Determination and Validation of Residual Stresses in CFRP/Metal Hybrid Components Using the Incremental Hole Drilling Method

Round 1

Reviewer 1 Report

The current research sought to investigate the delamination of CFRP/metal hybrid components caused by residual stresses using the incremental hole drilling method. The residual stresses might occur during the manufacturing process when drilling the hole in the composite component. It is a worthy research as prediction of residual stresses are often difficult and at the same time important. I believe the paper requires some major amendment at this stage as below:

1) My main criticism is regarding the methodology. The authors asserted that they employed the incremental hole drilling method to obtain the residual stresses while the only results the reviewer can observe is based on numerical investigation using ABAQUS finite element (FE) package. I strongly recommend authors reevaluate their methodology and explicitly discuss how they approached the problem. If the incremental hole drilling method was employed, how the theory was exploited should be explicitly demonstrated in the paper. 

2) The authors haven't provided detailed analysis as to how the FE simulation analysis was conducted. It's extremely important.The authors are encouraged to discuss how the FE simulation analysis was conducted. Some issues to be considered:

-How were the loading and composite material properties introduced into the FE software?

-How were the boundary conditions prescribed to your FE model? 

-What type of element has been selected from the material library? exp: shell elements if plane stress is considered for thin plates or solid elements for thick and/or moderately thick plates.

3) Have the authors adapted subroutine programming with a sequence of program instructions that perform residual stress evaluation? To my knowledge, the crack formula needs to be introduced into a FE software using subroutine programming codes. If yes, it must be discussed. 

4) It seems like the authors are unfamiliar with two leading research publications associated with fracture mechanics in engineering components made of isotropic and orthotropic/composite materials. I have provided them for the author's attention as below. The authors are expected to discuss the below references in the Introduction portion of their manuscript and explain what advantage their method has over the ones discussed in the papers below.

"Localized failure analysis of internally pressurized laminated ellipsoidal woven GFRP composite domes: Analytical, numerical, and experimental studies", Archives of Civil and Mechanical Engineering, Vol:19, pp:1235-1250.

"Effect of temperature on crack initiation in gas formed structures", Journal of Mechanical Science and Technology, Vol: 27, pp: 3745–3754 (2013).

5) Please explain what you mean by referring to the points "A" and "B"  associated with the dimension of the non-symmetric CFRP/metal hybrid component used for residual stress analysis by the HDM method.

6)The manuscript needs some revisions to correct some typos and improves its written expression.

If the authors respond to my comments carefully and apply them into the revised manuscript, the manuscript will then be acceptable for publication.  

Author Response

We thank the reviewers for their comments and suggestions. The changes we made are explained in detail in our response to the reviewers. The changes and additions made to address the reviewers’ concerns are marked with red font throughout the manuscript.

The current research sought to investigate the delamination of CFRP/metal hybrid components caused by residual stresses using the incremental hole drilling method. The residual stresses might occur during the manufacturing process when drilling the hole in the composite component. It is a worthy research as prediction of residual stresses are often difficult and at the same time important. I believe the paper requires some major amendment at this stage as below:

Responses: The authors thank the reviewer for her/his careful reading and in-depth evaluation of present work. It is well-known that the residual stresses could induce the delamination of CFRP/metal hybrid components when they are subjected to loading. Thus, the objective of the present work is to measure the residual stresses in hybrid components through incremental hole drilling method and validate the reliability of the results, rather than to investigate the delamination induced by residual stress in the incremental drilling process. We fully agree with the reviewer that in the incremental drilling process additional residual stresses could be caused and the delamination could be induced, particularly when improper drilling parameters are utilized. In the present work, a driller with the radius of 1mm and an orbital technique were applied, which enables avoidance of inducing new stresses in the material and eventually delamination.  

1) My main criticism is regarding the methodology. The authors asserted that they employed the incremental hole drilling method to obtain the residual stresses while the only results the reviewer can observe is based on numerical investigation using ABAQUS finite element (FE) package. I strongly recommend authors reevaluate their methodology and explicitly discuss how they approached the problem. If the incremental hole drilling method was employed, how the theory was exploited should be explicitly demonstrated in the paper. 

Responses: In light of the reviewer’s criticism, some parts of the text have been reworded for clarity and expanded for an improved explanation of the methodology of evaluating residual stresses in orthotropic materials and the calculation of calibration coefficients in ABAQUS, see lines 173-188. In present work, the methodology of the evaluation of residual stresses and the approach of calculating the calibration coefficients were in line with previous work conducted in the group [A. Magnier, Residual stress analysis in polymer materials using the hole drilling method-basic principles and applications, 2018, Phd Thesis]. In order to avoid making this paper lengthy, the authors decided not to rewrite all details, which have been described in [Magnier 2018], but to highlight the cited literature.

2) The authors haven't provided detailed analysis as to how the FE simulation analysis was conducted. It's extremely important. The authors are encouraged to discuss how the FE simulation analysis was conducted. Some issues to be considered:

-How were the loading and composite material properties introduced into the FE software?

Responses: The model was characterized by homogeneous orthotropic material properties and the ply orientation was defined in a local coordinate system, which is mentioned in lines 216-217. The relevant information regarding loading conditions is given in lines 217-223, the way of prescribing the boundary conditions is given in line 217-220. The in-plane stress components ,  and _xy are converted to  and  in a radial coordinate system for prescribing the loads at the hole boundary, enabling the application of a defined stress state equivalent to the released stresses within an increment during the drilling process.

-How were the boundary conditions prescribed to your FE model? 

Responses: In terms of the boundary conditions, relevant information is given in lines 220-223.

-What type of element has been selected from the material library? exp: shell elements if plane stress is considered for thin plates or solid elements for thick and/or moderately thick plates.

Responses: The element type used is detailed in line 215: eight-node solid elements of type C3D8R. 

3) Have the authors adapted subroutine programming with a sequence of program instructions that perform residual stress evaluation? To my knowledge, the crack formula needs to be introduced into a FE software using subroutine programming codes. If yes, it must be discussed. 

Responses: No subroutine programming for achieving the aim of modeling a crack was used, as this phenomenon was excluded in the present work. Some sentences have been added in lines 226-227 for providing the information regarding the “Remove” function implemented based on a Python script.

4) It seems like the authors are unfamiliar with two leading research publications associated with fracture mechanics in engineering components made of isotropic and orthotropic/composite materials. I have provided them for the author's attention as below. The authors are expected to discuss the below references in the Introduction portion of their manuscript and explain what advantage their method has over the ones discussed in the papers below.

"Localized failure analysis of internally pressurized laminated ellipsoidal woven GFRP composite domes: Analytical, numerical, and experimental studies", Archives of Civil and Mechanical Engineering, Vol:19, pp:1235-1250.

"Effect of temperature on crack initiation in gas formed structures", Journal of Mechanical Science and Technology, Vol: 27, pp: 3745–3754 (2013).

Responses: The authors thank the reviewer for the suggested papers. In present work, focus is on measuring residual stresses in hybrid composite, instead of modeling the localized failure by drilling. However, this direction is very interesting for future work. These two paper will be extensively discussed and cited in follow-up work.

5) Please explain what you mean by referring to the points "A" and "B"  associated with the dimension of the non-symmetric CFRP/metal hybrid component used for residual stress analysis by the HDM method.

Responses: The relevant information has been added in the caption of Figure 5. The residual stresses on the points “A” and “B” were measured and the results are shown in Figure 9. For symmetric laminates, the residual stresses in point A and B could be equal, however, this is not the case for unsymmetric laminates.

6) The manuscript needs some revisions to correct some typos and improves its written expression.

Responses: The whole manuscript has been carefully checked and corrected when necessary during revision.

If the authors respond to my comments carefully and apply them into the revised manuscript, the manuscript will then be acceptable for publication.  

Reviewer 2 Report

My comment are in the attached file

Comments for author File: Comments.pdf

Author Response

We thank the reviewers for their comments and suggestions. The changes we made are explained in detail in our response to the reviewers. The changes and additions made to address the reviewers’ concerns are marked with red font throughout the manuscript.

This research manuscript have a good quality but some thinks can be improved. The experimental method is not sufficiently describe. Some figures are difficult to read. My comment are below.

In the introduction line 101: the correct sentence is “is unbalanced” and the laminate [02/θ2]S “In [9],the residual stress distribution in unbalanced composite laminates [02/θ2]s (θ = 0°, 30°, 45°,

60°, 90°)”

The authors appreciate the reviewer’s comment. This typo has been corrected.

Figure 1 and equation 1 are not in adequacy in the equation ε3 correspond to the shear that is not the case for your rosette strain gauge. For a better understanding, the Y direction can be noted 2 and the shear direction noted 3.

This issue has been revised. Many thanks to the reviewer for her/his careful reading.

“Line 184: a special strain gauge with eight grids is employed in present work. The position of the grids 1 to 8 is shown in Figure 2(a), and eight different combinations of strain gauge grids are evaluated as highlighted in Figure 2(b).” These sentences according to the figure 2 (a) and (b) are not clear, because if you used a 8 grids strain gauge you don’t have to turn it. If you used a 3 grids strain gauge you have to turn it to have the entire cartography (like figure 2(a)).

Thanks for pointing at this inconsistency. Additional sentences have been added for clarification, see lines 193-196.

Section 3.1 Prepreg process:

• What is the dimension of the mold? Have you cut specimen after molding (it is possible that after cutting some manufacturing stresses are release)?

The mold size was 100 mm*250mm and the samples were cut from the fully processed plate after demolding for residual stresses measurements. Here, residual stresses could have been already released by machining. This fact now is highlighted in lines 278-279.

You have to put the time of the curing process.

The curing time was 18 minutes at 160°C and a constant pressure of 0.3 MPa. Relevant information has been added in line 266.

• C U255-0/NF-E322/37%, You must mention the source I think Sigrapreg®? 37% is the resin content?

The information the reviewer requested has been added in line 270.

• Have you measured the surface roughness?

According to the suggestion from the reviewer, the surface roughness values of CFRP and metal sides have been measured and the information has been added, see lines 274-276.

• The quality of the figure 5 (b) have to be improved, and what is the arrow A and B? Please inform us in the legend.

The relevant information has been added in the caption of Figure 5. The residual stresses at points “A” and “B” were measured and the results are shown in Figure 9. For a symmetric laminate, the residual stresses at point A and B could be equal, however, this is not the case for an unsymmetric laminate.

Figure 6: carbon fiber are perfectly cylindrical, in your view, the carbone fibres are oval, why? In the prepreg process you have to explain the stratification of your CFRP, number of ply and orientation. In this figure what is the two line between interface and CFRP.

The sample was cut neither along nor orthogonal to the fiber direction. Thus, the fibers can appear in different shapes eventually only affected by their orientation with respect to the cutting plane. In consequence, the oval shape of some fibers results from an angel between cutting plate and the fiber being different from 0 and 90 degree, respectively. Within single plies of the CFRP the fibers are not perfectly straight and aligned. Moreover, fiber waviness may appear in the composite. These aspects further can explain the two lines between interface and CFRP. The information of the stratification of the CFRP has been added in line 264.                        

Section 3.2

Line 290: What is the full range of strain gauge (+/- X%)?

The full range of strain gauge has been added in the manuscript, see line 307.

• Have you measured the load imposed by the drilling tool on the material (with a load cell)?

The load imposed by the drilling tool was not measured in present work. The authors thank the reviewer for this suggestion and direction for future work.

• What is the approximate strain? You have to explain it in the section.

This value now is provided in the caption of Figure 7.

• In figure 8 you forget a P in CFRP. For a better understanding explain why you axis are plot only between 2000 and 1200μm (because you drill between 0 and 800μm), maybe you can add just few word like exterior of CFRP at -2000, exterior of steel at +2000 and interface at 0μm.

The sample is comprised of 2mm CFRP and 2mm steel. Thus, residual stress values from 2000 μm to -2000 μm were plotted. In light of the reviewer’s comment the interface now is highlighted for clarity in Figure 8. 

In figure 8 please put a second Y axis for the stress of CFRP (I can’t see the values and the Sigma Y (dotted line)).

The residual stresses in Y direction in CFRP were not shown in Figure 8, but in Figure 9. The aim of Figure 8 is to provide a general information on the residual stress distribution on both sides of the sample.

Line 329: Can you think that residual stress can provide by the difference of thermal expansion coefficient (negative for carbone fibre and positive for the metal sheet) When your steel is without hybridization, what is the shape, is it curved ? It’s not mentioned in your manuscript.

Residual stresses in hybrid composites are mainly induced by the different thermal expansion coefficients upon cooling. The steel sample before hybridization was not curved. This information has been added in line 280.

Why you choose this angle for orientation? I think you can found the real angle fibre with the ovalization of your cabone fibre in the figure 6.

As suggested by the reviewer, by using SEM one can determine the exact angle between fiber direction and the strain gauge. The aim of Figure 6 is to study the effect of the orientation between the actual fiber direction and the strain gauge on the resulting residual stresses. Therefore, different angles were chosen and tested. 

Section 3.3

• Line 457: [8]. In both case (you wrote [8] .In)

These typos have been corrected.

• Figure 12: can you put the legend of the setup on the figure: where are the mass, the applied point nose (diameter point or line) and the boundary condition (embedded at 0mm?)

All relevant information has been added in Figure 12.

• Line 462 is loaded as shown in figure 12 not 11.

Thanks to the reviewer for careful reading. Thy sentence has been revised accordingly.

You mentioned many time the difference of thermal expansion coefficient. Have you measured it? I think it is a good experimentation to improve the results.

Currently, quantitative data are not available, however, will be determined in future work. The present argumentation only considers well established qualitative relations.

I am disappointed not to see a micrograph of your hole and strain gauge.

The micrographs of the hole and the strain gauge have been added to Figure 2 and Figure 15, respectively. Also, additional texts for detailing Figure 15 have been provided, see lines 512-522.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors have satisfactorily responded to my comments. This version of manuscript is acceptable for publication in my opinion. 

Reviewer 2 Report

Authors respond to my comments.

The manuscript will then be acceptable for publication