Creep, deformation and moisture redistribution during air convective wood drying and conditioning : air convective wood drying and conditioning

Abstract: Air convective wood drying is associated with a considerable risk for damage to the timber, especially checking and warping. This thesis contains an analysis of creep and deformation during wood drying related to stress development and susceptibility of checking. Also heat and mass transfer problems in the wood drying process are considered. Finally an investigation of different conditioning treatments as a way of relaxing residual stress and moisture gradients is presented. Related to the studies mentioned above, this thesis describes some useful techniques to evaluate deformation, moisture flux and redistribution by nondestructive and non-contact measurements. Moisture flux is evaluated by infrared thermography, surface moisture content by near-infrared measurements, displacement by white light speckle photogrametry and moisture pickup during steaming by computer tomography. The results show that during air convective drying the heat transfer from hot humid air to the wood starts a surface evaporation which differs between sapwood and heartwood but also between earlywood and latewood in the annual ring. This evaporation pattern is caused by the amount of water contained in these tissues of the wood and also by the pit aspiration, especially in the earlywood cells. Another result shows that Fickian diffusion theory for wood results in a mass transfer coefficient (surface emission factor) which is not in accordance with classical heat and mass transfer boundary layer theory. The mass transfer coefficient is one order of a magnitude lower than the ones obtained from classical theory. Checking susceptibility is closely related to stress development across the grain during drying. The tension stress at the beginning of the drying period develops rapidly causing a creep response of the surface. This mechanosorptive creep relaxes the tension stress enough to prevent checking in many cases. This effect is more pronounced at elevated temperatures. It is shown that high density and low temperature make wood more susceptible to checking, a property closely related to the value of strain at failure. Warping during drying is generally caused by the anisotropic shrinkage of wood. The study of cupping shows that it depends mainly on the differential shrinkage in tangential and radial direction but also that cupping is reduced to some extent by the creep of the surface layer. However during the conditioning treatment cupping increases to values near those reached after stress free drying. Finally conditioning tests in saturated water steam show that this treatment is an efficient way of relaxing residual drying stress as well as to equalize moisture variation. Also steaming offers an interesting method of fast and efficient heating prior to the drying period.