The ultrasonic velocity and elastic stiffness constant of rubber wood were studied using ultrasonic technique in three main symmetry axes and an angle rotating from the symmetry axes. The effects of moisture content were also determined in each case. A strong correlation was found between the velocity and the moisture content. As the moisture content increased, the velocity and the stiffness constant were found to decrease. The longitudinal direction showed the highest velocity and hence the elastic stiffness constant. Linear regression equations were obtained between velocity and grain angle with R 2 from 0.86 to 0.99.
Dielectric constant and dielectric loss factor of rubber wood have been studied at different moisture content, grain direction and frequency. Different dielectric dispersion mechanisms are also observed at different ranges of moisture content. The moisture content above the fiber saturation point does not contribute much to the dielectric properties following a single dielectric mechanism. But the moisture content below the fiber saturation point shows considerable effect on the variations of dielectric properties having different dispersion processes at different moisture content ranges. Based on the shape of the curves, five different moisture content ranges have been identified such as 1) 25 % and above, 2) 18-25 %, 3) 11-17 %, 4) 5-10 % and 5) below 5 %. It may be possible to explain all these dispersion processes by means of dielectric mechanism for dipole, quasi-dc and diffusive processes. Longitudinal direction shows a higher dielectric constant when compared to radial and tangential directions in ovendry condition. This dielectric anisotropy may be attributed to the microscopic, macroscopic and molecular structures of wood.
Ultrasonic scanning experiments were conducted on two species of pallet deckboards using rolling transducers in a pitch-catch arrangement. Sound and unsound knots, cross grain, bark pockets, holes, splits, decay, and wane were characterized using several ultrasound parameters. Almost all parameters displayed sensitivity to defects distinctly from clear wood regions-being greatest for unsound knots, bark pockets, decay, holes, splits, and less for sound knots and cross grain. This study supports our conjecture that on-line inspection of wooden pallet parts is possible using rollingtransducer ultrasonic inspection.
Summary
Dielectric properties from 1 to 18 GHz of rubber wood are modeled using generalized mixture equations and also with equations proposed by Weiner, Kraszewski, Looyenga and Landou, Lichtenecker. Dielectric properties were measured with an open-ended coaxial line-sensor in three structural directions longitudinal, radial and tangential and at different moisture contents. The dielectric constants were predicted well by the Weiner model for all structural grain directions and it was found that the degree of binding decreases with increasing frequency. However, the Weiner model cannot be used for predicting the dielectric loss factor at frequencies below 3 GHz. This may be due to the high conductive loss in this frequency region. The lower value of the exponents in generalized mixture equation was found suitable for fitting the experimental data as well as the Kraszewski equation. Values predicted by Lichtenecker equations are in well agreement with the experimental data at higher microwave frequencies. The prediction of dielectric loss factor using Kraszewski, Looyenga equations were not possible at frequencies below 3 GHz since it is dominated by conductive loss. Above 3 GHz, it was well predicted by Kraszewski and Looyenga equations.
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