A creep-fatigue test method in a molten salt environment

Abstract

The present application relates to a kind of creep-fatigue test methods in molten salt environment, it includes to be measured alloy material and is processed into integrated molten salt tank smooth bar sample;To the molten salt tank is filled with molten salt, so that sample gauge section is completely immersed in molten salt;The sample of assembly completion is transferred to fatigue testing machine and installs clamp and measuring device;Test container is evacuated and filled with inert gas, and temperature is raised to test temperature;Using axial strain control mode, trapezoidal wave cyclic load is applied to sample, only at the tensile peak strain setting, the test process is real-time collected test data, until sample reaches failure judgment condition.The present application relies on integrated molten salt tank structure and exclusive loading waveform, so that sample is continuously in molten molten salt environment, can simultaneously carry out creep, fatigue, corrosion coupling test, truly restore material service condition, effectively make up the defect that traditional test cannot simulate multiple damage synergism.

Description

Technical Field

[0001] This invention belongs to the field of material performance testing technology, and more specifically relates to a creep-fatigue testing method in a molten salt environment. Background Technology

[0002] Metal structural materials operating under high-temperature molten salt conditions are subjected to a complex environment involving the coupled effects of multiple factors, including high-temperature molten salt corrosion, cyclic alternating loads, and static continuous loads. They also endure the synergistic damage caused by molten salt corrosion, fatigue damage, and creep damage. The overall service performance of these materials directly determines the operational safety and service life of the related equipment. Therefore, accurately simulating actual operating conditions and conducting creep-fatigue-corrosion coupled performance tests on materials are crucial for material selection, service life prediction, and damage mechanism research.

[0003] Current traditional testing methods in the industry have significant shortcomings, making it difficult to accurately represent the true service condition of raw materials in high-temperature molten salt. Most existing fatigue, creep, and molten salt corrosion tests are conducted independently, meaning high-temperature creep tests, fatigue tests, and molten salt corrosion tests are performed separately, failing to reflect the actual working conditions where these three types of damage occur simultaneously and influence each other. Even some studies attempting to conduct tests combining corrosion and fatigue generally employ simplistic methods such as corroding before loading or coating the sample surface with molten salt before loading. This fails to ensure that the gauge length of the sample is continuously immersed in high-temperature molten salt during dynamic loading, thus failing to accurately simulate the synergistic damage process of molten salt corrosion, creep, and fatigue acting simultaneously. Consequently, the test results obtained deviate significantly from the actual service performance of the materials.

[0004] Currently, there is no unified and standardized technical system for creep-fatigue coupled testing in high-temperature molten salt environments. There are no unified and universally applicable operating procedures regarding specimen structure design and processing standards, molten salt preparation and environmental control methods, cyclic load waveforms, test parameter settings, data acquisition rules, and specimen failure judgment criteria. Different researchers and laboratories employ significantly different test schemes, resulting in a lack of comparability of various test data and making it difficult to establish a standardized material performance database. This severely restricts the progress of performance evaluation, damage law research, and long-life prediction of structural materials used in high-temperature molten salt environments. Summary of the Invention

[0005] To address the problems of existing technologies that cannot simultaneously perform coupled testing of creep, fatigue, and corrosion damage in a real high-temperature molten salt immersion environment, and that the test procedures are not standardized, parameters are uncontrollable, and data repeatability is poor, this invention aims to provide a creep-fatigue testing method in a molten salt environment.

[0006] The creep-fatigue testing method in a molten salt environment according to the present invention includes the following steps: S1, processing the alloy material to be tested into a smooth rod-shaped specimen with an integrated molten salt vessel; S2, filling the molten salt vessel with molten salt so that the gauge length of the specimen is completely immersed in the molten salt; S3, transferring the assembled specimen to a fatigue testing machine and installing fixtures and measuring devices; S4, evacuating the test container and filling it with inert gas, and heating it to the test temperature; S5, applying a trapezoidal wave cyclic load to the specimen using an axial strain control mode, setting a hold-load stage only at the peak tensile strain, and collecting test data in real time during the test until the specimen reaches the failure judgment condition. The present invention, by adopting an integrated specimen structure with an integrated molten salt vessel, combined with a trapezoidal wave loading method with a hold-load at the tensile end and no hold-load at the compression end, enables the gauge length of the specimen to be continuously immersed in molten salt, simultaneously achieving coupled testing of creep, fatigue, and corrosion damage, realistically reproducing the actual service conditions of the raw material, and overcoming the shortcomings of traditional step-by-step testing and surface salting methods that cannot simulate real working conditions.

[0007] In a preferred embodiment, in step S1, the gauge length of the smooth rod-shaped sample has a diameter of 5-10 mm, a length of 16-30 mm, and a surface roughness Ra ≤ 0.2 μm. The gauge length is sequentially treated with mechanical polishing, fine sandpaper polishing, manual polishing with polishing paste, and electrolytic polishing. This invention, by limiting the gauge length size and surface roughness and employing a multi-stage composite polishing process to process the gauge length of the sample, can eliminate the interference of surface processing defects on the test results, ensure the uniformity of surface conditions for different samples, and effectively improve the repeatability and accuracy of test data.

[0008] In a preferred embodiment, electropolishing uses a polishing solution composed of sulfuric acid, glycerol, and water, and the electropolishing process is carried out under a fixed voltage in an ice bath environment. This invention, by employing a specially formulated polishing solution and combining it with ice bath and fixed voltage electropolishing conditions, enables refined polishing of the gauge length, further reducing surface micro-defects and meeting the stringent surface quality requirements of molten salt coupled corrosion tests.

[0009] In a preferred embodiment, in step S2, the molten salt used is a fluoride molten salt or a chloride molten salt; the molten salt is prepared through precise weighing, powder mixing, raw material drying, vacuum impurity removal, high-temperature melting, cooling, and pulverization. This invention, by limiting the type of molten salt and employing multiple processes to purify it, can thoroughly remove moisture and volatile impurities from the molten salt, ensuring the stability of its composition and properties, and creating a pure and stable corrosive environment.

[0010] In a preferred embodiment, in step S3, the loading of the sample and molten salt is completed in a glove box protected by an inert gas atmosphere. After assembly, the gaps in the molten salt container are sealed with sealant before the entire container is transferred to the fatigue testing machine. By completing the loading under an inert atmosphere and sealing the molten salt container before transfer, this invention can isolate the molten salt from the outside air, prevent oxidation and deterioration of the molten salt during assembly and transfer, and maintain the consistency of the test environment.

[0011] In a preferred embodiment, in step S4, the operation of evacuating and filling with inert gas is repeatedly performed to displace the air inside the container; the heating rate is 3~5℃ / min, and the heating adopts a segmented temperature control mode, first rapidly heating to near the test temperature, and then switching the temperature control mode to rise to the test temperature. This invention, through multiple gas replacements combined with a limited heating rate and segmented temperature control, can fully remove the air inside the container, while slowing down the rate of temperature change, avoiding thermal shock to the sample and molten salt, and ensuring a uniform and stable temperature field in the test system.

[0012] In a preferred embodiment, in step S4, the inert gas inside the container is high-purity argon, and the temperature control deviation of the entire experimental system is controlled within ±2℃. After heating to the test temperature, the temperature is held until the molten salt is completely melted and reaches thermal equilibrium. By using high-purity argon as the protective atmosphere and strictly controlling the temperature control accuracy and setting a thermal equilibrium holding stage, this invention can suppress oxidation reactions, while ensuring that the molten salt is fully melted and the sample and molten salt temperatures are consistent, further improving the stability of the experimental environment.

[0013] In a preferred embodiment, in step S5, the loading strain rate is 0.001~1% / s, the total strain amplitude is 0.2%~1.2%, and the compression half-cycle of the trapezoidal wave cyclic load does not have a hold-load stage. By limiting the range of strain rate and total strain amplitude, and clearly defining the load waveform structure, this invention can flexibly adjust the proportion of creep damage and fatigue damage, simulate load states under various actual working conditions, and expand the applicability of the test method.

[0014] In a preferred embodiment, in step S5, the collected test data includes the number of cycles, peak stress, valley stress, and stress relaxation curve during the load holding stage; the failure criterion is a 30% decrease in the peak stress of the specimen during the relatively stable stage. This invention, by collecting mechanical data from multiple dimensions and using stress attenuation as a unified criterion for engineering failure judgment, can accurately capture the material damage evolution process, overcoming the limitations of traditional methods that solely rely on specimen fracture as the judgment standard, and achieving a refined evaluation of the material's service performance.

[0015] In a preferred embodiment, after the experiment, peak stress-cycle count curves and stress relaxation-time curves are plotted based on the collected experimental data to characterize the material's cyclic hardening, softening properties, and creep deformation behavior. By plotting these two types of characteristic curves and conducting corresponding analysis, this invention can intuitively quantify the interaction between creep and fatigue, comprehensively analyze the material damage evolution mechanism, and provide complete data support for material performance evaluation and life prediction.

[0016] In summary, this invention employs an integrated molten salt vessel sample structure combined with a specific trapezoidal wave loading method, allowing the gauge length of the sample to be continuously immersed in molten salt. This enables simultaneous coupled testing of creep, fatigue, and corrosion damage, realistically replicating the actual service conditions of raw materials and solving the problem that traditional testing methods cannot simulate the synergistic effects of multiple damages under molten salt conditions. Coupled with a dedicated system for sample surface treatment, molten salt purification, sealed transport, atmosphere and temperature control, loading parameter setting, and data acquisition and analysis, this invention effectively ensures the stability of the test environment, sample condition, and loading conditions, avoiding interference from external factors. It also allows for flexible adjustment of the proportion of creep and fatigue damage. Relying on unified failure judgment criteria and multi-dimensional data analysis methods, it accurately characterizes the material damage evolution law. The test data exhibits strong repeatability and comparability. The overall solution is standardized and widely applicable, providing reliable experimental support for performance evaluation and damage mechanism research of structural materials under molten salt conditions. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the overall process flow of the creep-fatigue testing method in the molten salt environment of the present invention.

[0018] Figure 2 This is a schematic diagram showing the structure and dimensions of the smooth rod-shaped sample of the integrated molten salt vessel used in this invention.

[0019] Figure 3 This is a schematic diagram of the trapezoidal wave loading waveform used in the experiment of this invention.

[0020] Figure 4 This is a schematic diagram of the cyclic stress response curves of the specimen under different holding times according to the present invention.

[0021] Figure 5 This is a schematic diagram of the stress relaxation curves of the specimen during the holding period under different holding times according to the present invention. Detailed Implementation

[0022] The present invention will be further described in detail below with reference to the accompanying drawings. This section is based on the complete technical solution of the present invention and is intended to illustrate the preferred embodiments of the present invention, and is not intended to limit the scope of protection of the present invention.

[0023] This invention provides a creep-fatigue testing method in a high-temperature molten salt environment, applicable to the performance evaluation and damage study of structural materials (such as GH3535 alloy) in molten salt reactors and molten salt energy storage systems under the synergistic effects of high-temperature molten salt corrosion, creep, and fatigue. Compared to existing technologies that separately conduct corrosion, creep, and fatigue tests, or employ methods such as corrosion followed by loading and surface coating with molten salt, this invention ensures that the gauge length of the sample remains immersed in high-temperature molten salt throughout the entire loading process, simultaneously completing the coupled testing of creep, fatigue, and corrosion damage, thereby maximally reflecting the actual service conditions of the raw materials.

[0024] like Figure 1 As shown, the creep-fatigue synergistic testing method in a high-temperature molten salt corrosion environment of the present invention includes the parallel preparation of molten salt and fatigue specimens. Molten salt preparation provides a molten salt medium that meets the requirements of the operating conditions and is the foundation for constructing the corrosion environment. Specimen preparation refers to processing a special specimen with a molten salt vessel adapted to the test of the present invention, providing a qualified test object for subsequent loading tests. The prepared molten salt and fatigue specimens are integrated and sealed in the sample assembly and transfer step, achieving the test condition of immersing the gauge length in molten salt, and then transferred to the test equipment. After sample assembly, the test environment establishment step is entered, constructing a stable high-temperature oxygen-free molten salt environment through vacuuming, filling with inert gas, and temperature control. After the environment is established, the creep-fatigue synergistic loading stage begins, using a special trapezoidal wave load to simultaneously realize the coupling effect of creep, fatigue, and corrosion damage, collecting complete mechanical response data and test results.

[0025] In the sample preparation step, the alloy material to be tested is processed into a smooth rod-shaped sample for use in an integrated molten salt vessel. For example... Figure 2As shown, in a preferred embodiment, threads are machined at both ends of the specimen to provide a threaded specimen. The external thread structure at both ends is used for direct threaded connection with the testing machine clamps, ensuring reliable fixation and coaxiality with the testing machine. In another preferred embodiment, stepped structures are machined at both ends of the specimen to provide a boss specimen. The cylindrical boss structure at both ends is adapted to a clamping device with slots, similarly ensuring reliable fixation and coaxiality with the testing machine. In addition to the clamping sections at both ends mentioned above, the overall specimen structure also includes a middle gauge length section and transition arcs at both ends of the gauge length section. The middle gauge length section is the core testing area for the specimen to withstand tensile-compression cyclic loads and is also the key part for subsequent contact with molten salt. The smooth transition arc is set between the gauge length section and the clamping section to avoid stress concentration that could cause premature fracture of the specimen outside the gauge length section, ensuring that fatigue damage only occurs within the gauge length section. To meet the requirements of molten salt immersion testing, this invention welds a molten salt vessel onto the specimen, allowing the gauge length to be completely immersed in molten salt. Simultaneously, it ensures that the clamping sections at both ends do not contact the molten salt, preventing corrosion. Compared to traditional standard specimens without a molten salt vessel, this integrated structure is specifically designed for molten salt immersion environments, eliminating the need for additional large salt bath installations and reducing the difficulty of equipment modification. Figure 2 As shown, the molten salt vessel is a cylindrical structure open at one end, with a flange at the opening. A seal can be achieved by engaging the sealing element with the sample or testing device, preventing leakage of the high-temperature molten salt and isolating it from external air to maintain a stable environment for the molten salt inside. In a preferred embodiment, the sample is a non-standard design, with a total length of 155-165 mm for the rod-shaped sample, a gauge length of 16-30 mm, and a gauge diameter of 5-10 mm.

[0026] The gauge length section of the precision-machined specimen underwent surface treatment: first, longitudinal mechanical polishing was performed to control the surface roughness Ra ≤ 0.4 μm; then, manual fine polishing was carried out sequentially using 4000# and 5000# sandpaper; subsequently, further manual polishing was performed using a viscous polishing paste containing 0.05 μm alumina and a polishing cloth; finally, electrolytic polishing was performed. The electrolytic polishing solution was prepared according to the ratio of 50% sulfuric acid, 40% glycerol, and 10% water. The electrolytic polishing process was conducted in an ice bath environment using a fixed voltage (e.g., 36V). After polishing, the gauge length section was observed under a 500x metallographic microscope; the absence of obvious scratches indicated acceptance, effectively preventing surface defects from interfering with the test results. After polishing, the specimen was ultrasonically cleaned sequentially using acetone and anhydrous ethanol, dried with cold air, and then placed in a desiccator for later use. The surface roughness Ra of the gauge length section after sequential mechanical polishing, fine sandpaper polishing, manual polishing with polishing paste, and electrolytic polishing was ≤ 0.2 μm. The combined process of multi-stage polishing and electrolytic polishing of the sample surface significantly improves the surface accuracy of the sample compared to the sample preparation method of ordinary mechanical polishing alone, ensures the consistency of the state between different samples, and enhances the repeatability of test data.

[0027] In the molten salt preparation step, in order to accurately simulate the high-temperature molten salt corrosion environment of actual working conditions, this invention adopts a self-prepared high-purity molten salt (precise weighing, powder mixing, raw material drying, vacuum impurity removal, high-temperature melting, cooling and pulverization), strictly controlling parameters such as molten salt temperature, composition, water and oxygen content. Compared with the method of directly using commercial molten salt, it can completely remove moisture and volatile impurities, ensure the stability of molten salt performance, and thus ensure the consistency of the corrosion environment.

[0028] Specifically, after cleaning the graphite crucible and its stainless steel jacket, it is dried in a 110°C drying oven for 48 hours. The molten salt raw material is weighed according to the target molar ratio using an electronic balance with an accuracy of 0.01g. The molten salt types used in this invention include fluoride molten salts and chloride molten salts, with a wider range of applications; FLiNaK molten salt is preferred. In a preferred embodiment, the molar ratio is LiF:NaF:KF = 46.5%:11.5%:42%. After weighing, the powdered raw material is thoroughly mixed and stirred evenly in a glove box, then placed into the graphite crucible, and the entire crucible is placed in a vacuum induction melting furnace. It is first dried at 400°C for 3 hours to completely remove moisture and volatile impurities; then the temperature is raised to 700°C and held for 4 hours to obtain a molten solid salt. The temperature fluctuation of the heating furnace is controlled within ±2°C to ensure the uniformity and stability of the molten salt composition. After the solid molten salt cools, it is pulverized and sealed for storage. During the formal test, 85g of molten salt powder was weighed and placed into the molten salt container of the sample.

[0029] The entire sample assembly and transfer process is completed within a glove box under inert gas protection. The prepared powdered molten salt is loaded into the sample's own molten salt container, ensuring the gauge length is completely submerged. The clamping sections at both ends of the sample are placed outside the molten salt container for subsequent connection to the testing machine. After assembly, to prevent air contamination of the molten salt during transfer, a protective cap is placed at the opening of the molten salt container, and the gap is sealed with an open elastic rubber ring. The sealed sample is then transferred to the fatigue testing machine, and the rubber seals are removed before quickly sealing the test reactor. Two deformation measurement brackets are respectively mounted on the protrusions of the upper and lower clamping sections of the sample. One end of the extension rod is fixed to the measurement bracket, and the other end is connected to a magnetic rod core. The overall deformation of the sample is calculated by the stroke difference between the upper and lower sets of displacement sensors. Compared to unsealed transfer and open-top salt loading, this invention uses a glove box assembly + sealed transfer method, effectively isolating air, preventing molten salt oxidation and water absorption, and maintaining a stable corrosive environment.

[0030] In the environmental setup step, the mechanical pump is first started to evacuate the sealed test container until the internal pressure is below 10 Pa. Then, high-purity inert gas is introduced; this invention preferably uses high-purity argon with a purity ≥99.999% and an oxygen content <10 ppm. The "evacuation-argon filling" operation is repeated three times to fully replace the air inside the container. After gas replacement, the temperature is increased under continuous argon flow at a rate of 3-5 °C / min, with a target test temperature range of 650-750 °C. The temperature is controlled in stages: first, the furnace rapidly heats the container to 50 °C below the test temperature, then the temperature is switched to vessel-based control mode, slowly increasing the temperature to the set test temperature. The temperature deviation of the entire control system is controlled within ±2 °C. After reaching the test temperature, the container is held for 40 minutes to allow the molten salt to completely melt and the sample and molten salt to reach thermal equilibrium. The cooling water circuit is kept running throughout the process. Compared to the conventional method of single-stage gas replacement and rapid heating, the multi-stage gas replacement and segmented precise temperature control scheme of this invention not only eliminates the problem of air oxidation, but also ensures that the molten salt is fully melted and the temperature field is uniform, providing a stable high-temperature corrosion environment for coupled experiments.

[0031] In the creep-fatigue synergistic loading and test termination steps, this invention employs an axial strain control mode to apply cyclic loads to the specimen. A trapezoidal wave is selected for the loading waveform, and the specific parameters are as follows: Loading rate: The strain rate is controlled, typically set to 0.001~1% / s; Total strain amplitude (Δε / 2): Selected according to the test objective, typically ranging from 0.2% to 1.2%; Holding time (t... h ): Maintain constant strain at the peak tensile strain, and set the holding time according to the research needs (e.g., 10s, 60s, 180s, etc.); Waveform symmetry: No holding is set for the compression half-cycle, and a triangular wave return is used.

[0032] like Figure 3 As shown, the horizontal axis represents time, and the vertical axis represents strain. The strain increases uniformly from 0 to the peak value ε. max This achieves positive tensile loading, bringing the specimen to the set maximum tensile strain; the strain is maintained at ε. max The load holding time is kept constant, and the duration is marked as t in the figure. h This is the core design of the present invention for controlling creep damage. By setting the holding load only at the peak of the tensile stress, the sample can undergo controllable creep deformation in a high-temperature molten salt environment. After the holding load ends, the strain decreases at a constant rate, passes through the zero strain line, and continues to decrease to the reverse compression valley value ε. min The compression process does not include any load hold or pause; it adopts a linear change form with direct drop. The strain changes from ε... minThe strain returns to the initial zero-strain position at a uniform speed, completing one full loading cycle. This cycle can be repeated to achieve cyclic fatigue loading. Compared to conventional fatigue loading waveforms such as sine waves and triangular waves without load holding, this invention uses a trapezoidal wave structure with load holding only at the tensile end. The load holding time t can be adjusted. h The contribution ratio of creep damage can be freely adjusted, enabling both pure fatigue tests and creep-fatigue coupled loading of different intensities. This allows for precise matching of the actual stress characteristics of materials under high-temperature molten salt conditions, and the acquisition of mechanical evolution data that better reflects actual service conditions.

[0033] During the experiment, the system collected and recorded the number of cycles N and the peak stress σ in real time. max Valley stress σ min The invention collects all test data, including stress relaxation curves during the load holding phase. Preferably, a unified test failure criterion is established: when the peak stress of the specimen decreases to a specified proportion (a 30% decrease in peak stress) compared to the load stabilization phase, the specimen is deemed to have failed, and the test is immediately terminated. After the test, based on the collected test data, peak stress-cycle number curves and stress relaxation-time curves under different load holding times are plotted to quantitatively analyze the cyclic hardening / softening characteristics and creep deformation behavior of the material. Compared to existing test methods where data evaluation indicators and failure criteria are inconsistent and horizontal comparisons of different test results are difficult, this invention, through multi-dimensional data acquisition and curve plotting combined with a unified failure criterion, can comprehensively analyze the interactive damage mechanism of creep, fatigue, and corrosion, providing more comparable and complete data support for material performance analysis and life prediction.

[0034] The method of this invention is applicable to various metallic materials such as GH3535, Hastelloy N, and Inconel 617, and can be used with fluoride molten salts such as FLiNaK and FLiBe, as well as chloride molten salts for testing. The test temperature can be extended to the range of 550~850℃ depending on the tolerance of the material being tested. The strain amplitude and tensile holding time during the test can be flexibly adjusted according to the actual working conditions. When using it, it is only necessary to ensure that the loading waveform is stable and that the molten salt does not decompose.

[0035] Example 1

[0036] In this embodiment, a solution-treated GH3535 alloy was used as the test material, with a gauge length of Φ6mm × 16mm. FLiNaK molten salt (LiF-NaF-KF, 46.5:11.5:42 mol%) was selected. The test temperature was set at 700℃, the total strain amplitude was 0.5%, the strain rate was 0.001% / s, the tensile holding time was set to 10s, and the protective atmosphere was high-purity argon with an oxygen content <5ppm. The test was conducted according to the complete above steps, with a trapezoidal loading waveform: tensioned at a rate of 0.008mm / s to the set strain amplitude of 0.5% and held for 10s, then compressed at the same rate to the reverse strain of -0.5% (without holding), and this cycle was repeated.

[0037] Test results: Sample fracture cycle life N f =2131 cycles.

[0038] Example 2

[0039] The test conditions in this embodiment are the same as those in Example 1, except that the tensile holding time is adjusted to 60s.

[0040] Test results: Sample fracture cycle life N f =332 cycles.

[0041] Example 3

[0042] The test conditions in this embodiment are the same as those in Example 1, except that the tensile holding time is adjusted to 180s.

[0043] Test results: Sample fracture cycle life N f =103 cycles.

[0044] like Figure 4As shown, the horizontal axis represents the number of cycles (Cycles), and the vertical axis represents the peak stress (unit: MPa, positive values ​​are tensile peak stress, and negative values ​​are compressive peak stress). The three curves showing the change of peak stress with the number of cycles correspond to Examples 1-3 with a total strain amplitude of 0.5% and tensile holding times of 10s, 60s, and 180s, respectively. The specimen with the red square curve (0.5%-10s) in Example 1 exhibits a typical three-stage characteristic of "slight cyclic hardening - long-term mechanical stability - late-stage cyclic softening": the initial stage is characterized by slight cyclic hardening (cycles 1-50), followed by a stable stage (cycles 50-1900), and the last 200 cycles show continuous softening with a basically constant softening rate, ultimately resulting in a fracture cycle life of approximately 2131 cycles. In Example 2, the blue circular curve (0.5%-60s) shows a shortened cyclic hardening phase, a significantly shorter mechanical stabilization period, and earlier cyclic softening: a slight cyclic hardening phase occurs from the 1st to the 50th cycle, followed by a brief stabilization phase from the 50th to the 180th cycle. From around the 180th cycle, continuous softening begins with a gradually increasing softening rate, resulting in an overall lifespan significantly lower than the 10s hold-load group, with a fracture cycle life of approximately 332 cycles. In Example 3, the green triangular curve (0.5%-180s) shows no obvious mechanical stabilization phase, but rapidly enters a continuous softening state after initial hardening, with a significantly accelerated stress reduction rate: rapid hardening occurs initially, followed by no stabilization phase, with cyclic softening immediately beginning at the 50th cycle, resulting in a fracture cycle life of only approximately 103 cycles. Therefore, this invention, by simultaneously collecting and plotting cyclic stress response curves with different holding times under high-temperature molten salt conditions, intuitively quantifies the influence of creep holding time on the cyclic hardening / softening behavior, mechanical stability, and fatigue life of materials, and clearly reveals the damage evolution process under the coupled effects of creep-fatigue-corrosion, providing comprehensive and comparable experimental data support for material performance evaluation, damage mechanism analysis, and life prediction.

[0045] like Figure 5As shown, Examples 1-3 correspond to three sub-plots for different holding times (t=10s, t=60s, t=180s). Each row contains three sub-plots, corresponding to the single-cycle stress-time curves for different number of cycles. The horizontal axis represents time (s), and the vertical axis represents stress (MPa). The stress changes in the 1st, 1000th, and 2000th cycles of Example 1 show that during the 10s tensile holding stage, the stress only shows slight relaxation, and the relaxation amount decreases slightly with the increase of the number of cycles, indicating that creep damage contributes little and the material has high mechanical stability: the stress decreases by about 31MPa during the 1st cycle, about 32MPa during the 1000th cycle, and about 25MPa during the 2000th cycle. The stress changes in the 1st, 160th, and 330th cycles of Example 2 show that during the 60-second tensile hold phase, stress relaxation is divided into two stages: rapid relaxation and slow relaxation. The relaxation amount gradually decreases with the increase of the number of cycles, indicating that creep damage gradually accumulates as the cycle progresses, and the material properties change. The stress decreases by about 53 MPa within 60 seconds of the 1st cycle (40 MPa in the first 30 seconds and 12 MPa in the last 30 seconds), the stress decreases by about 44 MPa during the 160th cycle, and the stress decreases to 30 MPa during the 330th cycle. The stress changes in the 1st, 50th, and 100th cycles of Example 3 show that during the 180s tensile hold period, stress relaxation exhibits a three-stage characteristic: rapid relaxation, steady-state relaxation, and accelerated relaxation. In the first cycle, the stress decreases by approximately 62 MPa within 180 seconds (32 MPa in the first 30 seconds and 30 MPa in the next 150 seconds). During the 50th cycle, the stress decrease increases to 77 MPa, and during the 100th cycle, the stress decrease increases sharply to over 117 MPa. In later cycles, due to crack propagation, a significant creep-fatigue acceleration phenomenon occurs during the hold period. This invention, by collecting and plotting stress relaxation curves at different hold times and cycle stages, comprehensively characterizes the creep deformation behavior and damage development law of materials under high-temperature molten salt conditions. It achieves multi-dimensional analysis of coupled creep, fatigue, and corrosion damage, overcoming the deficiency of existing technologies in simultaneously quantifying the contribution of creep damage, and providing direct experimental evidence for revealing the mechanism of coupled damage.

[0046] The three sets of embodiments described above are merely preferred examples. Combined with the standardized testing process, specialized sample structure, trapezoidal wave loading method, and environmental control scheme of this invention, the impact of holding time on material damage rate and fatigue life can be clearly verified. Compared to existing testing methods lacking unified standards and with inconsistent testing conditions, the standardized design of this invention allows for cross-sectional comparison of data from different test groups, significantly improving the reliability and practicality of the test results. This provides reliable experimental evidence for the selection of structural materials, performance evaluation, and damage mechanism research of nuclear reactors and molten salt energy storage facilities.

[0047] Comparative Example 1

[0048] Comparative Example 1 employed a method of "coating the sample surface with molten salt before loading," whereby the prepared molten salt was coated onto the surface of the gauge length of a smooth rod-shaped sample, and then cyclic loading was performed directly on a high-temperature fatigue testing machine. During the research and development process, it was found that this method was prone to molten salt loss, local accumulation, or detachment during heating and cyclic deformation, making it difficult to maintain a continuous and stable molten salt coating throughout the entire gauge length. This caused contamination and corrosion to the test vessel and sample fixtures. Furthermore, the amount and distribution of salt on different samples fluctuated significantly, resulting in an unstable corrosion environment, poor test repeatability, and an inability to accurately characterize the creep-fatigue-corrosion coupled damage behavior under continuous immersion conditions.

[0049] Comparative Example 2

[0050] Comparative Example 2 employs a step-by-step testing method of "static molten salt corrosion followed by mechanical loading." This involves first subjecting the sample to isothermal corrosion in molten salt, then removing and cleaning it before conducting fatigue or creep-fatigue tests. While this approach can reflect the impact of pre-corrosion on subsequent mechanical properties, the corrosion and loading processes are temporally separate. It fails to reflect the true process of crack initiation, propagation, and stress relaxation occurring simultaneously in a high-temperature molten salt environment. Therefore, it is difficult to use for evaluating the coupled damage mechanism under the simultaneous effects of molten salt corrosion, creep, and fatigue.

[0051] Comparative Example 3

[0052] Comparative Example 3 directly used commercially available mixed salt powder that had not undergone drying, vacuum impurity removal, or pre-melting treatment in the molten salt preparation stage, loading it into the molten salt vessel. During the research and development process, it was found that this method was more prone to atmospheric fluctuations during heating due to uneven molten salt composition, the release of residual moisture and volatile impurities, and even problems such as localized oxidation and unstable corrosion conditions, leading to increased dispersion in test results across different batches. In contrast, this invention pre-treats the molten salt through steps such as raw material drying, vacuum impurity removal, high-temperature melting, and cooling pulverization, which is more conducive to ensuring the consistency of molten salt composition, state, and experimental environment.

[0053] Comparative Example 4

[0054] Comparative Example 4 only performed conventional machining and ordinary sanding on the gauge length section of the sample during sample preparation, without further fine polishing or electropolishing. During the research and development process, it was found that the sample surface prepared using this method easily retained tool marks, scratches, and minor machining defects. Under the combined effects of high-temperature molten salt environment and cyclic loading, these surface defects easily became locations of localized stress concentration and preferential corrosion, leading to early surface crack initiation and intensified localized corrosion. Furthermore, the surface condition of different batches of samples was difficult to maintain consistently, increasing the dispersion of test results and hindering the accurate differentiation between the intrinsic material properties and surface processing conditions on cycle life and stress response. In contrast, this invention, through fine polishing and electropolishing of the gauge length section of the sample, effectively reduces surface micro-defects and improves the consistency of the sample surface condition, thus enhancing the repeatability and reliability of test results.

[0055] Comparative Example 5

[0056] Comparative Example 5 attempts two common extended test schemes in the loading waveform design: one is to use symmetrical triangular wave cyclic loading without load maintenance, and the other is to set load maintenance at both the tensile peak and compressive peak. The former can complete high-temperature fatigue loading and is suitable for studying the effect of pure fatigue load without considering the introduction and control of creep damage contribution; the latter will introduce a compressive load maintenance process that is not completely consistent with the target service state, which complicates the interpretation of stress response and is not conducive to quantitative analysis of the impact of tensile peak load maintenance on service life and stress relaxation behavior. In contrast, the present invention sets load maintenance only at the tensile peak strain, which is more conducive to highlighting the dangerous damage stage of components serving in high-temperature molten salt environments and improving the interpretability and comparability of test results.

[0057] As can be seen from the above comparative examples, this invention is not a simple patchwork of existing fatigue tests or molten salt corrosion tests, but rather a synergistic technical solution addressing three key issues: "stable molten salt environment control," "standardized test procedures," and "quantitatively controllable creep-fatigue loading." Only by combining the integrated molten salt vessel sample structure, molten salt and sample pretreatment, and trapezoidal wave loading with tensile end-only loading, can the actual service conditions of materials in high-temperature molten salt be realistically simulated, and coupled damage data with good repeatability and comparability be obtained.

[0058] Compared with the prior art, the present invention has the following beneficial effects:

[0059] (1) Based on the integrated molten salt tank sample structure, the gauge length of the sample is completely immersed in dynamic molten salt, and the molten salt is kept in a molten state throughout the test, which truly simulates the service environment of the structural material in the molten salt pile (molten salt corrosion and cyclic load coexist), overcoming the shortcomings of "corrosion before loading" or "smearing method" in maintaining a stable corrosion environment.

[0060] (2) By using the strain-controlled trapezoidal wave loading method and independently setting the tensile holding time, the contribution of creep damage to fatigue life can be quantitatively studied. The holding time can be set from 0 (pure fatigue) to hundreds of seconds, covering the creep-fatigue interaction range under typical working conditions.

[0061] (3) Smooth rod-shaped specimens are used, and the entire process is standardized and compatible with international standards such as ASTM E606 / ASTM E 2714, making it easy to compare test results across different systems. At the same time, standardized procedures are provided for the entire process from specimen and molten salt preparation, environment establishment, loading control to fracture analysis, ensuring data repeatability.

[0062] (4) This scheme relies on the design of a vacuum high temperature fatigue testing machine and a sample molten salt tank, which has low requirements for the corrosion resistance of fixtures and tooling. This method can be applied to other alloys and corrosive media, reducing the threshold for equipment maintenance and modification.

[0063] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of the invention. Various variations can be made to the above embodiments of the present invention. That is, all simple and equivalent changes and modifications made based on the claims and description of this invention fall within the protection scope of the claims. All aspects not described in detail in this invention are conventional technical content.

Claims

1. A creep-fatigue testing method in a molten salt environment, characterized in that, Includes the following steps: S1. The alloy material to be tested is processed into a smooth rod-shaped sample of an integrated molten salt vessel; S2. Fill the molten salt container with molten salt so that the gauge length of the sample is completely submerged in the molten salt; S3. Transfer the assembled specimen to the fatigue testing machine and install the fixtures and measuring devices; S4. Evacuate the test container and fill it with inert gas, then heat it to the test temperature; S5. A trapezoidal wave cyclic load is applied to the specimen using axial strain control mode, with a load holding stage set only at the peak tensile strain. Test data is collected in real time during the test until the specimen reaches the failure judgment condition.

2. The creep-fatigue testing method according to claim 1, characterized in that, In step S1, the gauge length of the smooth rod-shaped sample has a diameter of 5~10mm, a length of 16~30mm, and a surface roughness Ra≤0.2μm. The gauge length is then subjected to mechanical polishing, fine sandpaper polishing, manual polishing with polishing paste, and electrolytic polishing.

3. The creep-fatigue testing method according to claim 2, characterized in that, Electropolishing uses a polishing solution composed of sulfuric acid, glycerin, and water. The electropolishing process is carried out in an ice bath environment with a fixed voltage.

4. The creep-fatigue testing method according to claim 1, characterized in that, In step S2, the molten salt used is either fluoride molten salt or chloride molten salt; the molten salt is prepared through precise weighing, powder mixing, raw material drying, vacuum impurity removal, high-temperature melting, cooling and pulverizing processes.

5. The creep-fatigue testing method according to claim 1, characterized in that, In step S3, the loading of the sample and molten salt is completed in a glove box protected by inert gas. After the molten salt container is assembled, the gaps are sealed with sealant, and then the whole container is transferred to the fatigue testing machine.

6. The creep-fatigue testing method according to claim 1, characterized in that, In step S4, the operation of evacuating and filling with inert gas is repeated to replace the air in the container; the heating rate is 3~5℃ / min, and the heating adopts a segmented temperature control mode, first rapidly heating to close to the test temperature, and then switching the temperature control mode to raise it to the test temperature.

7. The creep-fatigue testing method according to claim 1, characterized in that, In step S4, the inert gas inside the container is high-purity argon. The temperature control deviation of the entire test system is controlled within ±2℃. After heating to the test temperature, the temperature is maintained until the molten salt is completely melted and reaches thermal equilibrium.

8. The creep-fatigue testing method according to claim 1, characterized in that, In step S5, the loading strain rate is 0.001~1% / s, the total strain amplitude is 0.2%~1.2%, and the compression half-cycle of the trapezoidal wave cyclic load does not have a load holding stage.

9. The creep-fatigue testing method according to claim 1, characterized in that, In step S5, the collected test data include the number of cycles, peak stress, valley stress, and stress relaxation curve during the load holding stage; the failure criterion is that the peak stress of the specimen decreases by 30% during the relatively stable stage.

10. The creep-fatigue testing method according to claim 1, characterized in that, After the experiment, peak stress-cycle number curves and stress relaxation-time curves were plotted based on the collected experimental data to characterize the material's cyclic hardening, softening properties and creep deformation behavior.

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