The drying stress and check development on high-temperature kiln seasoning of sapwoodPinus radiata boards

Abstract

In the first part of this paper, a model of moisture movement and strain development for a sapwood board being dried under high-temperature conditions was developed. In this second part of the set, the stress development inside the board is examined. It is postulated that the stresses of the board must satisfy both the force equilibrium and geometric compatibility condition over the cross-section. Thus, as the evaporative front recedes into the board, significant tensile stresses will be successively induced at these locations, which are subject to the relaxation of the mechano-sorptive and creep strains. During the first drying period, it is found that the mechano-sorptive strain will have a principal influence on the magnitude of stress, whereas the creep strain can cause the large tensile stress falling significantly just after it has reached the peak value. After that, the relief effects of mechano-sorptive and creep strains tend to become much weaker. Because of the enhanced creep and softened material behaviour, wood subject to high temperatures may not necessarily lead to excessive degrade. It is demonstrated that the present model is capable of describing all the important drying phenomena observed in practice, including case-hardening (plastic set) and stress reversal. By accounting for the relief effect of mechano-sorptive and creep strains, a smaller board total shrinkage is obtained than that of a pure elasto-plastic model ignoring these stress-induced strains.