Stress relaxation of wood during the elevating and lowering processes of temperature and the set after relaxation

Stress relaxation of water-saturated wood was examined at various temperatures and during the processes of elevation and lowering of temperature. The difference between relaxation at a lower temperature and that at a higher temperature was almost same as the increase in relaxation during temperature elevation. Similar results were obtained regardless of the elevating speed of the temperature and regardless of preheating the samples. On the other hand, relaxation behavior during temperature lowering was quite different from that during temperature elevation. The relaxation moduli during the process of lowering the temperature scarcely changed, although those at various constant temperatures decreased with increases in temperature. Marked sets occurred after relaxation measurements under temperature lowering, whereas only slight sets occurred during the process of elevating the temperature or at a constant temperature of 80degC. Furthermore, it was found that the sets that occurred during the process of temperature reduction were almost recovered when the samples were heated again in water. The mechanism of the relaxation behaviors observed in the present study under nonequilibrium temperature is discussed in relation to the occurrence of set after relaxation measurements.