The Role of Temperature in the Relief of Drying Stresses

In an attempt to clarify the roll of temperature in the relief of drying stresses, a study was designed, using red oak as the sample material, to ascertain: l) the extent to which moisture will condense on cool wood surfaces exposed to elevated temperatures with controlled humidity; 2) the diffusion of original cell-wall moisture and of condensate into wood in response to a temperature gradient; and 3) the degree of stress relief accompanied by direct and indirect effects of heating. The results of the study show that moisture can he added to wood surfaces by direct condensation, which occurs whenever the temperature of the wood is below the dew point of the surrounding atmosphere. The swelling which results from this condensation helps explain why stresses are relieved more rapidly when cold lumber is conditioned in a hot kiln. Condensation also helps explain why stresses are relieved when there is no elevation in relative humidity, only an elevation in temperature. In addition to causing moisture to condense on the lumber, an elevation in temperature creates a thermal gradient, which helps relieve stresses by other means: l) The compressive stress at proportional limit and the elasticity of the wood in the shell is reduced when the shell is heated. The strain to proportional limit is not changed appreciably. The weaker shell is strained in compression above the proportional limit by the elastic recovery from tension of the cooler and therefore stronger core. 2) Thermal expansion of the surface against the cold interior adds to the compressive set. 3) Moisture migrates from a warm to a colder surface. The heat and moisture migration may trigger separation of newly formed chemical bonds between molecular components of the deformed cell wall.

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