## 4 Experimental Results  
### Single Regular Defect  
The monitored aluminum plate with a 20 mm diameter circular flat-bottomed hole in the center was made so that its thickness loss was 50%, as shown in Figure 5a. The transmitter transducers were excited one by one, and the receiver transducers recorded all the received signals. Take the 10 th transmitter and receiver transducers as an example. The baseline guided wave signal and the guided  
Figure 4: (**a**) Structure health monitoring (SHM) system diagram. (**b**) Typical time trace from the experiment with chosen time window.  
wave signal passing through the defect are illustrated in Figure 5b. The monitored area was divided into \(80\times 80\) grids. The tomographic images reconstructed using the SIRT and our Equation (12) are shown in Figure 5c,d. From Figure 5c, it can be seen that the spatial resolution of the tomographic reconstruction with the SIRT was relatively low, and the distinct artifacts can be found. On the contrary, from Figure 5d, it can be found that the spatial resolution of the tomographic reconstruction with our Equation (12) was relatively high, and the visible artifacts in the image did not exist.  
Next, the monitored area was divided into \(20\times 20\), \(40\times 40\), \(60\times 60\) and \(80\times 80\) grids respectively. The tomographic images reconstructed using the SIRT are shown in Figure 6a-d. It can be seen that the tomographic images reconstructed using the SIRT had a relatively low spatial resolution. Even if the grid density was increased, both the reconstruction spatial resolutions and the edges of the corrosion area still kept fuzziness. By contrast, Figure 7a-d are the reconstruction images using our method. These tomographic images show that they have relatively high spatial resolutions. With the increasing of the grid density, the edges of corrosion area were gradually distinct. The number of grids with the thickness changing imaged by the SIRT and our method was statistically analyzed, and the statistics result is shown in Table 1. It can be seen that the number of grids calculated by our methods with different grid density was almost the same as the area covered by the actual corrosion ones.  
Figure 5: Tomographic reconstruction images for a 1.5 mm thick aluminum plate with a 20 mm diameter circular flat-bottomed hole. Thickness loss was 50% within the flaw. (**a**) The actual location and size of the flaw, (**b**) the baseline guided wave signal and the guided wave signal passing through the defect, (**c**) the tomographic reconstruction image using the simultaneous iterative reconstruction technique (SIRT), and (**d**) the tomographic reconstruction image using our method.  
Figure 6: Tomographic reconstruction images for a 1.5 mm thick aluminum plate with a 20 mm diameter circular flat-bottomed hole with different grid density using the SIRT. The thickness loss was 50% within the flaw. (**a**) The number of grids was \(20\times 20\), (**b**) the number of grids was \(40\times 40\), (**c**) the number of grids was \(60\times 60\), and (**d**) the number of grids was \(80\times 80\).  
Figure 7: Tomographic reconstruction images for a 1.5 mm thick aluminum plate with a 20 mm diameter circular flat-bottomed hole with different grid density using the proposed method. The thickness loss was 50% within the flaw. (**a**) The number of grids was \(20\times 20\), (**b**) the number of grids was \(40\times 40\), (**c**) the number of grids was \(60\times 60\), and (**d**) the number of grids was \(80\times 80\).