Análise da ocorrência natural de slip planes na parede celular de fibras da madeira de Eucalyptus grandis

Abstract

The slip planes are deformations that arise in the cellular wall of the wood fibers caused by compressive stresses, being responsible for the mechanism of initial structural weakening of the wood. The objective of this work was to analyze the natural occurrence of slip planes in the fibers of Eucalyptus grandis wood in terms of their characterization, distribution in the stem, associations with other characteristics of the wood and presence on the fiber profile. Three 28-year-old Eucalyptus grandis trees were used. Of the stems were discs cut at positions 0, 25, 50, 75 and 100% of commercial height. Each stem segment was accurately cubed and its mean apparent density determined. The relative masses of each quarter of the stem were determined according to the respective volumes and relative density apparent from the segments. Each disc was sampled in the compressed wood (central) and tractioned wood (external) regions of the stem, considering the biodynamic stresses that characterize an erect stem, for microfibrillar angle measurement and determination of slip planes angle (Asp), number of slip planes by millimeter (Nsp/mm) and slip planes index (Isp). The Isp was defined as the percentage expression of the sum of the slip planes heights in relation to the fiber length. To analyze the fiber profile with the presence of slip planes, confocal microscopy was used. It was possible to verify: 1) the base of the stem, which supports larger mass, presented higher Isp; 2) there was reduction in the Isp from the base to the top of the stem; in the compressed wood the reduction was from 20.77 to 7.82%, while in the tractioned wood the reduction was from 17.62 to 6.70%. This tendency of reduction was found for Nsp/mm only in the compressed wood, ranging from 69.23 to 29.48 slip plane/mm; 3) comparing the compressed wood with the tractioned wood, there was variation only in the Asp, with the compressed wood presenting 77,29º and the tractioned wood 76,14º; 4) by observation in 3D images of confocal microscopy, the slip plane region was raised at 75 nm in relation to the surface of the fiber.