Effect of Temperature and Moisture Content On: Internal Friction and Speed of Sound in Douglas-Fir

Vibration characteristics, such as internal friction, speed of sound, and log decrement of Douglas-fir were measured over a range of temperatures and moisture contents. The speed of sound and Young’s modulus decreased with increasing moisture content and temperature. Two basic mechanisms are suggested to explain this internal friction. One appears in perfectly dry wood and decreases with temperature much as does the viscosity of most liquids. The other mechanism appears only when hygroscopically bound water is present and increases with temperature and moisture content. The relationship between these mechanisms and their relative dependence upon temperature and moisture content result in internal friction minimums in the range of temperature between 0 and 200?F and moisture content from about 2 to 28 percent. At room temperature, an internal friction minimum appears at about 7 percent moisture content. Other data show a strength maximum at about the same moisture content. This may result from either one or two basic processes. The first process is based on the formation of many small internal cheeks as the fibers dried below 7 percent moisture content or the formation of many regions of local internal stress, both of which reduce the ability of the fibers to withstand external stress. The second possible process is based on the formation of the water monolayer during chemical absorption which may improve the bond between cellulose fibers. Values of log decrement showed minimums at various combinations of temperatures and moisture contents. There is a similarity at room temperature and below of the log decrement-temperature relation for the phenolic plastic and the driest wood. This suggests the possibility that the predominant mechanism of internal friction in wood that is completely dry may be the same as that in the non-hygroscopic plastic.

You must be logged in to download any documents. Please login (login accounts are free) or learn how to Become a Member