RHEOLOGICAL BEHAVIOR OF DOUGLAS-FIR PERPENDICULAR TO THE GRAIN AT ELEVATED-TEMPERATURES

The theological properties of Douglas-fir lumber under tangential tens ion and compression were investigated with a small testing machine in a pressure vessel. The strain fields for matched test samples were mon itored remotely with a high-resolution video camera. The required stea dy- and unsteady-state equilibrium moisture contents (EMCs) were achie ved by controlling the total pressure in the absence of air. The moist ure 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 indicate d that the MS strain is the dominant component of the stress-induced s train in drying of small wood samples under load. At 65.6 C, the MS st rain in compression along the tangential direction was about three tim es larger than that in tension under the same stress level and MC chan ge. 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 develo ped and fit to experimental data, and material parameters for various strain components were evaluated.