Lightweight coaches by integrated sandwich technology
- 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.
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.
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 .
Static and Fatigue Tests
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.
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 . 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.
This research and developement project was funded by the Commision for Technology and Innovation (CTI) in Switzerland.
- Bhayade, P.: Crash Analysis of Railway Carbody designed with Sandwich panel elements“, Winterthur and Essen, Universität Duisburg-Essen, Master Thesis, 2016Google Scholar
- Hobbacher, A.: Recommendations for Fatigue Design of Welded Joints and Components. In: International Institute of Welding, Paris, 2007Google Scholar
- EN 1999-1-3:2007: Eurocode 9: Design of aluminium structures — Part 1-3: Structures susceptible to fatigueGoogle Scholar
- Leutenegger, S. et al.: Der Aluminium-Wagenkasten 4.1: Leichtbau durch integrierte FSW- Sandwiches. Conference Rad Schiene Tagung, Dresden (Germany), 2017Google Scholar