## 5 Conclusions  
A synthetic aperture focusing technique method is proposed for circumferential scanning of plates using guided waves. The SH0 mode is selected for the inspection, which can be effectively generated by the magnetostrictive sandwich transducer (MST). The proposed imaging method is based on the exploding reflector model (ERM) by solving the wave equation in a polar coordinate system and deriving the relationships between guided wave fields at different radii in the frequency domain. Numerical and experimental validation was successfully performed for the circumferential scanning of the defects in an iron plate. The main findings are summarized as follows:  
1. The SH0 mode has the same detection sensitivity for the cross section of the plate with a uniform wave structure along its thickness; it is nondispersive in frequency and has little energy leakage for the inspection of plates contacted with liquids, which can be generated purely at the frequency below the cutoff frequency of the SH1 mode. The SH0 mode is ideal for the inspection of tank floors in service.
2. The MST used for SH0 wave generation can control the wave propagation direction. The main structure of the MST is designed for the SH0 mode at a certain frequency, and the divergence angle can be adjusted with the length of the transducer.
3. The angular bandwidth of the defect spot by the proposed method (1.5-2.5\({}^{\circ}\)) in the numerical simulations are found to be better than those by the traditional delay and sum (DAS) method (3.4-3.8\({}^{\circ}\)) for transducers with different lengths (2-150 mm), both of which have a significant improvement compared with those by the B-scan image (12-29.5\({}^{\circ}\)). The proposed method has better calculation efficiency than the DAS method, particularly for a large amount of data. No additional trailing occurs in the image of the proposed method with the increment of the angular step. In circumferential scanning, there should not be too many cycles of the excitation signal when the testing range is sufficient.
4. By the proposed method, the peak amplitude of the defect spot can be increased, providing a significant improvement for the defect spot compared with those by the B-scan and DAS methods.  
The CSAFT method is expected to be effective for the non-destructive scanning of tank floors in service to minimize the risks and costs of the maintenance operations. For future study, actual oil tank floors installed with accessories will be examined. Defects of different kinds, sizes, and numbers will be inflicted on the tank floor, which will be tested when the oil tank is empty and filled with different kinds of liquids in the field.  
[PERSON] and [PERSON] conceived and designed the models and methods presented; [PERSON], [PERSON], and [PERSON] helped with the conception and experiments; [PERSON] performed the simulations, experiments, and data analysis; [PERSON] and [PERSON] wrote this paper.  
The work was supported by the National Natural Science Foundation of China (Grant Nos. 61271084, 51275454, and U1709216) and the Technique Plans of Zhejiang Province (2017C01042).  
The authors declare no conflict of interest.