Exploring the internal deformation of wood under loading based on X-ray CT
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摘要:
目的 木材是重要的室内装饰和建筑用工程材料，具有易加工，强重比高等优点。作为生物质多孔材料，载荷作用下木材内部结构易发生变化，进而对其力学性能产生重要影响。研究载荷作用下木材内部结构变形可为理解其力学行为提供基础理论支撑。 方法 为探索压缩载荷作用下木材内部空间结构的演变规律，使用微型加载设备对实体木材（花旗松）和胶合材（杨木）试件进行横纹压缩，实时记录加压头位移和加载压力，加载过程中使用X射线断层扫描仪周期性扫描试件，对扫描结构进行三维重建，实现试件内部空间结构可视化，结合力学性能和内部空间结构演变解析实体木材和胶合材力学失效机制。 结果 实体木材在受载时，早材部分密度快速增加，载荷达到25.26 MPa时，晚材部分密度开始增加。早材管胞压溃是木材内部结构变化的主要原因，压溃路径与生长轮平行；晚材结构改变主要体现为树脂道压缩变形和木射线压裂。胶合材在受载时，局部形变集中是杨木内部结构变形的主要原因，尤其是大孔径导管极易压溃；另外，胶黏剂能够明显增加胶层区域杨木刚度，提高导管结构的稳定性。 结论 本研究解析了横纹载荷作用下实体木材和胶合材内部空间结构的演变规律，为优化木材加工工艺和指导木材的科学利用提供了理论基础。新型三维动态检测技术为解析木材力学行为的发生机制提供了新的思路。 Abstract:Objective Wood is easy to process and has high strength/mass ratio. It has been widely applied in the decoration and construction fields in solid wood as well as wood-based panel formats. As bio-based porous materials, the internal structure of wood is prone to change in compression conditions, which has an essential impact on the mechanical performance of wood based materials. Hence, it is important to understand the interaction between loading force and internal deformation of wood. This shall contribute to better understand and evaluate the mechanical performances of wood. Method In order to study the internal structure changes of wood, samples from Douglas fir and poplar based panels were prepared. X-ray computed tomography (X-ray CT) was used to periodically monitor the internal structure of samples when they were compressed by a customized loading cell. X-ray CT was able to visualize the 3D internal structural changes of samples during compression. The compression conditions of customized load cell could be controlled by both loading wedge movement and loading force. Based on the results from X-ray CT and loading cell, the mechanisms of mechanical performance of solid wood and glued panels were discussed. Result The stiffness of Douglas fir was much higher than that of poplar. Density increase in Douglas fir sample mainly occurred in earlywood and density increase in latewood was found only when loading reaching 25.26 MPa. Regional stress accumulation was the main cause of internal structure changes. Structural changes could be re-distributed as increasing the stress. The existence of glue can significantly enhance the stiffness of wood and we found that the morphology of wood vessels adjacent to the glue line was kept well during compression. Large vessels were prone to be collapsed during compression and route of collapsing was parallel to the growth ring. Internal structure changes in latewood mainly occurred as the squeeze of resin canals and the crack of rays. Conclusion The results obtained from this study could be used in optimizing the processing technology and application approaches of wood and wood based materials. Combining the X-ray CT and loading cell is a promising way to reveal the mechanisms of wood based materials under loading. In order to further reveal the internal structure changes in wood cell wall, studies with even better resolution are required. Hence, a possible solution is to combine X-ray CT with other techniques that can reach the nano-scale resolutions. -
Key words:
- wood /
- loading force /
- structure change /
- X-ray computed tomography
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图 1 加载条件下X射线断层扫描仪扫描试件示意图
Figure 1. Schematic overview for X-ray computed tomographyscanning a sample under loading force
图 2 载荷作用下花旗松木和杨木胶合材试件沿厚度方向的平均灰度值变化
Figure 2. Grey scale value changes of Douglas fir wood and poplar plywood along thickness direction under loading force
图 3 不同载荷下花旗松木试件内部空间结构变化示意图
Figure 3. Internal structure changes of Douglas fir wood samples under different loading forces
图 4 不同载荷下花旗松木试件生长轮附近区域内部结构变化示意图
Figure 4. Internal structure changes in regions adjacent to growth ring of Douglas fir wood samples under different loading forces
图 5 不同载荷下花旗松木试件晚材区域内部结构变化示意图
Figure 5. Internal structure changes in latewood regions of Douglas fir wood samples under different loading forces
图 6 不同载荷下杨木胶合材试件内生长轮附近结构变化示意图
Figure 6. Internal structure changes in regions adjacent to growth ring of poplar plywood samples under different loading forces
图 7 不同载荷下杨木胶合材试件导管压溃过程示意图
Figure 7. Internal structure changes of vessels in poplar plywood samples under different loading forces
表 1 4个位移条件下试件承受的载荷
Table 1. Loading force of the samples under 4 different displacement conditions
MPa 试件类型 Sample type 位移 Displacement/mm 0 0.5 1.0 2.0 杨木胶合材 Poplar plywood 0.01 3.32 4.28 7.88 花旗松木 Douglas fir wood 0.01 5.30 5.62 25.26 
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