Rheological Behavior of Douglas-Fir Perpendicular to The Grain at Elevated Temperatures
Keywords:
Creep, drying, Douglas-fir, mechano-sorptive effect, shrinkage, tension, compressionAbstract
The rheological properties of Douglas-fir lumber under tangential tension and compression were investigated with a small testing machine in a pressure vessel. The strain fields for matched test samples were monitored remotely with a high-resolution video camera. The required steady- and unsteady-state equilibrium moisture contents (EMCs) were achieved by controlling the total pressure in the absence of air. The moisture content (MC) was monitored with a quartz spring sorption balance. Creep at a constant MC of 10% and mechano-sorptive (MS) effect during drying were measured for temperatures up to 82 C.
The results indicated that the MS strain is the dominant component of the stress-induced strain in drying of small wood samples under load. At 65.6 C, the MS strain in compression along the tangential direction was about three times larger than that in tension under the same stress level and MC change. An increase in temperature from 32.2 C to 82 C led to an increase in MS strain under compression. Constitutive equations quantifying the combined effect of mechanical loading and moisture change were developed and fit to experimental data, and material parameters for various strain components were evaluated.
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