Abstract: [Objective] Standing trees of different species were selected as samples to study the stress wave propagation velocity pattern in the RL(radial and longitudinal)plane of wood so that the rule of stress wave propagation can be understood well, and the theoretical basis of three dimensional imaging of wood internal structure can be improved.[Method] Firstly, an analytical stress wave velocity model in RL plane of wood was derived. Then eight standing trees of different species(Cinnamomum camphora, Liquidambar formosana, Michelia chapensis, Liriodendron chinensis, Populus adenopoda, Platanus sp., Pinus sp., and Populus alba)at the university arboretum in Zhejiang Agricultural and Forestry University were selected as samples for nondestructive evaluation experiments. Arbotom detector was used to measure the stress wave velocity at different grain angles in the RL plane of specimens. For the healthy samples of standing tree, the regression analysis about the ratio of the velocity v_θ along the direction angle θ and the radial velocity v₀ was finished.[Result] In the RL plane of healthy trees, stress wave velocity increased with direction angle θ becoming larger, and the wave velocity along the radial direction was the smallest. The radial direction angle θ=0. When θ increased gradually, the stress wave velocity became faster and propagated parallel to the grain gradually. For the same direction angle θ, the stress wave velocity v_θ were different in different standing trees. Generally, the relationship between the direction angle θ and the ratio v_θ/v₀ approximated to a quadratic function v_θ/v₀≈kθ²+1(0≤k≤1), and k depended on the physical and mechanical properties of wood. For all the regression models of different samples of standing trees, the coefficient of determination R² was higher than 0.92. The regression results demonstrated the effectiveness of the analytical stress wave velocity model. In order to evaluate the effectiveness of above mentioned model, two other experiments on Platanus sp., with internal decay and log samples were finished. In the RL plane of Platanus sp., some stress wave propagation paths traversed through the decay area, and the wave velocities didn't accord with above mentioned mode for these paths. Similarly, after the test on a healthy Pinus sp., log and a healthy P.alba log, a rectangle hole was dug in the log, and ran the test again with same wave propagation path as before. Finally, based on the proposed stress wave velocity model, a four direction cross-testing method was presented, which could detect the location of internal defect of wood.[Conclusion] In the RL plane of healthy standing trees, the relationship between the direction angle θ and the ratio v_θ/v₀ approximated to a quadratic function v_θ/v₀≈kθ²+1(0≤k≤1). Experimental results on many standing trees of different species demonstrated the effectiveness of the stress wave velocity model. Further experiment and analysis showed that the proposed wave model could be helpful to detect the internal defects of trees.

Key words: stress wave, RL plane, propagation velocity, non-destructive testing, standing trees