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Lightweight Design worldwide

, Volume 11, Issue 3, pp 48–51 | Cite as

Lightweight coaches by integrated sandwich technology

  • Hanfried Hesselbarth
  • Simon Leutenegger
  • Markus Hartwig
Design Rail Vehicles
  • 205 Downloads

The Zurich University of Applied Sciences, together with Airex Composite Structures and Rapid Technic, used friction stir welding to join sandwich components with extruded profiles. The technology makes it possible to build lighter rail vehicles, comply with registration-relevant axle loads and minimize operating costs.

The Car Body as a Challenge

Manufacturers in the railway sector must pay attention to weight. A multitude of comfort and information equipement is required in today’s railway passenger cars. However, the allowed total weight per axle is still limited. In the near future, rail infrastructure usage fees will increasingly be based on the weight of trains. An innovative lightweight design is required. Sandwich element manufacturer Airex Composite Structures, the technology company Rapid Technic, and the School of Engineering of the Zurich University of Applied Sciences (ZHAW) launched a research project to develop the Integrated Sandwich Technology (ISTech) to meet the structural requirements for railway approval.

Railway fees based on the weight of the trains will be more common.

For the required life cycles of more than 40 years, railway car body structures are manufactured as welded constructions in steel or aluminum. The design using extruded aluminum profiles has proven particularly successful. The profiles are welded together to the assemblies sidewalls, roof and floor and then assembled to form the car body. This is inexpensive compared to the earlier aircraft-like construction methods with stringers and ribs. However, more material and thus more weight is installed than would be required for strength. Here, a sandwich construction with a surface layer thickness of around 1 mm offers great potential for saving weight.

Sandwich components are already used successfully in bus and streetcar structures, especially for large areas such as the roof structure. In these designs, the sandwich components are usually connected to the base frame structure using a an elastic face bond. Despite their low weight, sandwich assemblies can withstand high loads and thus significantly stiffen the car body when welded into the load-carrying structure. With this new technology, the extruded sections in selected areas, such as floor or ceiling structures, are substituted by welded sandwich panels, using the same welding connections to the adjacent modules as in the classical integral extrusion profile construction. This can save several hundreds of kilos per car body in weight without changing the general layout. The challenge was to develop a seal-welded cost-effective technology that can be modularly fitted into existing car body constructions.

Developed Design

In the first part of the project, the geometry of the transition from the sandwich section to the welded components of the car body was developed. Finite element calculations for heat input, stresses and deformations were investigated. Figure 1 shows the layout: the right side shows the sandwich core and the cover panels. These are glued together and framed with the edge profiles. This panel construction cannot be welded into the car body, as conventional welding would introduce too much heat into the panel and could destroy the adhesive and core material. Therefore, the connection profile is used. The left side shows the interface of an extruded section for the car body construction, connected to the panel on the right by Friction Stir Welding (FSW) seams that generate only half the usual heat input. By trimming the panel, the dimensions can be controlled with great precision. A PET foam with a density of 110 kg/m3, aluminum honeycombs with a density of 60 kg/m3, and aramid honeycombs with a density of 48 kg/m3 were investigated as sandwich core material.
Figure 1

Detailed view of the transition zone between car body frame structure (left) and sandwich panel (right) (© ZHAW)

The technology should be able to be modularly fitted into existing car body constructions.

Welding and Bonding Process

The bonds of a sandwich can be thermally destroyed by the high welding temperatures of the conventional MIG welding processes. Conversely, there is a risk of the strength of the weld seam being greatly reduced by adhesive components. However, in order to be able to use the weight advantages of sandwich components in highly loaded welded constructions, the bonding area has to be separated from the welding area and a welding process with as little heat input as possible must be used. Friction stir welding is especially suited for this, as it generates heat by using the frictional forces of a tool on the parts to be joined. The materials are exposed to temperatures below the melting point and kneaded together by a stirring motion.

In friction stir welding requires only a small amount of energy compared to a traditional weld. At Rapid Technic, the friction stir welding tool and the welding parameters were determined in preliminary tests, Figure 2. Temperature measurements in the welding tests confirmed the calculations: For all FSW welding tests, the temperature did not rise above 100 °C in the area 40 mm from the weld. This means that the adhesive of the sandwich bonding is not destroyed and no impurities due to the adhesive get into the weld seam. Tensile samples were produced from the test weld seams, Figure 3, and tested at the ZHAW. The results were in line with expectations: The friction stir welds generally have a slightly higher strength than a classically welded MIG V-type seam. Typically, the heat-affected zone is the failure location. The results showed that the combination of sandwich technology and friction stir welding technology is feasible.
Figure 2

Principle of the FSW process (left) and friction stir welding (right) (© Rapid Technic)

Figure 3

Micrograph of friction stir weld (© ZHAW)

Strength Estimation

Since a general feasibility was to be proven here, a metro car body was chosen as a typical reference vehicle, Figure 4. It has a 2.8 m wide body floor with a 500 kg/m2 surface load. The load case buffer pressure is also essential for the car body. For this arrangement, a calculation model and test setup were created. In the calculation model, the behavior of a classical structure of the floor with extruded sections was compared with the new integrated sandwich floor.
Figure 4

Typical body of a metro car, with sandwich panels (green) for the floor and roof (© ZHAW)

All investigations were accompanied by extensive finite element analyses. Initially, this was focused on an appropriate way of modeling. In a complete vehicle model, the discretization of individual details is possible only to a limited extent. The calculation model would lead to impractical calculation times. Therefore, detailed 3-D volume models of the joint geometry are compared to standard shell element modeling, and modeling rules for shell element models were derived.

The combination of sandwich and friction stir welding technology is feasible.

The static load cases showed no critical locations compared to the existing design with extruded profiles. When re-designing, it should be kept in mind that the smaller wall thickness of the sandwich area meansslightly more load for the rest of the structure. This is usually not a problem, but it must be taken into account in high-stress areas, such as door and window corners. In the fatigue load cases, no unusual stress locations occurred, either.

Additionally, buckling was analyzed. In addition, the variants of the car body with the three core materials were compared with the existing car body with extruded profiles for a crash load case. The crash behavior is calculated in an explicit simulation using an elastic-plastic material model and large deformations. The sandwich structure can absorb an equal or higher crash load, compared to the variants with extruded sections [1].

Static and Fatigue Tests

Component samples were produced from the manufactured test samples and loaded on a testing machine until failure. For this purpose, a device has been developed so that the correct bending moment is produced on the testing machine at the connection structure, Figure 5. In this case, a bending load is simulated, which corresponds to the real load in the car body at the transition of the panel to the car body structure. In the static tests, all samples with the three different core materials endured more than 2.2 times the required test load. The load drop occurred when the pressure-loaded lower cover plate began to buckle in the area next to the FSW weld seam. The welding seam itself remained intact.
Figure 5

Strength test sample on testing machine (© ZHAW)

In the fatigue test, a swelling fatigue load was applied. At the sweep width of about 10 MPa, which is the dimensioning factor, no damage was found in all samples after 1 million load cycles. In order to get an idea about failureunder fatigue loads, the load was increased and another million cycles were run until finally a failure occurred in all samples. The damage location was, as expected from the static tests, in the buckling cover plates, but also in the FSW weld seam and in the MIG weld seam. Some of the damage locations could be assigned to production-related weak points at the sample edges. The distributed damage pattern shows that the design of the FSW weld joint is no worse than the rest of the design. In order to get a good overview, an equivalent stress was calculated for each sample with the damage accumulation rule according to Miner [2] and compared with standard values. All results are better than the values specified in Eurocode 9 [3] for a classic V-shaped weld, Figure 6.
Figure 6

Equivalent stress range Äó for the fatigue tests samples, together with design limit and reference value according to Eurocode 9 (© ZHAW)

The sandwich technology enables a significant weight saving of up to 10 %.

Optimized Manufacturing Process

The influence of manufacturing tolerances on the strength of the assembly was also investigated. At the same time, the production technology was optimized in such a way that the integrated sandwich elements can be produced with as few work steps as possible. The additional costs, compared to a design with a conventional extruded profile in integral construction, are under 20 euros per kilogram saved. In a redesign, additional advantages of thermal and acoustic insulation and functional integration can be used.

Weight Advantages

The Integrated Sandwich Technology enables a significant weight saving in high-load structures. With unchanged high structural strength, weight reductions of up to 10 % are realistic due to the novel welding sandwich technology, if parts of the floor, the roof and areas of the side wall are designed in this way [4]. The basic design and the manufacturing process of the car body do not need to be changed. The developed lightweight approach reduces the operating costs for vehicles and rails and makes a significant contribution to energy savings.

Notes

Thanks

This research and developement project was funded by the Commision for Technology and Innovation (CTI) in Switzerland.

References

  1. [1]
    Bhayade, P.: Crash Analysis of Railway Carbody designed with Sandwich panel elements“, Winterthur and Essen, Universität Duisburg-Essen, Master Thesis, 2016Google Scholar
  2. [2]
    Hobbacher, A.: Recommendations for Fatigue Design of Welded Joints and Components. In: International Institute of Welding, Paris, 2007Google Scholar
  3. [3]
    EN 1999-1-3:2007: Eurocode 9: Design of aluminium structures — Part 1-3: Structures susceptible to fatigueGoogle Scholar
  4. [4]
    Leutenegger, S. et al.: Der Aluminium-Wagenkasten 4.1: Leichtbau durch integrierte FSW- Sandwiches. Conference Rad Schiene Tagung, Dresden (Germany), 2017Google Scholar

Copyright information

© Springer Fachmedien Wiesbaden 2018

Authors and Affiliations

  • Hanfried Hesselbarth
    • 1
  • Simon Leutenegger
    • 1
  • Markus Hartwig
    • 2
  1. 1.Zurich University of Applied Sciences (ZHAW)WinterthurSwitzerland
  2. 2.Airex Composite StructuresAltenrheinSwitzerland

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