关键词: 锯材, 低碳钢, 悬臂板, 弯曲振形, 顺纹, 横纹, 泊松比

Abstract: 【Objective】 Based on the first bending mode shape of cantilever slab, this work proposed a method for dynamic testing of lumber Poisson's ratio. In this paper, this method was used to measure Poisson's ratio of Sitka Spruce (Picea sitchensis) lumbers along and across grain on radial section and across grain on transverse section. Based on the testing results, the accuracy is analyzed with testing results of elastic modulus, calculated by substituting the first-order bending frequency, measured with cantilever plate specimen, into cantilever formula. Meanwhile, dynamic testing of mild steel plate was conducted to verify the correctness of the dynamic method of testing for lumber MOE. 【Method】Based on the theory of structural dynamics, free vibration of cantilever specimen of Sitka Spruce lumbers and mild steel can be stimulated by knocking, and the fundamental vibration should be reserved by filtering processing. Additionally, decaying curve of oscillatory waves for transverse and longitudinal strain of fundamental vibration should also be recorded and displayed. Besides, Poisson's ratio can be obtained from the ratio between transverse strain peak and longitudinal strain peak at the same time. 【Result】 It is seen that the positive (negative) peak in oscillatory wave curve for transverse strain is corresponding to that for longitudinal strain, meaning that the phase difference, between oscillatory wave curves for transverse strain and longitudinal strain, is 180°, or that transverse strain and longitudinal strain are in reverse. According to verification test of mild steel, the average value should be taken after calculating the ratios between peak-to-peak values read from the first channel and the second channel in oscillatory wave curve. Finally, the measurement of Poisson's ratio of low-carbon steel should be μ =0.28 (the standard value is 0.25-0.28). The ratio, between μ_LR, (Poisson's ratio of grain at radial section) to μ_RL(Poisson's ratio of stripe at radial section) is 10.6, which means the Poisson's ratio of grain at radial section is one order higher than that of stripe at radial section. Elastic modulus calculated with cantilever formula is 0.7% smaller than the actual one.【Conclusion】 The dynamic method for Poisson's ratio measurement with the first-order bending mode shape of cantilever plate is proved to be feasible, efficient and highly accurate; Poisson's ratio of grain at radial section of Sitka Spruce is one order higher than that of stripe at cross section, which represents the anisotropy of lumber; The elastic modulus is sufficiently accurate by substituting the first-order fixed frequency, measured with the cantilever plate of Sitka Spruce specimen, into cantilever theoretical equation.

Key words: lumber, mild steel, cantilever plate, bending vibration shape, parallel to grain, transverse to grain, Poisson's ratio