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.
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