Thermal/moisture-related stresses and fracture behaviour in solid wood members during forced drying: Modelling and experimental study

The acclimation and kiln-drying of wood is an important process in the manufacture of all wood products. Those working in the woodworking industry and dealing with the wood used in manufactured products are well aware of the difficulties encountered in the proper removal of the moisture content of wood without injury to the timber itself.

When solid wood products are dried from a green condition down to an average moisture content level close to the service life conditions of the final product, significant moisture-induced stresses and related fracturing can occur. The drying stresses arise because of internalconstraints that are strongly affected by the annual ring pattern, the moisture gradient over the cross section, differential shrinkage and the inhomogeneity of the material.

The objective of the work reported here was to investigate the behaviour of softwood during drying, in particular the stress and cracking that takeplace during kiln-drying. Both experimental and numerical work was carried out so as to obtain knowledge regarding stress, strain, mechano-sorption and crackbehaviours in wood during drying.The investigations aimed also at revealing how drying damagecan best be reduced through appropriable adjustments at the kiln-drying process and identification of the parameters adjustable in this context.

Sincekiln-drying is usually performed at temperatures of 40 to 90 °C, knowledge of how temperature and moisture affect the elastic, visco elastic and crack behaviours of wood are of paramount importance. The studies were carried out in several steps, each step providing knowledge used in connection with the further steps, the experimental results being used, for example, for verifying the numerical model, and vice versa.

The major highlights of the thesis can be described as follows: Disc drying was investigated experimentallyby measuring the strain field over the cross section as a whole throughout the drying period. A non-contact optical 3D deformation measuring system called Aramis was used to measure the strain field.

The strains were measured, along with a drying history that was generated, to verify a model that was used to simulate disc samples of the same type. The stresses were analyzed so as to clarify whether and when critical stress stateswere encountered during the drying process. The reversibility of the mechano-sorptive strains, i.e. the possibility of driving mechano-sorptive strains back to their original state, was studied then by use of the verified model.

Kiln-drying experiments were carried out, ina specially designed climate chamber under well-defined climatic conditions, allowing the climatic variation limits for crack initiation to be identified. The critical tensile stresses were determined by means of simulations and were compared with the levels of tensile strength found in tests of tensile strength carried out at the same temperature levels.

Tensile strength is highly dependent upon the temperature! Water flux in the tangential and in the radial direction at three different temperatures, and for three different humidities at each temperature, was studied. This provided information regarding the expected drying times under different climatic conditions.

In addition data from these studies were used to simulate three timber boards differing in the location of the pith. The boards behaved quite differently during the drying as regards both deformations and stresses. The variations in the initial (green) moisture content of the timber boards had a significant effect on the final stress state present after drying.