Fatigue treatment of wood by high-frequency cyclic loading

摘要

New scientific tools for investigating the qualitative and quantitative response of wood to mechanical loading under conditions close to those of mechanical pulping are needed to find ways of radically reducing energy consumption. At KCL a modulated loading device was developed to fulfill these requirements. The device allows the static load, the loading frequency and the amplitude to which wood is subjected to be varied over a wide range. Wood samples were cyclically stressed at large strain (1 mm) pulses and at frequencies in the range 100-1000 Hz. The temperature rise in the wood was measured with thermocouples inserted into the wood and on the wood surface using a thermographic camera. It was found that a high static load during cyclic loading increased heat generation in the wood. The cyclic loading frequency had less effect than the level of the static loading. However, low frequencies generated more heat at a specific number of impacts. The responses of heartwood and sapwood were significantly different in the fatigue treatment. The temperature rise was higher in heartwood than in sapwood. The wood underwent irreversible plastic deformations seen as heat generation and increased pore volume in the wood microstructure.