Auxeticity and Wood

Poisson’s ratio, one of the elastic constants, describes the ratio of lateral strain to axial strain when a material is subjected to stress. The range of Poisson’s ratios greatly varies in terms of material types and their structures. Furthermore, some materials present negative Poisson’s ratios which are attributed to auxeticity. Wood material has six Poisson’s ratios corresponding to the neighboring planes of the essential axes of Longitudinal, Radial, and Tangential. Due to the polar orthotropic nature of wood, six of them are different, and generally reported Poisson’s ratios are positive. Limited studies focused on the auxetic behavior of wood. This study was focused on the auxeticity evaluation of poplar wood by ultrasonic testing over annual ring inclinations (30°, 45°, and 60°). The elasticity (EL, ER, and ET), and shear (GLR, GLT, and GRT) moduli and Poisson’s ratios (μLR, μLT, μRL, μRT, μTL, and μTR) were calculated. According to the results, auxetic behavior was not observed for 30° samples. Furthermore, negative Poisson’s ratios were not seen in μTR and μRT in all inclinations. However, μLT and μTL in 45° and μLR and μRL in 60° presented auxetic behavior. Furthermore, higher than 1 Poisson’s ratio values were also observed, which is not common for wood material. Also, moduli were determined using a simple formula and stiffness tensor. Considerable differences were observed in elasticity moduli (up to -70 % for ET), while shear moduli were almost the same. By the increase in inclination, ultrasonic wave velocities were differently affected in termsof increases, decreases, and oscillations. However, the impact of inclination on velocities, and all elastic constantswere statistically significant. The coefficients of determination between density and Poisson’s ratios were close tozero.