Taxonomy of Gas Turbine Blade Defects

This work   presents a good collection of engine blade defects with root causes. The work   is in general well organized and written.

We thank the Reviewer for this favourable comment.

But it can be improved providing a better link between the first part   including Introduction and Section 2 and the rest. More specifically, from   section 3 onwards the authors clearly state that they are considering only   defects detected by visual inspection while in the first part there is   mention of post incident inspection with different methods. The authors should better justify why   they focus on visual inspection only and briefly describe which type of   defect is detectable through visual inspection and which is lost requiring   other means.

We briefly described the focus on visual inspection and the   detectable defects by visual means.  See new text in section 01.

Lines 18-20:   “The work provides a single comprehensive illustrated list of engine blade   defects, and a standardised defect terminology, which currently does not   exist in the industry.” 

To what I   know each industry has its internal definitions and regulations and so it is   not clear whether this work may be of help to the industry. It might be useful   to add a sentence specifying the   principal audience to which the work is destined to.

Thanks, we added the principal audience as suggested. See Section 1,   and also 4.1.

Lines 43-47: “During   engine maintenance all blades are visually inspected for defects or   indications of damages. This is either done by borescopic in-situ inspection   or by complete disassembly of the engine followed by on-bench piece part   inspection [12]. In both cases the inspector has to examine the blades,   identify any damage and decide whether a part is serviceable, repairable or   has to be replaced.”

This part is   not at all correct as written since visual inspection is only one step and   the first one of the inspection process. Generally, the visual inspection is   followed by other non-destructive testing techniques such as ultrasonic,   infrared thermography, etc. This aspect is mentioned after on line 50 “an   ongoing need for more effective and efficient inspection processes”  It   is better to modify the text to better   express these concepts.

Thank you for your comment. Indeed, borescope inspection (visual) is   the first step in the inspection process. However, a borescopic in-situ   inspection is not always followed by an on-bench inspection or other NDT methods,   because if no defect is found during borescope inspection the engine module   is not teared down.

Nonetheless, we have emphasised the point that visual inspection is   the first step in the inspection process and other methods are required to   detect flaws that cannot be detected by borescope/visual means.

See new text in section 1.

Figs.2-4     where possible especially on Fig. 2 stretch the rectangles to   make each word fit on a line

We tried to improve this issue as suggested. However, the software   (Protégé) we used to create the diagram does not allow changes to the   rectangles or hyphenation. 

The current paper deals with a systematic categorization of defects   and defect causes in gas turbines viz. mainly jet engines. It is based on a   comprehensive and thorough literature review and supplemented by own research   basically by exchange with experienced MRO facility personnel. For the first   time a systematic and unambiguous wording and categorization of defects and   causes as well as their links is presented with the help of an ontology. The   presented defects are limited to defects detectable by visual inspection,   which are the most relevant in MRO.
  Against the background of only a few and less comprehensive   approaches/publications in this field in the past I consider this publication   a big step forward in structuring the scientific discussion and thus, laying   the basis for a sound scientific approach to gas turbine MRO in the future. I   expect that this paper will have a big impact in terms of future scientific   publications in this field, which will refer to this structured approach.   Thus, I strongly recommend its publication provided deficits mentioned in the   following are fixed.

We thank the Reviewer for this favourable comment.

The possibly biggest achievement of this work is documenting the   links between defects, causes and root causes, respectively. I agree with the   authors that this offers the opportunity either to find root causes for   identified defects or – in the inverse direction – systematically searching   for defects after an incident.

Whereas the existence of defects and causes is sufficiently well   documented in the paper and/or are obvious, their links - which are basically   physically mechanisms - are currently not. However, from my point of view   scientific rigor requires that each of these links, which the authors   postulate, must be verifiable. At   least it should be clearly stated by the authors for each link based on what   reasons they decided to set this specific link. I suggested doing this by   references or footnotes for each cause within table A.1. Due to this   deficit only I request major revisions.

Identification of the cause-defect relationship was done by analysis   of the research literature, discussion with expert maintenance practitioners,   reference to the maintenance manuals [19][20][21], and general engineering   principles. Hence this set of relationships has a degree of validation, and   provides a sufficient representation of the complexity of the real situation.   In particular, it is possible to show cascade failures using this tabular   data, as shown in the next section.

However we do not claim that the list is perfectly comprehensive, nor   entirely validated. It merely represents what is commonly known about the   cause-defect relationship. While it may be ideal that each of these   relationships be verified, that would require new research and a changed   level of record-keeping in the industry (see Discussion). Nor have we   addressed the other ‘Damage Attributes’ identified above. Nonetheless in   principle such additional data could be added to the table as additional   fields.

The cause-defect list is believed to be adequate for the maintenance   audience since it was validated in that field, but we caution that it may not   be sufficiently exhaustive for the accident investigation audience.

Text to this effect has been added in section 4.3.

Furthermore, there are some minor formatting and spelling   issues/typos, which have to be corrected:

Thank you. We took the Reviewer’s comments into account and improved   the formatting and spelling. Additionally, some other minor changes were made   in accordance to observations by other Reviewers.

Line 80: “[…] that caused the event   to happen. […]”

Fixed.

Line 240: Table reference corrupt

Fixed corrupt reference link.

Figure 3 and 4: The figures are barely readable in the document as   the font size is too small. If I zoom in I cannot read it either as the   resolution is too low. Thus, I am wondering what purpose these figures of the   whole ontology have here. I suggest either just showing readable subsections   of the ontology within the paper and refer to the supplementary files for the   full picture. Or use a vector graphic of the full ontology so a reader can   successfully zoom in the PDF file. Or use a larger font size.

We apologise the poor quality of Figure 3 and 4. This is a   fundamental limitation of the software used. The software we used to create   the diagram (Protégé) does not allow exporting files into vector graphics.   Only jpeg and png format is available. However, we tried to improve the   quality by applying image enhancement methods in Photoshop.

Line 368/369: “[…] One of the challenges with this is the need to   collect better At present MROs do keep data such as how many engines had an   engine visit due to FOD. […]” è I do not understand this sentence. Please   rewrite and re-arrange it.

Thank you. Sentence was rewritten as suggested.

Line 406: The header should be Conclusions (plural) as you have   more than one conclusion.

Fixed.

Additionally, to these publication-relevant   revisions, I recommend to publish the original OWL source files as   supplementary files via the repository in addition to the highly resolved   figures of the ontology. I expect that this would increase the range and the   citations of the work significantly. Other researcher and practitioners most   likely will use and adapt it for their purposes.

We thank the Reviewer for his favourable comment and valuing the   ontology. We are happy to provide the OWL source files as supplementary files.

This paper presents a taxonomy system for assessing aero-engine blade   defects. Potential defects types and their causes are identified from the   literature and maintenance manuals and categorized. Then, the categorized   defects and causes are integrated with an ontology. The reviewer suggests   that this paper would be accepted after addressing the following issues.

We thank the Reviewer for this favourable comment.

1. The description of Table 1 is omitted in the manuscript.

A reference to Table 1 was added to the manuscript at appropriate place,   where it was further described.

2. The numbering of potential causes in the   cause-defect list in Table 4 and Appendix A are different from Table 3. For   example, in Table 4, the numbering of cause ‘Fatigue’ is 2. However, it is 6   in Table 3. The reviewer suggests that the numbering should be identical to   reduce confusion.

The numbering in Table 4 and Appendix A was done independent of Table   3. Reason being that not all causes lead to each defect and therefore would   lead to the following table:

Potential Causes:

1.1. FOD

1.2. KOD

1.3. Organic impact

3.1. Stresses caused by heat

3.2. Sudden overload

6.1. Thermal or mechanical creep

We evaluated both options and came to the   conclusion that the original numbering should be kept to avoid confusion.

3. In Section 4.3, the reference source   error is found.

Corrupt reference link fixed.

The results are important and useful for other researchers as well as   practitioners. The ontology software can be used to extend the new defect   examples from real world data base.

We thank the Reviewer for this favourable comment. Some minor changes   were made to accommodate observations by other Reviewers.

Some limitations revealed in the article can be good start point for   further research.

Thank you, we agree that there is potential for further research.

Page   10 / 240

extract   from that list is shown in Table 4 Error! Reference source not found.Error! Reference source   not found. below.

Corrupt reference link fixed.

Reference   duplicate

[17]   Laguna-Camacho, J. R. et al., "A study of the wear damage on gas turbine blades," Engineering Failure Analysis, vol. 61, pp. 88-99, 2016. doi: http://dx.doi.org/10.1016/j.engfailanal.2015.10.002.

[61]   Laguna-Camacho, J. R. et al., "A study of the wear damage on gas turbine blades," Engineering Failure Analysis, vol. 61, pp. 88-99, 2016/03/01/ 2016. doi: http://dx.doi.org/10.1016/j.engfailanal.2015.10.002.

Duplicated reference [61] merged with reference [17].