MSC Software Professional Services Team Enables Development of Patented, Energy Absorbing Bus Bumper System
The high up-front cost of product development was hindering the development of an innovative energy absorbing bus bumper system conceived by Talfourd-Jones Inc. (T-Ji), parts makers for leading original equipment manufacturers. By retaining the simulation services of MSC.Software Professional Services, T-Ji was able to test and prove the concept for the bumper system before investing in the tooling required to produce the technology.
Bill Cherry, president, said, "Without FEA, we couldn't have justified such a major investment in tooling to make our Reflex bumper system with a vertical honeycomb energy absorber a reality. These development projects routinely run into millions of dollars. The first milestone was, knowing our concept would work. Then we needed to make sure there were customers that would make our investment pay off."
A book of procurement standards for purchasing buses, created by the American Public Transit Association (APTA), ensures the quality of buses purchased by transit authorities. A transit system can purchase a bus that does not meet the APTA standards, however, considering the high cost of a bus, it is in everybody's best long-term interest to require that the standards are either met or exceeded.
Under the FMVSS 301, the bumper must protect the bus from any damage as a result of 6.5-mph direct impact into a rigid barrier and a corner impact from a common carriage. In addition, the bumper must return to pre-impact shape within 10 minutes of the impact. The primary system for meeting the impact requirements is through energy absorbing bumpers mounted on the front and rear. To meet this requirement, the Reflex bumper was designed to progressively collapse, cushioning the vehicle from a collision. Following impact, the bumpers return to their original shape without permanent damage or degradation in strength.
Traditionally, transit bus bumpers are designed from a functional perspective. The concept developed by T-Ji, was based on aesthetic design, using a swept-radius style to provide the visually pleasing characteristics. Then an energy absorbing system was designed to fit within the envelope of the styling requirements.
The rigid wall and corner impact collision tests are the two most severe APTA structural requirements for an energy-absorbing bumper. The key design requirements included:
Front bumper : 5 mph impact of bus at curb weight (35000 lbs) to a fixed flat rigid barrier perpendicular to the bus longitudinal center line.
Front Bumper : FMVSS 301 common carriage loaded to 4,000 lbs, direct impact at 6.5 mph and corner impact at 5.5 mph.
Rear bumper : 2 mph impact of bus at curb weight to a fixed flat rigid barrier perpendicular to the bus longitudinal center line.
Front Bumper : FMVSS 301 common carriage loaded to 4,000 lbs, direct impact or corner impact at 4 mph.
After an impact bumper system shall return to its original shape with no damage or degradation in strength within 10 minutes.
Bumper system shall be independent of every power system of the bus, corrosion resistant, be able to withstand repeated impacts within specifications, and shall not require service or maintenance over the rated service life.
Using a Unigraphics CAD system, the original bumper envelope design was generated, including the swept curvature and additional requirements, such as a no-step surface for a rear bumper. The geometry defined the real estate in which the energy absorbing system had to fit.
The main components of the bumper system include the support beam, energy absorbing material and protective skin. Polyurethane was used for the bumper skin, because of its excellent physical properties and T-Ji's successful experience using it in many other components. The energy absorbing material is a blend of several materials, which when combined foam.
In order to minimize the overall weight of the bumper system, the support beam was made from structural aluminum. By using a custom extrusion, the cross-section was optimized for impact loading. On a curved bumper, the initial impact occurs at a moment on the bumper curve. As compression increases, the loads spread away laterally, from the impact point. If the impact is not exactly perpendicular, the Reflex technology compensates as the lateral spread takes place. Current flat designs require an exact perpendicular hit to distribute the impact uniformly across the flat face of the beam. If a flat bumper impact is not exactly perpendicular, one or the other side-points take the entire impact and the risk of failure dramatically increases.
Because of the high dynamic nature of the loading and the large non-linear distortion of the energy absorber, an explicit finite element solution was required. The finite element model (mesh) was generated using the ANSYS Preprocessor.
Material data for the aluminum support beam was obtained from standard engineering material databases. However, physical characteristics required for the foaming material were non-existent. Typically, material specification sheets list a single modulus, which assumes the material will remain linear. Certain materials exhibit this characteristic over small distortions only. In order to properly characterize the material's non-linear stress/strain characteristics, the material supplier was required to perform tests. Using the supplier's data, a Mooney-Rivlin equation was generated for defining stress-strain characteristics.
To perform the simulations, detailed finite element models of the bumper were created in the ANSYS Preprocessor. A full bumper model was simulated for the corner impact tests, however for the direct rigid wall impact, symmetry was utilized to reduce the solution to a half model. A complete 3D model of the 4,000 lb common carriage was generated for the corner impact.
For initial concept evaluation simulations, shell elements were utilized to represent both the energy absorbing structure as well as the support beam. Once concepts were qualified as to their acceptability, detailed solid models were constructed of the energy absorber. This was required to capture the complex nature in which the material was deforming upon itself.
Results of the simulations performed on the initial concepts indicated complex deformed shapes for the design in the compressed position. From a repeatability standpoint, complex shapes are more difficult to control. In addition, any deviation in manufacturing and assembly can significantly impair performance. Mr. Cherry said, "We had the staff at MSC Software work with us to refine a more stable design for the energy absorbing system that would exhibit better repeatability characteristics and be easier to manufacture."
New concepts were generated as 3D CAD models for evaluation of manufacturability. The process of molding with foam seems simple, but is actually fairly complex. The materials are injected into a mold as in plastic injection molding. However, the mixture of the material and the curing process significantly impact the material properties. Any variations in materials or the curing process would change the properties of the energy absorbing system.
The material properties, model boundary conditions, and vehicle initial velocities were defined in the ANSYS finite element Preprocessor and an LS/Dyna 'K' file written out for the impact analysis. Impact solution times were in the order of 12 hours on a 1 GHz Pentium PC. After a series of designs were simulated, a honeycomb pattern was selected because of its unique potential. Mr. Cherry said, "Virtual testing was an absolute must, in view of the significant tooling costs. And by performing virtual tests (computer simulations) many more designs could be evaluated in a shorter period with significantly less cost."
Some of the specific benefits of the Reflex system design include:
Easy to adapt for a modular design, minimizing tool costs and allowing easy installation for the bus service departments.
Easily modified, so the design can be tailored to other energy absorbing requirements by modifying cell wall thickness.
Easy interface with support beam, by incorporating a simple tongue for insertion into beam groove.
Provides a strong vertical support for front bumper step requirements.
Based on the final design, an initial prototype was manufactured for both the front and rear bumper configurations. The rigid wall impact tests were performed, and both designs passed without a single issue. Mr. Cherry said, "We could not have made such a sizable investment on speculation that our design might work. In fact, our first design iteration didn't work. In those respects, FEA made it possible to do this project. As a result, we already have customers for our patented Reflex Technology energy absorbing bumper system and last year we won the Canadian Urban Transit Association (CUTA) award for Corporate Innovation."