# Algorithm Development for the Non-Destructive Testing of Structural Damage
## 4 Experimental Setup
### Microwave Imaging for Damage Detection  
The measurement system consisted of a computerized imaging system, including a millimeter-wave reflectometer with a single frequency oscillator, isolator, magic tee, detector, variable short, the OEWA, and a scanning mechanism providing a raster scan of the reflectometer over the sample under test. The open-ended flange-mounted rectangular waveguide probe was used as an open-ended waveguide antenna (OEWA). The OEWAs are robust, reliable, efficient, and relatively simple antennas used for crack detection [10; 21; 22; 23]. The OEWA irradiated the specimen under testing, and picked up the signal reflected from the specimen. This reflected signal was combined with the reference signal in the magic-tee, and the combined data was sent to the detector; the computer recorded its output voltage for every data point over the plane for each specimen. The images were generated through the recorded data in Matlab software, and processed with the proposed image processing algorithms. Figure 3 shows the measurement setup of this study.  
To highlight the utility of the single-frequency reflectometer with the OEWA, along with the proposed image processing approaches for imaging different structural materials, various samples were prepared and imaged, as follows.  
* Steel Plate  
The first specimen was a section of a 12 mm-thick A-36 steel plate with a few through-fatigue cracks that were created during cyclic loading. A W-band open-ended flange-mounted rectangular waveguide probe with aperture dimension of 2.54 mm \(\times\) 1.27 mm, and with a square (19 mm \(\times\) 19 mm) flange was used in this investigation. Two scanned areas, with dimensions of 50 mm \(\times\) 70 mm and 30 mm \(\times\) 30 mm, included cracks that were obtained at a standoff distance of d \(\sim\) 0 mm, d \(\sim\) 0.8 mm, and d \(\sim\) 1.7 mm.  
Figure 3: Measurement setup.  
Figure 2: Bending test set up: (**a**) concrete, (**b**) RC beam.  
* Composite Material  
The second specimen consisted of six equally thick layers of construction foam with dimensions of 240 mm by 240 mm by 90 mm, as shown in Figure 4a. Four metal discs (each with a diameter of 25 mm and a thickness of 2 mm), in two groups of two, were placed at interfaces between layers 1-2 and 5-6, respectively, as shown in Figure 4. Sample 1 was scanned with a 3 mm step size (along the x- and y-axes), while the complex reflection coefficient was measured at a frequency of 11.5 GHz (X-band) and a 30 mm standoff distance (the distance between the OEWA aperture and the top surface of Sample 1).  
* Aluminum Plate  
The third specimen consisted of an aluminum plate with dimensions of 150 mm \(\times\) 60 mm \(\times\) 10 mm, and was covered by a rubber layer. Two notches were made on the surface of the plate, both with a length of 15 mm and a depth of 1 mm, and with widths of 1 mm and 0.5 mm in notch 1 and notch 2, respectively, as shown in Figure 5. Two scanned areas were made, with dimensions of 40 mm \(\times\) 20 mm and 100 mm \(\times\) 40 mm, including notch 1, Figure 5a, and notches 1 and 2, Figure 5b, respectively. The step sizes (sampling) of 1 mm and 0.5 mm along the length and width of the notches, respectively, were used in this investigation.