Internal stress development within wood during drying: A master curve concept and its application on drying stress evaluation

Abstract

The generation of internal stress (IS) in flat-sawn and quarter-sawn rubberwood boards during drying has been investigated using an online restoring force (RF) technique, which restrained a half-split rectangular wood specimen. Particular attention was given to the longest IS reversal regime. The IS development proceeds faster in the flat-sawn specimens than that in the quarter-sawn specimens while the maximum IS magnitudes in both specimens are rather similar. By normalizing the IS reversal period, a master IS profile, the derivative of the measured RF to the IS reversal time ratio versus the IS reversal time ratio, is proposed. This master curve, exhibiting some degree of independence from wood orientation and drying temperature, shows variations correlated with the free water content in the wet zone and the dry/wet zone fractions. The process of IS reversal, unaffected by temporary unrestraint, advances as the dry zone expands inwards and ends when the wet zone disappears. Assuming mechanical equilibrium between the dry and wet zones, the IS in both zones can be estimated from the RF data. The maximum tensile IS in the dry zone, indicating a risk of surface checks, evolves at slightly lower magnitudes at higher drying temperatures and is lower in the quarter-sawn specimens. The IS relaxation in the dry zone, still largely taking place in the absence of the applied RF, highlights the main contribution of the dry/wet zone fractions, continuing to proceed without restraining, to the IS development. These findings, emphasizing the significance of dry and wet zones, should pave the way for a better understanding of the IS development within wood during drying.