Canadian Forest Service Publications

A comparison of nanoindentation cell wall hardness and Brinell wood hardness in jack pine (Pinus banksiana Lamb.). 2014. Vincent, M.; Tong, Q.; Terziev, N.; Daniel, G.; Bustos, C.; Gacitua Escobar, W.; Duchesne, I. Wood Sci. Technol. 48:7-22.

Year: 2014

Issued by: Laurentian Forestry Centre

Catalog ID: 35345

Language: English

CFS Availability: PDF (request by e-mail)

Available from the Journal's Web site.
DOI: 10.1007/s00226-013-0580-5

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Nanoindentation is a powerful tool for hardness testing on a very small scale. Since the technique was first introduced for studying wood cell wall mechanics, it has been integrated as an important tool for measuring the modulus of elasticity and hardness of wood cell walls. In this study, hardness measured with nanoindentation (nanohardness) was compared with hardness measured by the standard Brinell test method (Brinell hardness) on jack pine (Pinus banksiana Lamb.) wood. Nanoindentation was performed on both the S2 layer of the secondary cell wall and the middle lamella (ML) of early- and latewood fibers. Four annual growth rings were studied. The influence of growth ring and initial spacing on both measurements was analyzed. The relationship between Brinell hardness, nanoindentation measurements, and average ring density was also studied. Results suggest that Brinell- and nanohardness are controlled by different mechanisms and factors. The location of nanohardness measurements (i.e., S2 layer or ML) also influenced hardness differently. It was concluded that nanomeasurements are not an exact representation of wood mechanical properties conducted at the macro level because of the hierarchical structure of wood. The effect of other factors such as moisture or wood extractive content may also need consideration.

Plain Language Summary

Processing wood into various products – ranging from structural wood to pulp and paper and composite materials – requires a solid understanding of the mechanical properties of trees. The primary mechanical properties studied in this article are the elasticity module and the rupture module. The first is a measurement of wood rigidity when increasingly constrained conditions are applied without causing permanent deformation. The second is a measurement of the maximum force that a piece of wood can withstand before breaking.

These properties largely depend on the hardness of the wood, which itself is dependent on the cell wall composition of the wood fibres. The researchers wanted to determine if there was a relationship between the hardness of the cell wall and the spacing between trees in plantations. To do this, they used nanoindentation, a technique that determines the hardness of the cell walls of wood fibres using microscopic tools. The researchers then compared their results with larger-scale hardness measurements (macroscopic) on a tree slice. The results indicate that at the microscopic level, there is no significant relationship between the hardness of the cell walls and the spacing of trees in plantations. However, at the macroscopic level, the researchers found that increasing the spacing tends to slightly decrease the hardness of the wood. These data demonstrate that the mechanisms responsible for hardness differ depending on the scale used.