A method for testing the thermal mechanical fatigue crack growth of metal materials

Abstract

The present invention belongs to the technical field of material mechanical property testing, and discloses a method for testing the thermomechanical fatigue crack propagation of metal materials. First, the functional relationship between the crack length and the flexibility of the sample, as well as the functional relationship between the crack propagation driving force and the crack length are obtained by the finite element method; then, a crack is prefabricated using the constant load method under normal temperature conditions, and a thermomechanical fatigue crack propagation test is performed to obtain the test results; finally, the crack length corresponding to each cycle of the metal sample is calculated by the improved flexibility method, and the crack propagation rate is calculated in combination with the functional relationship between the crack propagation driving force and the crack length. The present invention can effectively simulate the working conditions of variable temperature and alternating load coupling that high-temperature components are subjected to during service, obtain the specific crack propagation rate and the evolution of the plastic zone in the crack propagation area, and provide important experimental data and theoretical basis for the damage tolerance design and safety verification of high-temperature structures under severe working conditions.

Description

Technical Field

[0001] The present invention belongs to the technical field of material mechanical property testing, and in particular relates to a crack propagation testing method for metal materials under complex thermal-mechanical coupling loading conditions. Background Art

[0002] In industries such as aerospace, electricity, and energy, many hot-end components face the combined effects of severe temperature fluctuations and cyclic mechanical loads during their service life, a phenomenon known as thermomechanical fatigue. Long-term loading from thermomechanical fatigue can lead to fracture failure of key components, and has become a major factor affecting the integrity of high-temperature structures, causing serious safety hazards and economic losses. Furthermore, due to limitations in detection methods and production economic considerations, many components already have microcracks during the manufacturing stage or early in their service life. These cracks will accelerate under the action of thermomechanical fatigue loads, leading to premature failure of the workpiece.

[0003] Currently, research on the thermomechanical fatigue crack growth behavior of metallic materials is relatively limited. Furthermore, the lack of a unified test method has led to significantly different, or even contradictory, test results obtained by different operators using the same material and the same loading conditions. Therefore, a unified and effective test method for thermomechanical fatigue crack growth in metallic materials is urgently needed to determine the crack growth behavior of metallic materials under complex thermomechanical loading conditions and provide the test data necessary for damage tolerance design and safety verification of equipment. Summary of the Invention

[0004] This paper proposes a method for testing the thermomechanical fatigue crack growth of metallic materials. This method addresses a technological gap in the field by evaluating the thermomechanical fatigue crack growth behavior of metallic materials under extreme service environments. The method effectively simulates the coupled conditions of variable temperature and alternating loads experienced by high-temperature components during service, providing a detailed picture of the crack growth rate and the evolution of the plastic zone within the crack growth region. This method facilitates in-depth discussions of the thermomechanical fatigue crack growth mechanism of metallic materials, helps ensure the consistency of test results, and provides important experimental data and a theoretical basis for damage tolerance design and safety verification of high-temperature structures under harsh operating conditions.

[0005] The above-mentioned object of the present invention is achieved through the following technical solutions:

[0006] The present invention provides a method for testing the thermomechanical fatigue crack growth of a metal material, comprising the following steps:

[0007] S1: Based on the size of the metal specimen, the finite element method is used to obtain the functional relationship between the crack length a and the specimen flexibility Ev / P under constant temperature conditions, as well as the functional relationship between the crack propagation driving force ΔK and the crack length a;

[0008] Moreover, natural surface speckles are formed on the surface of the metal specimen;

[0009] S2: Prefabricate cracks using the constant load method at room temperature;

[0010] S3: Measure the crack length before the test by optical method, and mark the relative test machine mounting position and high temperature extensometer installation position on the metal specimen;

[0011] S4: Install the metal specimen and the high-temperature extensometer on the testing machine; determine whether the test is a variable temperature test, if yes, jump to step S5, otherwise jump to step S6;

[0012] S5: Measure the temperature-thermal strain relationship of metal specimens;

[0013] S6: Set the loading parameters of the testing machine, including stress amplitude, stress ratio, stress waveform, temperature amplitude, temperature mean, temperature waveform, cycle period, and mechanical-temperature load phase angle;

[0014] S7: Starting a thermomechanical fatigue crack growth test, during which a surface natural speckle image of the crack growth area is collected in real time and real-time image distortion correction is performed; when the crack length reaches a predetermined length as monitored by the real-time collected surface natural speckle image, the test is stopped and the test result is obtained;

[0015] S8: Determine whether the test is a variable temperature test, if yes, jump to step S9, otherwise jump to step S10;

[0016] S9: Based on the temperature-thermal strain relationship obtained in S5, the test results obtained in S7 are compensated for thermal strain to obtain mechanical strain;

[0017] S10: Calculate the crack length corresponding to each cycle of the metal specimen using the improved compliance method, and combine it with the functional relationship between the crack growth driving force ΔK and the crack length a obtained in S1 to calculate the crack growth rate;

[0018] The calculation process of the improved flexibility method is as follows:

[0019] S101: Calculating stress data using the test results to obtain a mechanical strain-stress hysteresis loop, and intercepting a linear portion of an unloading segment of the mechanical strain-stress hysteresis loop in each cycle of the test;

[0020] S102: Calculate the mean of every four adjacent data points in the linear part. The elastic modulus of the metal sample at different temperatures is measured in advance. The modulus is converted using the average temperature of every four adjacent data points to obtain the elastic modulus E of the metal sample per unit time. i ; Use the mechanical strain of adjacent data points to subtract and obtain the deformation v of the metal sample per unit time i; Then a mechanical strain-stress curve of n points can be carried out n-3 times E i *v i Calculation; where i is the calculation order;

[0021] S103: Select the stress difference between point 1 and point n-3 as P, and obtain the compliance result ∑E under thermomechanical fatigue loading. i v i / P;

[0022] S104: Substitute the compliance result into the relationship between the crack length a and the compliance Ev / P of the metal specimen obtained in S1 to obtain the crack length.

[0023] Furthermore, in S1, P is the stress difference of the metal sample, v is the deformation of the metal sample, and E is the elastic modulus of the metal material.

[0024] Furthermore, in S1, the natural surface speckle is obtained by slow wire cutting.

[0025] Furthermore, in S2, the allowable upper limit of the prefabricated load used in the constant load method is: the yield strength of the metal specimen at room temperature multiplied by λ and then multiplied by 80%; where λ is the ratio of the stress amplitude in S6 divided by the yield strength corresponding to the highest temperature set for the test in S7.

[0026] Furthermore, S3 further includes measuring the length of the crack before the test by an optical method.

[0027] Furthermore, in S4, if an induction coil is used to heat the metal sample, it is necessary to ensure that the axial temperature gradient of the induction coil in the metal sample is less than 10°C.

[0028] Furthermore, in S5, the mechanical loads at both ends of the metal sample are set to be kept at 0, and a cyclic temperature load is applied to the metal sample by an induction heating device, and the temperature-thermal strain data is measured and recorded by a high-temperature extensometer.

[0029] Furthermore, in S5 , data of at least 20 temperature cycles are measured and average results of temperature and thermal strain are obtained.

[0030] Furthermore, in S7, the image of the natural speckle on the surface is acquired by an image acquisition device.

[0031] Furthermore, in S7 and S9, the test results include time, total strain, mechanical load, and temperature.

[0032] Furthermore, in S9, the thermal strain compensation is to divide each cycle of the test data obtained in S7 into a heating section and a cooling section. For the heating section and the cooling section, the total strain in the test results is subtracted from the thermal strain data corresponding to the heating section and the cooling section in the temperature-thermal strain relationship to obtain the mechanical strain.

[0033] Furthermore, S10 also includes using S7 to collect the natural speckle image of the surface and calculating the plastic zone and strain field at the crack tip by using digital image correlation technology.

[0034] Furthermore, in S10, the crack growth driving force ΔK is calculated by the crack length a, and then the crack growth increment is obtained by subtracting the crack length a of the adjacent cycles. The functional relationship between ΔK and the crack growth increment is the crack growth rate.

[0035] The beneficial effects of the present invention are:

[0036] The method of the present invention can accurately simulate the complex working conditions faced by high-temperature components during actual service, namely the dual coupling of alternating temperature and mechanical load, ensuring a high degree of consistency between the test environment and the actual application scenario, and providing a more practical test platform for the performance evaluation of metal materials in harsh environments. Through a carefully designed test process and advanced monitoring technology, the method of the present invention can accurately obtain the crack propagation rate and the details of the evolution of the plastic zone in the crack propagation area. These data not only quantify the fatigue damage process of the material, but also provide detailed data support for in-depth research on the mechanism of thermomechanical fatigue crack propagation. In response to the problem of inconsistent results that was prevalent in previous similar tests, the method of the present invention effectively improves the stability and repeatability of the test results by optimizing the test condition control, standardizing the operating procedures and introducing precision measurement technology, providing a more reliable test basis for scientific research and engineering applications. It not only helps researchers to deeply analyze the damage evolution mechanism of metal materials under thermomechanical fatigue, but also promotes the improvement and innovation of related theoretical models, providing a solid theoretical basis for the optimization design, damage tolerance design and safety verification of high-temperature structural materials. Given the widespread application of high-temperature components in key fields such as aerospace, energy and power, and petrochemicals, this method is of great significance for improving the safety performance, extending the service life, and reducing maintenance costs of equipment in these fields, showing broad application prospects and socio-economic benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present invention and therefore should not be regarded as limiting the scope of the present invention.

[0038] Figure 1Optical microscope images of (a) the natural speckle pattern on the surface and (b) the particle size in Example S1 of the present invention;

[0039] Figure 2 Analysis of the effect of hot air flow on image distortion using a quartz grid plate in Example S7 of the present invention: (a) room temperature, (b) 550°C, (c) during a heating process from 350°C to 550°C, and (d) during a cooling process from 350°C to 550°C.

[0040] Figure 3 (a) the standard grid scale plate and (b) the captured image used in Example S7 of the present invention;

[0041] Figure 4 Comparison of the effects before and after distortion correction in Example S7 of the present invention: (a) before distortion correction, (b) after distortion correction;

[0042] Figure 5 The process and effect of thermal strain compensation in Example S9 of the present invention: (a) temperature-thermal strain data in the cooling and heating stages, (b) total strain-stress hysteresis loop obtained in the test, and (c) mechanical strain-stress hysteresis loop obtained after thermal strain compensation.

[0043] Figure 6 This is a comparison chart of the crack length results calculated by the improved compliance method in Example S10 of the present invention and the results of the traditional optical direct observation method. DETAILED DESCRIPTION

[0044] The present invention is further described below with reference to the accompanying drawings and specific examples, but the examples do not limit the present invention in any form.

[0045] This example further illustrates the present invention by conducting a thermomechanical fatigue crack growth test on 316LN austenitic stainless steel.

[0046] S1: Based on the size of the metal specimen, the functional relationship between the crack length a and the specimen compliance Ev / P under constant temperature conditions, as well as the functional relationship between the crack extension driving force ΔK and the crack length a, are obtained through finite element simulation.

[0047] Where P is the stress difference of the metal specimen; v is the deformation of the metal specimen; and E is the elastic modulus of the metal material.

[0048] In this embodiment, the metal sample was cut by slow wire cutting to obtain natural speckles on the surface. The morphology of the natural speckles on the surface is similar to the particle size. Figure 1 As shown in the optical microscope images, the average size of the natural speckle on the surface is 15 microns. The slow wire cutting method can effectively ensure that the speckle does not detach as the metal material deforms during the test.

[0049] S2: Pre-cracks are created using the constant load method at room temperature.

[0050] When using the constant load method for crack prefabrication, the prefabrication load should be less than 80% of the maximum mechanical load in the test. Since the yield strength of metal specimens at room temperature is much higher than that of high-temperature tests, in order to improve the efficiency of crack prefabrication, the load can be set using the normalized result relative to the current yield strength. Specifically, the stress amplitude in S6 is divided by the yield strength corresponding to the highest temperature set in the test in S7 to obtain the ratio λ. The yield strength of the metal specimen at room temperature is multiplied by λ and then multiplied by 80% to obtain the allowable upper limit of the prefabrication load.

[0051] In this example, the yield strength of 316LN material at 550°C is 135 MPa, and at room temperature (25°C) it is 280 MPa. If the stress amplitude in the formal high-temperature test is 120 MPa, which is 88.89% of the corresponding yield strength, the crack pre-stress at room temperature should be less than 80% of this value (88.89%), or less than 71.11% of the corresponding room-temperature yield strength, specifically less than 199.11 MPa. Therefore, a cyclic loading range of 0-150 MPa was selected at a frequency of 2.5 Hz, with pre-stressing stopped when the crack length reached 1 mm.

[0052] S3: Measure the crack length before the test by optical method, and mark the relative testing machine mounting position and high temperature extensometer installation position on the metal specimen to reduce installation errors.

[0053] S4: Install the metal specimen and the high-temperature extensometer on the testing machine; determine whether the test is a variable temperature test, if yes, jump to step S5, otherwise jump to step S6.

[0054] If an induction coil is used to heat the metal specimen, the coil distribution should be adjusted before the actual test to ensure that it does not affect the installation of the high-temperature extensometer and the acquisition of images of the crack propagation area. Furthermore, the temperature distribution in the induction coil of the metal specimen should be tested in advance to ensure that the axial temperature gradient is less than 10°C to avoid affecting the accuracy of the test results.

[0055] S5: Measure the temperature-thermal strain relationship of the metal specimen: Set the mechanical load at both ends of the metal specimen to 0, and apply a cyclic temperature load to the metal specimen through an induction heating device. Measure and record the temperature-thermal strain data using a high-temperature extensometer.

[0056] In order to reduce the influence of temperature distribution, at least 20 temperature cycles should be measured to obtain the average results of temperature and thermal strain.

[0057] S6: Set the loading parameters of the testing machine, including stress amplitude, stress ratio, stress waveform, temperature amplitude, temperature mean, temperature waveform, cycle period, and mechanical-temperature load phase angle.

[0058] In this embodiment, the stress amplitude is set to 120 MPa, the stress ratio is 0, the stress waveform is a triangular wave, the temperature amplitude is 100°C, the temperature average is 450°C, the temperature waveform is a triangular wave, the cycle period is 60 seconds to complete one cycle, and the mechanical-temperature load phase angle is 180°.

[0059] S7: The thermomechanical fatigue crack growth test begins. During the test, the image acquisition equipment is used to collect real-time natural speckle images of the crack growth area and perform real-time image distortion correction. When the crack length reaches the predetermined length as monitored by the real-time natural speckle images, the test is stopped and the test results of time, total strain, mechanical load, and temperature are obtained.

[0060] Among them, for image acquisition equipment, the acquisition deviation caused by factors such as high-temperature oxidation, hot air flow and high-temperature self-luminescence of metal samples during the speckle acquisition process should be considered and corrected.

[0061] Before the test, the metal sample was kept at the highest temperature during the test for 2 hours to allow the surface speckle to be fully oxidized, reducing the impact of changes in the degree of oxidation on the image acquisition during the test. Using a quartz grid plate fixed on the surface of the metal sample, the effects of hot air flow and high-temperature self-luminescence of the metal sample on the image acquisition equipment were measured. Figure 2 As shown. Figure 2 As can be seen in the figure, the node distance of the quartz grid plate remains unchanged under various temperature fields, indicating that image acquisition is not affected by factors such as hot air flow and the sample's high-temperature self-luminescence. If the distance between grid nodes changes, adjustments should be made to the induction coil, heat dissipation equipment, and image acquisition device.

[0062] In addition, the optical distortion of the image acquisition equipment should be considered and corrected to ensure the accuracy of image acquisition. Figure 3 As shown, the standard grid scale plate is photographed using an image acquisition device and distortion correction is performed. The distortion correction effect is shown in Figure 3 shown.

[0063] S8: Determine whether the test is a variable temperature test, if yes, jump to step S9, otherwise jump to step S10.

[0064] S9: Based on the temperature-thermal strain relationship obtained in S5, thermal strain compensation is performed on the test results of time, total strain, mechanical load, and temperature obtained in S7 to obtain mechanical strain;

[0065] Specifically, if Figure 5 As shown in (a), the temperature-thermal strain data is divided into the heating part and the cooling part, and thermal strain compensation is performed on the heating part and the cooling part respectively. Each cycle of the test data obtained in S7 is divided into the heating section and the cooling section. For the heating section and the cooling section, the total strain in the test results is subtracted from the thermal strain data corresponding to the heating section and the cooling section in the temperature-thermal strain relationship to obtain the mechanical strain, as shown in Figure 5 As shown in (b)(c).

[0066] S10: Use S7 to capture the natural speckle image of the surface and use digital image correlation technology to calculate the plastic zone and strain field at the crack tip for post-test analysis. For the digital image correlation calculation, consistent calculation parameters should be maintained to ensure the accuracy of the results. These parameters include consistent sub-zone radius, sub-zone span, and calculation smoothness.

[0067] The crack length corresponding to each cycle of the metal specimen is calculated by the improved compliance method, and the crack growth rate is calculated by combining the functional relationship between the crack growth driving force ΔK and the crack length a obtained by S1.

[0068] The improved compliance method is based on the ASTM E647 standard and is suitable for calculating crack length based on the change in compliance of metal specimens under variable temperature conditions. The specific calculation process of the improved compliance method is as follows:

[0069] S101: Using the test results to calculate stress data, thereby obtaining a mechanical strain-stress hysteresis loop, and intercepting the linear portion of the unloading segment of the mechanical strain-stress hysteresis loop of each cycle in the test;

[0070] S102: Calculate the mean of every four adjacent data points in the linear part. The elastic modulus of the metal sample at different temperatures is measured in advance. The modulus is converted using the average temperature of every four adjacent data points to obtain the elastic modulus E of the metal sample per unit time. i ; Using the mechanical strain subtraction of adjacent data points, the deformation of the metal sample per unit time v can be obtained i ; Then a mechanical strain-stress curve of n points can be carried out n-3 times E i *v i Calculation; where i is the calculation order.

[0071] S103: Select the stress difference between point 1 and point n-3 as P, and obtain the compliance result ∑E under thermomechanical fatigue loading. i v i / P.

[0072] S104: Substitute the compliance result into the relationship between the crack length a and the compliance Ev / P of the metal specimen obtained in S1 to obtain the crack length.

[0073] Among them, the initial crack length obtained by the improved compliance method is compared with the initial crack length measured in S3 to verify the calculation accuracy.

[0074] Finally, the crack growth driving force ΔK is calculated by the crack length a, and the crack growth increment is obtained by subtracting the crack length a of adjacent cycles. The functional relationship between ΔK and the crack growth increment is the crack growth rate.

[0075] The crack length was directly measured by conventional optical observation and compared with the crack length calculated in this embodiment. Figure 6 As shown, it can be seen that the crack lengths calculated under different test conditions are consistent with the crack lengths directly measured by the traditional optical observation method, which proves the accuracy of the crack length calculation of the present invention.

[0076] Although the preferred embodiments of the present invention have been described above in conjunction with the accompanying drawings, the present invention is not limited to the above-mentioned specific embodiments. The above-mentioned specific embodiments are merely illustrative and not restrictive. Under the guidance of the present invention, ordinary technicians in this field can also make many forms of specific changes without departing from the scope of protection of the present invention and the claims. These all fall within the scope of protection of the present invention.

Claims

1. A method for testing the thermal mechanical fatigue crack growth of a metal material, characterized in that: The steps include: S1: Based on the size of the metal specimen, the finite element method is used to obtain the functional relationship between the crack length a and the specimen flexibility Ev / P under constant temperature conditions, as well as the functional relationship between the crack propagation driving force ΔK and the crack length a; Moreover, natural surface speckles are formed on the surface of the metal specimen; S2: Prefabricate cracks using the constant load method at room temperature; S3: Mark the relative test machine mounting position and high temperature extensometer installation position on the metal specimen; S4: Install the metal specimen and the high-temperature extensometer on the testing machine; determine whether the test is a variable temperature test, if yes, jump to step S5, otherwise jump to step S6; S5: Measure the temperature-thermal strain relationship of metal specimens; S6: Set the loading parameters of the testing machine, including stress amplitude, stress ratio, stress waveform, temperature amplitude, temperature mean, temperature waveform, cycle period, and mechanical-temperature load phase angle; S7: Starting a thermomechanical fatigue crack growth test, during which a surface natural speckle image of the crack growth area is collected in real time and real-time image distortion correction is performed; when the crack length reaches a predetermined length as monitored by the real-time collected surface natural speckle image, the test is stopped and the test result is obtained; S8: Determine whether the test is a variable temperature test, if yes, jump to step S9, otherwise jump to step S10; S9: Based on the temperature-thermal strain relationship obtained in S5, the test results obtained in S7 are compensated for thermal strain to obtain mechanical strain; S10: Calculate the crack length corresponding to each cycle of the metal specimen using the improved compliance method, and combine it with the functional relationship between the crack growth driving force ΔK and the crack length a obtained in S1 to calculate the crack growth rate; The calculation process of the improved flexibility method is as follows: S101: Calculating stress data using the test results to obtain a mechanical strain-stress hysteresis loop, and intercepting a linear portion of an unloading segment of the mechanical strain-stress hysteresis loop in each cycle of the test; S102: Calculate the mean of every four adjacent data points in the linear part. The elastic modulus of the metal sample at different temperatures is measured in advance. The modulus is converted using the average temperature of every four adjacent data points to obtain the elastic modulus E of the metal sample per unit time. i ; Use the mechanical strain of adjacent data points to subtract and obtain the deformation v of the metal sample per unit time i ; Then a mechanical strain-stress curve of n points can be carried out n-3 times E i *v i Calculation; where i is the calculation order; S103: Select the stress difference between point 1 and point n-3 as P, and obtain the compliance result ΣE under thermomechanical fatigue loading. i v i / P; S104: Substitute the compliance result into the relationship between the crack length a and the compliance Ev / P of the metal specimen obtained in S1 to obtain the crack length.

2. A method for testing the thermomechanical fatigue crack growth of metal materials according to claim 1, characterized in that: In S1, P is the stress difference of the metal sample, v is the deformation of the metal sample, and E is the elastic modulus of the metal material; in S1, the natural surface speckle is obtained by slow wire cutting.

3. A method for testing the thermomechanical fatigue crack growth of metal materials according to claim 1, characterized in that: In S2, the allowable upper limit of the prefabricated load used in the constant load method is: the yield strength of the metal specimen at room temperature multiplied by λ and then multiplied by 80%; where λ is the ratio of the stress amplitude in S6 divided by the yield strength corresponding to the highest temperature set for the test in S7.

4. A method for testing the thermomechanical fatigue crack growth of metal materials according to claim 1, characterized in that: S3 also includes measuring the length of the crack before the test by optical method.

5. The method for testing the thermomechanical fatigue crack growth of a metal material according to claim 1, wherein: In S4, if an induction coil is used to heat the metal sample, it is necessary to ensure that the axial temperature gradient of the induction coil in the metal sample is less than 10°C.

6. A method for testing the thermomechanical fatigue crack growth of metal materials according to claim 1, characterized in that: In S5, the mechanical load at both ends of the metal specimen is set to 0, and a cyclic temperature load is applied to the metal specimen by an induction heating device, and the temperature-thermal strain data is measured and recorded by a high-temperature extensometer; in S5, at least 20 temperature cycle data are measured and the average results of temperature and thermal strain are taken respectively.

7. A method for testing the thermomechanical fatigue crack growth of metal materials according to claim 1, characterized in that: In S7, the image of the natural speckle on the surface is collected by an image collection device.

8. A method for testing the thermomechanical fatigue crack growth of metal materials according to claim 1, characterized in that: In S7 and S9, the test results include time, total strain, mechanical load, and temperature.

9. A method for testing the thermomechanical fatigue crack growth of metal materials according to claim 1, characterized in that: In S9, the thermal strain compensation is to divide each cycle of the test data obtained in S7 into a heating section and a cooling section. For the heating section and the cooling section, the thermal strain data corresponding to the heating section and the cooling section in the temperature-thermal strain relationship are respectively subtracted from the total strain in the test results to obtain the mechanical strain.

10. A method for testing the thermomechanical fatigue crack growth of a metal material according to claim 1, characterized in that: S10 also includes using S7 to collect the natural speckle image of the surface, and calculating the plastic zone and strain field at the crack tip through digital image correlation technology; in S10, the crack propagation driving force ΔK is calculated by the crack length a, and then the crack length a of adjacent cycles is subtracted to obtain the crack propagation increment. The functional relationship between ΔK and the crack propagation increment is the crack propagation rate.

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