STRESS-RELAXATION IN WOOD - SCOTS PINE VENEER .2. QUANTITATIVE COMPARISON WITH THE PREDICTION OF A COOPERATIVE FLOW MODEL

Stress relaxation data obtained from samples of Scots pine veneer at different initial stress and humidity levels are presented. When plotted as stress (sigma) against log time (t) the relaxation curves appear to be the result of a superposition of a largely linear portion and a tail which can be described by a single time exponential. The linear part of the sigma against log t curve is reproduced using a modified version of a cooperative flow model. Agreement between theoretical and experimental data is excellent over the entire time range covered by the measurements. This includes the validity of the relation F almost-equal-to 0.1 DELTA-sigma between the slope of the linear part of the sigma against ln t and the stress decrease produced by the corresponding mechanism. This relation is valid independent of the sample orientation, humidity and other variables; earlier, it has been shown to possess general validity for both polymers and metals. On the whole, relative humidity (up to about 95%) influenced the relaxation process relatively little, an exception to this being samples cut in the 45-degrees direction where an appreciable increase in the relaxation strength with relative humidity was observed. The relaxation curves scaled in an approximately lienar fashion with the initial stress.