Microwave Non-Destructive Testing of Wood and Similar Orthotropic Materials

Abstract

A non-destructive method is described to estimate fiber (grain) direction, moisture density, and dry density of an orthotropic material such as wood, from measurements of the complex attenuation of microwaves transmitted through the material. The complex attenuation in an orthotropic material has a tensor character, similar to other tensor quantities such as stress and strain. The same transformation equations and Mohr’s Circle constructions apply to rotations of material axes. Bulk material properties can be identified from the principal attenuations, and the fiber (grain) direction from the principal directions. The orthotropic nature of the material increases the number of independent electrical properties that can be measured and gives useful opportunities for data combination and consistency checking. In a series of microwave attenuation measurements using wood samples, the standard error for identification of grain angle was 0.9° (total measured range =−90° to +90°, for moisture density 3.7 kg/m3 (total measured range = 40–50 kg/m3), and for dry wood density 16 kg/m3 (total measured range = 325–625 kg/m3).