Nanoendentações na parede celular: relações entre propriedades nanomecânicas e o fendilhamento na madeira de Eucalyptus grandis

Abstract

In this work, the relations between mechanical properties of the cell wall and the wood strength to the cleavage were studied, aiming to better understand the formation process of this type of crack, which is very common in Eucalyptus wood. The aim of the thesis was to clarify the relations between the wood strength to the cleavage and mechanical properties of the cell wall, vessels parameters, lignin content and density of the wood. For this, two trees of Eucalyptus grandis were felled at 22 years of age and test specimens for the cleavage test were produced to determine the wood strength to the appearance of this type of crack. From these specimens, tests were carried out to determine the hardness, modulus of elasticity and ductility ratio of the cell wall to evaluate its relation with the strength to the cleavage. Samples were also produced to determine the basic density, mean area of the vessel, percentage area of vessels and lignin content in wood, aiming the fitting of mathematical models that explain the variation of the wood strength to the cleavage. The results showed that higher basic density and lignin content is related with the increase of the wood strength to the cleavage. However, higher mean area of the vessel and percentage area occupied by the vessels on the transversal surface of wood is related with the decrease of its strength to the cleavage. The linear multiple regression model is able to estimate the wood strength to the cleavage as a function of the lignin content, mean area of the vessel and percentage area of vessels. The effective modulus of elasticity of the fibers cell wall is positively related to the strength to the cleavage of the wood. Similar to the effective modulus of elasticity, the hardness of the fibers cell wall has a positive relation with the strength to the cleavage of the wood. The nanomechanical characterization of the cells present in the E. grandis wood showed that it is reliably possible to estimate the cell wall hardness based on the reduced modulus of elasticity.