Evaluation of Fracture Strain for Cold Drawn Thin-Walled Steel Tubes via Small Round-Bar Tensile Test

Round 1

Reviewer 1 Report

This paper is generally good but the authors need to pay more attention to the consistency of the figures and the text. Some questions/suggestions are as follows:

1. The authors show the chemical components of the steel in Table 2 but the types of steel are not mentioned. It should be illustrated in AISI/JIS or other commonly used standards.
2. Fig. 2 shows the microstructure of the steel tubes after drawing and annealing. Did the authors also examine the microstructure of the steel in different stages (like before welding, before drawing, before annealing, etc.) to ensure that the differences in grain structures in the axis and hoop directions came from the axial elongation in the drawing process?
3. Fig. 3  is missing.
4. The thickness of the target steel tube after drawing was 1.8 mm and the shoulders of the small round-bar specimens cut from the drawn steel tube were 2.5 mm. Is there any reason why the authors would rather weld a dummy steel tube to the drawn tube for producing small round bar specimens than drawing a steel tube with a thicker wall or cutting specimens into a smaller size? Was the accuracy of the tensile test affected by the dummy steel tube?
5. The authors stated that the nominal stress s and cross-sectional reduction ratio curves ρ were shown in Fig. 8 (lines 231-232). However, the curves in Fig. 8 look like stress-strain curves. Please clarify.
6. Should the “Fig. 13” mentioned in lines 265-276 be Fig. 11? There is no Fig.13 in the manuscript. Please clarify the “Fig. 12” in line 276 as well.
7. As mentioned by the authors, the superior fracture strain evaluated in the hoop direction of tube C was an interesting phenomenon. Did the authors repeat the experiments using different tubes made of the same material?
8. In the lable of Fig.7, what it means by 15.3 thickness% clearance?
9. In Fig. 8, the x axis is true strain or nominal strain?

Author Response

Thank you for your kind reviewing our manuscript. We uploaded the revised manuscript and response letter.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

This study evaluates fracture of three variants of cold-drawn, heat-treated steel tubes under uniaxial deformation in axial and hoop directions. Overall, the study is of interest and worth of publication provided that a few important points be considered in the revision:

- It should be noted that the fracture strain measured from post-mortem specimens under microscope, is the average of strain over the whole cross-section. Locally, at each material point the strains are likely higher at the center of the specimen due to non-uniform distribution of strains and can only be revealed with finite element simulations.

- The method utilized in this study to extract local strains with the optical micrometer reminds the Reviewer of the Area Reduction Method (ARM) that is typically employed to extract hardening (stress-strain) response of materials to large strains (past the onset of diffuse necking). The authors are suggested to see the study by Abedini et al. (2022, Forces in Mechanics, 7, 100090) where the technique is described in more detail. As noted by the authors, it is reported by Abedini et al. (2022) that the aspect ratio of the cross-section is critical to be properly measured for effective application of the technique. The authors are suggested to reflect this point in the revised manuscript.

- The authors should note that the fracture strain derived from Eq. (2) is the “major principal strain” at fracture and should not be confused with the equivalent strain at fracture that is typically termed as “fracture strain”.

- The effect of non-proportional loading on fracture behaviour of materials is critical and should be noted when interpreting the results of this study with tensile specimens (see the study by Abedini et al., 2018, International Journal of Solids and Structures, 144-145:1-19). Upon plastic localization, the stress state starts to deviate from uniaxial loading and shifts towards the plane strain state with larger stress triaxiality than 1/3. In other words, the fracture strain measured in the current study is not the fracture strain under uniaxial loading since the stress state does not remain constant at 1/3 throughout deformation.

- Considering the comment above, perhaps using the hole tension test (see Roth and Mohr, 2016, International Journal of Plasticity, 79:328-354 & Abedini et al., 2018, International Journal of Solids and Structures, 144-145:1-19) was more appropriate for this type of application that aims at edge quality in the burring process. Hole tension tests suppress, or at least delay, the onset of plastic localization.

- Figure 3 seems problematic in the manuscript received by the Reviewer. Currently there is no picture associated with this figure and only the caption of the figure is present.

- It appears that the term “crack percolation” has be used by the authors to refer to void nucleation followed by coalescence of voids leading to formation of micro-cracks that subsequently grow and create macro-cracks. Please clarify this point.

Author Response

Thank you for your kind reviewing. We uploaded the response letter. Please see attaching file.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

This paper can be accepted with the revised version.

Reviewer 2 Report

I recommend publication of this revised manuscript in the journal of Metals.