Research and Simulation Analysis of Life Prediction in Notched Structures of DZ411 Alloy

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript addresses a relevant engineering problem by investigating the contrasting notch strengthening and weakening effects in directionally solidified DZ411 alloys and combines experimental testing with FE simulation. The topic is suitable for Materials. However, several concerns regarding clarity, completeness, and rigor must be addressed before publication.

1. The authors state that research on DZ411 with different notch features is "relatively scarce," but they do not clearly articulate what this paper contributes beyond existing studies (e.g., Zhang et al. [16–18], which already examined film-cooling holes in similar alloys). The novelty of studying both ring-notched and hole-containing geometries together should be stated more explicitly, and the gap in the literature should be better defined.
2. Figure 13 Caption Error Figure 13 is labeled "Ring-notched Specimen Model and Boundary Conditions," but it clearly shows the hole-containing thin tubular specimen model. This is a straightforward but significant labeling error that must be corrected.
3. The authors adopted creep strain thresholds of 5%–10% (ring-notched) and 10%–15% (hole-containing) as fracture criteria based on a comparison with smooth specimen elongation. However, the selection of these specific thresholds appears somewhat arbitrary. A clearer physical justification or reference to established fracture mechanics criteria should be provided. Why is 10% acceptable for one geometry but not for the other?
4. The study used only one temperature–stress condition per specimen type for the representative FE analysis. With only two simulation cases per geometry, the validation basis is narrow. The authors should either extend the validation to all experimental cases or explicitly acknowledge this limitation.
5. A direct, side-by-side quantitative comparison of the stress relaxation behavior between ring-notched and hole-containing specimens is missing. This is central to the paper's thesis; however, the two are discussed in separate sections without synthesis. A dedicated comparison figure or table showing stress relaxation rates, stress redistribution timescales, and final equivalent stress uniformity for both geometries would strengthen the paper considerably.
6. The abstract states that the subroutine "accurately simulates" creep behavior; however, the actual prediction errors can reach up to 40%. This characterization is overstated. "Within engineering accuracy" would be more appropriate and consistent with the body text.
7. The 930°C / 320 MPa row in Table 4 is formatted inconsistently, with multiple smooth specimen life values (37.97 and 28.07 h) listed without clear identification. It is unclear which smooth specimen values correspond to which notched specimens under this repeated-test condition. The table should be restructured for clarity.
8. The same formatting problem occurs in Table 6 for the 930°C / 320 MPa condition. The life ratio column also appears to list only three values for five notched specimen results, making comparison ambiguous.
9. The dispersion formula uses tc,maxt_{c,max} tc,max​ and tc,mint_{c,min} tc,min​ but the subscript "c" is not defined in this context (it might be confused with creep). Consider using clearer notation such as tmaxt_{max} tmax​ and tmint_{min} tmin​ for rupture life scatter.
10. Section 5.3 is titled "Creep Behavior Simulation of Hole-containing Thin Tubular Specimens," which duplicates the title of Section 5.2. Section 5.3 discusses model uncertainty and scatter; accordingly, the title should be changed (e.g., "Discussion of Model Accuracy and Experimental Scatter").
11. The FE stress/strain contour plots (Figures 9, 10, 15, and 16) use color maps; however, the legend values at the extremes (e.g., 3.6 MPa minimum in Figure 9a) appear physically questionable for a specimen under 540 MPa net-section stress. The authors should confirm that the color scale minimum values are correctly reported and are not artifacts of the boundary conditions.
12. The authors mention that no extensometer was used (Section 2.2). Although this simplifies the setup, it means that no creep strain vs. time experimental curves are available to validate the subroutine's strain predictions. This is a significant limitation that should be explicitly acknowledged in the discussion or conclusions.
13. Conclusion point (1) reads: "the endurance life of the ring-notched specimens in this study was significantly shorter" (referring to the hole-containing specimens compared with smooth specimens). However, the sentence structure in the conclusion mixes the two specimen types in a confusing manner. Please revise for clarity.

 

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The study addresses a relevant engineering problem, however, some aspects require improvement before publication.

The introduction is adequate but lacks a clearly defined research gap. While the authors state that studies on DZ411 with notch geometries are limited, the novelty remains incremental.

On the Experimental Methodology, no microstructural characterization (SEM, fracture surfaces, precipitates) is provided, which is a major limitation for a creep study.

The interpretation of “notch strengthening” and “notch weakening” is consistent with classical multiaxial creep behavior, however, the discussion remains mostly phenomenological. There is no deeper mechanistic explanation.

The creep model is clearly implemented and numerically consistent. However:

 

The model is purely phenomenological and not compared with other established creep laws.

A major concern is the reported prediction error of up to 30–40% in creep life estimation (Sections 5.1.3, 5.2.3, and 5.3) . This level of deviation is considered “acceptable” by the authors, but no justification, benchmarking, or literature support is provided.

 

Comments on the Quality of English Language

The manuscript requires moderate English revision. Issues include repetition of term, minor grammatical inconsistencies and some unclear phrasing in methodology and modeling sections.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Dear Authors,

The article submitted for review concerns a creep test performed on unusually shaped metal samples. The article demonstrates a high level of scientific merit and describes test results that contribute to expanding our understanding of the effect of notches and holes in the samples on the obtained creep endurance values. Simulations were performed, confirming the experimental findings. However, I noted several ambiguities that require correction or clarification. I provide my comments below.

1. The title of the article, which uses the phrase "Stress Relaxation," is misleading. The article covers creep tests (not relaxation tests) performed on smooth samples, notched samples, and thin-walled (tubular) samples. I suggest changing the title of the article.
2. Lines 110–111 indicate that the authors specifically designed a sample, the dimensions of which are shown in Fig. 1. Why weren't the dimensions recommended by the standards used? How does this sample differ from the shapes covered by the standards (how is it better)? The notch radius has a tolerance of ±0.005 mm. How (with what devices) were the dimensions of the samples measured after they were manufactured?
3. Lines 110-121: What was the value of the set force maintained until the specimens failed? How was this force selected or calculated? It would be helpful (but not required) to present creep graphs in the elongation-time coordinate system in the article. Such graphs provide a more complete picture of the phenomena occurring during creep.
4. Fig. 4. The figure on the right is missing the center line (axis of symmetry).

Kind Regards

Reviewer

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have revised the manuscript satisfactorily. I recommend it for acceptance.

Reviewer 3 Report

Comments and Suggestions for Authors

Dear Authors,

Thank you for your valuable and precise explanations and corrections. I have no further comments.

Kind regards

Reviewer