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ACS Omega
ACS Omega
ao
acsodf
ACS Omega
2470-1343
American Chemical Society

10.1021/acsomega.4c05289
Article
Effect of Low Temperature on the Fatigue Crack Propagation Behavior of Underwater Manned Vehicle Rudder Materials in Arctic Environments
Guo Wenyong
https://orcid.org/0009-0001-3389-152X
Yu Li
Wu Zhe
Zhang Yue
Cao Chenghao
Zeng Yan
Yu Liangwu *
https://orcid.org/0009-0004-0354-7156
Huang Jianing *
College of Power Engineering, Naval University of Engineering, Wuhan 430033, Hubei, P. R. China
* Email: 19871536905@163.com.
* Email: 1920191014@nue.edu.cn.
30 08 2024
17 09 2024
9 37 3892538935
05 06 2024
21 08 2024
16 08 2024
© 2024 The Authors. Published by American Chemical Society
2024
The Authors
https://creativecommons.org/licenses/by-nc-nd/4.0/ Permits non-commercial access and re-use, provided that author attribution and integrity are maintained; but does not permit creation of adaptations or other derivative works (https://creativecommons.org/licenses/by-nc-nd/4.0/).

During polar navigation, adverse environmental conditions like cold temperatures, fatigue, and corrosion can affect surface and underwater manned vehicles (UMVs). Understanding the fatigue fracture growth behavior of polar ship steel is crucial for ensuring safety. This study investigates the mechanical properties and fatigue fracture propagation of steel used in underwater vehicle rudders under various low-temperature conditions through experimental research. It compares and analyzes the static mechanical characteristics, fatigue crack growth rate, and fracture morphology of underwater manned vehicle rudder steels at different low temperatures. Findings show enhancements in yield strength, tensile strength, elastic modulus, and fatigue crack propagation life of steel 925A, steel 20#, and their welded parts under low-temperature conditions. The tensile strength of 925A steel, 20# steel, and their welded parts increases by 6.87%, 14.61%, and 12.55%, respectively, as the temperature decreases from 20 to −60 °C. The yield strength also increases by 14.17%, 29.09%, and 15.76%, respectively. Fatigue crack propagation rate experiments were conducted under different constant low-temperature conditions. This study offers direction for future modeling and experimental testing.

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pmc1 Introduction

1.1 Background

In recent years, the Arctic region has garnered significant attention from countries worldwide due to the escalating effects of global warming leading to the rapid melting of Arctic ice and snow. This heightened interest is primarily driven by the region’s rich natural resources, the potential economic and geostrategic advantages offered by Arctic shipping routes, its substantial influence on global climate change, and the considerable economic and scientific research opportunities it presents.1 The strategic importance, economic significance, scientific research potential, environmental conservation, navigational aspects, natural resource abundance, and other attributes of the Arctic are continuously expanding, prompting close monitoring by the international community.2 The majority of the Arctic region remains covered in thick ice throughout the year, with recorded summer temperatures in the Arctic waterway reaching up to 20 °C and winter temperatures dropping as low as −60 °C.3 Various countries and researchers are closely examining the endurance capabilities of polar surfaces and underwater vehicles, which are crucial for facilitating polar development and navigation.4−6

Due to the polar strategic environment and sea ice conditions, underwater vehicles are essential for polar strategy.7 In the rudder mechanism, the rudder blades and rudder stock may collide with sea ice during navigation, causing fatigue problems. Extreme conditions such as low temperature, alternating loads, and high corrosion in polar environments have a significant impact on the fatigue life of rudder materials for ships.8 To improve the safety and reliability of underwater vehicles in polar environments, this paper aims to study the low-temperature mechanical and fatigue properties of materials used for underwater vehicle rudders in polar low-temperature environments. Through experimental methods, their influencing factors and mechanisms are analyzed to determine low-temperature fatigue parameters for the design of underwater vehicles for polar navigation.

Fatigue failure remains one of the main causes of mechanical component failure, accounting for more than 80% of such situations.9 The fatigue crack propagation rate, as a fracture mechanical performance index of materials, is highly important for accurately evaluating the service life of components. According to the American Bureau of Shipping (ABS) regulations for low-temperature design in polar regions, the minimum design temperature for polar ships is −60 °C.10 Therefore, studying the fatigue strength of steel 925A used for underwater vehicle rudder systems at temperatures of −60 °C and above is particularly important.

1.2 Literature Review

By studying materials based on fracture mechanics, domestic and foreign scholars have researched the fatigue crack propagation performance of different steels. The relevant research investigated in the field of steel for polar ship structures includes: Wang et al.11 studied the mechanical properties and fatigue behavior of EH36 steel at temperatures ranging from room temperature to −60 °C. Zhao et al.12 compared the fatigue crack propagation rates of DH36 steel base metal and welded specimens at room temperature and −60 °C. Through experimental research, Qiao et al.13 systematically investigated the fatigue crack propagation behavior and mechanical property evolution of two commonly used shipbuilding steels, EH36 and eq 70, under the combined effects of Arctic low temperature and overload at temperatures ranging from room temperature to −60 °C. Zhang et al.14 conducted quasi-static and dynamic tests on EH36 steel within a temperature range of 20 °C to −60 °C. Bridges et al.15 performed experimental research on cyclic tensile fatigue tests of welded specimens made of AH36 and DH32 grade steels under conditions ranging from room temperature to −45 °C. Nguyen et al.16 predicted the progressive brittle damage of ship structures under different loading conditions from 0 to −50 °C and evaluated the structural strength during the damage evolution process.

They analyzed the influence of low temperature on the mechanical properties and fatigue crack propagation rate of steel used in ship structures. They studied the fatigue behavior of steel at low temperatures and the risk of brittle fracture in cold climate environments. The researchers emphasized the significance of understanding the mechanical behavior of ship materials under Arctic conditions to ensure the safety and structural integrity of ships navigating in cold environments.

In other structural steels, Liao et al.17 studied the fatigue crack initiation resistance of Q345qD steel through strain-controlled fatigue tests at room temperature and −60 °C. Kabaldin et al.18 discussed the fatigue failure of 20 steel at −30 °C low temperature and delved into the low-temperature fatigue failure mechanism. Sallaba et al.19 investigated the fatigue and fracture behavior of S500 welded steel joints in environments ranging from room temperature to −100 °C through a series of experimental methods. Lesiuk et al.20 analyzed the typical fatigue crack loading schemes of riveting cracks and bridge structural components, examined the process of fatigue crack propagation, and constructed a dynamic fatigue fracture diagram.

They all emphasized the importance of low-temperature fatigue behavior when using steel for structures such as bridges in cold regions. They explored the relationship between fatigue and fracture behavior of high-strength steel substrates and welded joints at low temperatures. The significance of understanding the fatigue crack propagation of low-carbon steel under mixed-mode loading conditions during long-term operation was studied. They highlighted the importance of material behavior in extreme environments for structural integrity.

Overall, the literature review indicates a growing interest in studying the effects of low temperatures on the fatigue properties of materials used in polar environments. Research on metallic alloys and steel used in polar ships provides valuable insights into the unique adaptations and challenges associated with polar and low-temperature fatigue.

However, in the Arctic region, the rudder mechanism of underwater vehicles is subject to the combined effects of environmental temperature, corrosion, and alternating loads, and there is currently a lack of research considering the fatigue crack propagation behavior of their rudder materials at low temperatures in the Arctic. One of the authors of this article, Yu, has previously analyzed the low-temperature fatigue performance of 925 steel.8 This article primarily compares and analyzes the mechanical and fatigue properties of 925A steel, 20 steel, and their welded components. Therefore, the parameters related to 925A steel may be reprocessed and applied. In this study, steel 925A, a commonly used material for polar special ships, and its welded parts with steel 20 were used as the research objects. Through experiments, the FCP of these two single materials and welded parts in polar low-temperature environments and their mechanical property evolutionary laws were systematically studied.

2 Experimental Section

2.1 Materials and Specimens

In this study, steel 925A8 and steel 20 for ship rudder and their welded components were used in the experiment. The experiment utilized materials provided by Wuchang Shipbuilding Industry Co., Ltd. The welding material used was ship welding 395, and the chemical composition parameters of the material are shown in Table 1. The size of the specimen used for the tensile test is shown in Figure 1. According to GB/T228.3–2019,21 considering the equipment and its clamping in the low-temperature chamber, the specimen used in this test was a long specimen with a circular cross-section and threaded connection. The size details of the compact tensile C(T) specimen are shown in Figure 2. According to GB/T6398–2017,22 the fatigue test used a compact C(T) specimen with a width of W = 50 mm and a thickness of B = 25 mm, which was prepared by wire cutting, drilling, and milling methods. The sampling direction of all the samples was strictly controlled in the rolling direction of the steel plate, ensuring that the grain flow direction of each sample was the same and that the fatigue performance of the material remained consistent.12

Figure 1 Specifications of tensile specimens (unit: mm).

Figure 2 Sample size of welded component C(T): (a) sample size diagram (unit: mm), where the shaded area represents the weld seam area, and (b) sample physical image.

Table 1 Chemical Composition of Steel 925A, Steel 20#, and the Welding Rod (Mass Fraction, %)

material	C	Si	Mn	S	P	Cr	Ni	Mo	V	N	Cu	
925A	0.13–0.18	0.17–0.37	0.30–0.60	≤0.015	≤0.020	0.90–1.20	2.60–3.00	0.20–0.27	0.03–0.08	 	≤0.25	
20#	0.17–0.23	0.17–0.37	0.35–0.65	≤0.035	≤0.035	≤0.25	≤0.25	 	 	 	≤0.25	
395 Welding rod	0.08–0.12	0.5–1.0	1.0–2.0	≤0.020	≤0.030	15.0–17.5	24.0–27.0	5.5–7.0	 	0.10–0.20	 	

The tensile test was conducted using a 100 kN ETM105D series universal testing machine, as shown in Figure 3(a), equipped with an EMC003A-2 high- and low-temperature environmental testing chamber. Liquid nitrogen was used for refrigeration, and the temperature was adjusted through an electromagnetic valve to ensure that the temperature change could be within a range of ±0.1 °C. The applicable temperature range was −70 to 150 °C. The fatigue crack propagation test was completed on an INSTRON electrohydraulic servo fatigue testing machine, as shown in Figure 3(b). The loading frequency was 10 Hz, the COD extensometer test range was −1 mm to 4 mm, and the temperature range of the 3119–607 environmental test chamber was −70 °C ∼ 350 °C, with liquid nitrogen used as the refrigerant. The above experimental equipment all met the testing requirements.23

Figure 3 Testing machine: (a) tensile and (b) fatigue experimental equipment.

2.2 Static Tensile Test

The ambient temperature in the Arctic region can decrease from 20 to 0 °C in summer and from −30 °C to −60 °C in winter.24 To comprehensively consider the influence of polar environmental temperature on the tensile performance of steel used for the main components of underwater vehicle rudder mechanisms, the temperature range of the experiment must cover the highest and lowest temperatures that may occur in the Arctic. Therefore, the experiment starts at room temperature (20 °C), with each decrease of 20 °C as a gradient. The experiment is repeated three times at each temperature, with an error margin of no more than 0.3%. The tensile tests at room temperature and low temperature followed the GB/T228.3–2019 standard, with a loading speed set at 5 mm/min. The test specimens were cooled in a sealed environmental test box and kept warm for at least 20 min before experiment.25 Some of the specimens after the tensile test are shown in Figure 4.

Figure 4 Fractured specimens after tensile tests.

2.3 Fatigue Crack Propagation Rate Test

The fatigue crack propagation rate (FCPR) experiment in this article used sine wave loading with a constant frequency of 10 Hz. The ambient temperatures for the experiment were 20 °C, – 20 °C, and −60 °C. For each material FCP experiment, three sets of repeated experiments were conducted. The fatigue crack propagation test was conducted using the constant load-increasing K method until the software monitored the fracture of the specimen and stopped the test. To illustrate the experimental results, this study used the average value of parallel specimens to quantitatively evaluate the FCP behavior and crack hysteresis characteristics. In addition, in some graphs used for comparison, the sample data closest to the average of parallel samples were selected to display differences.

2.3.1 Stress Intensity Factor Experiment

According to the standard GB/T 6398–201722 and ASTM E647–2015,26 the fatigue crack propagation stress strength factor threshold values (ΔKth) of 925A, steel 20, and two types of steel welded joints for the rudder were measured at three temperature points: room temperature (20 °C), – 20 °C, and −60 °C, with a stress ratio R = 0.1. However, at the scale of the material microstructure, ΔKth can be regarded as a function of the length of small cracks;27 here, only the macroscopic threshold for the propagation of long cracks was studied. The test used the same C(T) specimen and fatigue testing device as the crack propagation rate test. Due to high testing costs, only one valid specimen was tested under each working condition, resulting in a total of three standard C(T) specimens being tested. In the experiment, the K reduction program was used to determine the threshold value. As shown in equation 1, the descent force used to determine the descent force value is given by the ΔK calculation formula.28 By using the K reduction method, 2 mm initial cracks were prepared on each sample, and subsequent FCPR experiments were conducted using the constant load K increase method.131

Where ΔK0 and a0 are the initial stress intensity factor range and corresponding crack length at the beginning of the K reduction program, respectively, and the normalized K gradient C0 is taken as a constant of −0.1 mm–1.

2.3.2 Fatigue Crack Propagation Rate Experiment

In polar environments, the temperature difference between the above and underwater surfaces can cause temperature stress and temperature cycling stress in steel, which is a continuous load for polar ship structures.29 The experimental ambient temperatures were 20 °C, – 20 °C, and −60 °C, with a stress ratio of R = 0.1. The effects of different low temperatures on the FCPR behavior of steel 925A and steel 20, as well as their welded parts, were investigated. Some of the specimens after the FCP experiment are shown in Figure 5. The fracture surface of the specimens was packaged with cling film to prevent oxidation, which affects the microstructure of the fracture surface.

Figure 5 Fractured specimens after the FCP tests.

FCPR can be described in the Paris Law’s standard form as eq 2.30,312

where C and n are the material constants. According to GB/T 6398–2017, the seven-point incremental fitting method was used to fit the a-N curve and calculate the FCPR.32 The stress intensity factor range (ΔK) can be expressed as eq 3:33,343

In the formula, ΔP is the range of applied load; B is the thickness of the C(T) specimen; and α=a/W, where a and W represent the crack length and width of the C(T) specimen, respectively.

Then the curve of the crack propagation rate is drawn, the logarithms of its two sides are taken, and the linear equation 4 is obtained:4

3 Results and Discussion

3.1 Mechanical Performance Analysis

The stress–strain curves of steel 925A and steel 20 at five experimental temperatures are shown in Figure 6. Steel 20 had obvious yield points at each experimental temperature, which is consistent with the good plasticity of low-carbon steel. However, there was no obvious yield on the stress–strain curve of steel 925A, and there was almost no yield point at any temperature. As the stress and strain increased to their maximum values, i.e., the ultimate tensile strength, the curve rapidly decreased and eventually fractured. This is a type of ductile fracture.35

Figure 6 Stress–strain curves at different temperatures: (a) steel 925A and (b) steel 20.

The study analyzed key parameters such as yield strength, tensile strength, and elastic modulus of the material at various low temperatures. Figures 7 (a) - (d) present a comparative assessment of the mechanical properties of 925A steel, 20 steel, and their welded components at different low temperatures. The results indicate that as the temperature decreases from 20 °C to −60 °C, the tensile strength of 925A steel, 20 steel, and their welded parts exhibit increases of 6.87%, 14.61%, and 12.55%, respectively, while the yield strength shows increments of 14.17%, 29.09%, and15.76%, respectively. The elastic modulus also demonstrates an upward trend with decreasing temperature.36 The experimental findings in Figure 7 (c) further support this observation, revealing that the elastic moduli of the three materials experienced increases of 7.18%, 10.9%, and 7.32%, respectively. However, a slight rise in values is noted at 0 °C, with subsequent temperature reductions beyond −20 °C showing less significant changes. Despite a smaller initial value at 20 °C, the overall trend indicates an increase in elastic modulus with decreasing temperature. Figure 7 (d) illustrates the incremental changes in yield strength and tensile strength, with the smallest increment observed in the yield strength of 925A steel, indicating that it is not significantly affected by temperature.

Figure 7 Mechanical properties at different temperatures: (a) tensile strength (MPa), (b) yield strength (MPa), (c) elastic modulus (GPa), and (d) incremental comparison.

3.2 Analysis of the Fatigue Crack Propagation Rate

The a-N curves of steel 925A, steel 20, and their welded parts at different low temperatures (20 °C, – 20 °C, and −60 °C) in Figure 8 showed significant changes. The experimental results in Table 2 showed that as the temperature decreased from 20 °C to −20 °C, the FCP life of steel 925A, steel 20, and welded parts increased by 11.23%, 12.00%, and 22.78%, respectively. When the temperature dropped from −20 °C to −60 °C, the FCP life of steel 925A, steel 20, and welded parts increased by 29.83%, 59.01%, and 32.33%, respectively.

Figure 8 a-N comparison of steels at different temperatures: (a) steel 925A, (b) steel 20, and (c) welding parts of steel 925A and steel 20.

Table 2 Comparison of the Cycle Life N of Steel 925A, Steel 20, and Their Welds at Different Temperatures

specimens	temperature (°C)	cyclic number N (cycles)	ΔN	
925A	20	116442	 	
–20	129517	13075	
–60	168154	38637	
20	20	121415	 	
–20	135981	14566	
–60	216226	80245	
welding	20	193435	 	
–20	237501	44066	
–60	314289	76788	

Figure 9 depicts the double logarithmic plots of three materials at different temperatures, and linear fitting is used to calculate the Paris parameters, which are reported in Table 3. Figure 9 demonstrates that temperature has no major effect on the fatigue fracture propagation rate of 925A. The overall change rate of 20 steel is relatively fast, with a maximum ΔK value of roughly 1.68, which is lower than the other two materials, and the welding material is typically in the center. The difference in numerical values can be visually viewed using the parameters from Table 3. Explanation: Under varying low temperatures, the fatigue fracture propagation performance of different materials varies, resulting in varied fatigue life.37

Figure 9 Comparison of da/dN-ΔK of steels at different temperatures: (a) steel 925A, (b) steel 20, and (c) welding.

Table 3 Paris Parameters for the Crack Propagation Characteristics of Materials at Different Temperatures

materials	temperature (°C)	m	log C	R2	
925A	20	3.11012	–8.55301	0.97293	
–20	2.95344	–8.36779	0.96699	
–60	3.03567	–8.54219	0.97727	
20#	20	3.51554	–8.94727	0.94558	
–20	3.29147	–8.58809	0.94906	
–60	4.17104	–10.06299	0.96021	
weld	20	3.15834	–8.43407	0.96598	
–20	3.19941	–8.72743	0.97399	
–60	3.78439	–9.72902	0.98082	

3.3 SEM Analysis of the Fracture Morphology

3.3.1 Fracture Morphology of Tensile Specimens

The macroscopic fracture diagram in Figure 4 shows that under the action of axial tensile force, the steel 925A tensile specimen fractured laterally, with a relatively flat fracture surface and obvious necking phenomenon at the fracture surface. From a macro perspective, the sample conformed to the normal plastic deformation and fracture law of metal materials, and there was a necking phenomenon, indicating that the plasticity of the material was relatively high.11 The surface of the fracture was fibrous and rough, with a dark and metallic color, and featured fibrous and shear lips, which are consistent with the characteristics of ductile dimple fracture. It was preliminarily determined to be a ductile dimple fracture.38 The SEM images of the tensile specimens of two materials before fracture are shown in Figure 10.

Figure 10 SEM image of the specimen before fracture: (a) 925A and (b) 20.

SEM analysis was performed on the fracture surface after tensile and fatigue testing. Both steel 925A and steel 20 underwent a significant amount of plastic deformation before fracturing, causing the original grains to elongate or break, losing their initial size and shape. The fracture surface appeared gray, matte, and fibrous, with occasional traces of slip visible.11 The fracture exhibited a cup-shaped morphology.

Figure 11 shows SEM images of the fracture morphology of the steel 925A ((a1) ∼ (a5)) and steel 20# ((b1) ∼ (b5)) tensile specimens at different temperatures. The fracture surface consisted of three regions: fiber, radiation, and shear lip. The crack originated from the fiber zone and rapidly expanded to form a radiating zone. When the crack extended to the surface, it formed a shear lip representing a ductile fracture and finally formed a cup-shaped fracture.39 As shown in Figure 11, the proportion of the above three zones in the entire cross-section changed as the temperature decreased. Figures 11 (a1) ∼ (a5) show that the area of the fiber zone at the center of the steel 925A fracture gradually decreased, with obvious shear lips at 20 °C (circled area in Figure 11). Afterward, the shear lips gradually decreased, and at −60 °C, there were almost no shear lips, and the radial area became relatively flat, indicating a decrease in material plasticity. Figures 11 (b1) ∼ (b5) show that the radiation zone of steel 20 was not obvious, and the fiber zone was almost directly connected to the shear lip zone, especially at −60 °C; the fiber zone accounted for 80% of the area. For more images from the SEM analysis at high magnification, please refer to Supporting Information, Figures S1 and S2. Many micro pits of different shapes, sizes, and depths were distributed on the visible microscopic fracture surface at high magnification.

Figure 11 SEM images of tensile fracture surfaces of tensile specimens at different temperatures: (a1) 925A, 20 °C; (a2) 925A, 0 °C; (a3) 925A, −20 °C; (a4) 925A, −40 °C; (a5) 925A, −60 °C; (b1) 20#, 20 °C; (b2) 20#, 0 °C; (b3) 20#, −20 °C; (b4) 20#, −40 °C; and (b5) 20#, −60 °C.

During fracture analysis, the evaluation of component performance and determination of material toughness can be achieved by examining the morphology of the three zones present on the fracture surface and their respective proportions.40 A larger fiber area and shear strength typically indicate improved plasticity and toughness of the material. Conversely, a higher material strength is associated with a larger proportion of the radiation zone, suggesting reduced plasticity and increased brittleness. In such cases, the fiber zone diminishes in size.41

3.3.2 Fracture Morphology of the Fatigue Crack Propagation Rate Specimen

The macroscopic fatigue crack morphology and microscopic fatigue fracture morphology at various temperatures were analyzed. Figure 12 displays the fracture morphology of steel 925A at three distinct low temperatures. For additional SEM images of steel 20 and welded steel at different temperatures, please refer to the Supporting Information, Figures S3 and S4. The three stages of fatigue cracking, including crack initiation, slow crack propagation, and fracture, can be categorized into a fatigue source zone (I), a slow crack propagation zone (II), and an instantaneous fracture zone (III), respectively, based on the macroscopic fracture morphology.42

Figure 12 SEM image of the fracture morphology of the steel 925A fatigue crack propagation rate specimen at (a) 20 °C, (b) – 20 °C, and (c) – 60 °C.

On the left side of the orange line, there is a prefabricated crack zone from 2 mm to 3 mm in width. In the early stage of crack propagation, the crack propagation was not significantly affected by the toughness or brittleness of the material. The smooth morphological area within the blue solid line frame has a fibrous fracture, which exhibits ductile shear fracture characteristics. As the temperature decreased from 20 °C to −20 °C, the ductile fracture area in the blue area gradually decreased, indicating that the fracture characteristics gradually changed with temperature. When the temperature decreased to −60 °C, the ductile fracture zone in the cross-section decreased to the minimum. In the middle stage of crack propagation, the fracture surface became increasingly rougher with decreasing temperature. It is worth noting that in the fracture diagram at −60 °C, the roughness at the end of the second stage was significantly greater than that in the previous part. This indicates that the fatigue crack propagation rate had a sharp upward trend at this time, which is consistent with the conclusion drawn in Figure 8. As the cracks gradually expanded and the temperature decreased, the fracture morphology at low temperatures had more river-like patterns and cleavage steps than that at room temperature.

4 Conclusion

This article conducted static and fatigue crack propagation rate tests on materials used for the rudder of polar underwater vehicles in environments ranging from 20 °C to −60 °C. By analyzing and processing the test data, the following conclusions were drawn:(1) The static tensile mechanical performance parameters of 925A steel, 20# steel, and their welded components were tested. As the temperature decreased from 20 °C to −60 °C, the tensile strength of 925A steel, 20# steel, and their welded parts increased by 6.87%, 14.61%, and 12.55%, respectively. The yield strength also increased by 14.17%, 29.09%, and 15.76%, respectively. Lower temperatures are advantageous for enhancing the mechanical properties of metal materials.

(2) We tested the fatigue crack propagation rates of 925A steel, 20# steel, and their welded components. We obtained their a-N curves and established the log(da/dN)-logΔK relationship. The measured crack propagation rate aligns closely with the rate calculated by the Paris law. The experimental data can serve as a valuable reference for designing polar ships to withstand low-temperature fatigue and cold brittle fracture.

(3) Perform SEM analysis on the fracture surfaces of tensile specimens and C(T) fatigue specimens. As the temperature decreases, the fracture morphology shows more river-like patterns and cleavage steps compared to the fracture morphology at room temperature.

(4) 925A steel meets the requirements for low-temperature fatigue crack propagation rate in polar regions within −60 °C. However, welding components should be the focus of consideration at low temperatures. At extreme polar temperatures below −60 °C, preventing brittle failure of welded components has become a focus of fatigue design, and relevant research should be conducted in the future.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.4c05289.Additional details on high-magnification SEM images at different temperatures and two other materials (PDF)

Supplementary Material

ao4c05289_si_001.pdf

This Article was funded by the construction project of military training conditions in the 14th five-year plan (145BXL090035069X).

The authors declare no competing financial interest.

Acknowledgments

All the authors acknowledge the support provided to this study by Naval University of Engineering in the form of time and facilities.
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