This conference paper was submitted for presentation at the NAFEMS World Congress 2025, held in Salzburg, Austria from May 19–22, 2025.

Abstract

The adoption of wood-based laminates in high-performance applications is often hindered by their limited fracture toughness. While wood inherently is very strong along its grain (growth direction), it fractures in a brittle fashion, making it less suitable for absorbing energy. To address this, we propose a novel strengthening approach utilizing a stitching process adapted from technical textiles. This method employs industrial heavy-duty sewing machines to introduce through-thickness reinforcement, akin to tufting. However, while tufting relies on friction between the thread and the base material to improve the mechanical performance, stitching utilizes a second thread in order to prevent slippage and therefore further improves the mechanical properties, especially when the laminate is already fractured or delaminated. Such enhancements are particularly critical for wood-based laminates in safety related applications. To support this approach, we developed and validated simulation models for a double cantilever beam (DCB) and end notch flexure (ENF) configurations against experimental data. These numerical models represent the laminate structure as discrete orthotropic wood layers bonded by cohesive elements mimicking the adhesive. Importantly, the models account for the damage introduced by the stitching process, such as needle-induced perforations. These models are then used to evaluate the balance between this damage and the reinforcement provided by the stitching thread. A numerical parameter study was conducted to optimize stitching parameters, such as thread type, stitch density, and other sewing patterns. These studies identified configurations that maximize fracture toughness while minimizing damage, offering insights into the trade-offs involved. Additionally, simulations explored the potential benefits of oblique stitching, which improves mode II fracture toughness but may reduce mode I benefits. Further analyses included compression-after-impact (CAI) simulations, demonstrating that stitching significantly enhances structural integrity after damage. These results suggest that stitching-reinforced laminates could be viable for high-performance applications such as wooden aircraft structures and crash-resistant components in vehicles. Beyond the mechanical benefits, this stitching method supports the use of renewable, bio-based materials in industries traditionally dominated by synthetic composites. Our findings indicate that this stitching process can substantially enhance the fracture toughness of wood-based laminates, paving the way for their use in applications previously deemed impractical. This work underscores the potential of numerical modeling as a powerful tool for optimizing advanced material design and validating innovative reinforcement techniques.