Creep of heat-treated birch wood under long-term loading

Silver birch (Betula pendula Roth) is one of the most important deciduous pioneer woody plants in Central Europe. Birch wood has very good mechanical properties, but low resistance against the fungi attack. One of the possibilities is to increase its durability by thermal modification. In this paper, we tested the effect of the modification on a creep under long-term load. Birch wood was exposed to two temperature stages (170 and 190 degrees C) in an atmosphere of air mixed with water vapour. Own-developed patented " creep" units were used to load the test specimens in bending. The classical Burger's model was used to evaluate the rheological parameters, or primarily to evaluate the share of individual types of deformations in the total deformation during long-term creep of wood under constant climatic conditions. It was found that due to the thermal modification the creep factor decreases, i.e. the ratio of the total deformation to the initial (elastic) deformation. At the same time, the relaxation time decreases by up to 47.8% at the treatment of 190 degrees C. The stabilization of the elastic deformation in the course of time (viscoelastic) is therefore faster than in the untreated state, which is primarily related to the reduction of its share, by up to 19.0% when modified at 190 degrees C. The reduction of its share is quite clearly related to the reduction of the content of bounded water in the heat-treated wood. In reality, with increasing modification temperature, not only the total and the viscoelastic deformation decreases, but also the plastic one, at the expense of increasing share of the elastic deformation.