Investigation of mechanical behaviours of Eucalyptus nitens timber

This research focuses on the investigation of the mechanical properties of plantation Eucalyptus nitens timber and the understanding of its underlying failure mechanism above and below the fibre saturation point both experimentally and numerically. Fast-growing eucalypts are now considered potential building materials; one such re-source is plantation Eucalyptus nitens. To use such a resource as structural or engineered timber, that is to establish design codes and specifications, its compressive and bending strengths need to be studied as they are two important mechanical properties for structural elements in buildings, bridges and decks. For timber members when sub-merged in water or immersed in soil near the river but above the water table, water can be drawn into the wood by capillary action and lead to moisture contents (MC) above fibre saturation point, FSP, which is normally at MC of 25 - 30%. In this study, com-pression and four-point bending experiments were undertaken with a Universal Testing Machine to examine for the first time the anisotropic and/or nonlinear mechanical behaviour of fibre-managed plantation Eucalyptus nitens (E. nitens) samples for low and high MC. Here, low MC means that the MC is less than 15%, which is the normal situation for E. nitens timber as used in the building industry, and high MC means that the MC is above FSP, which is the extreme situation for E. nitens in structural applications when exposed to water. As the effect of the moisture dependent material properties are very difficult to examine separately using an experimental approach, sophisticated methods, such as finite ele-ment method (FEM) techniques combined with constitutive modelling, are needed to predict ultimate strength more accurately and to assess failure mechanisms in timber under complex loading situations. A newly developed constitutive model extended from an existing constitutive model, namely, a moisture-dependent anisotropic elasto-plasticity model, was proposed in order to display more realistically the short-term de-formation and more accurately the failure mechanisms in E. nitens timber with different moisture contents and varying grain angles. In this model, a criterion which combined the Hill yield criterion with the Drucker–Prager yield criterion was introduced to exam the anisotropic ultimate capacity of wood with consideration of differences in tension and compression. Variation of material properties of E. nitens timber with varying moisture contents is included in a new moisture-dependent anisotropic elasto-plasticity model in order to predict the short-term anisotropic material behaviour of E. nitens timber more accurately when moisture content changes. All these parameters can be derived from experimental data. A UMAT (User-defined mechanical material behaviour in ABAQUS) subroutine within the commercial software ABAQUS was coded to execute the constitutive model for compression, tension and bending. The observed agreement between numerical predictions and experimental investigations shows that the developed constitutive model can be used for E. nitens timber with varying moisture contents. The underlying anisotropic behaviour of E. nitens timber was also examined numerically. The key findings of this research were that fibre-managed plantation E. nitens timber was highly anisotropic with respect to grain angle and its ductility increased as MC increased. The E. nitens timber with high moisture content exhibited considerably larger deflections at lower maximum loads, while that with low moisture content showed quite abrupt failures at relatively higher ultimate loads. For both tension and compression, stiffness and strength were dependent on moisture content and loading direction, while the failure mode depended mainly on loading direction. That the anisotropy of E. nitens timber was dependent on moisture content sensitivity was proven both numerically and experimentally. The yield surface of E. nitens timber enlarged with a decrease in moisture content, while different failure strengths under tension and compression were found. Compared with moisture content, grain angle had a stronger effect on the anisotropic behaviour of E. nitens timber due to a difference in the stresses state. At a grain angle of 0°, the normal stress along the grain was dominant compared with the corresponding shear stress and the normal stress across the grain. At loading directions of 30°, 45° and 60°, shear stresses played an important role, and for 90°, the effect of normal stress across the grain was significant. Parametric investigation showed that Young’s modulus parallel to the grain and yield stress affected the stiffness and strength significantly when loads were at grain angles between 0° and 15°. Their effects weakened as the grain angle increased from 15° to 45° with almost no effect on compressive strength after 45°. The effect of shear modulus and shear yield stress along wood fibre became the largest contributing factor from 15° to 30°, while after 45° the radial Young’s modulus and yield stress took a dominant role in determining stiffness and strength, respectively, compared with the effects of longitudinal Young’s modulus and shear modulus. The risk of damage to an E. nitens member is expected to increase in high moisture conditions. The moisture modification factors of Eucalyptus timber at mean level are higher than those of the traditional construction material, Pinus radiata, implying that E. nitens was promising as a material to be used for compressive or bending members in the construction industry, especially in water saturated conditions, for example, in wood piling foundations of buildings near the coast for a long time. This research will provide basic data for E. nitens in structural applications, and the approaches introduced can be used in future research for assessing the vulnerability of E. nitens members in normal or fully water-saturated states for building and structural applications. The moisture dependent composite plastic constitutive model, by providing results for a comprehensive study of anisotropic plasticity material behaviour incorporating anisotropic and moisture effects, overcame the limitations of the von Mises model and the Hill model with nonlinear isotropic hardening, as they were not always appropriate for E. nitens timber. For example, experiments found that the load deformation behaviour changed from unsteady-state “softening” behaviour at compression parallel to the grain to steady-state “hardening” behaviour at compression perpendicular to the grain. The numerical investigation into the short-term response of E. nitens under compression, tension and bending with high and low moisture contents, compared with the experimental data, demonstrated acceptable agreement, showing that the constitutive model works well for Eucalyptus wood