A method for obtaining material low-cycle fatigue parameters based on uniaxial tension test

Abstract

The application discloses a method for obtaining material low-cycle fatigue parameters based on a uniaxial tension test, and relates to the technical field of material performance evaluation, and comprises the following steps: performing a uniaxial tension test on a standard sample of a material to be tested, obtaining a monotonic tension stress-strain curve of the material to be tested, and recording the Young's modulus of the material to be tested; based on Masing characteristics, mapping a plurality of preset strain loading conditions to the monotonic tension stress-strain curve one by one, and determining a stress amplitude and a plastic strain amplitude corresponding to each strain loading condition; adopting a stress control mode, performing a stress fatigue test on the standard sample by taking each stress amplitude as a loading condition respectively, until the standard sample fails, and obtaining a fatigue life corresponding to each stress amplitude; based on the plastic strain amplitude and the fatigue life corresponding to each strain loading condition, combining the Young's modulus, fitting a Manson-Coffin equation, and obtaining low-cycle fatigue parameters of the material to be tested, so that the test reliability under extreme conditions and the engineering requirement of rapid detection are taken into account.

Description

Technical Field

[0001] This invention relates to the field of material performance evaluation technology, and in particular to a method for obtaining low-cycle fatigue parameters of materials based on uniaxial tensile testing. Background Technology

[0002] In the field of low-cycle fatigue performance evaluation of materials, the Manson-Coffin equation is a fundamental theoretical model that quantitatively describes the relationship between the plastic strain amplitude and fatigue life of materials. However, accurately obtaining the characteristic parameters of this equation often requires conducting a large number of time-consuming cyclic fatigue tests.

[0003] In existing technologies, the industry typically employs strain control and utilizes high-temperature extensometers to implement steady-state cyclic loading, solving for relevant parameters by analyzing the hysteresis loop data during the stable phase. This approach suffers from the following problems: under conditions of large strain amplitude, the specimen experiences significant plastic deformation and radial shrinkage, leading to a continuous deterioration in the extensometer probe's contact state, directly limiting the accuracy of the feedback signal and control precision. To improve testing efficiency, existing technologies attempt to estimate relevant parameters using a single tensile test combined with empirical formulas; however, the predictive effectiveness of such methods highly depends on the matching degree of the material system, and the physical nature and inherent logical relationship between the parameters remain unclear. Furthermore, in practical industrial applications, traditional fatigue testing methods generally suffer from limitations such as long test cycles, poor adaptability to large deformation conditions, and large parameter dispersion, resulting in high time costs for material service performance verification and engineering selection decisions. Summary of the Invention

[0004] This invention provides a method for obtaining low-cycle fatigue parameters of materials based on uniaxial tensile testing. The technical problem it aims to solve is: how to provide a scheme for obtaining low-cycle fatigue parameters that can balance testing efficiency and data reliability under extreme deformation conditions.

[0005] This invention provides a method for obtaining low-cycle fatigue parameters of materials based on uniaxial tensile testing, wherein the material under test has massing characteristics, and the method includes: A uniaxial tensile test was performed on a standard specimen of the material to be tested to obtain the monotonic tensile stress-strain curve of the material to be tested, and the Young's modulus of the material to be tested was recorded. Based on the Masing characteristic, multiple preset strain loading conditions are mapped one by one to the monotonic tensile stress-strain curve to determine the stress amplitude and plastic strain amplitude corresponding to each strain loading condition. Using a stress control mode, stress fatigue tests are conducted on the standard specimen with each stress amplitude as the loading condition until the standard specimen fails, and the fatigue life corresponding to each stress amplitude is obtained. Based on the plastic strain amplitude and fatigue life corresponding to each strain loading condition, and combined with the Young's modulus, the Manson-Coffin equation is fitted to obtain the low-cycle fatigue parameters of the material under test.

[0006] Optionally, based on the Masing characteristic, mapping multiple preset strain loading conditions one by one to the monotonic tensile stress-strain curve, and determining the stress amplitude and plastic strain amplitude corresponding to each strain loading condition, includes: Based on the Masing characteristic, the envelope of the monotonic tensile stress-strain curve is equivalent to the envelope of the steady-state cyclic hysteresis curve of the material under test under cyclic loading. For each strain loading condition, determine the coordinate point corresponding to the strain amplitude of the strain loading condition on the abscissa of the monotonic tensile stress-strain curve. Draw a straight line perpendicular to the horizontal axis from the coordinate point, and determine the stress value at the vertical axis corresponding to the intersection of the straight line and the envelope of the monotonic tensile stress-strain curve as the stress amplitude of the strain loading condition. Starting from the intersection point, draw a diagonal line with the slope corresponding to the Young's modulus until it intersects the horizontal axis. The strain corresponding to the intersection point of the diagonal line and the horizontal axis is determined as the plastic strain amplitude of the strain loading condition.

[0007] Optionally, the stress control mode, which uses each stress amplitude as a loading condition to perform stress fatigue tests on the standard specimen until the standard specimen fails, and obtains the fatigue life corresponding to each stress amplitude, includes: The force value measured by the force sensor is used as a closed-loop feedback quantity to control the loading load to reach each stress amplitude. After reaching the stress amplitude, the standard specimen is cyclically loaded until the standard specimen fails, and the fatigue life corresponding to each stress amplitude is obtained.

[0008] Optionally, the loading frequency of the cyclic loading is 5Hz to 10Hz.

[0009] Optionally, before mapping multiple preset strain loading conditions to the monotonic tensile stress-strain curve one by one based on the Masing characteristic, and determining the stress amplitude and plastic strain amplitude corresponding to each strain loading condition, the method further includes: The strain loading conditions are configured with at least five different strain amplitudes, and the strain amplitudes are distributed in a gradient from low to high.

[0010] Optionally, before performing a uniaxial tensile test on a standard specimen of the material to be tested, obtaining the monotonic tensile stress-strain curve of the material to be tested, and recording the Young's modulus of the material to be tested, the method further includes: Prepare a smooth round bar specimen that meets the fatigue test standard, and use the smooth round bar specimen as the standard specimen.

[0011] Optionally, the step of performing a uniaxial tensile test on a standard specimen of the material to be tested, obtaining a monotonic tensile stress-strain curve of the material to be tested, and recording the Young's modulus of the material to be tested includes: A uniaxial tensile test is performed on the standard specimen at a constant strain rate that matches the loading conditions of the stress fatigue test. Stress and strain data are recorded, and the monotonic tensile stress-strain curve and Young's modulus are obtained based on the stress and strain data.

[0012] Optionally, the step of fitting the Manson-Coffin equation based on the plastic strain amplitude and fatigue life corresponding to each strain loading condition, combined with the Young's modulus, to obtain the low-cycle fatigue parameters of the material under test includes: Based on the plastic strain amplitude and fatigue life corresponding to each strain loading condition, the Manson-Coffin equation is curve-fitted to obtain the low-cycle fatigue parameters, which include fatigue strength coefficient, fatigue strength index, fatigue ductility coefficient, and fatigue ductility index.

[0013] Optionally, the preset multiple strain loading conditions include large deformation conditions with strain amplitude greater than or equal to 2.0%.

[0014] This invention provides a method for obtaining low-cycle fatigue parameters of materials based on uniaxial tensile testing. By utilizing the inherent massing properties of the material under test, the stress obtained from a single monotonic tensile test is... The strain curve is mapped to the parameters under cyclic loading conditions. Only one tensile test is needed to determine the stress amplitude and plastic strain amplitude under each target strain condition, thus replacing the traditional test path that relies on multiple sets of steady-state cyclic fatigue tests to acquire hysteresis loops with a uniaxial tensile test. Furthermore, based on this, a stress-controlled mode is adopted for fatigue testing, using the force value measured by the force sensor as the closed-loop feedback reference. This feedback is unaffected by the radial shrinkage and surface morphology evolution of the specimen during large plastic deformation, ensuring a stable and controllable loading waveform and highly repeatable test data. Moreover, the stress-controlled mode overcomes the bandwidth limitations of the extensometer response under traditional strain control modes, significantly increasing the testing frequency and greatly reducing the test cycle without significant specimen temperature rise. In summary, this scheme can effectively ensure the stability and accuracy of test data under large deformation conditions while significantly improving detection efficiency, thus balancing the engineering requirements of testing reliability and rapid detection under extreme conditions. Attached Figure Description

[0015] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the following description of the embodiments will be briefly introduced. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0016] Figure 1 A flowchart illustrating the method for obtaining low-cycle fatigue parameters of materials based on uniaxial tensile testing, provided in an embodiment of the present invention. Figure 2 This is a schematic diagram of the monotonic tensile stress-strain curve of the material under test in an embodiment of the present invention; Figure 3 This is a schematic diagram illustrating parameter mapping and plastic strain amplitude determination based on Masing characteristics in an embodiment of the present invention. Detailed Implementation

[0017] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] It should be understood that, when used in this specification and the appended claims, the terms "comprising" and "including" indicate the presence of the described features, integrals, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components and / or collections thereof.

[0019] It should also be understood that the terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention. As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” are intended to include the plural forms unless the context clearly indicates otherwise.

[0020] It should also be further understood that the term "and / or" as used in this specification and the appended claims refers to any combination of one or more of the associated listed items and all possible combinations, and includes such combinations.

[0021] As used in this specification and the appended claims, the term "if" may be interpreted, depending on the context, as "when," "once," "in response to determination," or "in response to detection." Similarly, the phrase "if determined" or "if [described condition or event] is detected" may be interpreted, depending on the context, as "once determined," "in response to determination," "once [described condition or event] is detected," or "in response to detection of [described condition or event]."

[0022] Please see Figure 1 This invention provides a method for obtaining low-cycle fatigue parameters of materials based on uniaxial tensile testing, applicable to materials exhibiting Massing characteristics. The Massing characteristic is a type of mechanical behavior feature of metallic materials during cyclic plastic deformation. Its core meaning is that, under symmetrical cyclic loading conditions, the envelope of the cyclic hysteresis curve formed by reverse loading essentially coincides with the monotonic tensile stress-strain curve of the material, and the geometry of the rising and falling branches is similar to the monotonic tensile curve, with an overall scale magnification factor of two. This characteristic establishes an intrinsic correlation between monotonic tensile mechanical behavior and cyclic fatigue mechanical behavior, allowing the stress-strain response of the material under cyclic loading to be deduced through a single monotonic tensile test, thus eliminating the need for time-consuming steady-state cyclic fatigue tests.

[0023] This embodiment obtains the monotonic tensile stress-strain curve of the material by performing a single uniaxial tensile test, and maps multiple preset strain loading conditions onto this curve to determine the stress amplitude and plastic strain amplitude corresponding to each condition. Subsequently, a stress-controlled fatigue test is conducted, and finally, the low-cycle fatigue parameters of the material are fitted according to the Manson-Coffin equation. This technical approach fundamentally changes the traditional paradigm of low-cycle fatigue testing that relies on strain control and contact extensometers, effectively solving the problem of extensometer signal inaccuracy under large deformation conditions. The method includes the following steps: S1, Perform a uniaxial tensile test on a standard specimen of the material to be tested, obtain the monotonic tensile stress-strain curve of the material to be tested, and record the Young's modulus of the material to be tested.

[0024] In practice, a uniaxial tensile test is performed on a standard specimen of the material to be tested. The uniaxial tensile test is conducted according to the specimen preparation and testing procedures in relevant fatigue testing standards. During the test, a tensile load is applied at a set constant strain rate until the specimen fractures or reaches a predetermined strain; this is not specifically limited in this invention. During the tensile process, the applied stress and corresponding strain data are recorded in real time to obtain a monotonic tensile stress-strain curve of the material to be tested, and the Young's modulus of the material to be tested is recorded simultaneously. The Young's modulus is calculated from the slope of the elastic segment of the stress-strain curve and is an important input parameter for calculating the elastic strain amplitude in the subsequent Manson-Coffin equation fitting.

[0025] In some preferred embodiments, before performing a uniaxial tensile test on the standard specimen of the material to be tested, obtaining the monotonic tensile stress-strain curve of the material to be tested, and recording the Young's modulus of the material to be tested, the method further includes: preparing a smooth round bar specimen that meets the fatigue testing standard, and using the smooth round bar specimen as the standard specimen.

[0026] In practice, this embodiment ensures the comparability and repeatability of uniaxial tensile tests and subsequent stress fatigue tests by standardizing the size and surface quality of the specimens. The specific implementation method is as follows: Before initiating the uniaxial tensile test, a sample is first taken from the blank of the material to be tested and prepared into a smooth round bar specimen that meets the fatigue testing standard requirements through machining. The smooth round bar specimen has a standard gauge length diameter, gauge length, clamping end size, and transition fillet radius. For example, the gauge length diameter is typically 6 mm to 10 mm, and the gauge length is 4 to 5 times the diameter; however, this invention does not specifically limit these dimensions. The specimen surface must be precision-machined or polished to ensure that the surface roughness meets the standard requirements, thereby eliminating the potential impact of surface machining marks on fatigue life. After specimen preparation, dimensional measurements and surface quality checks are required; only specimens that meet the requirements can be used as standard specimens.

[0027] In subsequent tests, multiple standard specimens prepared from the same batch were used for all operating conditions, with at least one specimen used for each condition. This standardized specimen preparation ensured consistency in the material performance testing, avoiding data discrepancies caused by differences in specimen geometry or surface defects.

[0028] This embodiment eliminates the interference of sample size and surface quality differences on the test results by preparing smooth round bar specimens that meet the fatigue testing standards. This makes the results of uniaxial tensile tests and stress fatigue tests highly comparable and repeatable, and also facilitates the comparison of test results with industry standard data.

[0029] In some preferred embodiments, the step of performing a uniaxial tensile test on a standard specimen of the material to be tested, obtaining a monotonic tensile stress-strain curve of the material to be tested, and recording the Young's modulus of the material to be tested includes: performing a uniaxial tensile test on the standard specimen at a constant strain rate matching the loading conditions of the stress fatigue test, recording stress and strain data, and obtaining the monotonic tensile stress-strain curve and the Young's modulus based on the stress and strain data.

[0030] In this specific implementation, by selecting a strain rate similar to that of the fatigue test, the mechanical behavior reflected by the monotonic tensile curve can more accurately approximate the material response under cyclic loading. At the same time, it ensures that the conversion process from the original recorded data to the final output parameters has a complete data chain. The specific implementation method is as follows: When performing a uniaxial tensile test on a standard specimen of the material to be tested, a constant strain rate is applied, the value of which is determined according to the loading conditions of the target stress fatigue test. The tensile strain rate and the average strain rate during fatigue cyclic loading can be on the same order of magnitude. For example, if the loading frequency of the fatigue test is 5 Hz and the strain amplitude is 2%, the corresponding average strain rate is approximately 0.2 s. -1 Therefore, a tensile test time of 0.001 s can be selected. - ¹to 0.1 s - ¹A constant strain rate within a certain range, the specific value of which can be determined according to material properties and standards, is not specifically limited in this invention. Selecting a strain rate similar to that of fatigue testing can make the strain rate sensitivity effect of the material during tensile testing consistent with that in cyclic fatigue testing, thereby improving the accuracy of the mapping results.

[0031] Furthermore, during the constant strain rate loading process, the displacement rate of the testing machine's crossbeam is controlled, while load data is acquired in real time via a force sensor and strain data is acquired in real time via an extensometer or displacement sensor. Stress and strain values ​​at each moment are continuously recorded at a predetermined sampling frequency until the specimen fractures or the strain reaches a predetermined value. The recorded stress and strain data are matched sequentially over time to form a stress-strain data point set.

[0032] Furthermore, after completing the tensile test, based on the stress-strain data point set, a monotonic tensile stress-strain curve of the tested material is plotted with strain value on the x-axis and stress value on the y-axis. During the plotting process, the data point set is smoothed or fitted to eliminate noise interference from the test process. Simultaneously, multiple data points from the elastic segment of the stress-strain curve (usually the linear portion within the proportional limit range) are used for linear regression, and the slope of the regression line is calculated. This slope is the Young's modulus. The Young's modulus, as a physical quantity characterizing the elastic stiffness of the material, is a key parameter for calculating the elastic strain amplitude in the subsequent Manson-Coffin equation fitting.

[0033] This embodiment uses a constant strain rate that matches the loading conditions of the target fatigue test for stretching, so that the strain rate effect of the monotonic stretching curve is close to the strain rate effect in the cyclic fatigue test, thereby improving the accuracy of mapping based on Masing characteristics and reducing the equivalent error introduced by the difference in strain rate.

[0034] S2, based on the Masing characteristic, the preset multiple strain loading conditions are mapped one by one to the monotonic tensile stress-strain curve to determine the stress amplitude and plastic strain amplitude corresponding to each strain loading condition.

[0035] In practice, after obtaining the monotonic tensile stress-strain curve, based on the masing characteristics of the material under test, multiple preset strain loading conditions are mapped one by one to the monotonic tensile stress-strain curve to determine the stress amplitude and plastic strain amplitude corresponding to each strain loading condition. The specific principle of the mapping is as follows: the monotonic tensile curve is regarded as the skeleton curve of cyclic loading. The stress value corresponding to any total strain amplitude on the monotonic tensile curve represents the stress amplitude required for cyclic loading under that strain amplitude; and the strain after deducting the elastic recovery part is the plastic strain amplitude. Further, the total strain amplitude corresponding to each of the multiple preset strain loading conditions is used as the abscissa value, and the stress value corresponding to that strain amplitude is found on the monotonic tensile stress-strain curve. This stress value is the stress amplitude under that condition; at the same time, the plastic strain amplitude under that condition is obtained by drawing a hysteresis loop from this point along the slope of Young's modulus to the horizontal axis.

[0036] In some preferred embodiments, before mapping multiple preset strain loading conditions to the monotonic tensile stress-strain curve one by one based on the Masing characteristic, and determining the stress amplitude and plastic strain amplitude corresponding to each strain loading condition, the method further includes setting at least five sets of strain loading conditions with different strain amplitudes, wherein the strain amplitudes are distributed in a gradient from low to high.

[0037] In practice, this embodiment ensures sufficient data points for subsequent Manson-Coffin equation fitting by setting up multi-gradient strain conditions covering the elastic to plastic range, thereby improving the reliability and accuracy of parameter solving. The specific implementation method is as follows: After completing the uniaxial tensile test and obtaining the monotonic tensile stress-strain curve, and before mapping, at least five strain loading conditions with different strain amplitudes should be set up. The strain amplitude refers to the total strain amplitude, which should extend from near the material's elastic limit to a larger plastic strain region. For example, strain values ​​of 0.2%, 0.5%, 1.0%, 1.5%, 2.0%, and 3.0% can be selected. These strain values ​​are distributed in a gradient from low to high, with low-end strain values ​​covering the region dominated by elastic strain, high-end strain values ​​covering the region of large plastic strain, and intermediate strain values ​​used for transition. Furthermore, setting up more than five loading conditions ensures sufficient degrees of freedom for subsequent fitting of the Manson-Coffin equation, enabling the regression analysis to obtain accurate fatigue strength coefficients, fatigue strength exponents, fatigue ductility coefficients, and fatigue ductility exponents.

[0038] Furthermore, the strain loading conditions can be pre-stored in a configuration file or manually entered based on material type and experience. After setting, each set of conditions is sequentially used as input data and entered into the mapping steps described in the above embodiments to calculate the stress amplitude and plastic strain amplitude corresponding to each condition. Since different materials may have different stress responses under the same strain amplitude, the stress amplitude of each condition is not uniformly distributed but is actually determined by the monotonic tensile curve. This ensures that the loading conditions of the subsequent fatigue test are completely matched with the cyclic performance of the actual material.

[0039] This embodiment ensures sufficient and reasonably distributed data points for fitting the Manson-Coffin equation by setting at least five sets of strain loading conditions with a gradient distribution from low to high. This significantly improves the solution accuracy and statistical reliability of the four key parameters, including fatigue strength coefficient and fatigue ductility coefficient, avoids fitting deviations caused by data points concentrated in a single region, and enhances the representativeness of parameters across the entire strain range.

[0040] In some preferred embodiments, the preset multiple strain loading conditions include large deformation conditions with strain amplitude greater than or equal to 2.0%.

[0041] In practical implementation, this embodiment explicitly includes large deformation conditions, ensuring that the method can cover fatigue performance evaluation under extreme plastic conditions, filling the technical gap in traditional strain control methods for testing failures in large strain ranges. The specific implementation method is as follows: After completing the uniaxial tensile test and obtaining the monotonic tensile stress-strain curve, at least one of the preset strain loading conditions is a large deformation condition with a strain amplitude greater than or equal to 2.0%. The strain amplitude of the large deformation condition can be selected as 2.0%, 2.5%, 3.0%, etc., and the specific value can be determined according to the material plasticity and testing requirements. Because the material undergoes significant plastic deformation under large deformation conditions, the extensometer in the traditional strain control mode is prone to slippage and loss of control, leading to invalid test data. However, in this method, due to the use of stress control mode and mapping based on Masing characteristics, the stress amplitude can be directly read from the tensile curve regardless of the deformation. Furthermore, the stress fatigue test is controlled by a closed-loop force sensor and is completely unaffected by the extensometer. Therefore, the test under large deformation conditions is also stable and reliable.

[0042] During the test, the stress amplitude corresponding to the large deformation condition (e.g., 370 MPa for 2.0% strain) was read from the monotonic tensile curve, and this stress amplitude was cyclically loaded until the specimen failed, thus obtaining its fatigue life. The obtained plastic strain amplitude and fatigue life data, along with data from other small strain conditions, were used to fit the Manson-Coffin equation, thereby truly reflecting the fatigue behavior of the material under extreme plastic conditions.

[0043] This embodiment effectively overcomes the technical difficulty of traditional strain control methods in obtaining effective data in the large strain zone due to extensometer slippage by including large deformation conditions with strain amplitude greater than or equal to 2.0% in the test range. This enables low-cycle fatigue testing to cover the full range of working conditions from elastic to severe plastic deformation, filling the technical gap in material fatigue performance evaluation under extreme load conditions. It is particularly suitable for the large load design requirements of key fields such as aerospace and nuclear power.

[0044] In some preferred embodiments, the step of mapping multiple preset strain loading conditions to the monotonic tensile stress-strain curve one by one based on the Masing characteristic, and determining the stress amplitude and plastic strain amplitude corresponding to each strain loading condition, includes: according to the Masing characteristic, equating the envelope of the monotonic tensile stress-strain curve to the envelope of the steady-state cyclic hysteresis curve of the material under test under cyclic loading; for each strain loading condition, determining a coordinate point on the abscissa of the monotonic tensile stress-strain curve corresponding to the strain amplitude of the strain loading condition; drawing a straight line perpendicular to the abscissa from the coordinate point, and determining the stress value on the ordinate corresponding to the intersection of the straight line and the envelope of the monotonic tensile stress-strain curve as the stress amplitude of the strain loading condition; starting from the intersection point, drawing a diagonal line with the slope corresponding to the Young's modulus until it intersects the abscissa, and determining the strain corresponding to the intersection of the diagonal line and the abscissa as the plastic strain amplitude of the strain loading condition.

[0045] In this specific implementation, this embodiment transforms the strain information on the monotonic tensile curve into loading parameters required for stress-controlled fatigue testing through precise geometric plotting and coordinate transformation, while also providing plastic deformation data for subsequent Manson-Coffin equation fitting. The specific implementation method is as follows.

[0046] First, based on the Masing characteristic, the envelope of the monotonic tensile stress-strain curve is equivalent to the envelope of the steady-state cyclic hysteresis curve of the material under test under cyclic loading. Here, the envelope refers to the complete trajectory on the curve. This equivalence stems from the Masing criterion, which states that the shape of the hysteresis loop after steady-state cycling of a metallic material is similar to the shape of a single tensile curve, but with a proportionally enlarged size. Therefore, the stress value corresponding to any strain point on the monotonic tensile curve can be directly regarded as the stress response value under cyclic loading with the same strain amplitude. This equivalence is the core physical basis for this invention to replace cyclic fatigue testing with uniaxial tensile testing.

[0047] Furthermore, for each strain loading condition, a coordinate point corresponding to the strain amplitude of the strain loading condition is determined on the abscissa of the monotonic tensile stress-strain curve. The strain loading condition is typically expressed as a total strain amplitude, such as 0.5%, 1.0%, 1.5%, 2.0%, etc. The corresponding point is found on the curve using this total strain amplitude as the abscissa scale.

[0048] Furthermore, a straight line perpendicular to the horizontal axis is drawn from the coordinate point. The stress value corresponding to the intersection of this line and the envelope of the monotonic tensile stress-strain curve is determined as the stress amplitude under the strain loading condition. This intersection point represents the stress level that the material needs to reach during cyclic loading under a specified total strain amplitude. This stress amplitude will serve as the loading target for subsequent stress-controlled fatigue tests. This operation utilizes real data points on the curve, rather than estimations based on empirical formulas, thus providing a clear physical meaning and a high degree of consistency with the actual material response.

[0049] Furthermore, starting from the intersection point, a diagonal line is drawn along the slope corresponding to the Young's modulus until it intersects the horizontal axis. The strain corresponding to the intersection point of this diagonal line and the horizontal axis is determined as the plastic strain amplitude of the strain loading condition. The slope of this diagonal line is consistent with the Young's modulus, and its physical meaning is: in the total strain amplitude, after deducting the elastic recovery portion, the remaining strain is the portion of the material that has undergone irreversible plastic deformation. This portion is precisely the plastic strain amplitude required in the Manson-Coffin equation, while the elastic strain amplitude can be obtained by subtracting the plastic strain amplitude from the total strain amplitude, or directly calculated from the Young's modulus and stress amplitude.

[0050] This embodiment combines the equivalent envelope principle of Masing properties with specific geometric mapping operations to achieve a seamless conversion from static tensile data to cyclic fatigue parameters. Because this mapping method is strictly based on the Masing physical criterion and does not rely on empirical correction coefficients, it has clear physical meaning and a wide range of applicable materials, avoiding the problem of large differences in fitting accuracy for different material systems caused by empirical estimation methods.

[0051] S3, using stress control mode, stress fatigue test is performed on the standard specimen with each stress amplitude as the loading condition until the standard specimen fails, and the fatigue life corresponding to each stress amplitude is obtained.

[0052] In practice, after determining the stress amplitude and plastic strain amplitude for each working condition, a stress control mode is adopted. Stress fatigue tests are performed on the standard specimen using each stress amplitude as the loading condition until the standard specimen fails, and the fatigue life corresponding to each stress amplitude is obtained. The stress control mode refers to using the loaded load as the control target, measuring and feeding back the force value in real time through a force sensor, and using a closed-loop control system to keep the actual load stable at the set stress amplitude level. Furthermore, since the force sensor is a rigid measuring element, its accuracy is not affected by the radial shrinkage or surface morphology changes of the specimen. Therefore, it can maintain a stable loading waveform even under large plastic deformation conditions, avoiding data failure caused by extensometer slippage in traditional strain control modes. During the fatigue test, the number of cycles at which the specimen fractures under each working condition is recorded, which is the fatigue life corresponding to that stress amplitude.

[0053] In some preferred embodiments, the stress control mode, which uses each stress amplitude as a loading condition to perform stress fatigue tests on the standard specimen until the standard specimen fails, and obtains the fatigue life corresponding to each stress amplitude, includes: using the force value measured by the force sensor as a closed-loop feedback quantity, controlling the loading load to reach each stress amplitude, and cyclically loading the standard specimen after reaching the stress amplitude until the standard specimen fails, and obtaining the fatigue life corresponding to each stress amplitude.

[0054] In practice, this embodiment uses the force value measured by the force sensor as a closed-loop feedback quantity to achieve precise control of the applied load, thereby ensuring the stability and reliability of the test process under each stress amplitude. It is especially suitable for large plastic deformation conditions. The specific implementation method is as follows: Before the stress fatigue test begins, the stress amplitude corresponding to a certain strain loading condition determined in step S2 above is used as the loading target for this test. The force applied to the standard specimen is measured in real time using a high-precision load cell, and its signal is amplified and converted from analog to digital before being input to the controller. The controller compares the measured force value with the target force value corresponding to the target stress amplitude, and adjusts the output of the loading mechanism using a proportional-integral-derivative (PID) control algorithm to make the actual load approach and stabilize near the target force value, forming a closed-loop control. The PID control algorithm is a commonly used feedback adjustment method in industrial control; its parameters can be tuned according to material properties and loading rate to ensure rapid response and stability.

[0055] Furthermore, during the test, the standard specimen is continuously cyclically loaded with the target force value. The loading waveform is typically a symmetrical sine wave or triangular wave, and the cycle frequency is determined by the test settings. Throughout the cyclic loading, the force sensor continuously feeds back the force value signal, and the controller adjusts the loading output in real time to ensure that the stress amplitude remains at the set value. When the specimen fractures due to accumulated fatigue damage, a sudden drop in load or excessive displacement is automatically detected, the test is stopped, and the number of cycles the specimen underwent under this condition is recorded. This number of cycles represents the fatigue life corresponding to that stress amplitude.

[0056] The stress control mode used in this embodiment is fundamentally different from the traditional strain control mode: In the strain control mode, the extensometer picks up the deformation signal of the gauge length of the specimen as the feedback quantity. However, when the specimen undergoes radial shrinkage under large plastic deformation, the contact pressure of the extensometer decreases, and the signal is easily distorted, leading to control failure. In contrast, in the stress control mode, the feedback quantity comes from a rigid force sensor and is not affected by changes in the specimen's geometry. Even if the specimen shrinks significantly or its surface morphology changes, the force feedback remains stable. Therefore, the test process is stable and controllable, and the data repeatability is significantly improved.

[0057] This embodiment implements stress control by using the force value measured by the force sensor as a closed-loop feedback quantity, so that the loading control is completely unaffected by deformation factors such as oxide scale peeling and radial shrinkage on the sample surface. Compared with the defects of extensometer signals being easily interfered with in the traditional strain control mode, the loading waveform of this embodiment is more stable and the control accuracy is higher. It significantly reduces the dispersion of test results between different samples of the same material and at different cycle stages of the same sample, and ensures high consistency in multiple repeated tests.

[0058] In some preferred embodiments, the loading frequency of the cyclic loading is 5 Hz to 10 Hz.

[0059] In practice, this embodiment significantly reduces the duration of a single fatigue test by employing a higher loading frequency under stress control mode, thus meeting the engineering requirements for rapid material screening and batch testing. The specific implementation method is as follows: Based on the stress control mode described in the above embodiments, the cyclic loading frequency is set to a value between 5 Hz and 10 Hz. The loading frequency refers to the number of complete tensile-compression cycles per second, measured in Hertz (Hz). The upper limit of this frequency is determined by the response speed of the force sensor and controller, while ensuring that the sample does not experience a significant temperature rise due to high frequency during loading, thus altering material properties. The most suitable loading frequency can be selected within the range of 5 Hz to 10 Hz based on the thermal conductivity and fatigue testing standards of the material under test. For example, for metal materials with good thermal conductivity, a high frequency close to 10 Hz can be used for testing; for materials prone to heat generation, the frequency should be appropriately reduced.

[0060] Furthermore, after setting the frequency, an alternating load is applied to the standard specimen at that frequency using the closed-loop control method described in the above embodiment. Since the stress control mode does not depend on the contact response of the extensometer, and the dynamic response bandwidth of the force sensor is much higher than that of a traditional high-temperature extensometer, the loading frequency can be easily increased to over 5 Hz. Furthermore, during the test, the specimen temperature is monitored; if the temperature rise exceeds a preset threshold, the frequency can be automatically reduced or the test paused to ensure data validity. The entire test cycle is significantly shortened due to the increased frequency. Under traditional strain control mode, the loading frequency typically does not exceed 1 Hz, while in this embodiment, by increasing the frequency by 5 to 10 times, the same amount of testing can be completed in a shorter time.

[0061] This embodiment increases the testing efficiency by 5 to 10 times by increasing the loading frequency to 5 Hz to 10 Hz under stress control mode. The time required for a single complete test is reduced from several days to several hours, effectively meeting the engineering timeliness requirements for batch material screening, rapid testing, and big data statistical analysis. At the same time, the combination of high-frequency loading and stress control ensures the stable output of the load waveform under high frequency, and the test quality is not reduced due to the increase in frequency.

[0062] S4. Based on the plastic strain amplitude and fatigue life corresponding to each strain loading condition, and combined with the Young's modulus, the Manson-Coffin equation is fitted to obtain the low-cycle fatigue parameters of the material under test.

[0063] In practice, finally, based on the plastic strain amplitude and fatigue life corresponding to each strain loading condition, and combined with the Young's modulus, the Manson-Coffin equation is fitted to obtain the low-cycle fatigue parameters of the material under test. Furthermore, the Manson-Coffin equation is a core, well-known theoretical model in the field of low-cycle fatigue, used to describe the power function relationship between plastic strain amplitude and fatigue life. By performing regression analysis on the plastic strain amplitude and fatigue life data under multiple conditions, the material constants in the equation are solved, thereby obtaining a complete set of low-cycle fatigue parameters.

[0064] This embodiment obtains a monotonic tensile stress-strain curve through a single uniaxial tensile test and uses a stress-controlled mode for fatigue testing. It completely eliminates the reliance on contact-type high-temperature extensometers, fundamentally avoiding the problems of extensometer contact instability, signal distortion, and data failure caused by radial shrinkage of the specimen under large deformation conditions. It significantly improves the stability and data reliability of low-cycle fatigue testing under large strain conditions, while greatly compressing the traditional testing cycle of several days to several weeks, thus balancing accuracy and efficiency.

[0065] In some preferred embodiments, the step of fitting the Manson-Coffin equation based on the plastic strain amplitude and fatigue life corresponding to each strain loading condition, combined with the Young's modulus, to obtain the low-cycle fatigue parameters of the material under test includes: performing curve fitting on the Manson-Coffin equation based on the plastic strain amplitude and fatigue life corresponding to each strain loading condition to obtain the low-cycle fatigue parameters, wherein the low-cycle fatigue parameters include fatigue strength coefficient, fatigue strength index, fatigue ductility coefficient, and fatigue ductility index.

[0066] In this specific implementation, the fatigue strength coefficient, fatigue strength index, fatigue ductility coefficient, and fatigue ductility index in the equation are solved by curve fitting of the data on plastic strain amplitude and fatigue life. The specific implementation method is as follows: After completing stress fatigue tests under all preset strain loading conditions and obtaining the fatigue life under each condition, the Manson-Coffin equation is fitted based on the plastic strain amplitude and fatigue life corresponding to each strain loading condition, combined with the Young's modulus. In the field of low-cycle fatigue, the Manson-Coffin equation is a core, well-known theoretical model characterizing the relationship between the plastic strain amplitude and fatigue life of a material. Its standard form is: Δε t / 2 = Δε e / 2 + Δε p / 2 = (σ f ' / E)(2N f ) b+ ε f '(2N f ) c Where: Δε t / 2 represents the total strain amplitude, which is half of the total strain change in each loading cycle; Δε e / 2 is the elastic strain amplitude, which is equal to the total strain amplitude minus the plastic strain amplitude, or it can be obtained by dividing the stress amplitude by Young's modulus; Δε p / 2 represents the plastic strain amplitude, which is the strain portion of the material undergoing irreversible plastic deformation, obtained through the mapping steps described above; E is the Young's modulus; σ f ' is the fatigue strength coefficient, which is the fitted value of the maximum elastic stress amplitude that a material can withstand under a single cycle of fatigue; b is the fatigue strength index, which describes the rate of logarithmic deceleration of elastic strain amplitude with respect to fatigue life; ε f ' is the fatigue ductility coefficient, which is the fitted value of the maximum plastic strain amplitude that a material can withstand under a single cycle of fatigue; c is the fatigue ductility index, which describes the rate of logarithmic deceleration of the plastic strain amplitude with respect to fatigue life. N f The fatigue life is the number of cycles required for the specimen to fracture under a given stress amplitude.

[0067] During the fitting process, the elastic strain component Δε is utilized. e / 2 = (σ f ' / E)(2N f ) b By taking the logarithm of the elastic strain amplitude and fatigue life data and performing linear regression, the fatigue strength coefficient σ can be obtained. f ' and fatigue strength index b; simultaneously, utilizing the plastic strain portion Δε p / 2 = ε f '(2N f ) c The fatigue ductility coefficient ε was obtained by taking the logarithm of the plastic strain amplitude and fatigue life data and performing linear regression. f And the fatigue ductility index c. In the regression process, standard statistical methods such as the least squares method can be used, and the correlation coefficient should be tested to ensure that the fitting results are statistically significant.

[0068] The low-cycle fatigue parameters finally obtained include the fatigue strength coefficient σ. f ', Fatigue strength index b, Fatigue ductility coefficient ε fAnd the fatigue ductility index c. These parameters are the core characterization quantities of the low-cycle fatigue performance of materials, and can be used to plot strain-life curves, predict the fatigue life of components, and evaluate material selection.

[0069] This embodiment solves all four core parameters of the Manson-Coffin equation by curve fitting of plastic strain amplitude and fatigue life data. This method can output a complete low-cycle fatigue performance model, which can not only be used to draw strain-life curves, but also provide direct quantitative basis for fatigue life prediction, material selection and reliability assessment of engineering components.

[0070] For example, in one embodiment, the monotonic tensile stress-strain curve of the material under test is as follows: Figure 2 As shown, the curve represents strain on the horizontal axis and stress on the vertical axis, reflecting the stress-strain variation of the tested material under uniaxial tensile load. The curve is linear in the elastic segment, and its slope represents the Young's modulus. This curve provides a data basis for subsequent parameter mapping based on Masing properties.

[0071] Furthermore, such as Figure 3 The diagram illustrates the geometric transformation process from strain loading conditions to stress amplitude and plastic strain amplitude. For the strain amplitude of the strain loading condition, a corresponding coordinate point is determined on the abscissa of the curve. A straight line perpendicular to the abscissa is drawn from this coordinate point. The ordinate value of the intersection point of this line and the envelope of the monotonic tensile stress-strain curve is the stress amplitude. Starting from this intersection point, a diagonal line is drawn with the slope corresponding to the Young's modulus until it intersects the abscissa. The strain corresponding to this intersection point is the elastic recovery component, and the strain from the origin to this intersection point is the plastic strain amplitude.

[0072] It should be understood that the sequence number of each step in the above embodiments does not imply the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present invention.

[0073] In the above embodiments, the descriptions of each embodiment have different focuses. For parts that are not described in detail in a certain embodiment, please refer to the relevant descriptions in other embodiments.

[0074] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, this invention is also intended to include these modifications and variations as long as they fall within the scope of the claims of this invention and their equivalents.

[0075] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in the present invention, and these modifications or substitutions should all be covered within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for obtaining low-cycle fatigue parameters of materials based on uniaxial tensile testing, characterized in that, The material to be tested has massing properties, and the method includes: A uniaxial tensile test was performed on a standard specimen of the material to be tested to obtain the monotonic tensile stress-strain curve of the material to be tested, and the Young's modulus of the material to be tested was recorded. Based on the Masing characteristic, multiple preset strain loading conditions are mapped one by one to the monotonic tensile stress-strain curve to determine the stress amplitude and plastic strain amplitude corresponding to each strain loading condition. Using a stress control mode, stress fatigue tests are conducted on the standard specimen with each stress amplitude as the loading condition until the standard specimen fails, and the fatigue life corresponding to each stress amplitude is obtained. Based on the plastic strain amplitude and fatigue life corresponding to each strain loading condition, and combined with the Young's modulus, the Manson-Coffin equation is fitted to obtain the low-cycle fatigue parameters of the material under test.

2. The method for obtaining low-cycle fatigue parameters of materials based on uniaxial tensile testing according to claim 1, characterized in that, Based on the Masing characteristic, multiple preset strain loading conditions are mapped one by one to the monotonic tensile stress-strain curve to determine the stress amplitude and plastic strain amplitude corresponding to each strain loading condition, including: Based on the Masing characteristic, the envelope of the monotonic tensile stress-strain curve is equivalent to the envelope of the steady-state cyclic hysteresis curve of the material under test under cyclic loading. For each strain loading condition, determine the coordinate point corresponding to the strain amplitude of the strain loading condition on the abscissa of the monotonic tensile stress-strain curve. Draw a straight line perpendicular to the horizontal axis from the coordinate point, and determine the stress value at the vertical axis corresponding to the intersection of the straight line and the envelope of the monotonic tensile stress-strain curve as the stress amplitude of the strain loading condition. Starting from the intersection point, draw a diagonal line with the slope corresponding to the Young's modulus until it intersects the horizontal axis. The strain corresponding to the intersection point of the diagonal line and the horizontal axis is determined as the plastic strain amplitude of the strain loading condition.

3. The method for obtaining low-cycle fatigue parameters of materials based on uniaxial tensile testing according to claim 1, characterized in that, The stress control mode is adopted, and stress fatigue tests are performed on the standard specimen using each stress amplitude as a loading condition until the standard specimen fails, and the fatigue life corresponding to each stress amplitude is obtained, including: The force value measured by the force sensor is used as a closed-loop feedback quantity to control the loading load to reach each stress amplitude. After reaching the stress amplitude, the standard specimen is cyclically loaded until the standard specimen fails, and the fatigue life corresponding to each stress amplitude is obtained.

4. The method for obtaining low-cycle fatigue parameters of materials based on uniaxial tensile testing according to claim 3, characterized in that, The loading frequency of the cyclic loading is 5Hz to 10Hz.

5. The method for obtaining low-cycle fatigue parameters of materials based on uniaxial tensile testing according to claim 1, characterized in that, Before mapping multiple preset strain loading conditions to the monotonic tensile stress-strain curve one by one based on the Masing characteristic, and determining the stress amplitude and plastic strain amplitude corresponding to each strain loading condition, the method further includes: The strain loading conditions are configured with at least five different strain amplitudes, and the strain amplitudes are distributed in a gradient from low to high.

6. The method for obtaining low-cycle fatigue parameters of materials based on uniaxial tensile testing according to claim 1, characterized in that, Before performing a uniaxial tensile test on a standard specimen of the material to be tested, obtaining the monotonic tensile stress-strain curve of the material to be tested, and recording the Young's modulus of the material to be tested, the method further includes: Prepare a smooth round bar specimen that meets the fatigue test standard, and use the smooth round bar specimen as the standard specimen.

7. The method for obtaining low-cycle fatigue parameters of materials based on uniaxial tensile testing according to claim 1, characterized in that, The step involves performing a uniaxial tensile test on a standard specimen of the material to be tested, obtaining a monotonic tensile stress-strain curve of the material to be tested, and recording the Young's modulus of the material to be tested, including: A uniaxial tensile test is performed on the standard specimen at a constant strain rate that matches the loading conditions of the stress fatigue test. Stress and strain data are recorded, and the monotonic tensile stress-strain curve and Young's modulus are obtained based on the stress and strain data.

8. The method for obtaining low-cycle fatigue parameters of materials based on uniaxial tensile testing according to claim 1, characterized in that, Based on the plastic strain amplitude and fatigue life corresponding to each strain loading condition, and combined with the Young's modulus, the Manson-Coffin equation is fitted to obtain the low-cycle fatigue parameters of the material under test, including: Based on the plastic strain amplitude and fatigue life corresponding to each strain loading condition, the Manson-Coffin equation is curve-fitted to obtain the low-cycle fatigue parameters, which include fatigue strength coefficient, fatigue strength index, fatigue ductility coefficient, and fatigue ductility index.

9. The method for obtaining low-cycle fatigue parameters of materials based on uniaxial tensile testing according to claim 1, characterized in that, The preset multiple strain loading conditions include large deformation conditions with strain amplitude greater than or equal to 2.0%.

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