Creep in chipboard. 11. The effect of cyclic changes in moisture content and temperature on the creep behaviour of a range of boards at different levels of stressing

Matched samples to those used in a previously reported investigation (under constant environments employing eight board materials and solid wood) were subjected to five different levels of stress under four different cyclic environments for periods of six months. Relative creep increased slightly and linearly with respect to stress within the range of stress levels adopted, as well as increasing with severity of the environment. All materials showed greater sensitivity to alternating humidity than to alternating temperature. The effects of the three prinicipal variables (stress level, environment, material), and the strong interactions among them, were quantified using analysis of variance and found to be similar to those under constant environments: the effect of environmental condition was found to be far greater on relative creep than that of either stress level or material. Averaging over all conditions and stress levels, relative creep increased progressively among timber, plywood, waferboard, chipboard and fibreboard, though this ranking order varied at different stress levels and environments due to the presence of strong interactions. At a stress level of 30% and a time of 43,200 minutes, the relative creep under alternating humidity of 30 in equilibrium with 90% was on average fourteen times greater than that under a constant humidity of 65% (averaging across the nine materials): however, there was considerable divergence among the materials in this ratio. At the same level of stressing, the mean relative creep for all materials under an alternating temperature of 10 in equilibrium with 30 degrees C was 1.6 times higher than at a constant temperature of 20 degrees C. The ranking order of materials in their sensitivity to alternating temperature was different to that under alternating humidity.