| With the rapid development of economy and society, wood resources have not been able tomeet the market demand increasingly. Nondestructive testing is an emerging technology forevaluation of wood, and it is of great importance for diagnosis of wood defects and improvingwood utilization. At present, great progress about stress wave nondestructive testing of wood hasbeen made by the scientists over the world. However, a complete stress wave propagation theoryin living trees has not built, and there is no uniform conclusion about this field. In this paper, weanalyze the stress wave propagation in the healthy living trees, and establish a theoretical model ofthe stress wave velocity. Experiments for different species were carried out, and the stress wavetomography was used to verify the theoretical model. The main work of this paper is as follows：1. The mathematical model between the intermediate stress wave transmission velocity andthe direction angle of healthy timber was proposed by mathematical derivation. When the crosssection was approximate to an ideal circle, there was a relation of quadratic function between thestress wave transmission velocity and the direction angle. Meanwhile, the ARBOTOM woodnondestructive testing instrument was also used for experiment research on the healthy gingko,12sensors were distributed around the tree and the stress wave transmission velocity between everytwo sensors was measured. A regression analysis was made on the average transmission velocityin the same direction angle on different sensors, the relation between the direction angle and stresswave transmission velocity was v T/vR0.221. The linear regression model between thedirection angle and stress wave transmission angle had higher fitness, with determinationcoefficient higher than0.95, proving the correctness of the theoretical model.2. In the cross section of different heights of twenty-two healthy trees of various species,different knocking forces were applied for a lot of experiments. The results showed that the treespecies, position of cross section and knocking force etc. would change the stress wavetransmission velocity, but the regression curve between the direction angle and stress wavetransmission velocity had always presented the relation of quadratic function. The fluctuation ofthe quadratic term coefficient K was small, stable between-0.2and0.2. The experiment resultshowed that the model proposed was not influenced by tree species, position of cross section andknocking force. 3. The camphor tree, acacia tree and Chinaberry disc with defect such as decay and crackwere detected, and the experiment result showed that due to the existence of hollowness, decayand crack, the fitted curve between the direction angle and stress wave transmission velocity didnot present the relation of quadratic function. In the defective position, the curve would sinkinwardly with a significant difference from the experiment on healthy trees. Therefore, the stresswave transmission velocity model proposed in this paper had a good guiding significance to thedetection of internal defect of trees. |
