Analysis of stress development during kiln drying of beech timber

Abstract The moisture content of timber must be reduced by drying before its use. This study aims to evaluate this cracking risk during the drying of beech wood samples using the Response Surface Methodology (RSM) and develop an empirical model describing the relationship between the occurrence of critical tensile stress during an early stage of drying and the air parameters commonly used in kiln-drying schedules of beech timber, i.e., dry-bulb temperature (DBT) and wet-bulb depression (WBD). Nine options of drying conditions were analyzed in the study according to the Central Composite Design (CCD) assumptions; they were combinations of three option of DBT (30 °C, 40 °C, and 50 °C) and three option of WBD (4 °C, 5 °C, and 6 °C). During the experiments, a tangential shrinkage of the wood samples was completely restrained with a load cell that measured generated tensile drying stresses. The results of the ANOVA analysis confirmed that the DBT is the only factor significantly influencing the tensile stress at failure (σ f ). The second-order effect between DBT and WBD has also been confirmed. The drying condition causing the highest risk of cracking was shown using the multiple contour plots of tensile stresses at failure and the moisture content at failure (MC f ). Reliability theory was used to predict the cracking risk of wood tissue. The analysis confirmed the dependence of the cracking risk of wood tissue on the drying conditions. In the case of DWT equal to 30 °C, the cracking risk increases as the relative humidity (RH) decreases. However, during drying beech wood samples at a temperature of 50 °C, decreasing the RH in the range corresponding to the increase of WBD from 4 to 6 °C, reduces the cracking risk, which is pointed out by the lower moisture content at failure (MC f ) of the wood samples.