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This study demonstrated the applicability of a nondestructive microwave testing method for the long-range inspection of pipe inner surfaces. Experimental verifications were conducted using pipes with lengths of 19–26.5 m, inner diameters of 11, 19, or 39 mm, and made of either brass or austenitic stainless steel. Microwaves were emitted from a probe attached to one of the pipe ends, and the reflections due to full circumferential wall thinning 50 mm in length and 1 mm in depth situated at various distances from the end were measured. The frequency range and the probe characteristics were decided so that microwaves propagating only in TM 01 mode affect signals. After signal processing to compensate for microwave dispersion, the measured signals showed clear reflections caused by wall thinning situated 23.5 m from the pipe end at which microwaves were emitted. This study also discussed the attenuation of microwave reflection with the distance between a probe and wall thinning. The experimental results agreed well with a theoretical model describing the attenuation of microwave propagating inside a pipe, although the model did not consider the reflection of microwaves due to a flaw, the dependence of the reflection on the frequency of microwaves, or mode conversion of the microwave due to the reflection. The agreement revealed that not only frequency but also the inner diameter, electromagnetic property, and surface roughness of a pipe are needed to evaluate the length of the pipe that can be inspected by the method. [ABSTRACT FROM AUTHOR]

• Microwaves in TM 01 mode are useful for detecting multiple thinning defects. • Two defects in a metal pipe were located in a single measurement. • A 1 mm deep flaw does not hamper the detection of other flaws behind that. In this study, the applicability of a nondestructive method using microwaves to the detection of multiple pipe wall thinning defects was investigated. In numerical simulation, a single full circumferential defect was introduced into a straight pipe of 19 mm inner diameter, and the effect of the defect on transmitted waves was investigated. The transmitted energy ratio of the TM 01 mode to the emitted wave was found to be 93% in average even in the presence of a 1-mm depth thinning. After that, experimental measurements were performed using a flanged brass tube with two artificial thinning defects. By applying signal processing to the measured S -parameters, clear reflections from the two defects were observed. By comparing the signals, we confirmed that the first full circumferential defect provides at most 12% amplitude attenuation of the reflected signal from the second defect, which should be related to the distance and size of the defects. [ABSTRACT FROM AUTHOR]

This study investigated the effect of a bend on microwave NDT (non-destructive testing) applied for rapid pipe inspections. The factors affecting the mode conversion of microwaves due to a bend were deducted theoretically and verified through numerical simulation. Both theoretical and numerical results revealed that the mode conversion at a bend was actually determined by the frequency normalized by the cut-off frequency of an arbitrary mode (f / f c), the ratio of the curvature radius to the inner diameter of the pipe (r / D) and the bend angle (α). Subsequent experimental evaluations were performed using four groups of pipes with different inner diameters and five different bends. The effect of a bend was quantitatively evaluated by comparing the reflection signals from pipe wall thinning in a straight pipe with those in a bent pipe. The results showed that the presence of a bend led to a decrease in the reflection signal behind the bend, and the extent of decrease also depended on r / D and α of the bend. Furthermore, the length or the depth of the pipe wall thinning did not noticeably affect the decrease in reflection signal, when the bend's dimensions were certain. [ABSTRACT FROM AUTHOR]

This study proposed a long-range non-destructive pipe inspection approach using linearly-polarized circular TE 11 mode microwaves, enabling rapid detection of both axial and circumferential cracks on the inner surface of a metallic pipe. The applicability of the TE 11 mode microwaves for crack detection was discussed in the light of the currents induced on the inner surface of a pipe due to the propagation of the TE 11 mode microwaves. A dual-bend structured mode converter was designed to propagate linearly-polarized TE 11 mode microwaves inside a pipe with a certain inner diameter, based on the theoretical and numerical analysis of mode conversion due to a bend. A 3.5 GHz working bandwidth of the mode converter was achieved with a conversion efficiency of the TE 11 mode of over 90%. Experimental verification was conducted using stainless-steel pipes with an inner diameter of 23 mm and a total length of 7 m. An axial and a circumferential slit were introduced to simulate cracks. The experimental results confirmed reflections from both the axial and circumferential slits, indicating that the TE 11 mode microwaves are applicable to crack detection. The results also confirmed the correlation between TE 11 mode polarization and its detection sensitivity toward slits with different orientations along the circumferential direction. Furthermore, the axial and circumferential slits situated at different longitudinal and circumferential locations in the pipe were successfully detected by orthogonally deploying the mode converter to change the polarization of the TE 11 mode by 90°. [ABSTRACT FROM AUTHOR]

This paper presents a multivariate probability of detection (POD) analysis method for long-range pipe inspection using microwave non-destructive testing (NDT). The proposed two-dimensional (2D) POD model considers both the size and location of pipe wall thinning (PWT) and is thus able to simultaneously characterize the detectability and detection range. First, the propagation of microwaves in a pipe with PWT was modeled through numerical simulation. The simulated S-parameters (S11) were converted into the time domain and were further processed to correlate the amplitude of the reflection signal with the corresponding PWT depth and location. The simulation results indicate that the logarithmic amplitude of the reflection signal is proportional to the PWT location and logarithmic PWT depth. Therefore, a prior multivariate regression model was established for the above three parameters. Second, an experiment was performed using a 14.5-m-long brass pipe. Twelve short pipes with six different inner diameters were used to emulate PWT and were deployed at eight different locations in the pipe. The experimental results suggest that each PWT can be detected with excellent repeatability, and show consistency with the simulation results. The processed experimental signals were subsequently employed for the regression model of the amplitude of the reflection signal, PWT depth, and location. Finally, a 2D POD model was constructed on the basis of the regression model and a dynamic-static threshold, combining in-pipe microwave signal attenuation with instrumental noises. The POD plot and contour give reasonable interpretations of the detection capability of this testing method against PWTs with different depths, considering the PWT locations. [ABSTRACT FROM AUTHOR]