Denoising of LCR Wave Signal of Residual Stress for Rail Surface Based on Lifting Scheme Wavelet Packet Transform

Round 1

Reviewer 1 Report

This paper proposes a denoising method based on LSWPT for  LCR Waves from Rail Surface. The introduction is providing a sufficient explanation of the problem and also a background on the state-of-the-art on noise reduction in this field. The algorithm is clearly explained in section two and the system schematics are explained in the manuscript which greatly helps the reader in understanding the algorithm. The claimed conclusion are backed up by the provided results and the comparison clearly shows the mertis in using this algorithm against the others.

One point that might be worth mentioning is that how would this method work in the presence of defects? I believe the main use of such techniques is to enable non-destructive testing and assesment of structural life. What would be the best way to utilize technique in such scenario to report on the health of the structure?

Author Response

Many thanks for your comments and suggestions and please find the submitted file.

Author Response File: Author Response.docx

Reviewer 2 Report

I found this manuscript (MS) interesting. The paper is properly structured, clearly written, and well-documented. Nonetheless, I am very surprised that this study has been proposed for publication in the journal Coatings. Indeed, the scientific content of the MS in not in the scope of the journal since it does not deal with any coating and is essentially focused on signal processing issues (admittedly, applied on surface residual stress characterization). Based on this: (i) I am not confident to be a relevant reviewer for this MS, and above all (ii) I strongly suggest to propose this MS for publication in another journal in the field of signal processing. If the authors are willing longer to publish their work in an open access journal from MDPI, I suppose it could be more positively assessed in the journal "Signals".

Author Response

Many thanks for your comments and suggestions. This manuscript focusing on surface residual stress is for the call of paper for the special issue of Advanced Nondestructive Evaluation and Characterization of Surface and meets the topics of the special issue.

Reviewer 3 Report

In the current paper, authors presented an attempt to denoise LCR wave signal of residual stress for rail surface based on lifting scheme wavelet packet transform. The paper lacks scientific analysis and discussion. It looks like reporting data without any comprehensive discussion. General comments that must be included to improve the value of paper: 

Introduction section:

1. The introduction section is not clear. The knowledge gap has been presented but reviewer is not sure what's new/ground-breaking in this work? Is this approach is new and novel? What makes authors work valuable in comparison to other work already published. There is no clear summary of introduction section.
2. The main difficulty in the practical applications of the ultrasonic stress measurement techniques is the relatively weak dependence of the wave velocity on stress. This dependence demands a very accurate measurement of wave pulse transit times. Thus, advanced electronic measuring systems and special ultrasonic probe systems, that could reduce the impact of acoustic coupling on the accuracy of measurements should be used. An additional difficulty is the influence of the texture on the measured transition times which should be considered during data analysis. The ultrasonic wave, that depends on the heads geometry and the thickness of the element, can reach the receiving head in various ways.

The transit time of ultrasonic waves during ultrasound measurements is affected by the stress, deformation and temperature. These factors should be considered during measurements and further analysis. It should be also mentioned, that the value of the elasto-acoustic constant depends on:

• the type of material in which the wave propagates;
• the type of wave;
• the direction of wave propagation relative to the direction of stress.

All these conditions should be included in the literature review and discussion sections.

Results and discussion section:

3. Results obtained in this section and each subsection separately should be supported by sufficient references especially when ‘results’ and ‘discussion’ sections are combined into one. There are no references used in this section. The results obtained were not compared or supported by similar work. I feel that too much experimental work was reported and not much discussed.
4. Performance verification of LSWPT for LCR wave subsection: the essential results from Fig 12 should be clearly presented. The denoised wave signals at time of 1.5 s should be enlarged or time axis should be shorten to clearly indicate the changes of wave.
5. In order to confirm the significance of the ultrasonic method applied, the ultrasonic apparatus should be calibrated on samples made of structural steel subjected to uniaxial stretching and compression in the range of 0-200 MPa for tensile and 0-100 MPa for compression.
6. Strain gauge measurements should be performed on the sample subjected to stretching in order to assess the effectiveness of ultrasonic methods.
7. It would allow to state, that ultrasonic measurements have the same or similar accuracy to strain gauge , which would be of great cognitive value for Coatings readers..

The paper is interesting at some point but I strongly suggest to provide more scientific discussion. Right now it looks like purely technical paper. Additional proofreading is also suggested (too long sentences, informal language).

Author Response

Many thanks for your comments and suggestions and please find the submitted file.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

The authors have positively responded to the reviewers’ expectations, especially by including more explanations on the ultrasonic equipment calibration protocol. The paper provides interesting advances in the detection and evaluation of residual stresses on the surface of railways through an innovative LSWPT denoising method that is also compared with other methods in terms of efficiency and calculation speed. It meets the topic of the special issue “Advanced Nondestructive Evaluation and Characterization of Surface” of the journal Coatings. The paper is properly structured, clearly written, well-presented and documented, and figures of quality are also provided. In y opinion, it could be accepted as is for publication in the journal Coatings.

Author Response

Thanks for your comment again, some spelling mistakes in the English language have been corrected.

Reviewer 3 Report

Thank you the authors for their effort to improve the publication. They declare have that added more discussions in the ‘Discussion and Conclusion’ section and cited relevant references which cannot be seen in any way in the revised manuscript. A significantly important amendment introduced to the Manuscript is only the adopted an absolute calibration method to calibrate the ultrasonic testing equipment. It is commonly known that the residual stresses in the surface layer of the bulk material or in the protective coating are created at specific technological process. So, the factors such as surface material texture and other mentioned by reviewer must be taken into account in the analysis of the residual stresses what confirms also the authors. The authors considerations focuses only to the topic of introducing the SLEEP method where suggest that this manuscript is limited the space to discussion the influence of the texture on the measured transition times and other factors mentioned by reviewer. At the same time, the authors are fully aware that the influence of various modes of ultrasonic, surface material texture, deformation, and temperature on the measurement of residual stress are extremely important factors. 

At this situation the manuscript is not adequate to the readers of Coatings journal and in my opinion should be published in another journal in the field of signal processing.

Author Response

Thank you very much for your comment again. We have added a description of the importance of residual stress measurement for coatings in the introduction section.

At the same time, the description of the influence of factors such as the texture of the material and the type of wave should be considered in the ultrasonic measurement of the residual stress, and the relevant references are marked.

In order to verify the effectiveness of ultrasonic measurement of residual stress, we designed an experiment in which the tensile specimen was bent and deformed to generate tensile stress, and the method of stress measurement by X-ray was used for comparison.

Round 3

Reviewer 3 Report

Accept after revision