Non-destructive Testing of Timber Using Near-field Measurements at Microwave Frequencies

Abstract

Timber is widely used in many modern structures. Continuous screening of wood for potential problem areas or detecting internal conditions is necessary for quality reasons. The industry commonly uses Non-destructive Testing (NDT) to evaluate wood properties for defects and discontinuities without causing damage or altering the original wood structure. This Ph.D. research focused on non-destructive testing using electromagnetic radiation. The proposed technique uses near-field measurements using an inward-directed cavity-backed slot antenna placed on the wood surface. Electromagnetic signals at microwave frequencies are used as a diagnostic tool for the non-destructive testing of structural timber beams. A narrative literature review was conducted to provide information about the uniformity in natural timber in situ. Defects and cracks in dried natural timber (relative permittivity 2–5) may cause structural weakness and enhanced warping in structural beams. Cavity-backed slot antennas were used to detect defects and cracks for a Pinewood beam. Microwave reflection (S11) and transmission (S21) measurements were recorded by moving the antenna on the kiln-dried wood surface. Measurements showed the variations caused by imperfections and defects in the wood. The kiln-dried Pinewood sample was saturated with water to increase its moisture content from 17% to 138%. Both parallel and perpendicular E-field measurements showed a difference of more than 15 dB above an open saw-cut slot in the water-saturated beam. Since wood is an anisotropic material of biological origin, a study was conducted to report the effect of annual rings on wood anisotropy. Near-field microwave scanning was measured for Pine, Hemlock, Red Cedar, and Hardwood laminated wood beams. The effect of annual rings in guiding the transmitted signal through wood fibers was observed using transmission measurements. Microwave guidance in transmission disappeared with increasing moisture content of the Pinewood sample. An enhanced antenna design with the feed port directly behind the rectangular slot allows maximum E-field coupling to the wood. It provides more space for electronics inside the aluminum box. The E-field in a rectangular wood piece was calculated using CST simulations at the resonant frequency of 7.4 GHz. The wood material was modeled as isotropic with a relative permittivity of 2 and low conductivity (10-6 S/m). A comparison between simulations and a lossless spherical wave calculation showed a significant change in E-field in the very near-field region of the antenna at distances smaller than 4 mm. Also, a numerical study was conducted to evaluate the Specific Absorption Rate (SAR) distribution in CST. The simulation explored the radiation pattern around an open cut in the timber. It was found that the attenuation followed closely to an inverse square law at the microwave frequency. Finally, the study explored whether an automated microwave system could detect anomalies, so that commercial devices could measure the reflection coefficient and hence define timber characteristics. The study evaluated a UHF Scalar Network Analyzer (SNA) for bolt detection. The electronics placed inside an aluminium box with a slot aperture generated a microwave signal to determine the reflection coefficient (S11). The SNA circuit detected bolts and cavities within a 30 mm range from the wood surface. The study provides an accurate, fast, low-cost structural-wood testing method using cavity-backed slot antennas for transmission and reflection measurements. The microwave measurements using the SNA, offer a simple, single-frequency non-destructive testing method of structural timber in situ when one or more plane faces are accessible for direct antenna contact.