## 4 Results and Analysis
### Enhancement Effect of VCF Weighting with Different Focus Depths  
The defect at the buried depth of 90 mm is selected to study the influence of focus depth with VCF weighting. The selected region is in a green rectangle, as shown in Figure 8. Both SNR of cracks at different focusing depths with DAS and DAS_VCF algorithms are calculated and compared with the SNR of cracks of the buried depth at 70 mm. The SNR curve is drawn, as shown in Figure 9.  
As shown in Figure 9, the variation trend between the SNR value of crack at buried depth 90 mm and 70 mm is the same, and the SNR value increase with the focus depth  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Algorithm** & \begin{tabular}{c} **SNR (dB)** \\ **70 mm** \\ \end{tabular} & \begin{tabular}{c} **60 mm** \\ **60 mm** \\ \end{tabular} & \begin{tabular}{c} **Focus Depth** \\ **50 mm** \\ \end{tabular} & \
\begin{tabular}{c} **40 mm** \\ \end{tabular} &
\begin{tabular}{c} **30 mm** \\ \end{tabular} \\ \hline DAS & 12.99 & 13.69 & 11.90 & 11.77 & 11.24 \\ DAS\_VCF & 23.76 & 25.36 & 23.16 & 22.58 & 19.03 \\ \hline \hline \end{tabular}
\end{table}
Table 4: SNR of cracks with DAS and DAS_VCF algorithms.  
Figure 9: Comparison of SNR curve for defects with a buried depth of 70 mm and 90 mm.  
increasing. With the VCF weighting, the SNR value of DAS_VCF is better than that of DAS. During the propagation processing, due to the beam's energy attenuation being serious at the large buried depth, makes the amplitude of defect echo received by the transducer is low and results in the SNR value of crack at the buried depth of 90 mm usually being lower than that of 70 mm in DAS images. Moreover, due to the focus region of the sound beam does not cover the defect by 90 mm, the phase consistency of defect echoes is dispersed. Hence, the VCF weighting effect of cracks at the buried depth of 90 mm is half that of the cracks at the buried depth of 70 mm.  
Unlike the TFM and CPWC, sector scan imaging performs the test by the focusing sound beam. The focusing effect of the actual physical focus point is essentially different from that of the post-processed virtual physical focus of TFM and CPWC. In addition, active focusing is the signal acquisition strategy of sector scan imaging. Therefore, compared with the signal acquisition strategy of plane wave and full matrix capture, the coherence effect of defects is different in sector scan imaging. In addition, extracting instantaneous phases before and after receiving delayed processing will also lead to different phase coherence effects. It can be determined that the artificial groove of bolt shank cannot be detected, or even sidewall images of the bolt shank cannot be observed with TFM and CPWC under the same experimental conditions. Our research is only on sector scan imaging based on VCF weighting. The primary purpose is to improve the resolution and signal\(-\)to\(-\)noise ratio of traditional phased array sector scan imaging. Furthermore, this paper has analyzed the application of VCF factors in sector scan imaging. It hopes to enlighten the readers who study the imaging of such complex components, and more readers can recognize the application prospect of sector scan imaging based on VCF weighting.