EFFECT OF RESTRAINED SWELLING ON WOOD MOISTURE CONTENT

Transcription

1 U.S. DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY MADISON, WIS. In Cooperation with the University of Wisconsin U.S.D.A. FOREST SERVICE RESEARCH NOTE FPL-0196 JULY 1968 EFFECT OF RESTRAINED SWELLING ON WOOD MOISTURE CONTENT

2 Abstract Swelling stresses are responsible for many of the defects that occur in wood and wood products during the drying process and in end-use situations. A rational approach to reducing these defects depends on a knowledge of the swelling stresses and the factors that affect them. Maximum swelling pressure and the time required to reach the maximum swelling pressure are shown dependent on the size of the sorption increment, on the temperature, and on the direction of restraint. It is shown that the equilibrium moisture content of wood restrained from swelling is lower than that of wood unrestrained from swelling. With a knowledge of the swelling stresses and the mechanical properties of the wood and wood products involved it may be possible to design more effective drying processes and to secure better performance for end products.

3 EFFECT OF RESTRAINED SWELLING ON WOOD MOISTURE CONTENT By WILLIAM T. SIMPSON, Forest Products Technologist and CHRISTEN SKAAR, Wood Products Engineer 1 2 Forest Products Laboratory, Forest Service U.S. Department of Agriculture ---- In the wood products industry it is generally understood that moisture content changes in wood cause stresses, whereas it is not generally understood that the reverse is true--that stresses in wood can cause moisture content changes. The theory relating stress to moisture content changes has been developed by Barkas, 3 and corroboration of the effect of swelling stress (restraint) on equilibrium moisture content has recently been discussed by Bello 4 who restrained both radial and tangential swelling of wood by using steel rings. The purpose here is to investigate the effect of restraint in the radial and the tangential directions on the time-dependent swelling stress developed by wood and on the equilibrium moisture content of the wood. Restraint was imposed in either the tangential or the radial direction rather than both directions simultaneously, and the magnitudes of swelling forces were recorded as functions of time. Restrained swelling tests were made between approximately 1 percent moisture content and equilibrium with each of three relative humidities (50, 70, and 80 percent) and two temperatures (80 F. and 130 F.). The swelling tests were made on red oak blocks 1-1/2 inches by 1-1/2 inches by 1/2 inch along the grain. 1 State University of New York, College of Forestry, Syracuse University, Syracuse, N.Y. 2 Maintained at Madison, Wis., in cooperation with the University of Wisconsin. 3 Barkas, W. W. The swelling of wood under stress. For. Prod. Res. Lab., Princes Risborough, England, 103 pp., Bello, E. Effect of transverse compressive stress on equilibrium moisture content of wood. Forest Prod. J. 18(2):69-76, FPL-0196

4 The blocks were restrained in either the radial or the tangential direction. Swelling pressure was measured with electrical resistance strain-gage load cells designed to prevent swelling and, at the same time, measure the swelling force. The specimens were dried to about 1 percent moisture content by heating in an oven at 80 C. for 24 hours. After removal from the oven, the specimens were wrapped in a vapor-barrier material, cooled, and weighed for initial moisture content before the restrained-swelling tests, The specimens were then inserted in the load cells in a. controlled environment chamber. The swelling force was recorded as a function of time on a strain-gage recorder. At the end of each swelling test, the specimens were removed from the load cell, weighed, and ovendried to determine moisture contents. Unrestrained specimens for controls were included in each test for comparison of moisture contents. Some results of the swelling tests are shown in figure 1 in which swelling pressure is plotted as a function of time for various combinations of temperature, equilibrium, relative humidity, and direction of restraint. The curves for 70 percent relative humidity are not shown because of space limitations, but they fall approximately midway between the 50 and 80 percent curves. It is apparent that the magnitude of the maximum swelling stress increases and the time required to reach the maximum stress decreases when the size of the sorption interval is increased. It is also apparent that the maximum swelling stress is greater in the tangential than in the radial direction. The mechanism controlling the swelling pressure-time relationship is complex, but a comparison between the radial and the tangential results serves to point out some of the more important factors involved. To oversimplify the assumptions could be made that the mechanical properties in the radial and the tangential direction are the same and are independent of time and moisture content, and that the amount of swelling per unit increase in moisture content is the same in each direction. When restrainedswelling begins, an elastic strain develops, and a swelling pressure proportional to the elastic modulus develops. The swelling pressure will increase until sorption is complete and will then remain at the same constant value for both the radial and the tangential directions. If the fact is now introduced that the elastic modulus in the radial direction is approximately twice that in the tangential direction, 5 the swelling pressure 5 Youngs, R. L. The perpendicular-to-grain mechanical properties of red oak as related to temperature, moisture content, and time. U.S. Dep. Agr., Forest Serv., Forest Prod. Lab. Rep. No. 2079, Madison, Wis., FPL

5 Figure 1.--Relation of swelling pressure to time for combinations of temperature, relative humidity, and direction of restraint. M

6 in the radial direction should be about twice that in the tangential direction. However, the amount of swelling per unit increase in moisture content in the radial direction of red oak is approximately half that in the tangential direction. 6 Therefore, for the same increase in moisture content, the elastic strain is only half as much in the radial as in the tangential direction, thus tending to compensate for the difference in the elastic modulus in the two directions. This may partly account for the reason that the radial and the tangential swelling pressures in figure 1 do not differ greatly. Another factor involved in the swelling pressure-time relationship is the decrease in the elastic modulus as the moisture content increases during sorption. Thus, each increment of strain contributes less to the increase in swelling pressure as sorption progresses. From Youngs' data, 5 the effect of moisture content on the elastic modulus does not appear to differ greatly between the radial and the tangential directions and, thus, would not seem to contribute much to the differences between the radial and the tangential swelling pressures. A further complication is the time-dependent nature of the mechanical properties and the difference in time dependency between the radial and the tangential directions. Preliminary work has shown, for example, that in oak, tangential creep proceeds at a faster rate than radial creep. The swelling stress begins relaxing as soon as restrained swelling begins and continues relaxing at a rate that depends on time, moisture content, and the magnitude of the swelling pressure. In figure 1 it is apparent that a point is reached where the relaxation rate becomes greater than the rate of buildup of swelling pressure, and, consequently, the swelling pressure decreases with time beyond this point. It is also apparent that, at the higher moisture contents, the rate of stress relaxation is greater in the tangential than in the radial direction. The differences in equilibrium moisture content between restrained and unrestrained swelling specimens are shown in table 1. All unrestrained swelling specimens attained a higher moisture content than the restrained specimens. Table 1 also shows that, with sorption starting at 1 percent moisture content, the larger the sorption interval, the greater the difference in equilibrium moisture content between restrained and unrestrained swelling. 6 U.S. Department of Agriculture (Forest Products laboratory). Wood Handbook, Agriculture Handbook No. 72, U.S. Government Printing Office, Washington, D.C., 528 pp., FPL

7 -- Table I. Differences in moisture content percentages between restrained and unrestrained specimens

8