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The performance of steel and aluminum alloys as structural materials is a subject of great debate. There are two main failure mechanisms by which structures may collapse and survival space may be lost. The first is by material failure, where the load exceeds the strength capacity of the material. The second mode of failure is by buckling. In this case the material remains largely intact but the structure collapses by folding up or crumpling.
Modern steel vehicle bodies are formed by skin-on-frame monocoque construction. The main structure comprises a thin skin supported by a framework of formed steel sections. When overloaded failure is usually initiated by local buckling, which then progresses to gross structural collapse and some local fracture.
An aluminum alloy structure built in the same manner as a steel one will tend to fail in a similar manner if it is overloaded. However, the most viable method of producing vehicle bodies in aluminum alloy is to manufacture them from large multicell extrusions joined lengthwise to form the main tubular structure. A structure formed in this way has double skins with close-spaced continuous internal bracing. When overloaded this structure will maintain its stability and fail predominantly by fracture, starting at the weakest point(s) []. The use of closed cell double-skinned longitudinal aluminum extrusions that can be welded to form the vehicle body has enhanced the efficiency of the manufacturing and assembly methods and at the same time resulted in highly rigid rail vehicle bodies. Such sections (shown schematically in Figure 5) have an inherently excellent resistance to impact loading that contributes to the crashworthiness of modern rail vehicles. This construction technique is enabled by a relatively new process called friction stir welding (FSW) . In fact, it has been found that a double-skinned train car made of aluminum longitudinal hollow extrusions behaves like a “rigid body” during a collision.
For this reason, impact energy absorbing zones are introduced at either end of the car to absorb the impact energy that would otherwise be transferred to the passengers, crew, and equipment.
Figure 5. Double-skinned closed-cell concept for car shell construction [].
Recent rail accidents (such as that which occurred in Eschede, Germany in 1998) have revealed a critical failure mode in aluminum rail vehicles in which the longitudinal welds joining the extruded sections, which form the vehicle body, appear to fail by fast fracture along the heat affected zone (HAZ)/weld metal (WM) interface. The term “weld unzipping” is commonly used to describe this type of failure.
In an aluminum alloy body, the welded joints joining the sidewalls to the floor are stronger in almost every respect than those in a typical steel-bodied vehicle. They have tended to fail in accidents because the surrounding structure is even stiffer and stronger, whereas in a typical steel body the sidewalls buckle before the attachment weld fails.
An inherent problem with fusion welding of heat treatable aluminum alloys, such as the 5000 and 6000 series used in train construction, is that the heat input during the welding process subjects the material to a localized solution treatment, which alters the microstructure adjacent to the weld, resulting in a reduction of the mechanical properties of the welded joint, as compared to those of the parent material. “Weld undermatching” is often used to describe this condition.
Numerical modeling performed to understand this “unzipping” phenomenon has shown good correlation with failure observed in an actual vehicle which was involved in a collision. Thickening of the aluminum sheet at the weld region is shown to eliminate the weld unzipping failure mode with the impact energy absorbed by controlled buckling of the structure [].
Friction stir welding (FSW) has the potential to improve the crashworthiness performance of aluminum rail vehicles which may fail by “weld unzipping”. Sapa and Hydro Aluminium (Scandinavian aluminum extruders) pioneered the commercial application of FSW in the manufacture of single-wall aluminum roof panels for rail equipment [].
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