Abstract: Comparison between flexural wood strength values obtained from three-point and four-point bending tests in wood were only reported in few previous works, but remains unclear. Thus, the aim of this study was to determine the relationship between wood strength (MOR) obtained by 4-point and 3-point bending tests in Eucalyptus. Two or three specimens were cut from the same scantling (thus considered as twin) and submitted to flexural vibration tests. Then 190 wood specimens were tested by 4-point bending tests and 138 twin specimens were submitted to 3-point bending tests. Wood strength determined from 3-point bending test presented significantly higher mean value (76,8 MPa) than that from 4-point bending test (73 MPa). A linear regression for converting MOR from 3-point to 4-point bending test was proposed for these Eucalyptus specimens: MOR4p = 0,889 × MOR3p + 5,14 in MPa (R²=0,74). The correlation of MOR in 3-point bending with density was higher (R²=0,57) than in 4-point bending (R²=0,45). The correlations between dynamic elastic moduli and moduli of rupture were similar (circa R²=0,57 for MOR3p and R²=0,65 for MOR4p).

Estudar a influência de um defeito sobre o comportamento dinâmico da madeira, a fim de detectar as heterogeneidades locais, é de grande importância em ensaios não destrutivos. As heterogeneidades naturais da madeira são orientadas em um volume. Entretanto, modelos unidimensionais ainda são utilizados para a caracterização dinâmica de vigas de madeira. Objetivou-se, neste estudo, investigar experimentalmente os efeitos da orientação e da posição de um defeito artificial nas frequências de vibração de flexão. Diferentes lotes de amostras de Fagus orientalis foram perfurados na direção radial, em cinco posições, ao longo do comprimento da amostra. Os ensaios dinâmicos em vibração de flexão livre foram realizados nas amostras antes e depois da perfuração nos planos de flexão longitudinal-radial (LR) e longitudinal-tangencial (LT). O comportamento em vibração de flexão livre foi diferente em função da posição e da orientação da heterogeneidade. Quando o eixo de perfuração estava no plano de flexão (LR), o enfraquecimento das frequências foi máximo, no local de um antinode de vibração. O deslocamento de frequência, apresentou comportamento contrário, ou seja, foi máximo na zona de um nó de vibração, quando o eixo de perfuração era ortogonal em relação ao plano de flexão (LT).

Studying the influence of defect on the dynamic behavior of wood in order to detect the local heterogeneities is of great importance in non-destructive testing of wood. The natural heterogeneities in wood are oriented in a volume. However, one-dimensional models are still used in dynamic characterization of wooden beams. The aim of this study was to experimentally investigate the effects of the orientation and position of an artificial defect on the flexural vibration frequencies. Different batches of Fagus orientalis specimens were drilled in the radial direction at five positions along the specimen. Dynamic tests in free flexural vibration were performed on the specimens before and after drilling both in the longitudinal-radial (LR) and longitudinal-tangential (LT) bending plan. The behavior in free flexural vibration was found to be different depending on the position and orientation of heterogeneity. When the drilling axis lies in the bending plane (LR), the weakening of frequency was maximal at the location of an antinode of vibration. On the contrary, the frequency offset was maximal in the place of a vibration node when the drilling axis was orthogonal to the bending plane (LT).

This work is part of a program that aims at studying the burr wood of thuja (Tetraclinis articulata). The goal of this work is to identify material symmetries of burr wood to improve its machining. To have a sufficient number of data and to limit the variability between samples, an ultrasonic experimental device, in direct contact on spherical samples, has been developed and improved. Until now, the geometry used in direct contact ultrasonic methods was either cubic or polyhedral allowing to obtain, on the same sample, 3 (cube) to 13 (polyhedron) measurements or usable data. By choosing a reasonable angular gap, the spherical geometry allows the ultrasonic velocity to be measured in 133 different directions on the same specimen. We present here the adaptation and development of the ultrasonic experimental device and results obtained on (i) aluminum chosen as a reference material, (ii) beech wood and (iii) burr wood of thuja.