Study On Diagnosis And Assessment Technology Of Stress Wave Tomography In Old And Famous Trees

Old and famous trees are the cultural heritage of human history, and they are called"living heritage","national treasure", but almost all the old and famous trees have different degrees of decay, hollow, fracture and other hazards, which have harmed old and famous trees. But the lack of effective detection technology and protection measures have caused big losses in China. Therefore, to find a rapid and effective diagnostic technique and assessment method has become the most important prerequisite for the protection of old and famous trees. Stress wave tomography is a kind of new nondestructive testing method. By detecting stress wave transmission time, calculating velocity, transforming matrix and reconstructing tomogram, the internal defects in the trunks would be shown directly by two-dimensional or three-dimensional color tomograms. Application of stress wave tomography to diagnose and assess internal defects of standing trees has achieved some progress in foreign countries, but in China a few researches in defect detection involve only in logs, therefore so far there has not been research reports published about the diagnosis and assessment of old and famous trees. In this paper, with the reputation of"living fossil tree", Hu Yang (Populus euphratica Oliv.) was selected as objective tree for in-depth, systematic study by stress wave tomography technology. The objective of this research is to ultimately use the technology to diagnose and assess old and famous trees in order to provide a scientific basis and advanced technology for the detection, protection and restoration of old and famous trees in China. The main results of this research are as follows:1. Single-path stress wave is a rapid and effective testing method for internal defects of tree trunks. When choosing the average velocity of healthy trees as assessment reference value, the healthy state of tree trunks can be evaluated by single-path stress wave, but it can only judge whether the trunks are healthy or not instead of accurately distinguishing the types of defects. In cross section of healthy tree trunks, with the increase of testing angles the velocity trend in clockwise direction rises first and then falls. Velocity (y) and testing angle (x) regression equation is: y=-29.088x2+349.63x+355.37 ( r =0.994 2 ).2. Stress wave velocity changes with the change of angles between sensors. The increase of angles made velocity change significantly. At 90 o , the stress wave transmission time and velocity did not significantly change with the increase of hollow diameter. At 180 o, the stress wave transmission time was rapidly increasing with the increase of hollow diameter, but in contrast the velocity was, rapidly decreasing. Successful simulation of wavefront transmission of healthy, hollow, cracking and decay cross-section of tree trunks has been done. Wavefront three-dimensional images were reconstructed through Kriging interpolation. The two-dimensional images made at the same time showed high similarity with the actual cross-section. The velocity of normal wood from pith to bark was higher than that of tension wood. Regression equation of velocity (y) and distance from pith to bark (x) is: y =276.71ln(x)+318.82 (r=0.9762). Velocity increased with the increase of tree age, and the correlation analysis showed that velocity and age were logarithmic relation and the correlation was significant.3. Stress wave tomography can successfully simulate the shape of tree trunks. The two-dimensional tomogram can effectively diagnose the decay and hollow of tree trunks. Color tomogram can directly show defects'location, size and shape. Three-dimensional cross section images from stress wave tomography can diagnose defects of tree trunks at different angles, and the test results and the actual situations showed obvious consistency. Decay and hollow were often difficult to be accurately distinguished. When the crack is small, the two -dimensional tomogram can accurately show the crack's position, but its shape and size are different with the actual ones.4. The velocity increases with the decrease of moisture content. Lower than the fibre saturation point, the velocity highly increased. Higher than the fibre saturation point, the velocity's increase was not obvious. The higher the moisture content, the more gently the velocity The impact of moisture content on wave velocity can be ignored when it is high. Increasing the number of sensors can improve detection accuracy in certain extent. Because different number of sensors should be selected for different testing purposes and requirements. Detection accuracy rate (y) and the number of sensors (x) have a significant positive correlation, the regression equation is: y=0.224x~3-3.8353x~2+23.949x+22.791 (x=6, 8,…, 24) .Error rate (y) and the number of sensors (x) have a significant negative correlation, the regression equation is: y=-0.0018x~3+0.0343x~2+0.2352x+0.7928 (x=6, 8,…, 24). Defect areas tested from random distribution, semicircle distribution and uniform distribution did not have significant difference with the actual areas. However, if accurate judgement about the defect size and location is made, uniform distribution should be selected.5. There is similarity between two-dimensional image of hardness and stress wave tomogram, and the positive correlation between velocity values of mesh and gridding hardness.6. 12 sensors are used to test defect area. The regression equation of the defect area ( ln a ') and the actual defect area (ln a ) is: ln a = 0.5018 + 0.9349lna', standard deviation unbiased estimateσ= 0.1567. The existence of decay and hollow in tree trunks made density, bending modulus of elasticity, bending strength and toughness of wood decreased by 11.4%, 9.5% and 14.3%, 22.9%, respectively, which led to serious harm to the strength of trunk, Therefore, strength loss should be the key index in assessing the hazard of trees. When usingt R =0.3 as a threshold of hazard assessment, threshold value range of color line was shown in stress wave tomogram. Hazard of standing trees can be assessed directly and rapidly according to the threshold region size. Wagener, Coder and Mattheck model can be used to effectively assess the hazard of tree trunks, but Mattheck model should be chosen to assess the hazard when the decay or hollow is not located in the centre of tree trunks.