Knowing the grain angle of structural wood members relative to the direction of loading is important in estimating their mechanical performance. The strength of a structural wood member can vary greatly depending on the orientation of grain, making the understanding of grain angle an essential requirement for machine grading and for structural analysis. Condition assessment of structural wood members using non-destructive evaluation (NDE) techniques (e.g., microwave/radar, ultrasonic, stress wave, and X-ray) is a major approach for existing wood structures. Among existing NDE techniques, synthetic aperture radar (SAR) imaging offers technical advantages include remote sensing and subsurface sensing. The objective of this paper is to use SAR imaging to quantify the change in grain direction of various size wood dowels relative to the imaging radar. Two wood specimens were produced each 14 in. long with diameters of 1.25 in. and 1.5 in. Each wood specimen was placed vertically inside an anechoic chamber and imaged at different orientations using a 10 GHz SAR system. It was found that the integrated SAR amplitude and amplitude distribution were affected by the grain orientation of the wood specimens. Further analysis was conducted by estimating the area of contour slices of the SAR images taken in the amplitude direction and were used to confirm the findings.
The density of wood is one of the most important physical properties when it comes to understanding its mechanical behavior. The strength of a wood specimen is directly related to the amount of wood material in a given volume, making the accurate determination of density essential for the analysis of wood structures. The use of non-destructive evaluation (NDE) techniques (e.g., microwave/radar, ultrasonic, stress wave, and X-ray) is a powerful approach for condition assessment of existing wood structures. Synthetic aperture radar (SAR) imaging, with its remote and subsurface sensing abilities, provides information about the mechanical properties of wood structures without obstructing their functionality. The objective of this paper is to use SAR imaging to determine the differences in density in a variety of different wood species. Five 14 in.-by-2 in.-by-0.75 in. wood specimens were manufactured. Each wood specimen was imaged vertically inside an anechoic chamber using a 10 GHz SAR system. It was found that SAR amplitude distribution was affected by the density of wood specimens. It was also found that the increase of wood density leads to the increase of contour area of SAR images.
Detection and quantification of moisture content inside wood (timber) is key to ensuring safety and reliability of timber structures. Moisture inside wood attracts insects and fosters the development of fungi to attack the timber, causing significant damages and reducing the load bearing capacity during their design life. The use of non-destructive evaluation (NDE) techniques (e.g., microwave/radar, ultrasonic, stress wave, and X-ray) for condition assessment of timber structures is a good choice. NDE techniques provide information about the level of deterioration and material properties of timber structures without obstructing their functionality. In this study, microwave/radar NDE technique was selected for the characterization of wood at different moisture contents. A 12 in-by-3.5 in-by-1.5 in. white spruce specimen (picea glauca) was imaged at different moisture contents using a 10 GHz synthetic aperture radar (SAR) sensor inside an anechoic chamber. The presence of moisture was found to increase the SAR image amplitude as expected. Additionally, integrated SAR amplitude was found beneficial in modeling the moisture content inside the wood specimen.
Synthetic aperture radar (SAR) imaging of construction materials offers civil engineers an opportunity to better assess the condition of aging civil infrastructures such as reinforced concrete (RC) structures. Corrosion of steel rebar in RC structures is a major problem responsible for their premature failure and unexpected collapse. In this paper, SAR imaging is applied to the quantitative assessment of corroded steel rebar in free space as the first step toward the use of SAR imaging for subsurface sensing of aging RC structures. A 10 GHz stripmap SAR system was used inside an anechoic chamber. The bandwidth of the radar system was 1.5 GHz. Steel rebar specimens were artificially corroded to different levels by regularly applying a mist of 5% NaCl solution for different durations of time in order to simulate the condition of natural corrosion. Two sizes (No. 3 and No. 4) of steel rebar were used in this research. Different orientations of steel rebar were considered. Corrosion level was determined by measuring the mass loss of corroded steel rebar specimens. From our results, feasibility of SAR images for the condition assessment of corroded steel rebar was experimentally demonstrated. It was found that the presence of surface rust on corroded steel rebar reduces the amplitude in SAR images. The SAR image of corroded steel rebar also exhibited a distribution of SAR amplitudes different from the one of intact steel rebar. In addition, it was also found that there is an optimal range for the condition assessment of corroded steel rebar in free space. In our experiment, the optimal range was determined to be 30.4 cm.
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