In the case of structural timber, the influence of the cross - sectional size and the length of the specimens on the determination of the dynamic mechanical properties of wood were studied. We selected spruce (Picea abies Karst.) specimens without visible anomalies, having cross section of 42 mm × 42 mm and 42 mm × 84 mm, and length of 4000 mm. The material was conditioned in the laboratory at 20 ° C and 50 % air humidity, and the density of the wood was determined gravimetrically. The dynamic mechanical properties of wood were determined by analyzing the longitudinal vibration of free supported specimens and by the time of flight of the ultrasound. Measurements were performed successively, in 8 steps, by shortening the specimens in a 500 mm interval, to a final dimension of 500 mm. The density of wood varied between 400 kg/m3 and 656 kg/m3, with little variability along an individual specimen (CV = 13.5 %; CV – Coef. of variation). The density of wood had no effect on the sound velocity (vf), with an average at 4564 m/s (CV = 5.8 %), and on ultrasound velocity (vUZ), where it was 5079 m/s (CV = 9.5 %). The sound velocity and the modulus of elasticity (Ef) determined in the longitudinal vibration were independent of the length of the specimens. In several cases, we observed an increase in the ultrasound velocity and consequently the ultrasonic modulus of elasticity (EUZ) with increasing length of the specimens. The ratio vUZ/vf and EUZ/Ef were greater than one, and increased with increasing cross-section and length of the specimens.
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