Document Type : Research Paper
Authors
1 Associate Professor of Forestry Department, Faculty of Natural Resources, University of Guilan, Iran
2 Professor, Faculty of Natural Resources , University of Guilan, Iran
Abstract
Background and purpose: Dynamic methods (non-destructive) work on the basis of creating tension without failure in the samples and save time and costs for evaluating mechanical properties. Knots or the encapsulated branches are the most important factor in the grading of wood. Therefore, the objectives of this research are: detection of knots by non-destructive methods, measuring the dynamic modulus of elasticity of beech log using stress waves, and the effect of knot size on the speed of stress waves Subsequently, the value of dynamic modulus of elasticity.
Materials and methods: The studied area of Shafarood series 11, which is located in the southern part of the Shafarood watershed, starts at an altitude of 500 meters and continues up to an altitude of 1650 meters. From this area, Five number of beech trees were cut and divided into three equal parts of three meters by transverse cutting, and according to the size of the knots, the properties of density, stress waves velocity Subsequently, the dynamic modulus of elasticity were measured. From the tangential image of the external indicator and the radial image of the internal indicator by using the Digimizer image4 software, the characteristics of each knot such as the diameter and angle of the knot were measured. For non-destructive detection of beech tree knot, ultrasonic tomography, stress wave velocity and CT scan methods were used. In the stress waves velocity method, a Fakopp microsecond timer device was used to measure the time of stress waves velocity. In the radiographic method as well to prepare CT scan images, A digital ground column device with two detectors made in France belonging to a medical clinic was used. To measure the density, 30 cylindrical stems with knots of small, medium and large classes were used, and according to the relationship between density and stress wave velocity, the value of dynamic elasticity modulus was calculated and evaluated. The diameter of the knot along the length of the stem, the density and the dynamic modulus of elasticity of the beech tree stem were measured. To analyze the data at the 95% confidence level, the one-way Anova test was used, and to determine the normality of the data and the homogeneity of the variance, the Kalmograph Smirnov and Leven tests were used. To compare the means, Tukey's test was used by using Spss software.
Results: In the detection of beech tree knot with the three mentioned non-destructive methods, the results show the existence of limitations in each method. As the ultrasonic tomography is not able to detect knots inside the beech tree trunk and the stress wave method also does not detect very small knots. The accuracy of the CT scan method in detecting the knot is high, but the cost of its detection is high. According to the growth stages of the beech tree, factors such as growth speed, branch encapsulation speed and natural pruning are different. The origin of the branch is pith of the stem. As a result, the number and size of the created knots are different along the tree. Therefore, most of the small knots are created in the main trunk of the tree. The diameter of the knot has also increased with the increase in the height of the stem. The knot has increased the density. The density results have shown that the samples with large, medium and small knots have significant differences. Considering that the ultrasonic tomography method is based on the reduction of density, it is not able to detect the knot. But in the radiography method, the parts with higher density are seen in light color and the parts with lower density are seen in darker color. The size of the knot is effective on reducing the speed of stress waves and thus reducing the dynamic modulus of elasticity of wood. In comparison with the samples without knots, the samples with small, medium and large knots reduce the modulus of elasticity by 20%, 50% and 70% respectively.
Conclusion: in general, among the non-destructive methods, only the CT scan method is able to detect small knots and evaluate the clear wood and defects. In comparison with the control sample (without knots), with increasing the size of the knot, the value of the density increases and the stress wave velocity decreases Subsequently, the dynamic elasticity modulus of the samples decreases.
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