Detection and differentiation of the beech tree knot (Fagus orientalis Libsky) by non-destructive stress wave method

The knot is the branch impact on the tree's trunk which reduces the quality of the wood. Knots or occlusion branches are the most important factor in the grading of woods. The purpose of this research is to detect and distinguish the size of the knot in the beech tree stem by non-destructive stress wave method. In this research, five one-meter stems were examined and according to the knot size, the stems were classified into four parts without knots, with small, medium and large knots. By using the FAKOPP microsecond timer device, stress waves were passed along the length of the stem and the width of the knots in the four parts of the stem. The amount of the stress waves velocity were calculated according to the transit time of the stress waves and also percentage of the Relative Velocity decrease of Stress waves were calculated. Analysis variance of the data showed between the four parts there is a significant difference at the 95% confidence level. The results showed that the size of knot has effective on the Relative Velocity decrease of Stress waves in the beech tree stem. As the knot gets bigger, its angle decreases with the main stem axis. consequently more length is placed in the stem, and the speed of its occlusion increases. In this method,very small knots whose percentage of the Relative Velocity decrease of Stress waves was less than 10% were not detected. Therefore, for small, medium and large knots, the range of Relative Velocity decrease of Stress waves were obtained 10-20%, 20-45% and more than 45%, respectively.