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

(1)模拟木材高温干燥力学行为的断裂力学试验的发展；(2)实际窑炉干燥过程中裂纹扩展特征；(3)干燥过程中内部水分分布的无损测量的改进；(4)裂纹扩展机理的断裂力学分析。1.制作了一种在100~120℃高温干燥过程中在木材试件中产生应力的装置。样品浸泡在硅油中并烘干。该仪器由一个玻璃电池和一个耐高温、耐高湿的称重传感器组成。在环境和高温条件下，断裂韧性分别为0.8MN/m^和0.18MN/m^&lt；在环境干燥条件下，裂纹通过晚期木材区域扩展时，应力有较大幅度的降低。但在高温干燥时略有下降。2.从…观察在玻璃槽中进行了较多的断裂力学试验，实际温度较高。窑炉烘干过程中，出现了以下裂纹发展过程。在干燥初期，木材后期区域会出现微裂纹。然后它延伸到两个早期的木材区域，最后生长到一个可见的格子。径向线上轻微移动的裂纹延伸到之字形的“稻田瓷器”。3.通过评估高频范围内电容的变化，研究了木材干燥过程中水分梯度的测量方法。电极在样品的横截面周围进行扫描。木材中的水分梯度被很好地估计出来了。4.木材裂纹尖端周围的力学状态可以用具有粘弹性柔度单元的力学柔度模型来表示。随着温度的升高，断裂韧性大幅度下降。这意味着在高湿度和高温度条件下蠕变的增加减少了在实际高温窑干燥过程中产生的应力。这一机理可以解释高温干燥过程中表面裂纹的抑制。裂纹的止裂是通过增加裂纹扩展阻力来实现的。“为什么在高温烘干时容易出现内部裂纹？”是下一个要解决的问题。较少

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