Dynamic testing and analysis of Poisson's ratio constants of timber

The objective of this work was to propose a method for dynamic testing of Poisson's ratio of dimension lumber. Based on the first-order bending mode shape of cantilever slab, Sitka Spruce was selected as research object using instruments including four-channel dynamic resistance strain gauge and CRAS vibration and a set of collection-analysis system for dynamic signal. Free vibration of cantilever specimen of Sitka Spruce 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. Obtained from the positive (negative) peak in oscillatory wave curve for transverse strain, Poisson's ratios along grain and across grain on radial section and that across grain on transverse section of Sitka Spruce specimens were measured. Then measured results were applied to analyse the accuracy of calculated modulus of elasticity obtained by plugging the first-order bending frequency measured by cantilever slab into equation of cantilever beam, and this method is proved to be robust and accurate. The method based on first-order bending mode shape of cantilever slab to measure dimension lumber Poisson's ratio is efficient and simple, with good repeatability and high accuracy. Besides, the same method was also applied to test Poisson's ratio of mild steel to validate the correctness of the method on wood. It is observed 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 degrees, or that transverse strain and longitudinal strain are reversed. 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. The results show that the Poisson's ratio along grain of Sitka Spruce specimen is one order higher than that across grain, which indicates the anisotropy of wood. The test results validated that the elastic modulus of the lumber can be accurately derived by substituting the first-order fixed frequency, measured with the cantilever plate of Sitka Spruce specimen, into the cantilever theoretical equation. This research can provide reference to practical measurement and assessment of mechanical properties of wood and steel.