Elastic moduli of Douglas-fir obtained by compression, bending and damped vibration

Douglas-fir as one of the important natural fiber reinforced composites has found wide applications in construction. Understanding the static and dynamic stiffness of this wood helps the design and safety assessment. In this paper, the elastic moduli of Douglas-fir were measured using both static and dynamic testing methods including uniaxial compression, three-point bending and damped vibration. The compression tests delivered a Young's modulus of 4.55 GPa. Under three-point bending, the longitudinal-radial modulus of elasticity was 7.35 GPa and the longitudinal-tangential modulus was 1.63 GPa. The dynamic moduli measured by the damped vibration were higher than the corresponding static values. The longitudinal-radial dynamic modulus of elasticity was found to be 15.3 GPa and the longitudinal-tangential modulus was 9.78 GPa. It is concluded that Douglas-fir is weaker and less stiff in the longitudinal-tangential direction than the longitudinal-radial direction. The microstructure and anatomical features are believed to be the major reasons for such differences. The crack propagation along the radial direction needs to fracture the strong wood fibers. While the crack propagation along the transverse direction is mainly through the debonding of the hemicellulose. The structural analysis and morphology observation support the conclusion drawn from the static and dynamic tests. The wood is much stiffer under dynamic loadings in both longitudinal-radial and longitudinal-tangential directions. The high deformation rate in the vibration is the major contributing factor to the higher modulus obtained from dynamic tests. • Douglas-fir natural composite was tested statically and dynamically. • Elastic moduli were measured by compression, bending and vibration. • The dynamic moduli were higher than the corresponding static values. • Douglas-fir is stronger and stiffer in the longitudinal-radial direction.