Fracture mechanics analysis of the crack development in structural timbers under high temperature drying

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Following four subjects were studied:(1)development of fracture mechanics test which can simulate mechanical behavior of wood under high temperature drying, (2)characteristics of crack propagation in actual kiln drying, (3)improvement of non-destructive measurement of inner moisture distribution during drying, and (4)fracture mechanics analysis of crack development mechanism.1.A specially designed apparatus which induced a stress in a wood sample during high temperature drying between 100 and 120°C was made. The sample was immersed in silicon oil and dried. The apparatus was composed of a glass cell and a load cell which was durable at high temperature and humidity. Fracture toughness was 0.8MN/m^<3/2> and 0.18MN/m^<3/2>, respectively for environmental and high temperature. A large decrease of stress occurred during crack extension through a late wood zone under environmental drying condition. However, a slight decrease occurred at high temperature drying.2.From the observations of the … More fracture mechanics test in the glass cell and actual high temperature. kiln drying, the following crack development process was shown. At the beginning stage of drying, micro cracks occur in a late wood zone. Then it extends to both early wood zones and finally grows to a visible check. Slightly shifted cracks on a radial line extended to a zigzag "Inazuma ware".3.A measuring method of moisture gradient in wood during drying was examined by evaluating the change of electric capacitance at high frequency range. An electrode was scanned around a cross section of a specimen. The moisture gradient in a wood was well estimated. Temperature dependence of capacity is required to be clarified in future application at conventional kiln drying.4.Mechanical condition around a crack tip in wood can be shown by a mechanical compliance model having a viscoelastic compliance element. The increase of temperature induces large decrease of the fracture toughness. This means that the increase of creep at high moisture and temperature conditions decreases a stress induced during actual high temperature kiln drying. This mechanism can explain the restraint of surface check occurrences at high temperature drying. Crack arrest occurs by increase of crack propagation resistance. "Why inner checks are apt to occur in high temperature drying?" is next problem to be solved. Less