Abstract:

Objective: To simplify the method of testing Poisson’s ratio of wood and improve its testing accuracy, a cantilever beam 1/3-span patch method is proposed. Method: The 1/3-span patch method for square cantilever beams is proposed for dynamic testing of Poisson’s ratio of wood based on the stress and strain analysis of the first-order bending vibration of the beam. Firstly, for three cantilever beam specimens with square cross-sections in the main direction, naely Picea asperata, Pinus sylvestris and Fagus sylvatica, with length-to-thickness ratios of 8, 10, 12, 16 and 20, the ANSYS 19 modal program block was used to calculate the first-order bending modal stress and strain, and the absolute value of the ratio of the transverse strain to the longitudinal strain, $ -{\varepsilon }_{y}/{\varepsilon }_{x} $ along the central line $ x/l $ of the cantilever beam surface, that is, the curve $ -{\varepsilon }_{y}/{\varepsilon }_{x}-x/l $, and the position of the transverse stress $ {\sigma }_{y} $ = 0 were determined. Secondly, based on the change characteristics of the $ -{\varepsilon }_{y}/{\varepsilon }_{x}-x/l $ curve of the square cross-section cantilever beam, which is a straight line in the range of x/l = 0.2?0.7 and the $ -{\varepsilon }_{y}/{\varepsilon }_{x} $ value on it is equal to the corresponding reference value of the Poisson’s ratio of the wood in the principal direction, a 1/3-span patch method for testing the Poisson’s ratio of wood was proposed (the strain gauges were pasted at l/3 of the distance from the cantilever end of the specimen). Finally, the Poisson’s ratios of Larix gmelinii in the tangential direction (LT) and radial direction (LR), Picea sitchensis in the transverse direction (TR and TL), and laminated veneer lumber (LVL) in the longitudinal and transverse directions were dynamically tested by the 1/3-span patch method. Result: The effectiveness of the 1/3-span patch method for dynamic testing of the Poisson’s ratio of wood was verified by the symmetrically loaded four-point bending beam method and the axial tension method (the Poisson’s ratios of Larix gmelinii, Picea sitchensis and LVL tested by the three methods are very consistent); the consistency of the Poisson’s ratio of wood tested by the 1/3-span patch method and the $ {\sigma }_{y} $ = 0 patch method was demonstrated experimentally (the Poisson’s ratios of Larix gmelinii and Picea sitchensis tested by the 1/3-span patch method were quite consistent with the $ {\sigma }_{y} $ = 0 method). Conclusion: In addition to the advantages of simple test operation and high test accuracy, the 1/3-span patch method of cantilever beam with square cross-sections also uses three square section cantilever beam specimens to dynamically test the Poisson’s ratio of six main directions of wood, such as LT, LR, RT, RL, TR and TL, which is superior to the patch method of $ {\sigma }_{y} $ = 0 of cantilever plate specimens. This method uses square section cantilever wood beam specimens with length-to-thickness ratio of 8?20, which is suitable for testing the six main Poisson’s ratio of wood, and the patch position is independent of the main direction of wood and the specimen size.

Key words: wood, Poisson’s ratio, square section, cantilever beam, 1/3-span patch method, dynamic testing