Fatigue treatment of wood by high-frequency cyclic loading

A new tool was used for investigating the qualitative and the quantitative response of wood at high static pressure superposed with a modulated cyclic loading. Wood samples were cyclically stressed with large compression pulses and at frequencies in the range 100-1000 Hz, i.e., at frequencies close to those used in production scale mechanical pulping. The temperature rise in the wood was measured both with thermocouples inserted into the wood and also with 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 after a specific number of impacts. The wood underwent irreversible plastic deformations accompanied by heat generation and increased pore volume in the wood fibre wall microstructure.