Strength and stiffness anisotropy of 3-D printed coarse root analogues for small-scale physical modelling

Plant root–soil mechanical interaction in the application of soil bioengineering such as tree and slope stability has been investigated via centrifuge modelling, utilising root analogues to replicate vegetated soils. Three-dimensional (3-D) printing can be used to model complex root architecture, but the nature of the layer-upon-layer printing process may lead to printed parts of differing tensile behaviour depending on orientation and, consequently, unrealistic simulation of root mechanical reinforcement. This study aimed to assess the strength and stiffness anisotropy of straight root analogues built at varying orientations via three different 3-D printing methods and compare the measured properties with those of real roots. The tensile strength ratios between horizontally- and vertically-printed samples were up to 3.90, 1.27 and 2.57 for fused deposition modelling (FDM), liquid-crystal display (LCD) and Polyjet methods, respectively. Stiffness anisotropy was also more significant in FDM. The relatively higher anisotropy in FDM-printed samples could overestimate the strength and stiffness of most roots in a hypothetical heart-shaped root system, depending on the diameter distribution. Such a physical model may be improved using 45° inclined Polyjet-printed rods.