Abstract
Guided wave transducers, such as electromagnetic acoustic transducers and piezoelectric transducers, generate multimode waves at a given excitation frequency in a cylindrical structure, making it difficult to detect flaws in such structures. To accurately identify the flaws, the transducers must be well designed to suppress the nonaxisymmetric modes. Instead of using the normal mode expansion (NME) method, a spatial Fourier transform (SFT) method is proposed to analyze source influence on the guided wave excitation in a cylindrical structure. A two-dimensional SFT is performed on the spatial distribution function of the surface loading applied to the cylindrical structure. The spatial distribution function is represented in a cylindrical coordinate system. The circumferential-direction SFT is carried out from the angular coordinate to the circumferential orders of the guided wave modes. The axial-direction SFT is carried out from the axial coordinate to the wavenumbers of the guided wave modes. The results of the two-dimensional SFT represent guided wave excitation capabilities for different circumferential orders and wavenumbers. The specific surface loading conditions on the outer surface of a pipe are analyzed to predict source influence on the guided wave excitation. The results are consistent with those obtained using the NME method. Experiments corresponding to the specific surface loading conditions are carried out on a stainless steel pipe. The results confirm the effectiveness of the SFT method.
Highlights
Cylindrical structures, such as rods and pipes, are widely used in petrochemical and transportation industries
To extract the nonaxisymmetric guided wave modes having circumferential orders n ≥ 1, a phase delay of nθk/2π is added to the received signal of each angular position before summation [32]. θk denotes the angular distance from the reference angular position
The proposed two-dimensional spatial Fourier transform (SFT) method can be used to analyze source influence on guided wave excitation in cylindrical structures. The advantage of this method is that only the spatial distribution function of an arbitrary surface loading condition on the cylindrical structure needs to be considered
Summary
Cylindrical structures, such as rods and pipes, are widely used in petrochemical and transportation industries. The ultrasonic guided wave technology has been used for inspecting cylindrical structures [3, 4]. The surface excitation sources formed by these transducers generate multimode waves comprising axisymmetric and nonaxisymmetric modes at the given excitation frequency because of the limitations of the structure, size, and other parameters of the transducers. The dispersion makes data analysis difficult for detection. It is advantageous for defect localization and focusing. To accurately identify the defects and improve the sensitivity, it is often desirable to obtain a single mode guided wave. In addition to transducers with a single element, transducers with multiple elements have been used to improve the guided wave mode and frequency selection performance [16].
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