Time dependence of Poisson’s effect in wood IV: influence of grain angle

Abstract

Off-axis tensile creep tests were conducted on woods taken from Japanese cypress and Kalopanax by changing the angle of load to the grain direction in the longitudinal–tangential (LT) plane. The dependence of the Poisson’s ratio and trend of the viscoelastic Poisson’s ratio on grain angle were investigated. The Poisson’s ratios were found to reach their extrema when the grain angle was around 30°. Moreover, the Poisson’s ratio in the LT plane was observed to be negative when the grain angle was in the range of 15°–45°. Comparing the experimental results with theoretical values obtained from the theory of orthotropic elasticity, it was revealed that, although the Poisson’s ratio reached an extremum in both cases, the specific values did not match, especially when the angle was between 15° and 45°. Furthermore, the temporal variation of the viscoelastic Poisson’s ratio was found to depend on the grain angle and the measurement plane. It also appeared to be affected by the Poisson’s ratio, showing an increasing tendency above a specific Poisson’s ratio (Japanese cypress: 0.196, Kalopanax: 0.102) and a decreasing tendency below it, regardless of the grain angle and measurement plane. Additionally, the increment in the viscoelastic Poisson’s ratio after 24 h of creep was observed to reach its extremum when the grain angle was around 30°. Finally, by improving the six-element Frandsen–Muszynski viscoelastic model, which simultaneously considers the longitudinal and transverse strains, an eight-element model was presented, and the trend of the viscoelastic Poisson’s ratio was well reproduced by this model.