Dislocation Emission and Crack Dislocation Interactions

Round 1

Reviewer 1 Report

Meaning of numbers in the third sentence is not clear.

Two ordinates in Fig. 10b look misleading.

Ref. numbers in Concluding Remarks should be corrected.

Refs. 57 and 60 are not mentioned in the text.

Author Response

Thanks for pointing out some typos in the paper, which have now been corrected in the revised manuscript. The numbers are in percent. We clarified the ordinates in the figure caption of Fig. 10 (b). We have modified the reference numbers in concluding remarks and  mentioned the ref. 57 and 60 in the revised version.

Author Response File: Author Response.pdf

Reviewer 2 Report

The fact that the experimental validation does not really validate the model is a point of concern.

A disrepancy of an order of magnitude is not something to be adressed in just one sentence.

This should be adressed in a revision of the paper / i.e. improvement of the model.

Author Response

We thank the reviewer for this comment. The reviewer raises a valid point regarding the discrepancy between the theory and experiments. We have now addressed this in the revised version, and have pointed out various factors that have contributed to this discrepancy and how to improve the theoretical models to address this issue.

Author Response File: Author Response.pdf

Reviewer 3 Report

The paper is a nice review in which the authors describe the experimental observations of dislocation emission, analyzing the influence of the dislocations in fatigue and fracture and estimating relevant parameters (plastic zone size, dislocation free zone and dislocation stress intensity factor. Although authors have not used the MDPI-Metals template, the paper is well written and the review is very interesting for the metal scientific community.
Some changes are proposed: 1) Since it is a review paper, Introduction should be extended; 2) Have you used some non destructive testing method to evaluate the cracking process, using for instance the acoustic emission technique by means of the elastic energy released as acoustic wave during dislocation emission and crack propagation? Could you evaluate this energy with your models?; 3) Please to remove the blank space between the quantity and the %, i.e. 30% instead of 30 %.

Author Response

Thanks for this interesting comment. We have made major changes in the revised version. Specifically, we have modified the introduction, discussion section,  concluding remarks and added some references. We have included the relevant references of dislocation emissions in the introduction. We did not extent the introduction because of the page limitation. In addition, we have modified the typos in the revised version.

In the present work, we have not used acoustic emission technique to study the dislocation emission and to evaluate the growth of the crack. However, it can be done as a future study to monitor dislocation emissions.

Author Response File: Author Response.pdf

Reviewer 4 Report

Title: Dislocation Emission and Crack Dislocation Interaction

In this manuscript, the authors show a review of experimental and analytical observations regarding dislocation activity around crack tips and the effects for crack propagation. The authors show very nice TEM pictures of the dislocation microstructure close to crack tips under loading. Furthermore, they explain a model to examine the emission of dislocations for specific loading configurations theoretically and compare the theoretical findings with experimental observations.

Although the subject is of high interest and scientific actuality, most of the paper has already been published in former publications of the authors. Therefore, the manuscript primarily shows a review on the former works of the authors and does not yield new findings and new insights.

This is accompanied by the fact, that the objective of the paper is not explained in the introduction, thus the reader is wondering throughout the paper, what the authors aim to show with the publication. The introduction does not give a thorough overview on the literature, but the literature is given throughout the paper, which also lower the understanding what the authors want to focus on. In addition, in the last paragraph of the introduction, the authors describe what they will show in the following sections. However, there is text missing in some sentences (objects, verbs) thus this is not understandable.

Throughout the sections 2-5, a fundamental problem is, that it is not clearly distinguished between what is new and what is taken from former works - particularly for the TEM pictures. There are also almost no explanations and details on how these pictures and results have been obtained. No setups, sizes and measures of specimen etc. are given (e.g. Fig 1b must be some in-situ picture, so it would be very interesting to have all the details). For the theoretical but also for the experimental parts, the used parameters are not properly introduced in many cases. Also the selection of the discussed models in totally unclear, besides they are related to the authors.

There is no final discussion in the manuscript. So, the reader is finally not sure, what he/she can or shall learn from the paper. The last sentence of the paper is “Such change in residual stress cannot be explained by the difference in dislocation density alone”. This is probably related to the results in Fig 11 (which is not referred to as it should be for a comprehensive discussion -  however, this is not the case in the whole section of concluding remarks). Maybe there is some hypothesis, the authors can imagine, what could be the reason?

Unfortunately, I have to suggest to decline the paper in the present form due to the issues mentioned above. However, I think the topic is very interesting and the former work of the authors is promising, so maybe the authors can rethink the focus of the paper and formulate their research question more properly.

Author Response

Thanks, we very much appreciate the reviewer’s comments. Yes, this manuscript has been submitted as a review of our work in this area.

We have included the relevant references of dislocation emissions in the introduction. We did not extent the introduction because of the page limitation. The objective of this brief review is

To establish that dislocation emission from cracks plays a major role in the fatigue process.To point out the methodologies and recent progresses in analytical formulation of the dislocation interactions, and the role they play in fatigue and fatigue processes. To present some very recent results pertaining to the changes in grain morphology, orientations and residual stresses as the materials undergo fatigue.   

Figs. 1 and 2 are TEM images obtained using in-situ experiments. We refer Ref. 11 for experimental details. We did not include everything because of the page limitation. Here we use Majumdar and Burns model to compare with the experimental observations as it deals with the mean position and number of emitted screw dislocations. The experimental observations (see Figs. 2 and 3) suggest they are mostly screw type.

We have now included a brief discussion and modified concluding remarks section. This is given below.

This review, we hope, establishes the fact that cracks are the primary source of dislocations. In fact, in our own work no secondary source of dislocation ahead of the crack was found. On the theoretical side, it appears that analytical modeling of the dislocation-crack interaction is much more complex than usually assumed. Firstly, there are more than one dislocation source at the crack tips. Secondly, the 3D nature of the process is hard to model and thirdly, the dislocations are curved. The superdislocation model introduced by Lin and Thompson may simplify some of the analytic difficulties. A relatively new result, i.e. the rotation of the grains ahead of the crack, even when the grains are not in the nano range ( £ 10 nm) was shown to be an important part of the fatigue process, and should be taken into consideration in making fatigue predictions.

The detailed picture of the deformation and fatigue processes has now been well described in literature. A semi-empirical understanding of various features involved in these processes is also available. Using empirical techniques and experimental methods, fairly accurate predictions can sometime be made. A macro picture of the process has also emerged, where dislocation density and dislocation configurations play a significant part. For an excellent,  but brief recent summary see  (Mugrabi, Ref    ).  For most of the purposes the above described knowledge of the phenomenon is sufficient.

In spite of the progress so far made, our basic understanding of many features of the processes are lacking. In the absence of the basic understanding, dislocation modeling so far has not become a predictive tool. It has been mostly used in many cases to provide  quantitative understanding of the experiments or give some rule of thumb for practical applications. We want therefore to strive a more basic understanding of the processes involved. In other words, we want to study the issues involved at a more fundamental level using modern analytical techniques and to use as far as possible rigorous mathematical techniques to analyze the dislocation configurations and their role.

Towards this goal, some fundamental studies have been made, which are briefly described in this review. In 2001, Riemelmosor et al gave  an overview of the dislocations modeling of cracks and in 2003 Pipan et al published a chapter in much greater detail on dislocation models of fatigue crack growth. Another review dealing with more applied aspects of the basic theoretical models, published in 2011 Bhat and Patibandia is also very useful.  A short review of 2D modeling, which is most common, is provided by Olarnritchinum in 2013.

One aspects of the modeling not mentioned in our review is the atomistic modeling of the fatigue process. This technique may be very useful in the future as the capacity of the computing process increases. A review of this technique applied to nanostructurally small cracks is available. In summary, much remains to be discovered and investigated before a comprehensive methodology for predicting fatigue from first principles can be made available.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Thanks for taking up the recommendations.

Reviewer 4 Report

There are no further suggestions from my side. The modifications significantly improved the manuscript.