Livonia, MI--The U.S. Army military transport truck, the M939/A1/A2 series, didn't have a good track record. In an accident study done by the U. S. Army's Tank and Automotive Command unit (TACOM), the 5-ton vehicle was repeatedly involved in rollover accidents due in part to brake lock-up or driver errors. To improve the truck's safety, the Army asked AM General Corp. to design, test, and manufacture a rollover protection structure that would be added to the truck's existing frame.
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AM General built a laboratory prototype based on the forces and impact energy that the frame and protection must withstand. When the initial T-shaped configuration failed because it was too flexible, engineers at AM General turned to virtual prototyping using Accupak/VE Mechanical Event Simulation with Linear and Nonlinear Stress Analysis software by Algor Inc. (Pittsburgh, PA), AM General engineer Michael Yan could apply loads sequentially and immediately see displacements and maximum principal stresses. He could also 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 rollover protection structure. The frame was made of welded stainless-steel tubes. "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 efficiency."
Because the welding alters the sectional properties of the tubing near the joints, Yan increased the density of elements in those regions. The final model ended up with more than 600 elements. Using Algor's Beam Design Editor utility, Yan tailored the sectional properties of the elements near the joints to replicate the characteristics of welded stainless steel.
Yan then set up a finite element analysis in Accupak/VE that simulated the failed prototype test. He applied boundary conditions to the top of the frame to simulate the frame's connection to the cab mounts.
Yan then studied deflection and maximum principal stress contours. The analysis showed which areas of the structure would experience the largest 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."
Over the course of one month, he tested 25 variations of the cab frame design, improving the 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 Yan. "If AM General had to produce 25 prototypes, it would have taken months and cost thousands of dollars."
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