Using dielectric orthotropy as an indicator of internal decay of wood members

Abstract

Several material properties of wood are directionally dependent. Mechanical properties such as tension, compression, and bending strength vary greatly for the same piece of wood when tested parallel to the grain (longitudinally), perpendicular to the grain and parallel to the annual rings (tangentially), or perpendicular to both the grain and rings (radially). For this reason, wood is often considered to be a cylindrically orthotropic anisotropic material. The orthotropy extends to dielectric properties of wood. However, in the presence of internal decay, the directional dependency of the dielectric qualities diminishes. By identifying and measuring the loss of directional dependency, the presence and extent of internal decay is evaluated. Ground penetrating radar (GPR) is used as the inspection tool. A bowtie, bistatic, dipole antenna with a center frequency of 2 GHz is used for measurement. The specimens are wood members of sizes typically used in bridge construction. The GPR technique has several advantages over existing non-destructive testing (NDT) techniques: it requires access to only one side of the specimen, no special methods are needed to couple the sensor to the specimen, and it uses a commercially available tool already accepted within the NDT industry. This study is part of a larger study to develop improved NDT tools for timber bridge inspection.

Citation

Senalik, C. Adam; Farber, Benjamin; Wacker, James P.; Wang, Xiping. 2022. Using dielectric orthotropy as an indicator of internal decay of wood members. In: Wang, Xiping; Ross, Robert J., eds., Proceedings: 22nd International Nondestructive Testing and Evaluation of Wood Symposium. May 24-27, 2022. Quebec City, Quebec, Canada. General Technical Report FPL-GTR-290. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory: 261-269.