湿度变化和荷载作用下胶合木曲梁的工作性能研究

发布时间：2018-07-10 00:50

本文选题：胶合木曲梁 + 湿度变化　；参考：《哈尔滨工业大学》2015年博士论文

【摘要】：胶合木(Glulam)是一种最常见的工程木产品(Engineered Wood Product),既保留了木材的天然纹理又具有良好的力学性能,因此广泛应用于木结构建筑中,且往往采用弧形受弯构件的形式(曲梁)。由于木材的横纹抗拉强度很低,胶合木曲梁在荷载作用下产生的横纹拉应力会导致横纹受拉破坏。在正常使用过程中,胶合木构件周围环境空气相对湿度会因气候变化和人类活动等影响而变化,吸湿和解吸过程使胶合木构件内部含水率不均匀分布导致梁内各点干缩湿胀变形不同,从而产生湿度应力。当胶合木曲梁受荷载和湿度变化的共同作用时,荷载产生的横纹拉应力与湿度横纹拉应力叠加,会降低胶合木曲梁的承载力。因此,开展湿度变化和荷载共同作用下胶合木曲梁的长期性能研究具有理论意义与实用价值。本文先对足尺东北落叶松胶合木曲梁进行了三分点加载抗弯试验,验证了胶合木曲梁发生横纹受拉脆性破坏的破坏模式,并获得了由该破坏模式决定的胶合木曲梁短期承载力,为长期承载试验提供了依据。然后在人工气候实验室内开展了胶合木曲梁环境湿度变化和荷载联合作用下的长期承载试验Ⅰ和长期承载试验Ⅱ,试验结果表明环境湿度变化引起的湿度应力与荷载应力叠加造成曲梁长期承载力下降明显。为获得有限元分析所需的材料参数,还开展了东北落叶松木材的Ⅰ型、Ⅱ型断裂韧性试验和横纹抗拉强度、顺纹抗压强度、顺纹抗剪强度及弹性常数试验。建立了考虑木材热胀冷缩、湿胀干缩、粘弹性蠕变、机械吸附蠕变、破坏模式、各向异性损伤和荷载持续时间效应的本构关系模型。采用简化的Luikov传热模型和Fick第二定律分别建立了温度和水分传输方程。采用Kelvin链模型描述木材的粘弹性蠕变,并采用与含水率变化率相关的机械吸附柔度矩阵来描述木材机械吸附蠕变。在木材三个材料主方向上建立了相互独立的强度准则,并引入9个损伤因子来描述木材的应变软化,从而实现了木材受拉或受剪发生脆性断裂、受压发生塑性变形的模拟。利用Fortran语言开发了用户材料子程序UMAT,实现了所建立的木材本构关系模型在有限元软件ABAQUS中的嵌入。采用ABAQUS建立了胶合木曲梁有限元模型,计算分析了胶合木曲梁短期承载力试验和长期承载试验,揭示了在试验的温、湿度条件下,湿度应力的幅值及分布规律,并实现了荷载和环境湿度变化共同作用下胶合木曲梁横纹受拉破坏的模拟并较准确地预测了破坏发生的时间。还基于线弹性断裂力学理论,建立了木材的开裂准则和裂纹扩展准则,采用扩展有限元法(Extended Finite Element Method)模拟了胶合木曲梁在荷载作用下横纹受拉破坏时裂纹的产生和扩展过程。提出采用湿度横纹应力积分来综合度量湿度横纹应力的大小和作用范围。以含水率影响深度来表示环境相对湿度变化导致梁内含水率剧烈变化的影响范围和含水率波动的衰减。通过有限元参数分析研究了胶合木曲梁横截面宽度、相对湿度变化周期、幅值和循环次数对湿度横纹应力的影响规律。提出了等效湿度横纹应力的概念,并提出了湿度横纹应力与荷载产生的横纹应力叠加的简化方法判断荷载和湿度变化共同作用下胶合木曲梁是否横纹受拉破坏的方法。分析了中国典型气候区代表性城市的年相对湿度变化产生的湿度横纹应力的幅值和变化规律,为在我国各典型气候类型区内考虑湿度应力影响的胶合木曲梁设计提供了参考。
[Abstract]:Glulam is one of the most common engineering wood products (Engineered Wood Product), which not only preserves the natural texture of wood but also has good mechanical properties. Therefore, it is widely used in wood structure and often used in the form of curved flexural members (curved beams). The tensile strength of the wood is very low, and the laminated beam is loaded. In normal use, the relative humidity of ambient air in the surrounding environment changes due to the influence of climate change and human activity during the normal operation. The moisture absorption and desorption process the uneven distribution of water content in the glued wood components, resulting in the different dry shrinkage and swelling deformation of each point in the beam. When the curved beam of the glued wood is subjected to the joint action of load and humidity, the stress of the transverse tensile stress and the tensile stress of the humidity will be superimposed, which will reduce the bearing capacity of the curved beam of the glued wood. Therefore, it is of theoretical and practical value to study the long-term performance of the curved beam of the glued wood under the joint action of humidity and load. In this paper, a three point loading anti bending test was carried out on the full scale northeastern Larix Larix glued beam, which verified the failure mode of the tensile brittleness of the glued Liang Fasheng cross pattern, and obtained the short-term bearing capacity of the laminated wood curved beam determined by the failure mode, which provided the basis for the long-term bearing test. Then it was opened in the artificial climate laboratory. The long-term bearing test I and long-term bearing test II under the combined action of ambient humidity and load on the curved beam of the laminated wood are shown. The results show that the superposition of the humidity stress and the load stress caused by the change of the ambient humidity causes the long bearing capacity of the curved beam to decrease obviously. Ye Song wood type I, type II fracture toughness test and tensile strength, CIS compression strength, CIS shear strength and elastic constant test. The constitutive relation model, which consider wood thermal expansion and shrinkage, wet expansion shrinkage, viscoelastic creep, mechanical adsorption creep, failure mode, anisotropic damage and load duration effect, is established. The temperature and moisture transfer equations are established by the Luikov heat transfer model and the second law of Fick. The viscoelastic creep of wood is described by the Kelvin chain model, and the mechanical adsorption flexibility matrix is used to describe the mechanical adsorption creep of wood. The independent strength of the three wood materials is established in the main direction of wood material. 9 damage factors are introduced to describe the strain softening of wood, thus the simulation of brittle fracture and plastic deformation of wood subjected to tension or shear can be realized. The user material subroutine UMAT is developed by Fortran language, and the embedded wood constitutive relation model is embedded in the finite element software ABAQUS. ABAQU S established the finite element model of glued wood curved beam, calculated and analyzed the short term bearing capacity test and long-term bearing test of the glued beam, revealed the amplitude and distribution of the humidity stress under the temperature and humidity conditions of the test, and realized the simulation of the tensile failure of the cross grain of the laminated wood under the joint action of load and the change of ambient humidity. The time of failure is predicted. Based on the theory of linear elastic fracture mechanics, the Cracking Criterion and crack propagation criterion of wood are established. The crack initiation and expansion process is simulated by the extended finite element method (Extended Finite Element Method). The grain stress integral is used to measure the size of the stress of the RH and the range of action. The influence depth of water content is used to indicate the influence of the change of the relative humidity of the environment and the attenuation of the fluctuation of water content in the beam. The width of cross section of the curved beam and the period of relative humidity change are studied by the finite element parameter analysis. The influence of the amplitude and the number of cycles on the stress of the RH is proposed. The concept of the equivalent humidity transverse stress is put forward, and a simplified method of superposition of the transverse stress of the RH stress and the load is put forward to judge whether the tensile failure of the curved beams of the glued wood under the action of the load and the humidity changes. The typical gas of China is analyzed. The amplitude and change law of humidity transverse stress produced by the change of relative humidity in the representative city of the weather district provides a reference for the design of laminated beam of glued wood considering the influence of humidity stress in the typical climate types of our country.
【学位授予单位】：哈尔滨工业大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TU366.3

【相似文献】