Rheology of wood - Part II. The swelling and shrinkage of wood under mechanial restraint

Abstract

The Swelling and shrinkage of wood under restraint which has many practical applications has been studied in the case of 14 species of Indian timbers varying in density from 0.30 to 0.97 g/cm³. Mechanical restraint of swelling in the radial direction can increase the tangential swelling from about 9 to 30% over that of the control in the tangential direction and with tangential restraint 15 to 43% of that of the free side. By repeated wetting and drying under mechanical restraint of one side the specimens compress and in 20 cycles a density of 1.17 g/cm³ was attained with Parishia insignis. As a general rule the compression shrinkage was found to decrease with density but the influence of other factors, like the presence of extractives, etc. cannot be eliminated. Interesting observations were also made on the total shrinkage, change in hygroscopicity and permanent set. Pterocarpus dalbergioides, Acacia catechu, Dalbergia sissoo, and Ougeinia dalbergioides which in addition to high density are rich in extractives showed a comparatively low shrinkage. Their preferance for tool handles can therefore be easily realized. Interesting observations were also made on the recovery of set, on the removal of stress and rewetting of the specimens. The percentage of recovery varied from 9.5% to 50% of the total set depending on the species and direction. Here also timbers rich in extractives like Ougeinia dalbergioides showed a small recovery. As is to be expected temperature was found to accelerate the process. Finally strength tests done on the specimens at the end of the cycles showed increase thus indicating possibilities of improving the properties of wood with simple equipment.