Relações entre atributos de qualidade físico-ultraestruturais da madeira de Tectona grandis e suas propriedades mecânicas

Abstract

The physical-ultrastructural characteristics of wood are normally considered attributes with variable incidence or with little-known effects on the mechanical properties of different species and their application in construction. Given this situation, a detailed study of the characteristics of the wood and its effects on its mechanical behavior becomes necessary. Thus, the objective of this study was to investigate the behavior and relationships of basic density (Db), microfibrillar angle (MFA) and occurrence of slip planes per millimeter (Qsp/mm) in the cell wall, with mechanical properties of Tectona grandis wood. For this purpose, wood samples from the internal, intermediate and external positions of the base of the stem of five 37-year-old trees from a seed plantation with a spacing of 3 x 2 m located in Belo Oriente - Minas Gerais State, Brazil were used. From the logs, central boards of ten centimeters thick were cut, aiming to obtain test specimens (CP) to evaluate the stiffness in compression parallel to the fibers (MOEc), in static bending (MOEf) and dynamic bending by the transverse vibration method (MOEd). Samples were also used to determine Db, Qsp/mm and AMF, in the last two, using polarized light microscopy on 30 fibers per sample. Additionally, slip planes were induced in static bending CPs under fixed end conditions, applying axial compressive pre-stressing at 30, 55 and 80% of the maximum resistance (RC) in order to obtain controlled data for analysis of their association with the mechanical properties. Average values of Db, AMF, Qsp/mm, MOEf, MOEd, MOEc and RC were, respectively, 0.52 g.cm-3, 15.13°, 2.23 QSP/mm, 12366 MPa, 14990 MPa, 9436 MPa, and 55.54 MPa. 40.99 and 36.59% of the variation of MOEf was explained, respectively and significantly, by Db and AMF; 17.16% and 10.39% of the variation of MOEd by AMF and Qsp/mm; 24.77 and 31.06% of the variation of MOEc by AMF and Qsp/mm; and 38.28 and 22.10% of the variation of RC by Db and Qsp/mm. A decrease in Db, AMF and Qsp/mm was determined in the pith-bark direction and a significant influence of the physical-ultrastructural characteristics on stiffness, under different loads. The AMF and Qsp/mm influenced, respectively, the MOEc and MOEd negatively; Db and AMF influenced the MOEf positively and negatively. While Db and Qsp/mm influenced RC positively and negatively. The increase in compressive pre-tensioning parallel to the fibers caused an increase in Qsp/mm; both pre-tensioning and slip planes negatively influenced MOEf and MOEd.