What could be more obvious for a university in southern Thuringia than research into wood as a material, as it forms a large part of the natural environment of the Thuringian Forest and is an important part of the regional industry? As a material, wood has hardly lost any of its relevance: it is a high-quality, renewable raw material that is robust and inexpensive on the one hand and versatile on the other. Even though wood products are highly ubiquitous, knowledge about the material and the complex challenges of processing it is still in short supply. Yet there is much to discover here.
A distinction must first be made between two ways of using the material: in addition to the use of solid wood, in which whole pieces of wood are sawn from the log and used as construction timber such as beams and boards, there is a variant in which components are manufactured using layers of thin sheets of wood, known as veneers. These veneer plywood products consist of individual sheets that are glued and tied together under high pressure. We will also be looking at veneer as a certain material and specifically at the aspect of dimensional stability under changing climatic conditions, which is the focus of the “FurForS” research project.
Manufacturing products such as chairs or veneer-based, elegant decorative surfaces in vehicle interiors requires some effort and knowledge of working with wood. Until now, many steps in production have been based on the experience of the employees: this is where science can help by deepening our understanding of veneer as a material. One open question in research, for example, is whether the behaviour of veneer plywood can be calculated and how its use and production can be optimized. But before we come to Professor Dietzel and his team from the Structural Mechanics research group at Schmalkalden University of Applied Sciences, our journey begins in a very traditional way: With the tree.
The long journey of wood
Veneer means 0.1 to 7 mm thin sheets of wood that are separated from the trunk using various methods. The word itself comes from a borrowing from 16th century French: Fournir meant ‘to equip’ and ‘to supply’ and referred to the application of fine, thin wood leaves to less valuable wood, i.e. a superficial refinement. Nowadays, these are subsumed under the term face veneer, which are used as layers of molded parts. Veneer sheets are layered, glued and assembled under high pressure: Not only does this create robust components, but by exploiting the directional properties of the veneer sheets, parameters such as flexibility and strength can be used in the design. Knowledge of the properties of the veneer sheets and their composition is crucial here.
However, not all trees are the same: as only the highest quality wood can be used for the subsequent processing steps, the selection of suitable trees is the first important step. Knotholes and similar defects in the wood grain must be avoided. A classification into A, B and C woods has been established, whereby only woods in the first category are suitable for veneers. The selection of the woods is based on the many years of experience of the specialists who have to assess the logs from the outside. In order to be able to use the raw material wood after the initial selection, a longer path must be taken.
The logs are first debarked and usually soaked or boiled. Once the material has become workable in this way, the log can be sliced, peeled or sawn in the next step. The three processes have different advantages and disadvantages and are therefore suitable for different types of wood and different purposes. For example, the minimum possible thickness of the veneers, the physical stress on the wood during the manufacturing process and the loss of material during the cutting process differ.[1] What all three processes have in common is that the stress causes material changes in the wood itself: In addition to moisture-induced swelling, this means, among other things, small cracks on the cut side of the veneer, which occur as a result of a necessary strong bending of the veneer during the cutting process. There is therefore a closed and an open side of the veneer, which exhibit different characteristics, for example in their reaction to moisture such as swelling and shrinkage. The structural change in the material must be taken into account in all subsequent processing steps.
Structure & mechanics
This is where the Structural Mechanics research group comes in: What happens in the wood due to the influence of various factors such as changes in moisture content and temperature? It makes sense not to start from the layered components – i.e. components manufactured from several veneer layers – but to first understand the individual veneer sheet and the effects. The knowledge gained from this can then form the basis for a description of the complex characteristics of connected layers or even entire ply components. However, it is precisely the connection and mutual influence of the veneer layers with different properties that makes this comprehensive description a complex challenge. In short, the aim of the project is a software-supported evaluation of the dimensional stability of veneer plywood materials.
The two sides of the veneer react differently to moisture: the fissures, i.e. the small cracks on the surface of the wood sheets, change the moisture transport within the wood and thus the processes of swelling and shrinkage on both sides. One consequence is the warping of the veneer. Another structural-mechanical factor is the fiber direction of wood growth: Different arrangements of the sheets, transverse or parallel, allow properties such as flexibility or strength to be controlled in the combination of different layers of veneer sheets. Again, the starting point for understanding the relationship is to look at the individual veneer. This understanding of the structural-mechanical effects of various environmental factors can then in turn be incorporated into the design of the products and guide the compositional alignment of the layers.
To visualize the impact of environmental effects on the wood, it is worth taking a look at a classic plywood chair: depending on the temperature, humidity and veneer characteristics, the shape of the chair changes and so, for example, does the inclination of the backrest. As a result, the same chairs always look slightly different in detail and give way differently, which is noticeable in parallel rows of chairs, for example, and is therefore perceived as a visual defect. In addition to these fields of application, the structural-mechanical description is also useful for optimizing production and exploring the limits of wood as a material.
The focus of the research group
In veneer manufacturing, many processes are still based on pure experience with the material wood. Without detracting from this individual achievement, there is potential for optimization as a result of technical progress. Professor Dietzel’s team in Schmalkalden is striving to make the structural-mechanical behavior of the veneer predictable in order to pave the way for this progress. On the one hand, the existing numerical models are to be extended to include the specifics of veneers and, on the other hand, relevant parameters of dimensional stability are to be identified
When a changeover is made in production today, it still requires a lengthy adjustment process that follows a trial-and-error approach. The changeover is time-consuming and material-intensive, which contradicts the principle of efficiency. It would be beneficial to use models here that offer decision-making aids and estimations, thus shortening the adaptation process. Furthermore, structural-mechanical insights can also be used to optimize design processes such as bending or flexibility.
Even though wood as a material is widely used in its various forms, only a few projects are dedicated to it. At Schmalkalden University of Applied Sciences, the Structural Mechanics research group has been researching the possibilities of mathematically describing the physical properties of veneer as a raw material for 18 years now. The research group combines aspects of basic research with application-oriented functional integration in the material wood.
The FurForS cooperation project
In the FurForS project, the research group cooperates with the TU Dresden, and thus the two few players in veneer research. At the Dresden Institute for Natural Materials Technology, the solid wood/veneer research group led by Prof. André Wagenführ is dedicated to topics relating to wood in its natural form, as well as wood composites. In cooperation with HSM, Dresden concentrates on the small-scale analysis of veneer sheets, i.e. the microscopic changes in the structures that result, for example, from treatment in the climate chamber.
However, the Schmalkader team is developing calculation approaches, carrying out modeling and assessing parameters and factors that have a significant influence on structural mechanics. The aim here is to develop calculation models that can be adapted to the individual properties of veneer plywood. The mediation between a generalization and an individual adaptation requires a high computational effort of the model, which increases with the addition of several layers. This approach combines structural mechanics with computer science, whereby the focus remains on material science.
In summary, the various project partners, HSM and TUD as well as Kreutzfeldt GmbH & Co. KG and GbR Lie-Design, have come together with the aim of predicting the changes in shape and residual stress of plywood products using model-based forecasts. The models are valid for both production and later use, and can achieve an optimization performance in both respects, for example in terms of efficiency in the manufacturing process. The common goal of the project partners is to further optimize the use of wood through a deeper understanding of the material and to further explore the limits of its application. The aim is to move from the experience used to date to calculation models that deepen our understanding of the material through simulations and numerical illustrations.
Location at the HSM
Professor Andreas Dietzel was appointed to the professorship of “Design, Production Metrology and CAD” at the Faculty of Mechanical Engineering in fall 2021 and has been involved in the Structural Mechanics research group ever since. This area is by no means new territory for him, as he completed his doctorate at Schmalkalden University of Applied Sciences and Ilmenau University of Technology on the topic of “Model-based determination and evaluation of the shaping limits of copper beech veneer” under Professor Hendrike Raßbach.
Even though the bureaucratic effort involved in acquiring third-party funding for applied research projects is a major challenge for universities, Professor Dietzel and his two research assistants – Dr. Dennie Supriatna and M.Eng. Daniela Pachatz – a motivated team that is now involved in the FurForS project.
[1] The handout “Veneer in interior design” contains much more detailed information on this. (Cf. Initiative Furnier + Natur e.V., Furnier im Innenausbau. Definitionen – Eigenschaften – Verarbeitung – Anwendungsbeispiele“, Dresden 2011, S. 8 – 13)