Experimental Analysis on the Impact of Current on the Strength and Lifespan of a Ni-Ti Element

Round 1

Reviewer 1 Report

Upon reviewing the manuscript, it seems that the authors have performed an application developed in the research laboratory for determining and monitoring the behavior of a material element with Ni-Ti shape memory. There are several crucial concerns in this work which should be addressed before it is suitable for publication, As the authors will note from my comments, Altering the manuscript structure to address such concerns earlier will improve the flow and readability of the paper.

• In abstract, “respectively its fatigue resistance”, what does the “respectively” mean?
• In page 2, line 76-79, “ɛ_t - the total relative elongation, ɛ_p- the permanent relative elongation”, ɛ_t and ɛ_p can not match figure 1, please be consistent.
• In page 3, line 85, the letter corresponding to Equation 2 has an error.
• In page 3, line 106-108, it is suggested to explain why you chose 0.4A and 0.6A, in this paper, these values considered as a percentage of 10% and 15%, these are too close, the amount of data is also insufficient.
• In page 3, line 116, in table 1, what does “PT1” mean?
• In page 5, line 150, in figure 4, there is no 7’, please be consistent.
• How many replicates were used for nitinol fatigue resistance test?

Comments for author File: Comments.pdf

Author Response

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Reviewer 2 Report

There are too many self-readings
The introduction is quite unclear and jumps from one idea to another. Each paragraph should support a single idea in order to have more clarity. More bibliographic references need to be entered. The introduction refers only to the shape memory (thermal) actuators. In the introduction must be presented the current state of the Ni-Ti elements.
Chapter 2 needs to be redone. The chapter starts too abruptly. You should introduce some notions of Thermomechanical Behavior. Pseudoelasticity and superelasticity must be defined. It shows some parameters such as the mathematical degree of elongation recovery and energy storage efficiency that are no longer used anywhere.
Chapter 3 needs to be redone. It's not obvious what it's referring to. I suppose they are the parameters used in experiments. The methodology for carrying out the experiments and how much would be the accuracy (i.e., reversed with chapter 4) should be introduced.  Figures 2 and 3 why have to be presented in the results. A conclusion should be drawn. What is the connection between the title of the chapter and the content?
Chapter 4 should be reversed with Chapter 3.
Chapter 5 should explain the parameters in Table 2 and their importance in the experiment. Sometimes test/rest period is used other times test/rest time. What is the connection between the number of attempts and temperature? In Chapter 5 a part of Chapter 3 on the comparison of experimental theory should be inserted.
Further research should be introduced in the conclusions. The dynamics of the actuator are presented only declaratively. What is the mathematical model? What is the specific field of use of Nitinol?

I recommend the publishing of the article after the improvement of the content.

Author Response

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Reviewer 3 Report

This paper studies the fatigue behaviour of a NiTi spring under cyclic heating-cooling. Due to the following fatal drawbacks, it has little/no scientific value.

(1) Unreliable tests

The paper claims to study the shape memory effect of NiTi under heating-cooling cycles. To do that, a DSC (Differential Scanning Calorimetry) must be conducted on the NiTi sample first to determine the following characteristic phase transformation temperatures: Ms (martensite start temperature), Mf (martensite finish temperature), As (austenite start temperature), Af (austenite finish temperature). Then the initial state of the NiTi spring can be determined: if the room temperature is below Mf, then the spring is in the martensite state; if the room temperature is above Af, then the spring is in the austenite state. If the temperature is in between, its state is undetermined, i.e., martensite or austenite or mixture (i.e., coexisting martensite and austenite phases). After that, the heating-cooling cycle can be determined: the maximum heating temperature of the sample must be higher than Af and the minimum cooling temperature must be lower than Mf to ensure there is cyclic martensitic phase transformation. Unfortunately, all these information is missing, which makes all the tests highly unreliable! The missing details are listed below:

• What is the initial state of the NiTi spring before each test? Martensite? Austenite? Mixture?
• What is the state of the NiTi spring at the end of heating for each test cycle? Martensite? Austenite? Mixture?
• What is the state of the NiTi spring at the end of cooling for each test cycle? Martensite? Austenite? Mixture?
• Line 75 ~ 76 sigma_i & sigma_d: from Figure 1, they refer to stress plateau. However, in tests, they seem to be the applied current. But authors claim here that they are voltage. What exactly are these parameters?
• Line 77 ~ 78 E1 & E2: from Figure 1, they represent line slopes. But the authors claim that they are energies. Unclear definitions.
• Line 103 ~ 104: the NiTi spring is elongated to “8 cm” (80 mm), but its initial length is only 20 mm (see Line 60). The resulted tensile strain is enormous, i.e., 300%! I’m afraid the NiTi spring is in the plastic region under such high strain level. Have the authors checked that the spring is always in the elastic region, which is the requirement of all the tests in this paper?
• Line 116 Table 1: more details are needed for this mathematical model The definitions of all parameters in the model is missing: what are xe, xi, T? How is this mathematical model derived? Any references? Based on which theory? Without reliable sources, this model is unreliable!
• Line 137: both “relaxed” and “compressed” states are mentioned. What is the exact initial state of the spring? Relaxed (i.e., without force) or compressed? If compressed, what is the initial compressive strain and force? Without the detailed information of the initial state, the forces measured in the tests are unreliable!
• Line 154: what is “ 7’ ”?
• Line 163: what is “n-1”?
• Line 164: is the rest period of 10 seconds long enough for the spring temperature to cool down below the martensite finish temperature? From Figure 8 in Line 226, the final cooling temperature is still so high, i.e., 43.4 °C! I’m afraid at this temperature, the material is still in the austenite or the mixture state.
• Line 198 Fig. 6: is the measured force due to the pure thermal expansion or the temperature-induced phase transformation? I’m afraid this force is not due to the phase transformation, which is aim of the tests, but due to the thermal expansion instead! The measured force at the beginning cycles (~ 4 N) in Fig. 9 is higher than the measured force (1.6 N) in Fig. 6. Taking look at corresponding heating temperatures, i.e., 94.3 °C from Fig. 8 vs 54.9 °C from Fig. 5, we can find that the measured force is almost proportional to the temperature increase (= heating temperature – ambient temperature) of the sample, which is quite typical for thermal expansion! If the measured force is due to the phase transformation, since the phase transformation strain is constant (i.e., does not depend on the final heating temperature), the resulted force should also be constant!

(2) Unsupported conclusions

• Line 260 “Joule heating of active elements is more efficient than…”: unfortunately, in the paper, the authors didn’t present any test to compare Joule heating with other heating methods to draw this conclusion!
• Line 265 ~ 268 “50% current of the maximum current …can be used…”: due to the unreliable tests, this conclusion is also unreliable!
• Line 269 ~ 273 “85% current … decreases …”: due to the unreliable tests, this conclusion is also unreliable!
• Line 281 ~ 291: all the conclusions in this paragraph have no test support from the paper!

(3) Bad English writing

I must say the reading this paper is very tiresome and difficult due to the bad writing. Some examples are given below (I do have time to list all the errors since they are numerous!):

• Line 9 ~ 12 the very first sentence in Abstract: but this is not a sentence! Strange structure! Need to rephrase!
• Line 53 ~ 55: not a complete sentence, need to rephrase.
• The authors do not know how to use “respectively” or perhaps they totally do not understand this word! Every usage of this word in this paper leads to misunderstanding, e.g., Line 14, 100, 107, 144, 162.
• Line 124 & Line 127: double “theoretical”
• Line 135 ~ 136: not a sentence, need to rephrase
• Line 144 ~ 148: really do not know what the author are writing, need to rephrase.
• Line 249 ~ 252: really do not know what the author are writing, need to rephrase.
• Line 277: what is “control their”?
• Line 281 ~ 291: messy structures! Need to rephrase!

Author Response

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Round 2

Reviewer 1 Report

The authors have done a great job of revising the manuscript. A few minor issues need to be addressed.

1、Page 2 and 3，line93, 96, 109, according to the revised introduction, it is suggested to renumber references.

2、Page 3，line 123, figure 1, it is suggested to change the Ep and Et on the X-axis to the ɛ_t and ɛ_p

Author Response

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Reviewer 2 Report

My comments are:

• self-citation: more than 9 bibliographic references are too much.
• the bibliographic references 6, 7, 8,9  are not used in text and can be erased
•  the bibliographic reference 1 can be erased
• the references 10 -12 are linked with a conclusion that could be made in only one article.
• figure 7 results from the citation 10-12 or is an original work?
• future work is missing 

Author Response

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Reviewer 3 Report

Thanks for the authors' responses for clarifying some issues. 

Author Response

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