DRYING STRESS AND CHECK DEVELOPMENT IN THE WOOD OF 2 OAKS

This study was designed to relate the checking behaviour of drying woo d as observed under light and electron microscopes to moisture gradien t, stress development, and mechanical properties of drying wood during early stages of drying. Woods used were northern red oak (Quercus rub ra) and narrow-leaved oak (Cyclobalanopsis longinux). Wood was dried f rom the green condition at 40, 60, and 80 degrees C as wafers. Wafers were sealed to retard drying from the transverse surfaces. Moisture gr adient was measured during drying on thin slices removed from the tang ential faces. Drying stresses were estimated for the same times and dr ying conditions by finite element modeling using the ABAQUS program mo dified to take into account time-related deformation. Stiffness and st rength in tension perpendicular to the grain in the tangential directi on were determined at 20, 40, 60, and 80 degrees C for specimens in th e green conditions and at 15% moisture content. These observations ind icate multiple origins of deformations and failures as drying stresses quickly reached levels of maximum tensile strength of the wood. The m ultiseriate rays play a key role as deformations and failures develop within their walls and between them and adjacent fibres. Early failure s in the uniseriate rays and in thick walled fibres contribute to chec k development as failures move out from them through the structural el ements of the wood. Observations of cell deformation and failure were on wafers dried without humidity control in order to induce checking, and should be applied only with caution to interpretation of the dryin g behaviour of lumber being dried commercially at similar temperature conditions.