Prediction of Fatigue Crack Initiation under Variable Amplitude Loading: Literature Review
Round 1
Reviewer 1 Report
This review paper deals with the various development of fatigue theories for modeling crack initiation in metals under variable amplitude loading (VAL). This survey is definitely worth undertaking since VAL fatigue crack initiation is of tremendous interest to engineers and scientists. However, there are some major issue that must be addressed before this paper can be accepted for publication in Metals, and they are as follows:
1. There is a large gap in terms of reviewing latest models capable of modeling fatigue, in particular, the works of Mozafari et al. [1-3], which are metal plasticity models based on modeling the smooth transition between microplasticity and macroplasticity, and these models have been shown to be able to model low cycle to high cycle fatigue under uniaxial and multiaxial loading conditions via FEM simulations, and they have also been used to model VAL fatigue experiments to good accord. A more comprehensive review paper on fatigue would contain a discussion of such modeling and simulation approach for predicting fatigue life.
[1] (2010) Proceedings of the Royal Society A vol. 466 pp. 423-438.
[2] (2020) International Journal of Fatigue vol. 134 art. 105477.
[3] (2022) Mechanics of Materials vol. 164 art. 104116.
2. In Page 15, it is stated that CDM is a relatively new branch of mechanics. It is not relatively new, and the citation of Kachanov [115] is to a wrong source. Please rephrase this statement and correct the citation.
3. The scales in Fig. 10 are not clear.
4. In Equation 2, please state explicitly that M is the Taylor factor. It depends on the crystallographic texture, and not the choice of material. This statement needs to be corrected.
Author Response
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Author Response File: 
Author Response.pdf
Reviewer 2 Report
(1) The paper is a literature review of fatigue crack phenomena in metallic structures, with particular regard to crack onset/initiation and variable amplitude loading conditions. The article is organized into thematic subsections. It starts with an overview of the crack initiation assessment approaches and the fundamentals of the crack initiation phenomena. Successively, the fatigue crack initiation phenomena associated with the microstructure aspects are described, and the available approaches and methods for the prediction of the crack initiation response and life are discussed, considering analytical and theoretical formulations, also including statistical aspects. The following sections of the paper are more specific and are related to (a) the crack initiation assessment considering variable amplitude loading conditions, (b) the fatigue crack initiation prediction through experimental tests, and (c) the influence of the temperature on the crack initiation. The conclusions summarize the review path as well as the key findings and discussion remarks.
(2) The paper provides useful descriptions and discussion remarks regarding the fatigue crack initiation phenomena in metallic structures. The contents are appropriate and cover in a relatively appropriate manner the past and current literature. Therefore, the paper might potentially contribute to the literature. However, the Authors should implement revisions since some issues affect the manuscript. Among the identified issues, the sequence of the contents should be improved, according to a more clearly logical and/or thematic flow. Furthermore, repetitions and redundancy should be avoided in the text, and the style should be more concise and strictly technical. The review comments and related revision recommendations are provided in the attached reviewed manuscript report.
Comments for author File: Comments.pdf
Author Response
Please refer to the attached file.
Author Response File: Author Response.pdf
Reviewer 3 Report
The research works presented and reviewed in the present paper covers a wide range of aspects and has profound significance in science and engineering. It is suggested that the authors consider the follow issues and seriously improve the manuscript before publishing. Appropriately increase the references of the researches of the immediate past decade.
1. The literature review focuses on the prediction of fatigue crack nucleation under variable amplitude load in the abstract. But the introduction and discussion of the effect of variable amplitude load on fatigue crack nucleation, in the text, seem not to be important point and not to be systematically displayed.
2. The experimental study of fatigue crack nucleation is introduced in this paper. However, the fatigue crack nucleation has a great relationship with the specimen form used in the test. What are the differences of the fatigue crack initiation laws among the standard tension / compression fatigue samples, torsion samples, notch samples, crack samples and other specimens? What are the effects of stress state and stress strain gradient in different samples? These issues were involved in the researches of many scholars, but they seem not to be explicitly discussed in the review articles.
3. The research work on fatigue crack initiation is mainly reviewed and introduced, but there is no explanation on how to define the fatigue crack. It is suggested that the author should supplement or improve the discussion on the following aspects which closely related to the topic of this paper: (1) The determination of crack initiation is related to the scale concerned by the research method, while the research involved in this paper (including various formulas) involves different scales, and the author has not made corresponding explanation. (2) how to distinguish micro cracks from macro cracks? (3) for the mentioned test, what method is used to identify the number of cycles corresponding to crack initiation, and what method is used to observe the cracks on the surface and inside the sample?
4. Equations (11)~(15) are often not aimed at crack initiation, but correspond to the full fatigue life of the material (including crack initiation process). These introductions are not very close to the topic of this paper, and contradict the previous statement on crack initiation. It suggests explaining them in combination with the topic of this paper. Moreover, these equations are mainly given for the case of symmetrical cyclic loading (average stress is zero), and should also be explained.
5. Equations (16) and (17) are generally not aimed at crack initiation. It seems that this discussion should also be explained in consideration of the theme of the paper.
6. This paper introduces the relevant research of FIP based on crystal plasticity finite element method (CPFEM). The determination of FIP and how to test its rationality and effectiveness is a very complex problem, because the evolution of the reasonable FIP to cycle may have nonlinear characteristics. Most studies seek FIP through the simulation with only several or several tens or hundreds of cycles, and only a few studies seek FIP and test its rationality through the simulation of the whole process of the fatigue cycle. It suggests that the author should pay attention to it in the article.
Author Response
Please refer to the attached file.
Author Response File: Author Response.pdf
Reviewer 4 Report
The reviewed paper titled “Prediction of Fatigue Crack Initiation under Variable Amplitude Loading: Literature review” is an interesting work constituting a compendium of knowledge on fatigue cracks, especially recommended to scientists starting fatigue research.
The authors did not avoid minor errors and inaccuracies. Notes for reflection and minor correction of the work:
There is no information about the types of fatigue cycles, e.g. constant stress, one-sided stress, zero pulsating stress, pendulum stress, etc.
No information about Wohler fatigue diagrams
Influence of type and frequency of loads on fatigue strength
Author Response
Please refer to the attached file.
Author Response File: Author Response.pdf
