Effect of press closing speed on stress relaxation and set recovery of densified alder wood

One of methods to overcome the problem of the set recovery in densified wood, is decreasing the press closing speed which leads to stress relaxation during densification. In the present study, the effect of press closing speed (at three levels of 0.2, 0.4 and 0.6 mm/min) at constant compression ratio of 45% was investigated on stress relaxation behavior and set recovery of alder wood. Results showed that with increasing press closing speed, stress created in densified wood was increased and the reason was attributed to the breakage of covalent hydrogen bond and relaxation of stress at lower press speed which led to decreasing of the set recovery. Two stages were observed in stress relaxation curves. At the first stage, the relaxation rate was high, attributed to the slippage of wood molecules and breakage of intermolecular hydrogen bonds. At the second stage, the stress relaxation curve became flat, represented the decrease of the stress relaxation rate. The Results of time-press closing time superposition showed that the master curve was able to predict the stress relaxation behavior to almost 380 hours (380 times of test time). Since a smooth master curve was not achieved by just horizontal shift and a vertical shifting was also required, it can be state that regarding high temperature and moisture content of woods, the material showed a complex thermorheological behavior. With increasing press closing speed from 0.2 to 0.6 mm/min, the set recovery value was increased up to 20%. A linear relation between the residual stress after one hour of stress relaxation and set recovery was found.