Rheological Behavior of Douglas-fir As Related to the Process of Drying

ABSTRACT

Large inelastic strain occurs inside a piece of lumber during drying. The strain consists of several components such as elastic, plastic, creep, shrinkage and mechano-sorptive effect. The drying behavior of the whole board is determined by the behavior of the individual components and their interactions. Whereas limited investigations have been made on those strains under moderate conditions, there is a lack of comprehensive research aimed to examine the behavior at elevated temperatures and to incorporate the various strain components into a process model. This research provides experimental data for various strain components of small wood samples and an analytical tool for evaluating the drying behavior of full-size boards.

Small test specimens of Douglas-fir were loaded tangentially in both tension and compression under constant and varying moisture conditions at different temperatures. Experiments were conducted using a small testing machine contained within a pressure vessel. The strain fields for loaded and unloaded test samples were measured using a high resolution video camera. The required moisture change at controlled temperatures was achieved by controlling the total pressure in absence of air with saturated steam. Moisture content was monitored by a quartz spring sorption balance.

The total deformation due to loading and moisture change was decomposed into instantaneous, creep, shrinkage and mechano-sorptive components. Constitutive equations for each component were developed as a function of stress, temperature, moisture, time and moisture change. These equations were incorporated into a process model to simulate the development of stress and strain in large pieces of lumber during drying.

A slicing method was used to measure the distribution of moisture and strain through the thickness of full-size boards at different stages of drying. The process model was used to predict drying stress and strain based on the measured moisture distribution and material properties. The effect of drying conditions and types of wood on the development of drying stress was demonstrated. The predicted drying strains under different drying conditions were compared with the corresponding measurements.

Key Wards and Phrases:

• constitutive equations
• creep
• mechanosorptive effect
• modelling
• strain
• stress
• wood dry