Abstract
Full-matrix capture (FMC)-based ultrasonic imaging provides good sensitivity to small defects in non-destructive testing and has gradually become a mainstream research topic. Many corresponding algorithms have been developed, e.g., the total focusing method (TFM). However, the efficiency of the TFM is limited, especially in multi-layered structures. Although the appearance of wavenumber algorithms, such as extended phase-shift migration (EPSM) methods, has improved imaging efficiency, these methods cannot be applied to cases with oblique incidence. Therefore, a modified wavenumber method for full-matrix imaging of multi-layered structures with oblique array incidence is proposed. This method performs a coordinate rotation in the frequency domain to adapt it to the oblique incidence. It then utilizes wave-field extrapolation to migrate the transmitting and receiving wave field to each imaging line, and a correlation imaging condition is used to reconstruct a total focused image. The proposed method can deal with any incident angle without precision loss. Moreover, it inherits the computational efficiency advantages of the wavenumber algorithms. The simulation and experimental results show that the proposed method performs better in terms of accuracy and efficiency than the TFM. Specifically, it is nearly 60 times faster than the TFM when processing an FMC dataset with a size of 4096 × 64 × 64.
Highlights
Complex structures such as welds, joints, and pipe bends are critical components in massive buildings, bridges, and oil pipelines, where internal defects pose considerable threats [1,2,3,4]
The proposed method introduces an accurate coordinate transformation relation, so that it can deal with any incident angle without precision loss. It can be used in many applications, such as immersion detection of large objects with slightly curved surfaces and oblique incidence inspection of welds with an angled wedge
The proposed algorithm provides an accurate method for compensating for the oblique incidence of the linear array and achieving oblique incidence full-matrix imaging with a significant efficiency advantage compared with the time-domain total focusing method (TFM)
Summary
Complex structures such as welds, joints, and pipe bends are critical components in massive buildings, bridges, and oil pipelines, where internal defects pose considerable threats [1,2,3,4]. Complex structures such as welds and joints usually require oblique incidence detection to obtain excellent inspection coverage [20,21] In this situation, the TFM needs to use the Fermat principle, calculating the refraction points at each interface to accurately estimate the wave propagation time in the multi-layered medium [18]. The proposed method performs a coordinate rotation in the frequency domain to extrapolate the original wave field, which is parallel to the linear array, to the virtual measurement line that is parallel to the object’s surface Both transmission and reception wave-field extrapolation are performed in the rotated coordinate system, and an imaging condition is applied to obtain a total focused image of the multi-layered structure.
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