Application of the Proposed Fiber Optic Time Differential BOCDA Sensor System for Impact Damage Detection of a Composite Cylinder

Round 1

Reviewer 1 Report

In this paper, the authors present the improvement of the two-dimensional damage mapping technique of composite materials using the BOCDA technique applying a 16 Gbps light signal of eight times higher modulation speed to improve the spatial resolution and use the time differential (TD) - BOCDA to enable measurement without an optical delay line. The results are interesting; however, there are some issues that need to be clarified:

1. Section 2 "The fabrication and the damage mapping method of a composite cylinder with an optical fiber" needs to be written with greater clarity and detail. In the third line of the first paragraph of this section the authors describe the cylinder configuration and present the layers as "[90°1/OF/90°1/+-20°1/90°3/+-20°1/90° 3/+-20°2/EPDM]T." a more detailed description of what differentiates each layer would make reading easier.
2. How was the polyimide coating provided? Please provide more information.
3. Throughout the text, the term "longitudinally" is used to refer to two distinct directions, along the arc of the circumference and along the axis of the cylinder. It would be better to use it just for the direction along the axis.
4. On page 3, the authors present the length of the fiber in one turn around the cylinder "786 mm", however this value diverges a little from the length of a circle with a diameter of 251.6 mm (diameter shown in figure 1). Why is there this difference? The longitudinal distance of the cylinder shown in the text, 1.8 m, also differs from that shown in figure 1, 1920 mm.
5. In figure 1, the authors present the cut of the layers with a cross section; however, the cut is in the longitudinal section. In the inset of figure 1 the acronym CFRP also appears. What does it mean?
6. In section 4, the symbol for "area" 1, 2, 3 and 4 is unformatted. Additionally these regions have a length dimension and should not be called "area".
7. In the impact measurements the authors used energies of 5, 10, 15 and 20 J. What is the detection limit, in energy, of the system? In figure 4 it is possible to see a noise pattern in the signal that limits the detection of impacts of small magnitude. This aspect could be discussed.
8. In the discussion of Figure 6 the author’s state, "two points (5J) had insufficient damage area in the two-dimensional view". Why? Is there any reason? What is the detection limit on this 2D map?

Author Response

We, authors, appreciate very much for your kind review on our manuscript titled on "Application of the proposed fiber optic time differential BOCDA sensor system for impact damage detection of a composite cylinder."

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

The authors presented a distributed strain detection BOCDA system to measure impact damage of a cylinder composite.

The improvement over previous publication is mainly by a using higher modulation.

The novelty is limited despite the results being positive.

I have one question and one comment for the authors. 1. For Fig. 7c, the reconstructed strain map show zero deformation in the center of the impact. Is that expected? 2.

In page 5 and 6 where the authors discuss different area in Fig. 4, the character denoting the area number is not showing correctly. 

Author Response

We, authors, appreciate very much for your kind review on our manuscript titled on "Application of the proposed fiber optic time differential BOCDA sensor system for impact damage detection of a composite cylinder."

As your comments, we revised our manuscript and answered your questions as following :

1. For Fig. 7c, the reconstructed strain map shows zero deformation in the center of the impact. Is that expected?

--> Around the impact area, the total integrated residual strain is proportional to the applied impact energy. After impacting the composite cylinder, the deformation around the impact center was recovered sometimes zero because the deformation was almost point-centric. This point-centric deformation was generated by the semi-hemispherical impactor tip. Therefore, the residual strain around the impact center was almost recovered. Fig. 8(a) is a good example for this explanation, where the largest deformation is shown at the center and slight deformation occurs between the center and the edge. So sometimes the deformation can be fully suppressed at the center of the impact. This is why we added these sentences for the explanation of Fig. 8 like “It is very interesting that this is very similar to the deformation of the surface of a liquid shaped by a thrown object. Our previous experiment also observed similar results close to this phenomenon [12]”.

2. In page 5 and 6 where the authors discuss different area in Fig. 4, the character denoting the area number is not showing correctly.

--> Thanks for this comment. In order to avoid such erroneous notation errors, we corrected all circled number notations with parenthesized number notations, like (1) and (2) instead  and ‚. These are clearly indicated in the revised manuscript.

Round 2

Reviewer 1 Report

I agree with the publication of the manuscript.