Detection of a defect on the back of a pipe by non-contact remote measurements

Pipe inspection is generally executed with ultrasonic pulse echo testing where a small range of pipe wall under an ultrasonic transducer can be evaluated in one measurement. Costly and laborious point-by-point testing is required if a whole range of a pipe should be inspected. The author has investigated fast defect imaging for a plate-like structure using a scanning laser source (SLS) technique as an efficient defect inspection technique. Although the imaging technique is feasible in non-contact remote measurements, only a plate cross-section under the laser irradiation surface can be evaluated. This study describes detection of wall thinning on the back of a pipe using resonance of guided wave propagating in a pipe circumference by non-contact remote measurements with the SLS technique. The narrowband elastic waves are generated in a pipe by modulating laser pulses with fiber laser equipment. When the modulation frequency is in harmony with the resonance frequency of a circumferential guided wave, the distribution of the frequency spectrum peak obtained with the SLS technique become identical to the resonance pattern of the circumferentially guided wave mode. The distributions are distorted for a pipe with wall thinning on the back indicating that this technique has a potential for detection of defects on the back of a pipe. Journal of Pressure Vessel Technology


INTRODUCTION
Non-destructive evaluation (NDE) using elastic waves generated by laser irradiation to industrial material is widely known as laser ultrasonics [1][2][3]. In laser ultrasonics, a broadband ultrasonic wave in the frequency of MHz range generated by a laser pulse with a pulse width of the order of ns is generally used for inspecting and evaluating materials. Because a broadband pulse wave having superior characteristics in resolution of time and space is easily generated in industrial materials at a high peak power using commercially available laser equipment such as a Nd: YAG laser, it has become standard in laser ultrasonics.
However, a broadband pulse wave cannot be used under experimental conditions at low signal to noise ratio (SNR) because it is easily buried in noise. Also, increasing the maximum power of laser pulse for improving SNR causes damage on a surface of a material due to ablation. In order to improve SNR at low maximum power, the authors developed narrowband elastic wave generation using fiber laser equipment that has become popular in laser processing [4][5][6]. Fiber laser equipment can generate laser pulses at a high repetition rate on the order of a few hundred kHz or MHz.
Modulating such high repetition laser pulses can output intermittent laser pulse trains, which results in generation of elastic waves corresponding to the modulation signal. The authors experimentally demonstrated in [6] that the use of long duration modulation signal significantly improves SNR in frequency domain, and this technique was applied to defect imaging by an SLS technique.
In the SLS technique, a laser generates an elastic wave, and the laser source is scanned over a material while the elastic wave is detected at a fixed position.
This leads to stable measurements of multiple waveforms. Moreover, it was experimentally proved that the SLS technique is effective for surface crack detection.
Kromine et al. [7] and Sohn et al. [8,9] detected surface cracks with a high precision by Rayleigh wave measurements with SLS. Takatsubo et al. [10] created a movie of ultrasonic waves propagating on the surface of a material from waveforms obtained by the SLS technique and proved that defects can be detected from the wave distortions at the defects. Dixon et al. [11] observed ultrasonic wave enhancement when the laser source was located in the vicinity of a surface crack, and Clough et al. [12,13] detected a surface crack with high precision using this phenomena.
The authors have investigated defect imaging using multiple waveforms measured by SLS [4][5][6], [14][15][16][17]. This defect imaging technique uses the phenomena that the generation energy of a flexural wave varies with plate thickness and defect depth near the laser source, and, therefore, this imaging technique can be applied to a complex plate structure in which there are no straight paths from a laser source to a receiver [4]. In reference [5], we developed a fast imaging system for a plate-like structure using a feature of SLS that allows multiple waveforms to be detected stably by a fixed receiver. In order to measure flexural vibration of a plate from a remote position, a laser Doppler vibrometer (LDV) is required as a receiving devise. Because an LDV reduces SNR significantly compared with such contact devices as ultrasonic transducers and piezo-electric transducers, the narrowband wave generation by a fiber laser was adopted to improve SNR, and defect imaging from a remote position was realized [5,18].
Since this imaging technique has prominent features of fast remote non-contact imaging, images of inner defects as well as surface defects of plate-like structures are obtained as with optical and infrared cameras.
Moreover, the authors proved that defect imaging with SLS, called Elastic wave camera (E-camera), can be applied to curved plates such as pipes. Defect images were experimentally obtained for straight and branched pipes [18]. However, this imaging technique using an E-camera can be applied only to plate regions on which a laser can be irradiated. In the case of pipework, half of the circular cross-section can be evaluated using an E-camera. Namely, the defect imaging with an E-camera system still has the issue that the other half region on the back of the pipe cannot be evaluated.
This study investigates detection of wall thinning on the back of a pipe using resonance of circumferential guided waves generated by laser. In the second chapter, guided waves propagating in the circumferential directions are discussed with dispersion curves and wave structures on a pipe cross-section. In chapter 3, the pipe specimen used and the experimental system, including fiber laser equipment and LDV, are described. Finally, experimental results are shown in chapter 4.

GUIDED WAVES PROPAGATION IN THE CIRCUMFERENTIAL DIRECTIONS
Defect imaging by SLS uses flexural vibration generated by laser irradiation onto a plate and a pipe [19]. The propagation mode in a plate with flexural vibration is only the fundamental anti-symmetric mode, called the A0 mode, in a small frequency- , and, then, the displacement distribution does not depend on θ and becomes symmetric with respect to the pipe center, as shown in Fig. 2 , and, then, the displacement distributes in the cosine function with one period in one round of the circle, as shown in Fig. 2 (b). This is described in detail in the author's previous papers [23,24].
The cutoff frequencies shown by the arrows in Fig. 1 provide infinite values in phase velocities, zeros in group velocities, and infinite wavelengths in the longitudinal direction. This means that the harmonic waves at the cutoff frequencies have uniform distribution in the longitudinal direction and become standing waves in the circumferential direction, as shown in Fig. 2. In other words, the cutoff frequencies denote resonance frequencies of the circumferential guided waves. Nishino et al.
developed a precise measurement technique of pipe thickness using the circumferential standing waves [25].
In the experiments shown later, wall thinning is detected using the characteristics that wall thinning disturbs the circumferential resonant modes at the cutoff frequencies.

EXPERIMENTAL SET-UP AND PIPE SPECIMEN USED
In the experiments described here, a fiber laser and an LDV were used for generating and detecting elastic waves, as shown in Fig. 3. The fiber laser equipment used in this experiment can generate laser pulses at a high repetition rate, and the laser output can be controlled by external modulation signals. In our previous studies [4][5][6], the use of the fiber laser and external modulation signals enabled us to generate longduration burst waves in plate-like structures. Because the use of a burst wave improves SNR in the frequency domain, defect images were successfully obtained using such a low sensitive receiving device as an LDV [6,18]. The same system was used in this study.
The repetition frequency of the fiber laser was 580 kHz, and the external modulation signals were rectangular burst waves with a duration time of 2 ms at frequencies of 5 kHz to 40 kHz. Waveforms were recorded with an analog-digital converter for 20 ms after the moment of laser shot, and then the peak values of their Fourier spectra were obtained using a personal computer.
Aluminum alloy pipes with an outer diameter of 100 mm, pipe wall thickness of 3.0 mm, and length of 500 mm were used as pipe specimens. An intact pipe with uniform thickness was used in section 4.1, and a pipe specimen with artificial wall thinning inside the pipe was used in section 4.2. The artificial defect as shown in Fig. 4 was made with a lathe, in which the tip of a tool rotated in a diameter of 65 mm, and the rotation center gradually moved away from the pipe center to adjust the minimum thickness of the wall thinning to 1.5 mm. The distance between the test pipe and the experimental system was about 3.0 m in these laboratory tests. Although the distance must be longer like 10 m or 20 m in a practical use, there seems to be no crucial problems to execute such remote measurements, because the LDV supports such distances and elastic waves can be generated when a laser spot is appropriately focused on the pipe surface.

Resonance patterns in an intact pipe
Guided waves in an intact pipe with uniform thickness were measured using the non-contact experimental system. As shown in Figs Fig. 5 (a), and the other one is the left edge and the right edge, as shown in Fig. 5 (b).
For the pipe support arrangement shown in Fig. 5 (a) Fig. 6 (a) gray, due to the large noise. However, a distinct frequency spectrum peak can be observed at 10 kHz in Fig. 6 (b) gray. In the frequency spectrum of Fig. 6 (b), a large peak appearing around 21 kHz is intrinsic noise generated in the LDV, and the noise is largely contained in the waveforms of Fig. 6 (a). At 14.3 kHz, the waveform exceeding the noise is obtained as shown in Fig. 6 (a) black. The waveform maintains a high amplitude over the whole time range of 20 ms even though the laser output stopped at 2 ms. This implies that resonance in the pipe was detected at 14.3 kHz.  In the pipe support arrangement of Fig. 5 (a), the values of the frequency spectrum peaks in the frequency range of the modulation frequency±0.25 kHz were plotted when the frequency of the modulation signal was changed from 5 kHz to 40 kHz with 0.1 kHz increments. A large peak was obtained at 14.3 kHz, which indicates that the waveform and its frequency spectra, shown with black lines in Fig. 6 (a) and (b), are under the resonance condition. From Fig. 7 (a), the resonance frequencies are 7.2 kHz, 10.5 kHz, 18.6 kHz, 23.4 kHz, 28.7 kHz, and 34.4 kHz, and the peak around 21 kHz is the effect of intrinsic noise, as seen in Fig. 6 (b). These frequencies correspond to the cutoff frequencies of n=5 to 11. Namely, these resonances are caused by the standing waves in the circumferential direction.
The frequency spectrum peak value was plotted in Fig. 7 (b) when a pipe support was located at the same longitudinal position as the generation and receiving points shown in Fig. 5  In Fig. 7 (b), the modes with an antinode at the pipe support did not resonate due to the energy leakage into the pipe support. However, the modes with a node at the pipe support resonated without influence of the pipe support.

FIG.7
Next, distributions of the frequency spectrum peak were obtained in Fig. 8 when a laser source was scanned over the whole pipe at the resonance frequencies. The pipe supports and the receiving point were located at the same positions depicted in Fig. 5 (b). The laser source was scanned over a range of 520 mm × 110 mm at 2 mm increments, and the peak values of the frequency spectra obtained at all of the rastering positions were plotted in gray scale. A small gray square appearing at the right edge of the region is due to the retro-reflection tape for LDV detection. The values at the point 20 mm apart from the tape correspond to the values at the frequencies in Fig. 7 (b). A horizontal striped pattern stretching over the entire pipe is clearly observed in Fig. 8 (a).
This pattern, consisting of five antinodes and four nodes, indicates the resonance mode of n=5, which agrees well with the mode in Fig. 2 (f). Although a horizontal strip pattern can also be seen in Fig. 8 (b), the intensity is small in the whole region due to the energy leakage into the pipe support, as seen in Fig. 7 (b). In Fig. 8 (c), the right half region consists of 7 antinodes and 6 nodes, which indicates the resonance mode of n=7. Fig. 8 (g) also shows the resonance pattern for the corresponding circumferential order. In Fig.   8 (d)-(f), the horizontal strip patterns were not clear because other resonances also occur at those frequencies.
Now, we focus on the horizontal strip patterns for detection of defects on the back of a pipe using the SLS. Although the horizontal patterns were seen in Fig. 8 (c) and (g), the intensities become smaller at the further ranges from the receiving point. These results show that the resonance modes of n=7 and 11 become gradually smaller at remote distances between the laser source and receiver. As shown in Fig. 9, the elastic wave generated by a laser shot propagates in all directions. Among them, only circumferentially propagating waves resonate at the cutoff frequencies. These modes do not propagate in the longitudinal direction at the cutoff frequencies. However, considering the modes at slightly higher frequencies, these modes can propagate with a very small group velocity and maintain resonance in the circumferential direction. These modes are spirally propagating modes in both circumferential directions. When a laser source and a receiving point are not close, the received waveforms are the spirally propagating modes. When they resonate in the cross-section of a pipe, the horizontal strip patterns are observed. Because the stripe bands become larger for smaller circumferential orders, n, the spirally propagating modes maintain resonance in spite of the inclination of the wave plane. On the other hand, for higher circumferential orders, even a small inclination in the spirally propagating modes suppresses the resonance.
Comparing the experimental results of Figs. 8 (a), (c), and (g), the horizontal patterns are localized more at the receiving point for higher order modes due to the features of circumferential modes.
Hereafter, detection of artificial wall thinning is discussed for the circumferential orders n=5 (7.2 kHz) and n=7 (14.3 kHz), in which the horizontal strip patterns were largely obtained in Figs. 8 (a) and (c).

Resonance patterns in a pipe with a defect on the back
The horizontal resonance patterns generated by guided wave modes propagating in the circumferential directions were discussed above. It can be predicted that the circumferential resonance patterns are changed due to obstacles such as defects and pipe supports. Therefore, in this section, distributions of frequency spectrum peak are discussed for a pipe with wall thinning on the back.  As shown above, resonance patterns were distorted due to the presence of wall thinning. This proves that the distortions of the image in the resonance patterns enable the detection of defects on the back of a pipe using the SLS. Considering the experimental results where the circumferential locations of wall thinning did not change the distortions of the resonance patterns, it can be concluded that the circumferential position cannot be located using the processing techniques described here. However, resonance patterns may be affected by the circumferential position of defects considering the results in section 4.2, where the locations of a pipe support and nodes and antinodes of a circumferential mode affect the resonant amplitude. Using the circumferential modes appropriately, a circumferential position of defects may be predicted with further study.

5．Conclusions
This study discussed detection of a defect on the back of a pipe using resonances in the pipe circumference with the SLS measurement. First, it was confirmed that narrowband burst waves generated by modulated high-repetition laser pulses induced resonance in the pipe circumference at the cutoff frequencies of the guided waves in the pipe. In the frequencies, distributions of the frequency spectrum peak obtained by the SLS corresponded to the resonance patterns of the circumferential guided waves.
The resonance patterns were distorted at the area where a defect was located on the back of the pipe, which is useful for defect detection in a pipe.

ACKNOWLEDGMENT
I would like to thank all the workshop staff at Kyoto University for their effort in the production of pipe specimens. This work was supported by JSPS KAKENHI, Grant Number 26282094, 17H02052, 18K18920, and Chubu Electric Power Co., Inc.