Machine Design

Will the crash zone crumple? FEA tells

Safety requirements for new train cars in Great Britain call for a crash or crumple zone to absorb the impact of a collision. But designing a working crash zone is no small feat. It must be strong enough to withstand day-to-day loads and still absorb the energy of a 37-mph collision, including 2,000 kN of longitudinal compression, vertical loads, fatigue, and compression and lifting forces.

Class 390 Pendolino trains will carry Alstom's crumple zone on each car.
The crush zone was modeled in Catia and meshed in Altair's Hypermesh. It's engineered to carry 4,500 kN in longitudinal compression, along with operating and maintenance forces.
Abaqus/Explicit simulated a collision to crush the crumple zone. Time stepping through the analysis allows a slow motion examination of what ordinarily happens in the blink of an eye.
The physical prototype of the crumple zone was crushed in a hydraulic press. It shows good agreement with digital simulations.

The zone must collapse in a controlled manner so passenger zones do not. Safety regulations stipulate that a car's crash zone must absorb as much as 3 mj of energy. (For comparison, an automobile running into a wall at 35 mph absorbs roughly 0.11 mj).

Rail-car manufacturer Alstom turned to Alcan Mass Transportation Systems, Zurich, for design and testing of a front and rear crash zone. "Each metal fixture of the crash zone has to absorb as much energy as possible before failing," says Alois Starlinger, head of structural analysis and testing at Alcan MTS. A European rail standard still being written specifies using finite-element analysis to verify that designs are structurally sound and crashworthy.

Starlinger's team used Abaqus FEA software from Abaqus Inc., Pawtucket, R.I., to simulate designs in collisions before actual collision tests. The goal was to verify that the final design would undergo plastic buckling at a fairly constant and predetermined rate.

The structure's numerous internal walls have enough space between them to allow buckling on impact. The team used a cross section of the car to create a 3D model of the crash zone, complete with wall thicknesses, joints, and welds.

Using Altair Engineering's HyperMesh software as a preprocessor, the engineers exported the 3D model of the crash zone to Abaqus/Standard. A quasi-static analysis simulated loads the structure would normally carry. A European standard for railway vehicles allows for local stress peaks, small zones of stress beyond the structure's yield stress, as long as they are small and do not cause significant residual damage or deformation.

Analysts performed 1D and 3D simulations to qualify the structure for operation and maintenance. "One-dimensional analysis starts by assuming cars are stiff, single-mass points," says Starlinger. "We assume certain deformation characteristics for the coupler elements, the buffers, and crash and passenger zones."

Engineers simulated head-on crashes, starting at the front coupling of the rail car, measuring the transfer of force through the crash and then passenger zones. The engineers then transferred the model from Abaqus/Standard into Abaqus/Explicit, a nonlinear FEA program. Automated transfer made short work of moving the approximately 120,000-element model.

A buckling-analysis tool in the nonlinear program let the team investigate how aluminum would fold on impact. The nonlinear FEA uses a time-step method. At each interval, the software calculates the amount of buckling, contact, and other quantities. Time stepping also lets the team take sequential "snapshots" of the metal as it deforms. The digital process lets the team vary wall thicknesses, try different cutouts, add curvatures, and change welds to ensure the structure deforms at a controlled and steady rate, front to back.

Modeling the transfer of forces inside the structure involved a general-contact feature in the nonlinear software. General contact lets users easily define complex contact scenarios. For example, it will handle the case of multiple walls of an egg crate crumpling under impact. "The feature automates a time-consuming, surface-by-surface process," Starlinger says.

After analyses revealed a successful design, Alcan MTS built a full-scale prototype. Aluminum alloy AA 6008, selected for the crumple zone, has the ductility to absorb crash energy. Alcan engineers heat treated finished parts to a cycle designated T7 to further increase ductility and placed the structure in a large hydraulic press. "This was an extremely slow-motion process," says Starlinger. "What happens in seconds in an actual crash takes hours in this quasi-static testing." Test results determined the load deflection curve under compression. The analysis results agree well with the physical test.

The next step, he says, "is to incorporate knowledge from FEA and other tools into the regulations. For example, existing stiffness requirements for railroad-car couplings predate analysis software. We will use the lessons learned in analyses to make trains safer and update the safety requirements."

Make contact

Abaqus Inc.
Pawtucket, R.I.
(401) 727-4200

Altair Engineering
Troy, Mich.
(248) 614-2400
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