Item View

Publication: Forest Products Journal
Authors: McMillen J M
Reference ID: 5(1):71-76
Publish Year: 1955
Member Download Price: $0.00
Member Physical Price: $0.00
Title: Drying Stresses in Red Oak
The effect of relative humidity variation on tensile and compressive stresses perpendicular to grain, during drying of 2-inch red oak at 110°F. was investigated. Red oak was shown to experience the same general pattern of internal stresses during drying as was previously found in sweetgum heartwood, blackgum, magnolia, and other hardwoods. This stress pattern is such as to cause surface and end checking and external set during the first stage, internal set during the second stage, honeycombing during the third stage, and it leaves the wood in a casehardened condition. Maximum tensile strains in outer slices and maximum compressive strains in center slices were approximately the same order of magnitude for the different humidity schedules tested, but these maxima and reversals of stresses occurred earlier with more rapid rates of relative humidity reduction. In red oak, it is possible to re-establish near maximum tensile strain in the outer zones by abrupt reductions of relative humidity after the maximum has been passed. Corresponding increases in compressive strains can occur in the interior zones. The specimens in this study did not check under such conditions. Set, or permanent deformation, occurred in the drying of oak, both a set tending to decrease shrinkage in the outer zones and a set tending to increase shrinkage in the interior zones. The magnitude of the external set was proportional to the severity of the relative humidity schedule. Shrinkage of the whole boards occurred from the start of drying and continued on a steady rate until accelerated during the period between stress reversal in the outer zones and the first maxima of the reversed stresses; thereafter shrinkage resumed a steady rate. The shrinkage rate was abruptly increased by an abrupt lowering of the relative humidity during the first part of the drying. Shrinkage, tensile strain, and external set all started when only a small portion of the wood was below the fiber saturation point. Moisture content in the center was still above the fiber saturation point when maximum internal tensile strain occurred.

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