Defects Detection and Localization in Underwater Plates Using Laser Laterally Generated Pure Non-Dispersive S0 Mode
Round 1
Reviewer 1 Report
Manuscript ID: applsci-424955
Title: Defects detection and localization in underwater plates using laser laterally generated pure non-dispersive S0 mode
Authors: Qingnan Xie, Chenyin Ni, and Zhonghua Shen
This paper presents a damage detection and localization method for plates immersed in water, which utilizes the S0 Lamb mode by the laser lateral generation method. The authors enabled the detection and localization of defects by detecting mode conversion and resonance generated at defects.
The topic seems to be interesting for readers who engage in non-contact ultrasonic non-destructive evaluation. The paper is well written in general, but there are several things to care about:
a) In Sec. 2.1, the mode selection for the defect detection in underwater plates is discussed based on the dispersion relation in non-immersed, free plates. Is it reasonable? The dispersion relation of quasi-Scholte waves cannot be obtained from free plates, and the use of the dispersion curves for underwater plates seems to be straight-forward. If unavailable, it will be fine to show by finite element simulation that the phase velocities of Lamb waves are almost invariant.
b) When generating the S0 mode by laser, how did you tune the frequency of the emitted wave to 5 MHz (1 MHz*mm)? Probably the details may have been described in previous works, but some explanations seem to be necessary for readers in this paper too.
c) From Line 220, there is a description that “the width of the notch can be estimated utilizing the top generation configuration…”, but “utilizing the lateral generation configuration” appears to be accurate.
d) In a plate with two notches, the localization of the left and right notches seems to be possible from the mode-converted reflected A0 mode and the short resonance, respectively. Basically, however, why is the short resonance observed for not all notches? In Fig. 9(b), which corresponds to the case of three notches, the reason for which the short resonance is not observed is explained as not sufficiently large scanning step, but is not the scanning step the same in all cases of single, double, and triple notches? Why does this difference happen?
e) In this work, the proposed method is applied to very thin aluminum plates. To emit the S0 mode efficiently, the size of the laser spot is adjusted to the plate thickness. Is this technique available for relatively thick plates, for instance, plates of thickness 2-5 mm?
Author Response
Dear reviewer,
We appreciate reviewers very much for taking so much time to concern our manuscript (applsci-424955) and make comments. Your hard works impress us a lot. Those comments are valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made correction which we hope meet with approval.
The point-by-point answers to the comments and suggestions are listed below.
Reviewer 1
Point 1: In Sec. 2.1, the mode selection for the defect detection in underwater plates is discussed based on the dispersion relation in non-immersed, free plates. Is it reasonable? The dispersion relation of quasi-Scholte waves cannot be obtained from free plates, and the use of the dispersion curves for underwater plates seems to be straight-forward. If unavailable, it will be fine to show by finite element simulation that the phase velocities of Lamb waves are almost invariant.
Response 1: Thank you for your advice. The dispersion curves of Lamb waves are almost invariant, which has been calculated in other researches, actually. As you suggested, I have added the explanation and references (ref. 27, 31) in line 120-123 in the revised manuscript.
Point 2: When generating the S0 mode by laser, how did you tune the frequency of the emitted wave to 5 MHz (1 MHz*mm)? Probably the details may have been described in previous works, but some explanations seem to be necessary for readers in this paper too.
Response 2: In our work, Lamb waves are generated by the laser with a wave length of 1064 nm and a pulse width of 10ns. Using this laser source, the center frequency of the emitted wave is obtained to be ~5MHz in experimental measurements in the plates with different thickness but the same material. In this experimental condition, it is hard to change the pump laser, so we choose the suitable thickness of aluminum plate to generate the S0 wave at the non-dispersive region. I have added the explanation in line 175-176 (line 178-179 using “Track Changes” function) in the revised manuscript.
Point 3: From Line 220, there is a description that “the width of the notch can be estimated utilizing the top generation configuration…”, but “utilizing the lateral generation configuration” appears to be accurate.
Response 3: Thank you for your pointing out this mistake. I have corrected it in the revised manuscript.
Point 4: In a plate with two notches, the localization of the left and right notches seems to be possible from the mode-converted reflected A0 mode and the short resonance, respectively. Basically, however, why is the short resonance observed for not all notches? In Fig. 9(b), which corresponds to the case of three notches, the reason for which the short resonance is not observed is explained as not sufficiently large scanning step, but is not the scanning step the same in all cases of single, double, and triple notches? Why does this difference happen?
Response 4: In the experiments except the width measurement, the scanning step is 0.5 mm. That means the probe laser scans point by point on the upper surface of plate, while the defect is on the other surface.
The defect width is ~0.5 mm, weather the short resonance can be observed depends on the relative position between the probe point and the defect. When the probe point is just on the projection of defect on the upper surface, the resonance can be observed. When the projection of defect on the upper surface is at the interval of probe points, the resonance can not be observed. This has randomness when the scanning step is not small enough. Even in the same sample with multiple defects, it is hard to ensure that the resonance can be observed at each defect.
Point 5: In this work, the proposed method is applied to very thin aluminum plates. To emit the S0 mode efficiently, the size of the laser spot is adjusted to the plate thickness. Is this technique available for relatively thick plates, for instance, plates of thickness 2-5 mm?
Response 5: For relatively thick plates, the proposed method is also applicable. In this case, the laser spot should be adjusted to a proper size and a suitable laser should be chosen. The plate is such thin in this work, one reason is that it is more convenient to using the thin plate (thickness of ~0.2 mm) to make the generated S0 at the non-dispersive region (fd ~1MHz·mm), because the center frequency of S0 mode is ~5 MHz using the pump laser. Another reason is that for thin plate, it is much more convenient to use laser excitation because the traditional transducers normally have relatively large size and need to contact with the sample. Besides, it is hard to ensure the symmetry using transducers.
Author Response File: 
Author Response.docx
Reviewer 2 Report
The paper deals with the propagation of S0 mode in an underwater plate by lateral laser source irradiation. Numerical model has been used to support the experimental investigation.
Paper is well written and the state of the art is described appropriately. However, some minor revisions and considerations have to been considered before publication.
1- In order to improve the readability of the paper, a description of the lateral source irradiation method should be made;
2- Page2, Lines 71-74. Authors wrote: “When the generating laser irradiates the plate surface, both symmetric modes and antisymmetric modes can be generated, while the symmetric modes normally have small out-of-plane displacement because of the antisymmetric excitation”. This information is often mentioned in the text. What do authors mean for “because of antisymmetric excitation ”? Symmetric modes for their nature are characterized by in-plane displacements, while antisymmetric modes by out-of-plane displacements.
3- Page 2, Line 75: Authors wrote: “This A0 mode Lamb wave increases the difficulty in signal analysis of the S0 modes[25,26]”. What do authors mean for “increases the difficulty in signal analysis”?
4- Page 2, Line 82: Authors wrote: “In this paper, we extend our previous work by combining the laser…”. Introduce the reference of the previous work, please.
5- Page 4. The FE model description is too poor. Improve it, please. A Figure of the FE model can improve the readability of the paper. Figure 3 is not clear. The size of the modelled aluminium plate seems to not match with the real plate one. Information about number of elements, FE analysis formulation should be provided.
6- Page 4, Lines 137-139. Authors wrote: “The generation of both S0 and A0 mode are simulated by loading the displacements respectively at the left side of aluminum plate, including in-plane and out-of-plane components, at the corresponding fd”. According to the experiment, the generation of Lamb waves occurs through a laser focused into a circular spot.
7- Page 7, Line 258. “an 5 mm” replace with “a 5 mm”
8- Page 8, Line 272. Check “cam”, please.
9- Page 8, Line 273. Replace “It’s” with “It is”, please. Reviewer suggests reading carefully the whole paper.
Author Response
Dear reviewer,
We appreciate reviewers very much for taking so much time to concern our manuscript (applsci-424955) and make comments. Your hard works impress us a lot. Those comments are valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made correction which we hope meet with approval.
The point-by-point answers to the comments and suggestions are listed below.
Reviewer 2:
Point 1: In order to improve the readability of the paper, a description of the lateral source irradiation method should be made;
Response 1: Thank you for your advice, short explanation have been added in line 165-167 (line 168-170 using “Track Changes” function) in the revised manuscript.
Point 2: Page2, Lines 71-74. Authors wrote: “When the generating laser irradiates the plate surface, both symmetric modes and antisymmetric modes can be generated, while the symmetric modes normally have small out-of-plane displacement because of the antisymmetric excitation”. This information is often mentioned in the text. What do authors mean for “because of antisymmetric excitation”? Symmetric modes for their nature are characterized by in-plane displacements, while antisymmetric modes by out-of-plane displacements.
Response 2: Sorry to make the reviewer confused. After carefully rereading the text, we find “antisymmetric excitation” is not accurate, it should be ‘asymmetric excitation’, which means the case that the source is asymmetric with respect to the middle plane of plate. For example, a pulsed laser irradiates on one surface of plate, in this case, though symmetric modes can be generated, normally they will have small amplitude. The word has been corrected and a short explanation has been added in line 72 in the revised manuscript.
Point 3: Page 2, Line 75: Authors wrote: “This A0 mode Lamb wave increases the difficulty in signal analysis of the S0 modes [25,26]”. What do authors mean for “increases the difficulty in signal analysis”?
Response 3: When propagating distance is not long enough, S0 and A0 mode have not separated yet. In this case, the large wave packet of A0 mode in time domain caused by the strong dispersion, may make it difficult to identify the S0 mode or other reflections. The description in previous manuscript is not clear, I have modified in line 75-77 in the revised manuscript.
Point 4: Page 2, Line 82: Authors wrote: “In this paper, we extend our previous work by combining the laser…”. Introduce the reference of the previous work, please.
Response 4: Our previous work presented a method to generate pure non-dispersive S0 mode using laser lateral generation in a thin aluminum plate in dry condition. Using this method, S0 mode is enhanced and A0 mode is restrained effectively. I have added the description in line 83-85 (line 84-86 using “Track Changes” function) in the revised manuscript.
Point 5: Page 4. The FE model description is too poor. Improve it, please. A Figure of the FE model can improve the readability of the paper. Figure 3 is not clear. The size of the modelled aluminium plate seems to not match with the real plate one. Information about number of elements, FE analysis formulation should be provided.
Response 5: Thank you for your advice, I have modified this part in line 138 and line 142-151 (line 139, line 143-153 using “Track Changes” function) in the revised manuscript.
Point 6: Page 4, Lines 137-139. Authors wrote: “The generation of both S0 and A0 mode are simulated by loading the displacements respectively at the left side of aluminum plate, including in-plane and out-of-plane components, at the corresponding fd”. According to the experiment, the generation of Lamb waves occurs through a laser focused into a circular spot.
Response 6: The simulation is performed to get the attenuation curves of S0 and A0 mode in the underwater plate. In this simulation, a series of S0 and A0 mode at different fd (0.5 to 1.5 MHz·mm) is calculated. For generating S0 and A0 mode, using laser source and loading the displacements of S0 and A0 mode are all feasible, but compared to using laser source, loading the displacement of S0 and A0 mode at each fd is more accurate and saving time.
Point 7: Page 7, Line 258. “an 5 mm” replace with “a 5 mm”
Point 8: Page 8, Line 272. Check “cam”, please.
Point 9: Page 8, Line 273. Replace “It’s” with “It is”, please. Reviewer suggests reading carefully the whole paper.
Response 7-9: Thank you for pointing out these mistakes, I have modified in the revised manuscript.
Author Response File: Author Response.docx
