Strategically placed nylon seams can compensate for the weaknesses of wooden laminates. The process significantly increases the load-bearing capacity and opens up new possibilities in vehicle, sports equipment and timber construction.
Wood laminates are used in many different ways, for example in the manufacture of skis and snowboards or in components for vehicle interiors. However, their weight advantages for lightweight construction also have disadvantages. They are significantly less resilient perpendicular to the grain and when force is applied perpendicular to the surface (peeling load), individual layers of wood can easily become detached (delamination). A team led by Florian Feist at the Vehicle Safety Institute at Graz University of Technology (TU Graz), together with the W.E.I.Z. innovation centre and partners from industry, has now developed a method of sewing wood veneers that counteracts delamination in particular. In tests, the stitched veneers withstood four times the force of peeling loads and the fracture energy in peel-mode was fourteen times greater than with unstitched veneer laminates. In addition to applications in vehicle construction and in the sports and leisure industry, it can also be used in the furniture and construction sectors – for example to produce foldable bridge elements or collapsible benches.
Potential adhesive substitute
“Our idea was to reinforce wood where it was really needed,” says Florian Feist. “With targeted sewing, we can supplement adhesives or laminating resins or even replace them in some cases. The principle is based on steel reinforcements (rebars) familiar from civil engineering. Just as the reinforcements in concrete absorb tensile forces, the seams in timber can take up critical forces. This has a particularly positive effect in the event of peeling loads and significantly delays the detachment of the laminate plies.”
The researchers systematically investigated the interaction between wood, yarn, needle geometry and the conditioning of the materials. They had to find needles that would displace the wood fibres rather than cut through them, and a triangular needle tip turned out to be the best solution. Nylon was ultimately chosen for the yarn because it provides a good trade-off between stiffness and strain-to-failure. Sewing is carried out using standard industrial sewing machines, with a standard sewing speed of 1 metre per minute and top speeds of up to 2.5 metres per minute. The stitched laminates could be up to 20 mm thick, with connections with other materials also being possible – including metal sheets. Sewing technology offers a time advantage over gluing or laminating, especially for smaller workpieces in the sports and leisure industry, as curing times account for a large proportion of the production time.
Use as rebate or joint
The sewing technique has proven itself in mechanical tests. As mentioned, the maximum load-bearing capacity under peel loads – i.e. forces perpendicular to the surface – increased fourfold and the absorbed fracture energy up to the failure of the component was fourteen times higher. Only under shear loads, which cause relative sliding of the individual layers, there were no significant improvements. Inserting very stiff threads at an angle would have advantages here, but this would make the entire production process very complex.
The technology also opens up new possibilities in structural design. For example, flexible wooden joints can be realised by sewing on fabrics that serve as rabbets or joints. Together with the University of Innsbruck, such concepts have already been realised in demonstrators, for example in a foldable and therefore transportable bridge or a foldable bench that combines stability and mobility.
Big advantages on small surfaces
“The possibilities that our technology offers for the design of sports equipment or components in vehicle interiors, for example, are extremely exciting,” says Florian Feist. “However, it must be understood that stitching is less suitable for large-area reinforcement, but should be used specifically on small areas exposed to high peel stress. The advantages clearly outweigh the disadvantages there.”
Partners in this research project were the needle manufacturer Groz-Beckert, Fill Machinenbau, Weitzer Woodsolutions and the W.E.I.Z. innovation centre, whose ongoing research projects will be consolidated within the Wood Vision Lab in future. The project was funded as part of the Think.Wood programme tendered by the Austrian Research Promotion Agency (FFG), which is implemented by the Federal Ministry of Agriculture, Forestry, Climate and Environmental Protection, Regions and Water Management as part of the Forest Fund.