A testing apparatus and method for stress corrosion testing of magnesium alloy C-ring specimens.

Abstract

This application belongs to the field of magnesium alloy and stress corrosion testing technology, specifically relating to a test method suitable for stress corrosion testing of magnesium alloy C-ring specimens. The test method includes the following steps: S1: preparing a magnesium alloy C-ring specimen; S2: calculating the radial compression amount using the following formula (1): Formula (1); S3: loading the magnesium alloy C-ring specimen; S4: corroding the magnesium alloy C-ring specimen using a corrosive liquid; in step S3, the bolt and nut used to load the magnesium alloy C-ring specimen are made of non-metallic materials, particularly PA66. This application also provides a test apparatus for implementing the above test method. The test method described herein accurately matches the characteristics of magnesium alloys, provides an evaluation that is more in line with actual working conditions, eliminates crevice corrosion interference, improves test accuracy, has high data reliability, and the results can be verified by slow strain rate tensile testing.

Description

Technical Field

[0001] This invention relates to the field of magnesium alloys and stress corrosion testing technology, specifically to a testing device and method suitable for stress corrosion testing of magnesium alloy C-ring specimens. Background Technology

[0002] Magnesium alloys, due to their excellent specific strength, biocompatibility, and biodegradability, have shown great potential in aerospace, medical devices, automotive lightweighting, and electronic components. However, magnesium alloys are highly chemically reactive, and their corrosion resistance is a particularly prominent weakness: they are susceptible to corrosion in environments containing Cl, such as seawater in the ocean, road salt solutions in contact with automotive chassis, and medical fluids. - In typical service environments, magnesium alloys are highly susceptible to stress corrosion cracking (SCC), which can lead to sudden component failure. This problem has become a core bottleneck restricting the transition of magnesium alloys from laboratory research and development to large-scale engineering applications.

[0003] In engineering practice, the service loads on magnesium alloy components often exhibit a "relatively stable or slowly changing" characteristic. These loads include, for example, static loads borne by ship decks and long-term physiological pressures experienced by implants. Therefore, static tensile stress simulation is used to accurately evaluate the performance of magnesium alloys under these conditions. - SCC sensitivity under environmental conditions is a key prerequisite for ensuring service safety. Among the many SCC testing methods, the C-ring specimen has become a widely used and effective method due to its advantages such as providing uniform and controllable tensile stress, ease of operation, accurate stress quantification, moderate size, compliance with standardization requirements, and ease of observation.

[0004] However, the industry currently lacks a C-ring test standard suitable for magnesium alloys. Therefore, there is an ongoing need in the field to develop a test apparatus and method suitable for stress corrosion testing of magnesium alloy C-ring specimens. Summary of the Invention

[0005] The purpose of this application is to provide an accurate and verifiable test method for stress corrosion testing of magnesium alloy C-ring specimens. A further purpose of this application is to provide a test apparatus for performing the above-described test method.

[0006] In existing technologies, to overcome the lack of a suitable C-ring test standard for magnesium alloys, the current national standard GB / T 22640-2023 "Test Method for Stress Corrosion Sensitivity of Aluminum Alloys," which is designed only for aluminum alloys, is often directly applied to magnesium alloy testing. This standard stipulates that "bolts, nuts, and washers should be made of metals of the same metal family as the test specimen." For example, aluminum alloy loading devices are used when testing aluminum alloy specimens.

[0007] However, after extensive experimentation, the inventors discovered that magnesium alloys have a much higher corrosion activity than aluminum alloys, and a "crack microenvironment" forms at the contact interface between the magnesium alloy loading component and the C-ring sample: Cl - This accumulation at the site accelerates localized electrochemical corrosion, forming corrosion pits. Such crevice corrosion not only masks the true SCC cracking signal but can also lead to misdiagnosis, attributing crevice corrosion-induced failure to SCC failure, rendering the test results completely worthless and unable to provide a reliable basis for magnesium alloy material selection and component design. The design of "homogeneous metal loading components" in existing standards can induce severe crevice corrosion at the contact point between magnesium alloy samples and bolts / nuts, interfering with test results.

[0008] In a preferred embodiment, the present invention uses PA66 nylon to manufacture bolts and nuts. On the one hand, the strength of PA66 nylon meets the circumferential stress loading requirements of the test; on the other hand, nylon is an insulating material, does not undergo galvanic reaction with magnesium alloy, and does not participate in the corrosion process, thus eliminating the influence of crevice corrosion at the source, making the test results more realistically reflect the stress corrosion performance of magnesium alloy itself, and significantly improving accuracy.

[0009] To address the aforementioned technical problems, this application provides the following technical solution.

[0010] In a first aspect, this application provides a test method for stress corrosion testing of magnesium alloy C-ring specimens, the test method comprising the following steps: S1: Preparation of magnesium alloy C-ring samples; S2: Calculate the radial compression using the following formula (1); Formula (1); in: This indicates the radial compression of the C-ring, in millimeters. f represents the test stress, and the unit is megapascal (MPa). t represents the wall thickness of the C-ring, in millimeters; D represents the mean diameter of the C-ring, which is equal to the outer diameter of the C-ring minus the wall thickness of the C-ring, in millimeters; E represents the elastic modulus of the material, and the unit is megapascal; Z represents the correction factor, which is the correction factor defined in the constant pressure strain test section of the national standard GB / T 22640-2023 for C-ring specimens; S3: Loaded magnesium alloy C-ring specimen; The magnesium alloy C-ring specimen is loaded using the stress ring loading device described in the loading support structure section of the national standard GB / T 22640-2023. The stress ring loading device loads the magnesium alloy C-ring specimen by means of bolts and nuts. When the magnesium alloy C-ring specimen reaches the predetermined compression amount, the bolts are tightened until the outer diameter of the magnesium alloy C-ring specimen after force is applied reaches the calculated value. S4: Use a corrosive liquid to corrode the magnesium alloy C-ring sample. During the test time, observe and record whether the C-ring cracks and the cracking time. In step S3, the bolt and nut are made of non-metallic materials. The strength of these non-metallic materials meets the circumferential stress loading requirements of the test, and they do not undergo galvanic reactions with the magnesium alloy, nor do they participate in the corrosion process. In one specific embodiment, the correction factor Z can be obtained by referring to the correction factor diagram shown in Figure 20 of the national standard GB / T 22640-2023.

[0011] In one embodiment of the first aspect, the non-metallic material is polyamide.

[0012] In one embodiment of the first aspect, the non-metallic material is nylon 66.

[0013] In one embodiment of the first aspect, in step S3, the bolt screw-in section has a sufficiently long thread, the reserved length being at least 3 times the radial compression.

[0014] In one embodiment of the first aspect, the magnesium alloy C-ring sample is an open annulus with a cross-section of 60°, an outer diameter of not less than 16 mm, an outer diameter to wall thickness ratio of 10-16, and a ring width of 15-25 mm.

[0015] In one embodiment of the first aspect, the magnesium alloy C-ring sample has an outer diameter of 32 mm, a wall thickness of 2.5 mm, an outer diameter to wall thickness ratio of 12.8, and a ring width of 20 mm.

[0016] In one embodiment of the first aspect, in step S4, the temperature is 27 °C, and the corrosive medium is a 3.5% NaCl solution.

[0017] In one embodiment of the first aspect, the testing method further includes the following steps: S5: Immediately after the test, remove the magnesium alloy C-ring sample, rinse it with deionized water, and then use nitric acid to remove the surface corrosion products. Rinse for a predetermined period of time until the corrosion products are removed. After removing the corrosion products, immerse the magnesium alloy C-ring sample in deionized water again, and add silver nitrate solution to the final rinse water. If no white precipitate is produced, it indicates that the chloride ions have been washed away.

[0018] In one embodiment of the first aspect, the magnesium alloy C-ring sample, after being rinsed with deionized water for 2 min and having a nitric acid concentration of 1.40 g / cm² to remove corrosion products, is then immersed in deionized water for 2 min.

[0019] In a second aspect, this application provides a testing apparatus suitable for stress corrosion testing of magnesium alloy C-ring specimens, used to implement the testing method for stress corrosion testing of magnesium alloy C-ring specimens as described in the first aspect, including the use of a stress ring loading device as described in the loading support structure section of national standard GB / T 22640-2023, wherein the bolts and nuts used in the stress ring loading device are made of polyamide material. In one specific embodiment, the specific construction of the stress ring loading device is as described in national standard GB / T 22640-2023. Figure 6 As shown.

[0020] Compared with the prior art, the positive effects of the present invention are as follows: (1) Accurately match the properties of magnesium alloys to make the assessment more in line with actual working conditions. The existing GB / T 22640-2023 standard only applies to aluminum alloys and is not suitable for magnesium alloys. Magnesium alloys are chemically reactive and have higher stress corrosion susceptibility compared to aluminum alloys, especially in most aqueous solutions containing Cl. - The corrosion rate in the solution is very high, while aluminum alloys are relatively more corrosion-resistant and have a lower corrosion rate than Cl in aqueous solutions. - With lower sensitivity, this invention designs test parameters (such as corrosive medium and loading method) based on the material properties of magnesium alloys, which can more accurately and reasonably characterize the stress corrosion sensitivity of magnesium alloys in actual service environments.

[0021] (2) Eliminate the interference of crevice corrosion and improve the accuracy of the test. The design of "homogeneous metal loading components" in existing standards can induce severe crevice corrosion at the contact point between magnesium alloy samples and bolts / nuts, interfering with test results. This invention uses PA66 nylon to manufacture bolts and nuts. On one hand, PA66 nylon's strength meets the required circumferential stress loading for the test; on the other hand, nylon is an insulating material, does not undergo galvanic reaction with magnesium alloys, and does not participate in the corrosion process. This eliminates the influence of crevice corrosion at its source, allowing the test results to more accurately reflect the stress corrosion resistance of the magnesium alloy itself, significantly improving accuracy.

[0022] (3) The data is highly reliable and the results are verifiable. This invention compares slow strain rate tensile tests (SSRT, strain rate 10) to... -6 s -1The results of the comparison and verification showed that if the loading component was made of magnesium alloy of the same series as the C-ring, the corrosion rate would be faster due to crevice corrosion, and the C-ring test results would deviate significantly from the SSRT results. However, if the loading component was made of PA66 nylon, the magnesium alloy C-ring test results would be highly consistent with the SSRT results, which greatly improved the credibility of the test data and ensured the reliability of the evaluation conclusions. Attached Figure Description

[0023] Figure 1 The dimensions of the C-ring are shown, with an outer diameter OD = 32 mm, a thickness t = 2.5 mm, and a ring width B = 20 mm. (Note:) Figure 1 The left image is a front view of a magnesium alloy C-ring sample; Figure 1 The right figure is a side view of a magnesium alloy C-ring sample.

[0024] Figure 2 This displays the dimensions of PA66 nylon bolts and nuts. Among them: Figure 2 The left image shows the front view of a PA66 nylon bolt and nut; Figure 2 The image on the right is a side view of a PA66 nylon bolt and nut. Figure 1 and Figure 2 All of these are disclosed in the national standard GB / T 22640-2023.

[0025] Figure 3 Showing a picture of the C-ring.

[0026] Figure 4 Showing a picture of the PA66 nylon loading component.

[0027] Figure 5 The diagram shows the state of the Mg-9Gd-3Y magnesium alloy C-ring (PA66 loading element) in Example 1 under a stress of 165 MPa. The results show that after 20 days of testing, there was no crevice corrosion, and only localized stress corrosion cracking occurred. Figure 5 The test results for the three parallel samples are shown.

[0028] Figure 6 The diagram shows the state of the Mg-9Gd-3Y magnesium alloy C-ring (homogeneous loading component) in Comparative Example 1 under a stress of 165 MPa. The results show that after 10 days of testing, a corrosion pit with a depth of 0.6 mm appeared at the contact point, and the crack was initiated by the corrosion pit, which is not true stress corrosion cracking. Figure 6 The results shown are from three parallel samples.

[0029] Figure 7 shows the SSRT stress-strain curves of magnesium alloy Mg-9Gd-3Y in air and 3.5% NaCl solution. In this figure, the curves show that the tensile strength and fracture time in the solution are lower than those in air, which are typical characteristics of stress corrosion.

[0030] Figure 8 shows the SSRT stress-strain curves of magnesium alloy AZ91D in air and in 3.5% NaCl solution. In this figure, the curves show that the tensile strength and fracture time in the solution are lower than those in air, which are typical characteristics of stress corrosion.

[0031] Figure 9 shows the SSRT stress-strain curves of magnesium alloy WE43 in air and in 3.5% NaCl solution. In this figure, the curves show that the tensile strength and fracture time in the solution are lower than those in air, which are typical characteristics of stress corrosion. Detailed Implementation

[0032] Unless otherwise stated, implied from the context, or as is customary in the art, all parts and percentages in this application are based on weight, and all testing and characterization methods used are concurrent with the filing date of this application. Where applicable, any patent, patent application, or disclosure relating to this application is incorporated herein by reference in its entirety, and its equivalent patent families are also incorporated herein by reference.

[0033] The terms “comprising,” “including,” “having,” and their derivatives do not exclude the presence of any other components, steps, or processes, regardless of whether such other components, steps, or processes are disclosed in this application. To eliminate any doubt, unless expressly stated otherwise, all compositions using the terms “comprising,” “including,” or “having” in this application may contain any additional additives, excipients, or compounds. Conversely, except for those necessary for operational performance, the term “substantially constitutes…” excludes any other components, steps, or processes described below with respect to that term. The term “consisting of…” does not include any components, steps, or processes not specifically described or listed. Unless expressly stated otherwise, the term “or” refers to the individual members listed or any combination thereof.

[0034] In one specific embodiment, this application provides a test method for stress corrosion testing of magnesium alloy C-ring specimens. The test method is similar to that specified in national standard GB / T 22640-2023, the main difference being that the standard specifies the use of "bolts, nuts, and washers made of metal materials of the same metal family as the specimen" to load the specimen, while this application uses non-metallic materials to prepare the bolts and nuts for loading the magnesium alloy C-ring specimens. Details not described in detail herein can be found in that standard.

[0035] In one specific embodiment, this application provides a method for testing magnesium alloy C-rings, wherein the nut bolt is made of PA66 nylon material. The key characteristics of PA66 nylon are strength, insulation, and chemical stability. The method is carried out according to the following steps: (1) Shape and size refer to Figure 1 and Figure 2 The cross-section is a 60° open annular ring with an outer diameter OD of not less than 16 mm. The ratio of outer diameter to wall thickness is 10-16, and the recommended ring width B is 15-25 mm. Machining should be performed according to the dimensions recommended in Table 1. The optimal outer diameter is 32 mm, the optimal ring width is 20 mm, and the optimal ratio of outer diameter to wall thickness is 12.8. In this case, the wall thickness t is 2.5 mm.

[0036] Table 1 Recommended Sizes for C-Rings

[0037] To avoid galvanic corrosion caused by contact between the bolt / nut and the C-ring, the bolt and nut should be made of nylon material with good insulation and sufficient strength. The bolt thread must be of sufficient length, ideally at least three times the radial compression. The radial compression is calculated as shown below. The inner diameter of the nut should match the outer diameter of the bolt to ensure uniform force distribution during tightening. The samples should be cleaned and dried with anhydrous ethanol before the experiment.

[0038] (2) Calculation of radial compression The test stress f is determined according to the product standard or the agreement agreed upon by the supplier and the buyer. After determining the value of f, the radial compression of the C-ring is calculated using the following formula (1). Formula (1); in: This indicates the radial compression of the C-ring, in millimeters. f represents the test stress, and the unit is megapascal (MPa). t represents the wall thickness of the C-ring, in millimeters; D represents the mean diameter of the C-ring, which is equal to the outer diameter of the C-ring minus the wall thickness of the C-ring, in millimeters; E represents the elastic modulus of the material, and the unit is megapascal; Z represents the correction factor, which can be obtained by referring to the correction factor diagram shown in Figure 20 of the national standard GB / T 22640-2023; (3) Loading steps When the C-ring reaches the predetermined compression, tighten the bolts. Use a digital vernier caliper (accuracy 0.001 mm) to measure the outer diameter OD of the C-ring on both sides of the force-applying hole in real time until the outer diameter OD of the C-ring after force application reaches the calculated value. To ensure the authenticity and reliability of the experimental data, each stress group is repeated three times. Simultaneously, the samples are tested in a periodic immersion test chamber at a temperature controlled at 27 ℃. The corrosive medium is a 3.5% NaCl solution. The preparation method is as follows: Dissolve 3.5 g ± 0.1 g (accurate to 0.1 g) of NaCl in 96.5 mL of water, and adjust the pH of this test solution to 6.4-7.2 with sodium hydroxide solution (50 g / L) or hydrochloric acid solution.

[0039] The inter-immersion time during the experiment was 1 hour, including 10 minutes of soaking and 50 minutes of drying. The experiment lasted for 20 days. During the experiment, the occurrence and timing of cracking of the C-ring were observed and recorded.

[0040] (4) Post-processing methods To remove corrosion products and clearly observe the matrix cracks and fracture surfaces, the sample was immediately removed after the test and rinsed with deionized water for 2 minutes. Then, nitric acid (1.40 g / cm²) was used to remove the surface corrosion products for approximately 30 seconds until the corrosion products were removed. After removing the corrosion products, the sample was immersed in deionized water again for 2 minutes. Silver nitrate solution was added dropwise to the final rinse water. If no white precipitate (AgCl) was formed, it indicated that the chloride ions had been removed. Finally, the sample was cleaned with anhydrous ethanol and dried.

[0041] In another embodiment, this application provides a testing apparatus suitable for stress corrosion testing of magnesium alloy C-ring specimens, for implementing the testing method for stress corrosion testing of magnesium alloy C-ring specimens as described in the first aspect, including the use of national standard GB / T 22640-2023. Figure 6 The stress ring loading device shown in the figure has bolts and nuts made of polyamide material.

[0042] Example The technical solution of this application will be clearly and completely described below with reference to the embodiments of this application. Unless otherwise specified, the reagents and raw materials used can be purchased commercially. Experimental methods in the following embodiments that do not specify specific conditions are performed according to conventional methods and conditions, or according to the product instructions.

[0043] In the following examples, the slow strain rate tensile test was conducted in accordance with the national standard test standard GB / T 15970.7-2017.

[0044] Example 1: As-cast Mg-9Gd-3Y magnesium alloy (T6 aging) 1. Select cast Mg-9Gd-3Y magnesium alloy under T6 aging condition. Take samples from the magnesium alloy in the high-axis direction and machine C-rings: OD=32 mm, t=2.5 mm, B=20 mm; machine PA66 nylon bolts and nuts according to design specifications, and assemble after pretreatment. Actual samples of the nylon bolts, nuts, and C-rings are shown below. Figure 3 and Figure 4 As shown.

[0045] 2. The test stress f was determined to be 180 MPa, 165 MPa and 150 MPa according to the product standard. The radial compression was calculated and the results are shown in Table 2.

[0046] 3. Load according to the above loading steps and conduct a periodic immersion test (27 ℃, 3.5% NaCl solution, pH=7.0, 20 days).

[0047] 4. Slow strain rate tensile test (SSRT): The tensile specimen is immersed in a 3.5% NaCl solution at a rate of 10... -6 s -1 The magnesium alloy was stretched at a strain rate until fracture, while a control test was conducted in air. The fracture stress and fracture time were determined. A stress of 0.6σy was applied to the C-ring, where σy refers to the magnesium alloy's stress at a strain rate of 10... -6 s -1 Yield strength measured under strain rate tension. The C-ring is tested for 20 days; a fracture time exceeding 480 hours indicates that it did not fracture by the end of the test and passed the test.

[0048] 5. In the C-ring test, no cracking occurred after 20 days under 150 MPa stress, the average cracking time was 16 days under 165 MPa stress, and the average cracking time was 8 days under 180 MPa stress; this is highly consistent with the SSRT results, such as... Figure 5 As shown, the sample under 165 MPa stress showed no obvious crevice corrosion, and the cracks were typical stress corrosion cracking.

[0049] Table 2. Mg-9Gd-3Y magnesium alloy C-ring (nut and bolt made of PA66 nylon) and slow strain rate tensile test ( =10 -6 s -1 Experimental results in 3.5% NaCl solution

[0050] Example 2: As-cast AZ91D magnesium alloy (T6 aging) Taking as-cast AZ91D magnesium alloy as an example, nuts and bolts are manufactured using PA66 nylon. The experimental procedure is as follows: 1. Sample preparation: T6 aged AZ91D magnesium alloy was selected, and a C-ring was machined with OD=32 mm, t=2.5 mm, and B=20 mm; PA66 nylon loading parts were assembled. 2. The test stress f was determined to be 70 MPa, 30 MPa and 10 MPa according to the product standard. The radial compression was calculated and the compression results are shown in Table 3. 3. The corrosion environment and operation are the same as in Example 1; 4. In the C-ring test, no cracking occurred after 20 days under stresses of 10 MPa and 30 MPa, and the average cracking time was 12 days under stress of 70 MPa; the deviation from the SSRT results was small, there was no interference from gap corrosion, and the results were reliable.

[0051] Table 3. AZ91D magnesium alloy C-ring (nut and bolt made of PA66 nylon) and slow strain rate tensile test ( =10 -6 s -1 Experimental results in 3.5% NaCl solution

[0052] Example 3: As-cast WE43 magnesium alloy (T6 aging) Taking as-cast WE43 magnesium alloy as an example, nuts and bolts are manufactured using PA66 nylon. The experimental procedure is as follows: 1. Select T6 aged WE43 magnesium alloy to machine C-rings: OD=32 mm, t=2.5 mm, B=20 mm; assemble PA66 nylon loading parts; 2. The test stress f was determined to be 115 MPa, 100 MPa and 85 MPa according to the product standard. The radial compression was calculated and the results are shown in Table 4. 3. The corrosion environment and operation are the same as in Example 1; 4. In the C-ring test, no cracking occurred after 20 days under stresses of 85 MPa and 100 MPa, and the average cracking time was 14 days under stress of 115 MPa. The deviation from the SSRT results was small, verifying the applicability of this method to different grades of magnesium alloys.

[0053] Table 4. WE43 magnesium alloy C-ring (nut and bolt made of PA66 nylon) and slow strain rate tensile test ( =10 - 6 s -1 Experimental results in 3.5% NaCl solution

[0054] The following comparative examples use "magnesium alloys of the same series as C-rings" to make bolts and nuts (following the standard approach of GB / T 22640-2023). Other test parameters are consistent with the corresponding examples to verify the defects of the existing methods.

[0055] Comparative Example 1: As-cast Mg-9Gd-3Y magnesium alloy (homogeneous loading component) 1. Select cast Mg-9Gd-3Y magnesium alloy under T6 aging and sample the magnesium alloy from the high-axis direction. Select a C-ring with an outer diameter (OD) of 32 mm, a thickness t of 2.5 mm, and a ring width B of 20 mm. The nut and bolt are made of Mg-9Gd-3Y magnesium alloy (of the same series as the C-ring).

[0056] 2. The test conditions were the same as in Example 1, with test stresses f of 180 MPa, 165 MPa and 150 MPa respectively. The radial compression was calculated and the compression is shown in Table 5.

[0057] 3. Severe crevice corrosion occurred under all stress conditions (corrosion pit depth ≥ 0.5 mm at the contact point between the bolt and the C-ring, such as...). Figure 6 As shown in the figure); the average cracking time under 150 MPa stress is only 10 days (20 days without cracking in Example 1), which deviates significantly from the SSRT results and cannot truly reflect the stress corrosion performance of magnesium alloys.

[0058] Table 5. Mg-9Gd-3Y magnesium alloy C-rings (nut and bolt materials are Mg-9Gd-3Y) and slow strain rate tensile tests ( =10 -6 s -1 Experimental results in 3.5% NaCl solution

[0059] Comparative Example 2: As-cast AZ91D magnesium alloy (homogeneous loading component) Taking as-cast AZ91D magnesium alloy as an example, nuts and bolts are manufactured using materials homologous to C-rings. The experimental procedure is as follows: 1. Select cast Mg-9Gd-3Y magnesium alloy under T6 aging and sample the magnesium alloy from the high-axis direction. Select a C-ring with an outer diameter (OD) of 32 mm, a thickness t of 2.5 mm, and a ring width B of 20 mm. The nut and bolt are made of AZ91D magnesium alloy (of the same series as the C-ring).

[0060] 2. The test conditions were the same as in Example 1, with test stresses f of 70 MPa, 30 MPa and 10 MPa respectively. The radial compression was calculated and the compression is shown in Table 6.

[0061] 3. Crevice corrosion appeared under 30 MPa stress, with an average cracking time of 15 days (20 days without cracking in Example 2); the average cracking time under 70 MPa stress was 5 days (12 days in Example 2), which deviated significantly from the SSRT results, indicating distorted results.

[0062] Table 6. AZ91D magnesium alloy C-ring (nut and bolt material is AZ91D magnesium alloy) and slow strain rate tensile test ( =10 -6 s -1 Experimental results in 3.5% NaCl solution

[0063] Comparative Example 3: As-cast WE43 magnesium alloy (homogeneous loading component) Taking as-cast WE43 magnesium alloy as an example, nuts and bolts are manufactured using materials homologous to C-rings. The experimental procedure is as follows: 1. Cast Mg-9Gd-3Y magnesium alloy under T6 aging was selected, and samples were taken from the high-axis region of the alloy. A C-ring with an outer diameter (OD) of 32 mm, a thickness (t) of 2.5 mm, and a ring width (B) of 20 mm was chosen. The nut and bolt were made of WE43 magnesium alloy (of the same series as the C-ring). The sample was cleaned with anhydrous ethanol before the experiment.

[0064] 2. The test conditions were the same as in Example 1, with test stresses f of 115 MPa, 100 MPa and 85 MPa respectively. The radial compression was calculated and the compression is shown in Table 7.

[0065] 3. Crevice corrosion occurred under 100 MPa stress, with an average cracking time of 8 days (20 days without cracking in Example 3); the average cracking time under 115 MPa stress was 6 days (14 days in Example 3), which deviated significantly from the SSRT results and could not be used as an evaluation basis.

[0066] Table 7 WE43 magnesium alloy C-ring (nut and bolt material PA66 nylon) and slow strain rate tensile test ( =10 -6 s -1 Experimental results in 3.5% NaCl solution

[0067] Figure 5 The diagram shows the state of the Mg-9Gd-3Y magnesium alloy C-ring (PA66 loading element) in Example 1 under a stress of 165 MPa. Figure 6 The diagram shows the state of the Mg-9Gd-3Y magnesium alloy C-ring (homogeneous loading element) in Comparative Example 1 under a stress of 165 MPa. Figure 5 It can be seen that the magnesium alloy C-ring of Example 1 exhibits no gap corrosion, only localized stress corrosion cracking, while the magnesium alloy C-ring of Comparative Example 1 shows corrosion pits with a depth of 0.6 mm at the contact point. Figure 6 As shown, the crack was initiated by corrosion pits, not by actual stress corrosion cracking.

[0068] also, Figures 7 to 9 The stress curves of magnesium alloys subjected to slow strain rate tensile testing in Examples 1-3 are shown. The curves in the figures show that the tensile strength and fracture time in solution are lower than those in air, which are typical characteristics of stress corrosion. This further verifies the accuracy of the stress corrosion test method for magnesium alloy C-ring specimens described in this paper.

[0069] The above description of the embodiments is intended to enable those skilled in the art to understand and apply this application. It will be apparent to those skilled in the art that various modifications can be easily made to these embodiments, and the general principles described herein can be applied to other embodiments without creative effort. Therefore, this application is not limited to the embodiments described herein, and any improvements and modifications made by those skilled in the art based on the disclosure of this application without departing from the scope and spirit of this application are within the scope of this application.

Claims

1. A test method suitable for stress corrosion testing of magnesium alloy C-ring specimens, characterized in that, The testing method includes the following steps: S1: Preparation of magnesium alloy C-ring samples; S2: Calculate the radial compression using the following formula (1); Equation (1); wherein: represents the radial compression of the C-ring in millimeters; f represents the test stress, and the unit is megapascal (MPa). t represents the wall thickness of the C-ring, in millimeters; D represents the mean diameter of the C-ring, which is equal to the outer diameter of the C-ring minus the wall thickness of the C-ring, in millimeters; E represents the elastic modulus of the material, and the unit is megapascal; Z represents the correction factor, which is the correction factor defined in the constant pressure strain test section of the national standard GB / T 22640-2023 for C-ring specimens; S3: Loaded magnesium alloy C-ring specimen; The magnesium alloy C-ring specimen is loaded using the stress ring loading device described in the loading support structure section of the national standard GB / T 22640-2023. The stress ring loading device loads the magnesium alloy C-ring specimen by means of bolts and nuts. When the magnesium alloy C-ring specimen reaches the predetermined compression amount, the bolts are tightened until the outer diameter of the magnesium alloy C-ring specimen after force is applied reaches the calculated value. S4: Use a corrosive liquid to corrode the magnesium alloy C-ring sample. During the test time, observe and record whether the C-ring cracks and the cracking time. In step S3, the bolt and the nut are made of non-metallic materials. The strength of the non-metallic materials meets the requirements of the circumferential stress loading required for the test, and they do not undergo galvanic reaction with the magnesium alloy, nor do they participate in the corrosion process. The bolt screw-in section has a sufficiently long thread, with the reserved length being at least 3 times the radial compression.

2. The test method as described in claim 1, characterized in that, The non-metallic material is polyamide.

3. The test method as described in claim 2, characterized in that, The non-metallic material is nylon 66.

4. The test method according to any one of claims 1-3, characterized in that, The magnesium alloy C-ring sample is an open ring with a cross-section of 60°, an outer diameter of not less than 16 mm, an outer diameter to wall thickness ratio of 10-16, and a ring width of 15-25 mm.

5. The test method of claim 4, wherein, The magnesium alloy C-ring sample has an outer diameter of 32 mm, a wall thickness of 2.5 mm, an outer diameter to wall thickness ratio of 12.8, and a ring width of 20 mm.

6. The test method of any one of claims 1-3, wherein, In step S4, the temperature is 27 °C, and the corrosive medium is a 3.5% NaCl solution.

7. The test method of any one of claims 1-3, wherein, The testing method includes the following steps: S5: Immediately after the test, remove the magnesium alloy C-ring sample, rinse it with deionized water, and then use nitric acid to remove the surface corrosion products. Rinse for a predetermined period of time until the corrosion products are removed. After removing the corrosion products, immerse the magnesium alloy C-ring sample in deionized water again, and add silver nitrate solution to the final rinse water. If no white precipitate is produced, it indicates that the chloride ions have been washed away.

8. The test method of claim 7, wherein, The magnesium alloy C-ring sample was rinsed with deionized water for 2 minutes, with a nitric acid concentration of 1.40 g / cm², and then immersed in deionized water for 2 minutes after removing the corrosion products.

9. A testing apparatus suitable for stress corrosion testing of magnesium alloy C-ring specimens, used to implement the testing method for stress corrosion testing of magnesium alloy C-ring specimens as described in any one of claims 1-8, comprising using a stress ring loading device as described in the loading support structure section of national standard GB / T 22640-2023, characterized in that, The bolts and nuts used in the stress ring loading device are made of polyamide material.

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