When Stadler Rail Group in Bussnang, Switzerland, received an order from the Netherlands for 43 of the latest generation of an articulated light rail car called GTW, the company faced a challenge: The cars had to meet crashworthiness standards the country had adopted in advance of European Community approval. Among the requirements was that the cars protect passengers better during a 36 km/h (22.4 mph) front-end collision between two units with a vertical offset of up to 40 mm.
There were two reasons for this requirement: Head-on impacts could easily include a small offset if two otherwise identical cars had differing amounts of wheel wear or braking inclination, and a recent numerical simulation of an offset collision indicated the previous design of a crash module, a safety device on the front of the train car, might not protect passengers during impact.
"Numerical simulation suggested that the crash module could undergo global shear deformation and fail at the fixation point, falling off the front structure," says Alois Starlinger, Stadler’s head of structural analysis, testing and certification. In a worst-case scenario, one car could climb over another, causing severe damage.
A finite-element model created in Abaqus software simulates two GTW light rail cars colliding head-on.
Stadler produces hundreds of light and commuter rail vehicles per year. Its products must meet stringent requirements governing safety equipment, strength of rail vehicles, and protection of passengers and crew from crashes.
FEA Software Essential
To satisfy the safety requirement, Stadler Rail designed a crash module with an anti-climb feature. Engineers validated the module design through a combination of dynamic physical testing and simulations in finite element analysis software.
After a physical test, Stadler’s crash modules demonstrate that they can absorb sufficient energy to protect passengers.
The crash module is a slightly tapered rectangular tube. Partitions divide the module into chambers that provide stability to counter eccentric forces. Horizontally aligned teeth on the front of the module are designed to engage the teeth of a similar module on an oncoming rail car and prevent climb. Once the teeth have engaged, the rest of the crash module is optimized for controlled structural deformation from the front to the back. Targeted slots on the sides create intentional weak points that initiate buckling to absorb energy. In developing the design, engineers built on lessons learned from producing crash modules for previous generations of the GTW car.
For the design, the engineers selected an aluminum alloy, AW 5754, that combines low-yield strength with good plastic-forming characteristics, enabling it to undergo large deformations without fracturing. An important goal was to create modules that could absorb up to 900 kilojoules of a crash impact while decelerating the train at a rate of 5 g or less as far as was practical.
Stadler extracted information from its own materials database, and engineers incorporated the data into a digital model. They calibrated the metals simulation by extracting aluminum samples from a production model of a crash module and testing the samples to create stress-strain curves. By comparing these curves to results generated by computer simulations, the engineers were able to fine tune the behavior of the FEA analysis so that it closely matched the real-world characteristics of the aluminum alloy in a crash module.
Next, the engineers built a model of the crash module and analyzed its behavior on impact, again running nonlinear dynamic simulations.
Anti-Climb Device Successful
The simulation results correlated well with physical dynamic tests. The anti-climb teeth prevent either train unit from moving over the other, and the module body undergoes controlled deformation to absorb 1.1 megajoules. Aluminum buckling decelerates the train unit at an average of 1.25 g.
"Our goal was to achieve an overall compressive strength for the train unit to 1,500 kilonewtons, without undergoing any yield and deformation in the passenger structure," Starlinger said. "In fact, our crashworthiness engineering improved the compressive strength to about 3,600 kN, with only small amounts of plastic deformation in the passenger zone. And we proved out the anti-climb device against offsets as high as 80 mm."
The crash module went into production eight months after the contract was signed, and the cars went into operation 10 months later, probably a record for starting a design from scratch in passenger train service, Starlinger said.
According to Stadler, the GTW is one of the most successful vehicles in the light railcar market. Almost 500 units have been produced. The company plans to continue to make each new train design safer than the last. Starlinger sees finite-element software as an important part of that process. "In its own way," Starlinger says, "FEA is now as essential to ensuring train safety as brakes are."
[Adapted from “No Climbing” by Mechanical Engineering Staff, Mechanical Engineering, February 2008.]
In its own way, FEA is now as essential to ensuring train safety as brakes are.
Alois Starlinger, head of structural analysis, testing, and certification, Stadler Rail Group
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