This study was designed to relate deformations and failures in drying wood as observed under light and electron microscopes to moisture distribution, stress development, and mechanical properties of drying wood during early stages of drying. That behavior is interpreted in terms of implications for wood drying. Woods used were northern red oak (Quercus rubra) from southwestern Virginia, U.S.A., and narrow-leaved oak (Cyclobalanopsis longinux) from southern Taiwan, R.O.C. Wood was dried from the green condition at 40, 60, and 80°C as boards and as wafers.
Checking began very early in drying, even at 40°C, a common temperature for initial stages of commercial kiln drying. This was first evident in C. longinux, the denser of the two woods, with both transwall and intrawall failures of the thick-walled fibers, followed by checking along the edges of compound multiseriate rays and along the uniseriate rays. As drying progressed, large checks developed in the center of multiseriate rays and latewood pores deformed and buckled. Raising the drying temperature of C. longinux markedly accelerated the onset of cell deformation, failure, and cell separation; this also increased the severity of drying defects relative to those in Q. rubra under comparable conditions.
In Q. rubra at 40°C the most pronounced effects were in the ray parenchyma of the multiseriate rays where deformation began very early in drying. A few small horizontal checks developed early and soon closed. Procumbent cells became more rounded and disrupted at the middle lamella. At 80°C, separation between cells at the edge of multiseriate rays developed soon after drying began and checking was soon apparent. These observations indicate multiple origins of the deformations and failures that lead to checking. Clearly the multiseriate rays play a key role as deformations develop within their walls and between them and adjacent fibers. Somewhat surprising is the early role played by thick-walled fibers, especially in C. longinux, which showed very early failure. Early failures in the uniseriate rays also contribute to check development.
As drying temperature is increased stresses due to elastic deformation of the wood increase more rapidly than the accompanying time-related factors of creep and stress relaxation. This implies a relationship between checking tendency early in drying and the time rate of development of creep and stress relaxation, which tend to reduce stresses. Moisture distribution and shrinkage tendency are also critical early in drying and probably account for the differences in drying behavior of the two species used here. The multiplicity of sources of failure initiation reflect the heterogeneous structure of these woods and imply that observations made in this study might be broadly applicable to refractory hardwoods.
Key words: drying stresses, cell deformation, wood ultrastructure, microscopy.
Correspondence: Hsiu Hwa Wang, Department of Forest Products Technology, National Pingtung Polytechnic Institute, Pingtung, China-Taipei 91207