Shrinkage Stresses in Externally Restrained Wood

Measurements were made of the forces developed in the tangential grain direction of aspen (Populus tremuloides) and red oak (Quercus rubra) while thin sections of these woods were restrained from tangential shrinkage during moisture content change. Test specimens were dried from at or above 30 percent moisture content to moisture contents of 16, 12, 8 or 4 percent within a temperature range of 50? to 150?F. When green aspen was dried under restraint, resulting forces varied with time in much the same manner as did dimensional change during unrestrained shrinkage. Graphs of force or shrinkage versus time formed sigmoid curves. Mode of cessation of force accumulation seemed to rely on drying rate. Force increases ended more abruptly at high rates of drying than at low rates. Maximum restrained shrinkage stress in aspen decreased with increasing E.M.C. conditions and with increasing temperature. An ultimate stress was attained in aspen before moisture equilibrium was established at all but the lowest E.M.C. content conditions. Significant shrinkage forces were measured while the specimens were drying from higher moisture contents down to their theoretical fiber saturation points. Oak specimens failed mechanically at or within large rays before attaining 35 percent moisture content. Forces developed during shrinkage were compared with values from static tensile tests. The maximum stresses attained by aspen and oak during drying from at or above the fiber saturation point to E.M.C. conditions of 4 percent at 85?F. were less than half the static tensile strengths of these woods tested at 4 percent E.M.C. The apparent strains developed during this same drying were equal in the case of aspen to the maximum strains found in static tests of green material. For oak, the strain to failure during drying was approximately half the strain found in static tests of green material. Strains developed at maximum drying stress were about the same regardless of whether the initial moisture content was at or above the fiber saturation point.

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