Effect of anisotropy on acoustoelastic birefringence in wood.

The ultrasonic velocity of shear waves propagating through radial direction of a wood plate specimen, transversely to the loading direction, was measured. By rotating an ultrasonic sensor, the oscillation direction of the shear waves was varied with respect to the wood plate axis and loading direction. The relationship between shear wave velocity and oscillation direction was examined to discuss the effect of anisotropy on the acoustoelastic birefringence in wood. The results obtained were summarized as follows. When the oscillation direction of the shear wave corresponded to the tangential direction of the wood specimen regardless of the stress direction, shear wave velocity decreased markedly and the relationship between shear wave velocity and rotation angle tended to become discontinuous. That is, when the shear waves oscillated in the anisotropic axis of the wood, the shear wave velocity peaked unlike in the case of oscillation in the stress direction. In an isotropic material (acrylic, aluminum 5052), on the contrary, when the shear waves oscillated in the stress direction of the specimen, the shear wave velocity peaked regardless of the main-axis direction of the specimen. On the basis of the discussion of these results, the ultrasonic shear wave propagating in wood under stress is confirmed to be polarized in the anisotropic axis of the wood.

Near-infrared spectroscopic study of the physical and mechanical properties of wood with meso- and micro-scale anatomical observation.

Estimation of the density along with the tensile strength of wood within both the elastic and plastic deformation ranges, represented as modulus of elasticity (MOE) and ultimate tensile stress (UTS), respectively, were performed using near-infrared (NIR) spectroscopy. A partial least squares (PLS) analysis was applied to the measurements of density, MOE, and UTS, and resulted in a high accuracy of prediction, independent of wood species. The correlation coefficient between the NIR spectra and criterion variables, and the regression vector resulting from the PLS analysis, suggested that the characteristic absorption bands were strongly related to the predictability of each property. In the case of softwood, absorption bands due to intra-molecular hydrogen-bonded OH groups in the crystalline regions of cellulose, which are oriented preferentially in a direction parallel to the cellulose chain, might strongly affect the tensile strength of softwood. Hardwoods have much more complex and variable structures than softwoods; therefore, it was supposed that the key factor governing the tensile strength in hardwood would be the interaction between the three principal constituents (i. e., cellulose, hemicellulose, and lignin) of wood.