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Reviewer #2: The manuscript intends to assess fatigue damage and cracking mechanism of CFRP composite with 45 degrees off-axis on the basis of the residual tensile strength methodology. Results of stress-strain of several static loading for both composites are well presented. The manuscript however requires to improve its literature survey and reference section. The Author needs to address recent research developments in literature. The reviewer included some of those and recommends the author to include in the revised manuscript. Comments are given as below:
1. The abstract does not clearly represents the manuscript." Off-axis tensile fatigue assessment based on residual strength for the unidirectional 45○ off-axis carbon fiber-reinforced composite at room temperature" is suggested as a revised title.
2. The first line of abstract, change "The tensile fatigue behaviors of unidirectional carbon fiber-reinforced thermoplastic and thermosetting laminates were examined at room temperature. " to " The tensile fatigue behavior of unidirectional carbon fiber-reinforced thermoplastic and thermosetting laminates was examined at room temperature. ".
3. In the abstract change "Observation" to "Examination".
4. In the last sentence of abstract, change "…while no interfacial/matrix damage in resins during low fatigue loading…" to "…while no interfacial/matrix damage in resins was detected during low fatigue loading…".
5. Under Introduction section, before the last sentence to the end of the second paragraph, the author needs to add following statement and references:
Varvani-Farahani and coworkers [*-*] have studied damage progress in FRP composites in three stages of (i) matrix cracking, (ii) fiber-matrix interface cracking and (iii) fiber fracture over life cycles. They mapped these stages and formulated for various off-axis loading conditions.
* A Varvani-Farahani, H Haftchenari, M Panbechi, A fatigue damage parameter for life assessment of off-axis unidirectional GRP composites, Journal of composite materials 40 (18), 1659-1670
*A Varvani-Farahani, A Shirazi , Prediction of stiffness degradation and damage of unidirectional GRP composites under fatigue cycles, Science and Engineering of Composite Materials 14 (3), 197-204
6. Based on data presented in figure 8, the author could show the degradation of modulus E in each hysteresis loop over 30 cycles. This could show how noticeable is the drop in modulus of elasticity over cycles for both composites.
7. In page 3, eliminate entire "However, all above investigations were concentrated on the stress-fracture cycles (S-N) relationship and corresponding model for predicting the arbitrary off-axis fatigue behavior based on fatigue results of three directional including 0 , 90 and ( : any one other direction) unidirectional laminates.". You instead include:
The (0/90) layup configurations of FRP composites were discussed and formulated through involvement of each composite laminate and were found in good agreement with experimental data .
* A Varvani-Farahani, A Shirazi ,A fatigue damage model for (0/90) FRP composites based on stiffness degradation of 0 and 90 composite plies, Journal of Reinforced Plastics and Composites, Vol. 26 (13), pp.1319-1336 (2007).
8. Equations (1)-(4) are statistical relations. Author needs to explain the damage mechanism based on a physics of damage progress in composite micro-constitutes and provide their corresponding equations. For example, the general formulation for residual strength is yet to present.
9. It is very enlightening if the author present micro-crack formation and cracking mechanism in the micro-constituents of composites in figures 12-14.
10. Conclusion section requires to highlight some keynote points on 45 off-axis composites described in discussion part of the manuscript.
In short, the manuscript includes valuable results however it requires to improve its literature survey and reference section. The manuscript is recommended for publication after fully addressing above suggested comments.
Reviewer #3: This is an interesting paper that has addressed the difference in fatigue life as well as in damage mechanism between the matrix-dominated fatigue behaviors of CF/PA6 and CF/Epoxy unidirectional composites. It may be considered for publication in the journal after minor revision that takes into account the following comments.
(1) The authors should check the statement of the first and second lines of the third paragraph in the introduction.
(2) The initial moisture contents of specimens before testing should be described in the text.
(3) The residual tensile strength of the CF/PA6 specimens that survived after fatigue loading is substantially equal to the initial value, while their tensile modulus is much lower than the initial value. The text should include a detailed discussion about the mechanism responsible for the observation in the unidirectional CF/PA6 composite.
(4) Why no SEM photos of the specimens fatigue loaded at a stress that is lower than the conditional fatigue limit identified? They should have demonstrated a very small number of debonding at the fiber/matrix interfaces in the composites.
Reviewer #4: The paper contains some inconsistencies. Some of them shold mandatorily corrected. The details of my comments are listed below
1) In the "Experimental " section (lines 29-31) the sentence "The mechanical properties.......were similar", is wrong just looking at the data. For instance the modulus and the strength epoxy resins is about 30% higher than PA6. More importantly, epoxy resins are brittle materials whereas PA6 is a ductile polymer.
2) In the "Results and Discussion" section (lines 14-25, pg.5) the substantial difference between the two systems is that epoxy resin wets the fibers at the pre-polymer stage while PA6 is already a polymer during fiber impregnation process and its ability to diffuse among th fiber bundles is much less than that of unreacted epoxy resin. In addition , please consider also that chemical shrinkage is a source of flaws in in crosslinking resins due to the considerable tensile stresses arising at the fiber resin interface.
3) In the "Results and Discussion" section (lines 44-48, pg.5) it is not clear which criteria is used to define the fatigue limits of both systens under study. Is it the "apparent" flatness of fatigue life curves? Please, give an explanation different from that reported in lines 47-48. For instance, given the loading condition and fibers orientation, is 4x10^6 a common magic number for both thermoplastic and thermosetting carbon fiber composites? If so, let me mention that I never heard anything similar, so far.
4) At page 7 line 19, the word "concluded"should be replaced with "reported".
5)At page 7 from line 20 to 37 the discussion is not completely correct. As stated in the introduction the strength is a statistical function. It would be very interesting if the authors try to evaluate the Weibull parameters of the residual strength in comparison with those of the static strength. It is widely reported that, despite the apparent similarity shown in figure 10, the fatigue damages imply a reduction of both the the shape and the scale factor (in respect to the parameters obtained for the static strength) when the residual strength distribution is reported in a Weibull plot. This aspect is of fundamental importance when reliability is of concern.
I am reporting some papers that help the discussion to take into account the statistical aspects of fatigue and residual strength.
- K.I. Tserpes, P. Papanikos, G. Labeas, Sp. Pantelakis Fatigue damage accumulation and residual strength assessment of CFRP laminates Composite Structures 63 (2004) 219-230
- R. D. B. Sevenois, W. Van Paepegem, Fatigue Damage Modeling Techniques for Textile Composites: Review and Comparison, Applied mechanics reviews (2015), 67 (2), .020802-(1-12)
-Chou P.C., Croman R., 'Residual strength in fatigue based on the strength-life equal rank assumption', Journal of. Composite Materials (1978), 12, pp. 177-194.
-Whitworth H.A., 'Evaluation of the Residual Strength Degradation on Composite Laminates Under Fatigue Loading', Compos Structures, 2000, 48, pp. 261-264.
- A. D'Amore, M. Giorgio, L. Grassia, 'Modeling the residual strength of carbon fiber reinforced composites subjected to cyclic loading', International Journal of Fatigue 78, pp. 31-37 (2015)
- A. D'Amore, L. Grassia "Constitutive law describing the strength degradation kinetics of fibre-reinforced composites subjected to constant amplitude cyclic loading" Mechanics of Time-Dependent Materials Volume 20, Issue 1, February 2016 ISSN: 1385 (Print) 1573-2738 (Online), -(2016)
- DOT/FAA/AR-10/6 (2011) "Determining the Fatigue Life of Composites Aircraft Structures Using Life and Load-Enhancement Factors" http://www.tc.faa.gov/its/worldpac/techrpt/ar10-6.pdf