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

高温干燥结构木材裂纹扩展的断裂力学分析

基本信息

批准号：
14560132
负责人：
SOBUE Nobuo
金额：
$ 2.18万
依托单位：
Shizuoka university
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
2002
资助国家：
日本
起止时间：
2002 至 2003
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/en/grant/KAKENHI-PROJECT-14560132/
关键词：
High temperature drying Crack development Fracture mechanics Crack control Fracture toughness Crack propagation mechanism Structural timbers 破壊靭性値木材乾燥

项目摘要

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

主要研究了以下四个方面：(1)建立模拟木材高温干燥力学行为的断裂力学试验；(2)实际窑干中裂纹扩展特性；(3)改进干燥过程中内部水分分布的无损测量方法；(4)裂纹发展机理的断裂力学分析。制作了一种特殊设计的装置，在100至120°C的高温干燥过程中对木材样品产生应力。样品浸在硅油中干燥。该装置由一个玻璃电池和一个在高温和高湿条件下经久耐用的称重传感器组成。环境和高温条件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，裂纹通过后期木带扩展时应力大幅度减小。高温干燥条件下略有下降。从玻璃池断裂力学试验和实际高温下的观察结果来看。窑内干燥后，裂纹发展过程如下。在干燥开始阶段，后期木区出现微裂纹。然后它延伸到两个早期木材区，最后生长到一个可见的检查。在径向线上有轻微位移的裂缝延伸成锯齿状的“Inazuma ware”。研究了一种通过评价高频范围内电容变化来测定木材干燥过程中水分梯度的方法。在标本的横截面周围扫描电极。木材中的水分梯度估计得很好。在今后的常规窑炉干燥应用中，还需要明确容量的温度依赖性。木材裂纹尖端周围的力学状况可以用具有粘弹性柔度元素的力学柔度模型来表示。温度升高导致断裂韧性大幅降低。这意味着在高湿度和高温度条件下，蠕变的增加减少了实际高温窑干燥过程中引起的应力。这一机理可以解释高温干燥时表面裂纹的抑制。裂纹的止裂是通过增加裂纹扩展阻力来实现的。“为什么在高温干燥时容易发生内检？”是下一个要解决的问题。少