Aging and Durability of Wood

Erina Kojima (Department of Forest Engineering)

Research Field: Timber mechanics

Research theme：Mechanical performance of aged wood at cell wall level

Background

In Japan, timber has long been used as a building material in such as temples and shrines (Fig. 1). Some of them still retain their original appearance even after a thousand years or more. This is empirical evidence of the superior strength and durability of timber. I am conducting research to evaluate these empirical values scientifically.

Fig. 1. An old temple during renovation

Importance of this Theme

Understanding the mechanical durability of wood will make it clear that how to maintain and manage wood as a structural member, and how to use it for longer term uses. I believe that this will be helpful in passing on the culture of the many traditional buildings that exist in Japan. In addition, the long-term use of timber as a building material maximizes the carbon storage effect. Therefore, I believe that this research theme is very important from both cultural and environmental perspectives.

Interesting Points

Unlike many other building materials, wood is a natural material, and its mechanical performance is subject to variability. Furthermore, wood has a very complex hierarchical structure. The annual ring structure is based on the different growth rates of cells (earlywood and latewood), and these cells have a three-layered structure. In addition, the layers are made up of three chemical components. It is the combination of these complex structures that provides the bearing capacity of wood as a building material. It is interesting to scientifically evaluate the variability of wood and hence to derive useful knowledge for building construction.

Key Findings so far

I have been conducting research on the mechanical durability of wood, focusing on the mechanical behavior of cellulose in the cell wall. X-ray diffraction (XRD) measurements under uniaxial loading were collected to determine the behavior of the cellulose (Fig. 2). In this study, the specimen was subjected to heat treatment (as a pseudoaging treatment) to understand its behavior. As a result, there was a delay in load transmission and a decrease in deformation capacity. It is believed that aged wood has a different fracture mechanism from new wood, arising from the presence of microscopic cracks that cannot be visually confirmed due to use over time. We believe that the mechanical behavior of the cell walls captured in this study may shed light on some of the mechanisms underlying the appearance of these microcracks.

Fig. 2 XRD measurement set up under loading.

For more information, see

Kojima et al. (2021) Wood Science and Technology, 55,955-969
https://doi.org/10.1007/s00226-021-01263-z

Kojima et al. (2020) Journal of Materials Science, 55, 5038-5047
https://doi.org/10.1007/s10853-020-04346-7

Future Perspective

To help achieve a carbon-neutral world, much research and development on the structural use of timber is being conducted globally. New technologies are being developed, and high-rise wooden buildings are being constructed that will astonish the world. Timber, as a structural member, will be exposed to a complex stress environment for a long period, conditions which would not be tolerated by conventional wooden buildings. By accumulating basic knowledge on the durability of wood and applying it to the actual building environment, including the evaluation of the durability of building components and joints, I hope to contribute to a new approach to the evaluation of the durability of buildings as a whole while including both traditional and innovative buildings.

In addition to durability, I would like to clarify the various mechanical properties of wood and find effective new uses, thereby helping to build a sustainable, recycling-oriented society that actively and sustainably exploits wood.