The compressive behaviour of beech and birch at different moisture and temperature conditions along the grain

As demand grows to replace petroleum-derived materials with more environmentally friendly materials, the use of wood in high-performance applications in the automotive industry has received increasing attention. Understanding the behaviour of wood during forming processes is an essential step towards its successful implementation in the automotive sector. This study accordingly investigates the compressive response and failure behaviour of two hardwood species (beech and birch) at different moisture and temperature levels. The experimental tests were conducted at 20, 60, 100 and 140 °C on samples with various moisture levels, ranging from completely dry to wet conditions. The stress–strain curves were analysed, and the failure modes were examined using visual observations and microscopic analyses. The results demonstrated that, when beech and birch were tested in standard laboratory conditions, the dominant mode of failure was the kink band, where the fibres buckled in an approximately sigmoidal shape. However, when the wet wood was tested at temperatures of 60 °C and above, the stress–strain curves showed long stress plateau region as well as strain hardening. We hypothesise that, at elevated moisture and temperature levels, the cell walls, promoted by the improved flexibility of the polymer network, start to fold, hence, the load stabilises or even slightly decreases. This corresponds to the plateau stage in the stress–strain curves of beech and birch. With further loading, the distance between the folds becomes rather small and compaction of the folds (or densification) apears to occur, corresponding to the observed strain–hardening stage. This hypothsis is supported by the concertina-like folds seen in the scanning electron microscope (SEM) graphs of post-tested specimens. Here, we present and discuss the effect of rays, as well as cell wall/lumen ratio, on the compression behaviour of wood, and the stability of the forming process.