," said Mr. Yan. "Using another element type on a model with such slender members would have resulted in a very high number of elements. Since the number of elements affects the processing time, beams offered the advantage of processing efficiency."

Since the process of welding would alter the sectional properties of the tubing near the joints, Mr. Yan increased the density of elements in those regions, resulting in a model with more than 600 elements. Using Algor's Beam Design Editor utility, he then tailored the sectional properties of the elements near the joints to replicate the characteristics of welded stainless steel.

Mr. Yan then set up a finite element analysis in Accupak/VE that simulated the failed prototype test. Boundary conditions were applied to simulate the frame's connection to the cab mounts. First, a 26,000 lb. lateral load was applied to the top of the frame. When this lateral load was released, a vertical load of about 53,000 lb. was applied. After the vertical load was released, a longitudinal load of over 21,000 lb. followed. The frame had to absorb energy as well as force throughout the load applications. For example, the frame needed to absorb 18,772 ft.-lb. of energy during the first load.

After the analysis was complete, Mr. Yan studied deflection and maximum principal stress contours. The analysis revealed which areas of the structure would experience the large deflections and highest stresses. Mr. Yan could then target those areas for modification. "Animated analysis replays show how the design will react in time under loading conditions and I can see the results step-by-step," said Mr. Yan. "The results of an Accupak/VE analysis are very obvious."

Mr. Yan then modified the frame to avoid deformation into the driver and passenger area. Over the course of one month, he tested 25 variations of the cab frame design, improving upon the Algor model each time by adding cross members to reinforce the frame and thickening necessary parts.

"Using Accupak/VE saved a huge amount of time and money," said Mr. Yan. "If AM General had to produce 25 prototypes, it would have taken months and cost thousands of dollars."

Physical and Field Testing Planned for New Rollover Protection Structure

Mr. Yan's modified design is scheduled for prototype testing in the summer of 1998. Although the testing procedure will be the same as for the first test, Mr. Yan's design is stronger than the original model. With successful analysis results behind it, the frame is expected to pass laboratory testing. "In my experience, successful analysis results lead to success in the laboratory," said Mr. Yan. "I fully expect the prototype to pass the laboratory test."

When the prototype passes its laboratory test, AM General will immediately manufacture an additional 20 units for both physical and field testing. This may include an armored variant for field testing in Bosnia, an area with rugged topography. TACOM plans to modify other M939/A1/A2 series trucks on an as-needed basis. They also plan to install the rollover protection structures on trucks in use where conditions such as rough terrain make rollover accidents more likely.

This series of images shows results from the analysis of AM General's final design. The displacement is exaggerated for easier viewing.

Algor's Monitor software enables engineers like Mr. Yan from AM General to study displacements (top) and maximum principle stress (bottom) during analysis processing or in post-processing. Mr. Yan studied deflection to determine how to avoid deformation into the driver and passenger area.