NDE Terahertz Wave Techniques for Measurement of Defect Detection on Composite Panels of Honeycomb Sandwiches

Round 1

Reviewer 1 Report

The manuscript entitled “NDE Terahertz Wave Techniques for Measurement of Defect Detection on Composite Panels of Honeycomb Sandwiches” by Kwang-Hee Im et al. presents an experimental method for detecting defects in optical structures by using THz electromagnetic waves. The quality of the language and the overall flow of the manuscript need major revision. It is difficult to follow and understand what authors are trying to say. In addition, the introduction should describe better and include more references. Some technical terms are used in a wrong way. For example: “the importance of THz wave” there is not just one THz wave; “the fundamental characteristics of the THz wave”, can the author explain instead of the frequency and the wavelength what else is considered as a fundamental characteristic of an electromagnetic wave? Undefined terms are used such as the “specimen”, “T-ray time domain”, “time of flight”, “shape delay time”. The equations and the figures are not described sufficiently well; e.g. the specimen thickness is defined at least four times; what is the difference between “V_air” and “Ca”. There is sufficient description of the materials that are used in this study. For example, “carbon-skin”, “glass-skin”, “honeycomb panel”. I assume that the term “skin” denotes a thin layer of a material, nevertheless it should be explained. What is a “honeycomb panel”? Is this a material with hexagonal atomic crystal structure such as graphene or hexagonal boron nitride? The research presented by this manuscript might be publishable, however, this is not obvious because the manuscript is not well and clearly written. I am in the unpleasant position to reject this manuscript for publication in MDPI electronics.

Author Response

Attached a file herewith.

Thank you.

Author Response File: Author Response.pdf

Reviewer 2 Report

In this manuscript, the authors presented the experimental results of honeycomb sandwich composite materials and proposed a method for measuring the refractive index (n) of various materials using terahertz waves. The characteristics of T-ray were studied and analyzed.. My comments on this manuscript are as below:

1. The method of measuring refractive index mentioned in the manuscript, how to judge whether it is correct?
2. On page 5, in line 154 the author mentions “Here, a preferred sample is positioned at a place where focal points of emitter and receiver are matched to conduct the experiment. The inclination angle of the T-ray lens is set to 16.6ᵒ”. Why do you choose the focal points of emitter and receiver are matched to conduct the experiment? Why do you set its angle to 16.6ᵒ?
3. Terahertz technology has a very important application in related fields. The author should enrich the latest relevant literature. “In particular, T-ray is critically important in areas of security devices used in airports, medical imaging, polar liquids, various industrial facilities, and spectroscopic evaluation on composites”, such as: “Physica E: Low-dimensional Systems and Nanostructures 117 (2020) 113840; IEEE Access 8, 85154-85161”. “The resonance frequency (∆f) can be expressed as follows when the shape delay time and inclined T-ray path are tracked”, such as: “IEEE Photonics Journal 12(3), 2020, 4500708; Results in Physics 16, 2020, 102952.”

Author Response

Attached a file herewith.

Thank you.

Author Response File: Author Response.pdf

Reviewer 3 Report

The research is interesting and the paper is well structured.

There are some comments and questions about the text:

1) Line 104: Is "t" missed in Eq.1?

2) Line 115: The sine argument is lost.

3) Lines 115-133: You use constructions "d refers to ...", "v refers to a ...", "Ca refers to a ..." about 3-4 times. Why? Especially after equations where you do not use these notation. It is enough to enter the definition once.

4) Line 137: You have already entered the abbreviation "THz TDS" on line 39.

5) Line 143: I believe that "wavenumber range" is worth reformulating.

6) Line 145-146: "TDS system can measure transmission or reflection" is may be also better to reformulate.

7) Check the THz receiver on the Figure 3.

8) Lines 181 and 192: A time difference - 40 ps? I can see a time difference > 83 ps on the Figure 5.

9) Line 191: "time/ps?"

10) Line 192: "t=5.79 mm". Is "t" a thickness ?

11) Line 196: "T-rays T-ray"

12) Line 229: "Using =10 S/m". Is there something missed?

Author Response

Attached a file herewith.

Thank you.

Author Response File: Author Response.pdf

Reviewer 4 Report

This manuscript by Im et al. reports on a use of terahertz wave for non-destructive inspections of defects in a honeycomb sandwich composite panel using conducive carbon fibers.  They observed predefined defects in the material under various directions of electric field for glass fiber based composites due to the insulating property of the fibers.  In contrast, clear E-field angle dependence in the signal to noise ratio for detecting defects was observed for the carbon fiber based composites suggesting an important role of the asymmetric conductivity in the fibril conductors.  Eventually, they confirmed that the angle should be 90 degrees between the E-field and the axial direction of the carbon fibers to obtain optimal signal to noise ratio for the detection of defects in the honeycomb sandwiches.

While the work is solid providing a technique useful for non-destructive inspection of composite materials, the data presented are not consistent in places.  I will recommend publication of this article after the authors address the following points:

1. Can the method be used to analyze the direction of the fibers? How about influence of the properties of the fibers used including the diameter, length, and the conductivity?  If the carbon fibers are randomly oriented, can the method still be used for detecting defects?
2. In Fig. 5 caption, delta_t is said to be 40 ps while that in the graph is obviously longer than 40 ps. Please clarify the discrepancy here.  There is also “?” in the label that should be removed.
3. While the conductivity in the radial direction of a carbon fiber is higher than that in the axial direction, the conductivity of composite materials is reported to be opposite as explained in page 7. The authors should briefly explain what cause the discrepancy.
4. Figure 6a: Peak-to-peak amplitude is said to be 1.9 but the data shows much smaller value. Please check and correct.
5. English writing needs to be polished. There are also many typos throughout the manuscript.  For example, “in order confirm” in the abstract; “Trays Tray” on page 7; “Using = 10 S/m” and “Figure 6(a) shows where the angle” on page 8.

Author Response

Attached a file herewith.

Thank you.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Most of my comments have been addressed by the authors and the revised manuscript has been improved. This study provides interesting experimental results. The method that is presented by this work can be useful to detect defects in optical structures by using THz electromagnetic waves. In the revised paper the text is more clearly written, the equations are described better, and the technical terms are explained better than the first version of the manuscript. Now I am in the pleasant position to suggest this manuscript for publication in MDPI electronics.

Reviewer 4 Report

The authors have revised the manuscript following the comments raised.  Optional: English writing may better be improved further before publication.