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

本论文主要研究了以下四个方面的内容：（1）模拟木材高温干燥力学行为的断裂力学试验方法的开发;（2）实际窑干过程中裂纹扩展的特性;（3）干燥过程中木材内部水分分布的无损测量方法的改进;（4）裂纹扩展机理的断裂力学分析。将样品浸入硅油中并干燥。该装置由玻璃传感器和耐高温高湿的测力传感器组成。室温和高温下的断裂韧性分别为0.8MN/m^<3/2>和0.18MN/m^<3/2>。在环境干燥条件下，裂纹扩展过程中，应力大幅度降低。然而，在高温干燥时略有下降。 ...更多信息 在玻璃池和实际高温下进行断裂力学试验。窑干燥后，显示了以下裂纹发展过程。在干燥初期，木材后期区域出现微裂纹。然后，它延伸到两个早期的木材区，最后增长到一个可见的检查。径向线上的微位移裂纹延伸成锯齿形的“Inazuma器皿”。3.通过测量高频电容的变化，研究了木材干燥过程中水分梯度的测量方法。电极在样本的横截面周围扫描。很好地估计了木材中的水分梯度。在今后的应用中，需要澄清传统窑干燥时容量与温度的关系。4.木材裂纹尖端附近的力学状态可以通过具有粘弹性柔度单元的力学柔度模型来表示。温度的升高导致断裂韧性的大幅度下降。这意味着在高湿度和温度条件下蠕变的增加降低了在实际高温窑干燥期间引起的应力。这一机理可以解释高温干燥对表面裂纹的抑制作用。裂纹止裂是通过增加裂纹扩展阻力来实现的。“为什么在高温干燥中容易发生内部裂纹？“是下一个需要解决的问题。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