Radial mechanical properties of high-temperature dried Norway spruce (Picea abies) wood

Abstract

Wood drying experiments were conducted in which the temperature and the drying rate were controlled independently. The mechanical properties of dried wood in radial loading were analysed in relation to drying parameters. Mass loss, due to thermal degradation of the wood structural components, occurred predominantly in slow high-temperature drying processes. Despite the higher mass loss, slowly dried wood showed similar radial strength and stiffness to rapidly dried wood. The formation of irreversible hydrogen bonds (hornification) within the wood structure may compete with the effects of mass loss on the radial mechanical behaviour of wood. However, both the mass loss and the hornification resulted in wood specimens with lower hygroscopicity. Application of slow high-temperature drying to reduce microscopic cell-wall damage, caused by anisotropic shrinkage of cell-wall layers, did not seem to affect the radial mechanical properties of wood. The effects of stress relaxation within the wood cell wall on the mechanical behaviour of wood may be offset by the degradation of structural components along with drying. Radial mechanical properties may be improved by rapid high-temperature drying up to high final dryness.

Keywords:

• Cell-wall damage
• drying
• hornification
• mass loss
• mechanical properties
• wood

Acknowledgements

The authors thank Mrs Reeta Stöd, Mr Hannu Koivunen and Dr Henrik Heräjärvi, from the Finnish Forest Research Institute (Metla), for their efforts in arranging and preparing the wood material for mechanical testing. Funding from the North Karelia Regional Fund of the Finnish Cultural Foundation is gratefully acknowledged.