Negative Poisson’s ratio: A ubiquitous feature of wood

Abstract

Most materials contract laterally when stretched axially i.e. they have a positive Poisson's ratio. Negative Poisson's ratios (NPR, also auxetic) are largely limited to single crystals or to artificial meta-materials such as honeycombs, foams and composites, which does limit their applications. This meta-study shows that NPR is abundantly present in an extremely common and useful category of natural materials, woods. This effect is so ubiquitous that 87 out of 123 measured hardwood samples and 58 of 62 softwood samples exhibit the property. In wood, NPR occurs predominantly in quite narrow off-axis directions, with values as low as − 3.32. This effect is chiefly attributable to the tubular structure of the wood cells. This suggests that low-cost, large-scale auxetic structural parts can be obtained by cutting low to medium density timber in specific off-axis directions, with potential benefits in a wide range of structural and construction applications.