The effect of heat treatment on the mechanical properties of fir (Abies alba Mill) was researched. We selected visually 45 fir boards - samples, nominal dimensions 4000 x 120 x 45 mm, and ranked them by density in 9 classes. Samples were then heat treated in an industrial vacuum process in company Silvaprodukt, with four different end temperatures (170 °C, 190 °C, 210 °C, 230 °C), a specimen from each class was left untreated as a control. The mechanical properties were studied with non - destructive methods, with a frequency response method in transverse and longitudinal direction, and by measuring the speed of ultrasound. With finite element method we made a numerical model of the response to an external mechanical disturbance. The loss of mass of the samples was increasing exponentially with increasing thermal treatment temperature. Modulus of elasticity changed the most in longitudinal direction. With increasing temperature of heat treatment the modulus of elasticity initially grew, but then at the highest temperatures dropped again. Significant trend changes of stiffness in the transverse direction of timber, was due to the high variability not confirmed. Frequency response by transverse vibrations confirmed lowering modulus of elasticity with increasing vibration modes. With increasing difference between the stiffness in each vibration mode we confirmed major structural inhomogeneity in the heat treated fir. The numerical model of the mechanical response showed comparable results to the response of real specimens.
|
