This article discusses that event-simulation software can help engineers improve the safety of a military transport vehicle, while reducing the number of prototypes and shortening the design cycle. US Army's Tank and Automotive Command (TACOM) unit brought the truck's frame design to AM General with recommendations for a rollover protection structure. 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. AM General stress analyst performed finite element analysis with Mechanical Event Simulation, including both Linear and Nonlinear Stress Analysis software from Algor Inc. of Pittsburgh. When the prototype asses 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 M 939/A1 / A2 series trucks on an as-needed basis; they will install the rollover protection structures on trucks in use where conditions like rough terrain make rollover accidents more likely.
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 vehicle manufacturer’s top priority is to respond to field experience in order to improve safety. When the U.S. Army’s Tank and Automotive Command (TACOM) unit conducted an accident study, it indicated that military transport trucks in the M939/A1/A2 series were repeatedly involved in rollover accidents, causing injuries and fatalities. As a result, TACOM turned to one of the truck’s original designers, AM General Corp. (AM General) of Livonia, Mich., 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 length of the design cycle.
The M939/A1/A2 series is one of the Army’s five-ton military vehicles that have been in the field since the 1980s. These truck models are generally used to trans. port troops and supplies. A recent accident study conducted by the U.S. Army indicated that these models experienced a relatively high number of accidents, due in part to brake lockup 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: addition of a rollover protection structure that would protect cab occupants in 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 with existing vehicle components, the rollover protection structure had 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 which are produced for the U.S. Army and the U.S. Navy. AM General also manufactures the Hummer, the civilian sibling of the military’s HMMWV (High Mobility Multipurpose Wheeled Vehicle) that 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 using computer simulation.
TACOM provided AM General with information about the forces and impact energy that the frame and rollover protection structure must withstand, which was based upon government standards and field experience. AM General then developed a laboratory prototype test. During the laboratory test procedure, they applied a series of loads to the prototype’s fixed frame to simulate the impact that loads experienced in a rollover scenario. The frame design would be a success if it absorbed the energy of impact and did not deform into the driver and passen-ger 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 virtual prototype testing on the computer to save time and money.
Reduced Prototype Testing
AM General stress analyst Michael Yan performed finite element analysis with Mechanical Event Simulation, including both Linear and Nonlinear Stress Analysis software from Algor Inc. of Pittsburgh (www.algor.com). Yan used Accupak/VE to apply loads sequentially and see resulting displacements and maximum principal stresses, as well as to analyze nonlinear effects such as large deflections and plastic deformations.
Using the design team’s 3-D solid AutoCAD model, Yan created a beam/truss element model of the truck’s cab frame with the 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 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, 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.
Next, Yan 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 lbs. was applied. After the vertical load was released, a longitudinal load of over 21,000 lbs. followed. The frame had to absorb energy as well as force throughout the load applications. For example, the frame needed to absorb 18,772 lbs.-ft. of energy during the first load.
After the analysis was complete, Yan studied deflection and maximum principal stress contours. The analysis revealed which areas of the structure would experience the large deflections and highest stresses. 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 Yan. “The results of an Accupak/VE analysis are very obvious.”
Yan then modified the frame to avoid deformation into the driver and passenger area. During one month, he tested 25 variations of the cab frame design, improving upon the Algor model each time by adding crossmembers to reinforce the frame and thickening necessary parts.
“Using Accupak/VE saved a huge amount of time and money,” said Yan. “If AM General had to produce 25 prototypes, it would have taken many months and cost thousands of dollars.”
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, 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 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 will install the rollover protection structures on trucks in use where conditions like rough terrain make rollover accidents more likely.