Within the project, DLR was involved in the conception, design, optimisation, simulation, testing and validation. The aim of the research project was to qualify beechwood-based multi-material systems for use in vehicle structures of the future. The aim was to create economically and ecologically attractive alternatives to existing material solutions.

Motivation for the use of wood

Wood has very good specific mechanical characteristics and is comparable in the fibre direction with typical technical materials such as aluminium or magnesium. In addition, wood is a renewable raw material that binds CO₂ during its growth. By using beech wood, short transport routes and good availability within Europe can be guaranteed.

Use of materials in the project

As part of the research project, the use of beech wood veneers was investigated in more detail. Various concepts were developed, ranging from the use of the material to reinforce thin sheet metal to structural components made entirely of wood that were only locally reinforced with sheet metal. A total of three different reference assemblies were analysed - a functionally integrated door impact beam for a car, a door wing for a rail vehicle and a segment of the side wall of a rail vehicle. More detailed information on the composition of the project consortium can be found at holz-im-auto.de

Challenges when using wood

The use of wood is associated with a variety of challenges. These include, for example, scattering material parameters, the dependence of material properties on environmental conditions such as humidity and, in particular, the simulation of the material. The project also focussed on wood/wood and steel/wood bonding, which must meet the high requirements of the automotive industry (e.g. cathodic dip coating, process times) and rail vehicle construction (e.g. fire protection). Solutions were developed and validated through design measures and scientific analyses of interactions. To this end, a comprehensive test programme was carried out to characterise wood-based multi-material systems. Furthermore, a suitable simulation methodology was developed and the material models were modelled using the characteristic values obtained. This created a sound basis for the development of wood-based concepts.

Functionally integrated door impact beam for a car

It was shown that the intrusion of the pile in the door with the wood-based functionally integrated internal door structure was only just under 2.6 % higher than the reference door. However, it must be taken into account that the timber hybrid door also absorbed around 5.9 % more energy than the reference.

The developed wood-based door interior structure consists of almost 90 % laminated veneer lumber and impressively demonstrates the potential of wood in safety-relevant assemblies that are exposed to extremely high mechanical loads.

Reference assembly rail vehicle "Door"

For the rail vehicle door assembly, a modular wood-intensive approach was realised based on the existing construction method. Individual hat profiles that can be used for both flat and slightly curved structures form the core of the structure. These elements are combined with very thin aluminium sheets (0.6 mm) that are reinforced with wood veneers. The developed concept made it possible to use significantly simpler and narrower aluminium profiles. The wooden components can be manufactured cost-effectively and with simple tools. It was also possible to reduce the thickness of the door leaf from 38 mm to 32.6 mm. Overall, a weight reduction of 15% and a substitution of aluminium components with wood was achieved. In a first step, the concept was simulated at the DLR for various load cases such as "misuse" and wind loads and then built as a prototype at Schaltbau Bode and successfully tested on a component test bench.

One of the central issues in the conception and design was the moisture absorption of the wood during operation. An in-situ measurement method was developed specifically for this purpose and, using generic components in a climate chamber, it was determined how the moisture is distributed in the component. It was shown that even at high temperatures and humidity (+60°C and 80% relative humidity), no critical wood moisture levels occur. Delamination or other damage to the composite was also not observed after a large number of climate cycles (20 cycles according to PV1200 from -40°C to +80°C).

Reference assembly rail vehicle "side wall"

A side wall segment from Alstom was selected for the rail vehicle reference assembly. For this, the approach of a homogeneous wood sandwich core with metallic cover layers was pursued. More complex geometries can also be realised by reshaping the veneer layer composite and subsequent planking.

The developed side wall segment was fully simulated. One of the most critical load cases is the so-called double cantilever seat. For this, a cantilever arm is screwed into the side wall and two seats are attached to it. Due to static and dynamic loads, the side wall has to absorb and dissipate very high forces. Suitable joining methods also had to be selected and validated. Within the project, in addition to the simulative verification by the DLR, it was also possible to provide proof on the test bench at Alstom that the permissible deformations were not exceeded.