Comparison of Specimen Geometries for Measuring Through-Thickness Tensile Mechanical Properties of Fibre-Reinforced Polymer Composites

Round 1

Reviewer 1 Report

General comment:

The paper is well within the scope of the journal and shows comprehensive results of through-thickness (T-T) tension experiments and a comparison to linear elastic finite element analyses results. The subject of investigation is oriented to provide confidence to the experimental methods and results. The paper is of high quality, well written and focussed. From the reviewers point of view only minor revisions are needed regarding: emphasising that only tension experiments are focussed, consideration of tolerances and discussion of machining influences (description and analysis) and the choice of stresses for failure analysis.

Due to quality of the paper only a view comments are given in the following.

Comments:

• Introduction:
  • From the reviewers point of view it would help to emphasis right from the beginning and later repeatedly that only tension stresses \sigma_33 are considered. It is given in the headline but, for instance, not in the abstract. Complete through thickness characterisation surely requires more tests (compression, shear etc.).
• Experimental testing
  • Please provide, if possible, also the individual equations used to calculate the stresses from the experimental force reading.
  • Could you provide requirements for the machining processes? Did you check the specimen geometry after machining?
  • Please discuss the influence of machining tolerances and machining induced damage. This may help to understand differences between strength values in terms of volume/surface-ratios later.
• Finite Element Analysis
  • The presented approach seems to be valid. More explanations should be spent to the choice of stress reading and their analysis:
    • Are peak stresses also taken from the mid plane element set or from the global set? What about the overstressed areas at the radius (e.g. Spool A, B) far away from the mid plane?
    • Do you expect multiaxial stresses due to Poisson’s ratio effects in the stacking? Could you please provide multiaxiality ratios or could you apply fibre composite related strength criteria?
    • Which criterion has been used to stop the calculations? How sound is the comparison between FE-stresses and experimental strength?
  • Figure 13 and the related discussion on deformation measurement “somewhere” in the testing machine may be deleted. The comparison based on strains is sufficient.
• Conclusions
  • Please include machining influences and requirements to your conclusions.
  • Refer to the choice of stresses and failure criteria as well.
  • Maybe volume/surface-ratios could also improve the discussion.

Author Response

Please see attachment

Author Response File: Author Response.docx

Reviewer 2 Report

The article is written nicely, and it is easy to read. In the article a comparison of different through-thickness methods has been performed through experimental testing and finite element analysis.

My comments and questions are as in follows: 

1. Is the addition of the FEA what makes this article unique? As authors have cited there is already a review of experimental testing of the through-thickness methods. 

2. (optional) In page 9 of 19, line 163: Could you explain what model does Abaqus use to make the properties valid for the whole geometry instead of layer by layer? 

3. In the FEA, a concentrated load was applied. That makes the simulation simpler, however, for realistic purposes should there a distributed load be used instead? 

4. In page 10 of 19, line184 and 185, it is mentioned that  the failure was either  intra- and inter-laminar failure. Which ones are intra and which ones are inter. Could you visualize it with a fracture surface picture? 

5. The addition of fracture surface analysis to investigate better the failure mechanism would be a good addition. 

6. Since FEA analysis was performed, has there been a mesh refinement study to make sure that the results did not change drastically? 

7. In the indirect geometries FEA results there is a discrepancy of predicting displacement results. One of the reasons for that authors claim that there is a lot of stress concentrations. Using finer elements in those regions, maybe could give different results? 

8. In page 17 out of 19, line 302, maybe the fracture surface analysis could provide more information on the big difference? 

Author Response

Please see attached

Author Response File: Author Response.docx