Thermomechanical fatigue crack growth rate test method based on stiffness and crack tip strain field

Abstract

The application discloses a thermal-mechanical fatigue crack propagation rate testing method based on rigidity and crack tip strain field, comprising the following steps: firstly, a relationship function between normalized crack length and normalized rigidity is established through finite element calculation; secondly, DIC speckle preparation is carried out for observing crack propagation and strain field analysis; then, a thermal-mechanical fatigue test is carried out, crack length is calculated based on rigidity change results of the test, an a-N relationship curve is drawn combined with DIC synchronous observation results, and comparative analysis is carried out; finally, crack propagation rate is calculated through a three-point moving average secant method, and the size of a plastic zone at a crack tip is determined according to DIC strain field analysis, so that the accurate test of the crack propagation rate is realized. The application can effectively test the fatigue crack propagation behavior under the thermal-mechanical coupling effect, and accurately obtain the thermal-mechanical fatigue crack propagation rate combined with the crack tip strain field measurement method, so as to provide test data support for damage tolerance design and structure integrity evaluation of high-temperature pressure-bearing equipment.

Description

Technical Field

[0001] This invention relates to the field of fracture mechanics performance parameter testing technology, and in particular to a method for testing thermomechanical fatigue crack propagation rate based on stiffness and crack tip strain field. Background Technology

[0002] With continuous innovation in materials manufacturing and equipment processing technologies, key equipment in the petrochemical and nuclear power sectors is gradually developing towards multi-parameter collaborative control, large-scale integrated manufacturing, and long service life with high reliability to meet the sustainability and stability requirements under extreme operating conditions. As core pressure-bearing components of high-temperature and high-pressure systems, equipment such as the inner walls of hydrogenation reactors and cracking furnace tubes in the petrochemical field, and reactor loop piping and steam generator heat transfer tubes in nuclear power systems, endure mechanical cyclic loads (internal pressure fluctuations, fluid shocks) during long-term service, while also dealing with periodic thermal loads caused by temperature changes. The cumulative fatigue damage effect caused by this thermo-mechanical cyclic load coupling significantly accelerates crack initiation and fracture failure processes, and is a key factor limiting the service life of process equipment.

[0003] Currently, there is no standardized system for testing thermomechanical fatigue crack propagation rates. Traditional fatigue crack propagation rate testing methods do not consider the impact of temperature fluctuations on material properties and equipment accuracy, resulting in significant errors in measurement results under thermomechanical fatigue testing conditions. Therefore, it is urgent to develop an accurate and reliable method for characterizing thermomechanical fatigue crack propagation rates, systematically considering the influence of thermo-mechanical coupling effects on fatigue crack propagation behavior, and seeking fatigue crack propagation rate characterization parameters applicable to thermomechanical load spectra. This would provide experimental data support for damage tolerance design and structural integrity assessment of high-temperature pressure-bearing equipment. Summary of the Invention

[0004] To address the technical problems existing in the prior art, this invention proposes a thermomechanical fatigue crack propagation rate testing method based on stiffness and crack tip strain field, which enables dynamic observation of crack length and real-time capture of crack tip strain field, thus overcoming the shortcomings of traditional fatigue crack propagation rate testing methods under thermo-mechanical coupled field.

[0005] To achieve the above objectives, this invention provides a method for testing the thermomechanical fatigue crack propagation rate based on stiffness and crack tip strain field, comprising:

[0006] Based on the full-size finite element model of the fatigue crack propagation specimen, the stiffness evolution characteristics under different crack lengths were calibrated, and the relationship function between normalized crack length and normalized stiffness was constructed.

[0007] DIC speckle patterns were prepared on the sample surface, and crack propagation and strain field analysis were observed.

[0008] Establish the elastic modulus-temperature reference curve of the uncracked specimen, determine the reference value of the uncracked stiffness, and determine the thermal strain reference required for thermo-mechanical decoupling through thermal strain testing and zero stress measurement.

[0009] Thermomechanical fatigue crack propagation tests were conducted, and the crack length was calculated based on the stiffness change results of the tests. The aN relationship curve was plotted in conjunction with the DIC synchronous observation results and compared and analyzed.

[0010] The crack propagation rate was calculated using the three-point moving average secant method. The size of the crack tip plastic zone was determined based on the DIC strain field analysis, and the crack propagation rate results were correlated to achieve the test of the thermomechanical fatigue crack propagation rate.

[0011] Preferably, the relationship function between the normalized crack length and the normalized stiffness is:

[0012]

[0013] In the formula, a is the crack length, W is the net cross-sectional width at the notch root, and E... FE,crk and E FE,uncrk The stiffness is defined as uncracked stiffness and cracked stiffness, respectively, with F representing the functional relationship.

[0014] Preferably, DIC speckle patterns are prepared on the sample surface, and crack propagation and strain field analysis are observed, including:

[0015] Speckle patterns were prepared by adjusting the spray gun air pressure and spraying distance. The areal density was calculated and the spatial distribution randomness was analyzed based on digital image correlation (DIC) technology. Image distortion was corrected using a calibration plate.

[0016] Preferably, the thermal strain reference required for thermo-mechanical decoupling is determined by thermal strain testing and zero-stress measurement, including:

[0017] Thermocouple wires are welded to the gauge length of the specimen, and the fixture installation reference line and the high temperature extensometer positioning point are marked. The specimen and the high temperature extensometer are installed according to the installation reference line and the high temperature extensometer positioning point. The elastic modulus is tested at different temperatures. A reference curve of the elastic modulus of the uncracked specimen changing with temperature is established through a series of temperature control cycles, and the reference value of the uncracked stiffness is determined.

[0018] Based on the uncracked stiffness reference value, thermal strain testing and zero stress measurement are performed to obtain thermal strain data of the sample during temperature loading cycles, and to determine the thermal strain benchmark required for the thermo-mechanical decoupling.

[0019] The elastic modulus test requires completing at least a preset number of elastic loading and unloading cycles at several characteristic temperature points, and the temperature gradient of the sample in the direction perpendicular to the heating direction during the thermal strain test does not exceed a preset temperature range.

[0020] Preferably, the thermomechanical fatigue crack propagation test is performed, and the crack length is calculated based on the stiffness change results of the test, including:

[0021] Set the loading parameters, data acquisition frequency, and test termination conditions for the thermomechanical fatigue test, conduct the thermomechanical fatigue crack propagation test, and simultaneously acquire speckle images of the sample surface and observe the crack propagation length until the test termination conditions are met.

[0022] Based on the thermal strain data during the temperature loading cycle, a thermal strain compensation method is used to perform thermo-mechanical decoupling on the nominal total strain measured by the high-temperature extensometer, separating the mechanical strain component from the thermal strain component.

[0023] Combining the mechanical strain components and synchronous stress data, a nominal stress-mechanical strain hysteresis curve is plotted. The stiffness evolution data of the linear portion in the initial stage of unloading is extracted. The cracking stiffness is determined by piecewise linear fitting. The cracking stiffness is then substituted into the relationship function between the normalized crack length and the normalized stiffness to calculate the crack length.

[0024] Preferably, acquiring speckle images of the sample surface and observing the crack propagation length includes:

[0025] The magnification of the high-speed camera was preset and the scale was calibrated. The speckle image of the sample surface was acquired in real time through the high-temperature DIC system and the crack propagation length was observed simultaneously.

[0026] Preferably, the method for determining the crack stiffness through piecewise linear fitting is as follows:

[0027]

[0028] In the formula, i is the current fitting order, n is the preset maximum number of fitting iterations, and E crk,i E is the local crack initiation stiffness of the segmented interval corresponding to the current fitting order. crk This refers to the crack stiffness.

[0029] Preferably, the three-point moving average secant method is as follows:

[0030]

[0031] In the formula, (a i N i () represents the current data point. The crack propagation rate corresponding to the data point, (a i+1 N i+1 ), (a i1 N i1 All of them are adjacent data points.

[0032] Preferably, the size of the crack tip plastic zone is determined by setting DIC calculation parameters, including fixing the area of ​​the calculation region, the size of the subset, the correlation coefficient threshold, and the Gaussian low-pass filter.

[0033] Compared with the prior art, the present invention has the following advantages and technical effects:

[0034] (1) This invention establishes a thermomechanical fatigue crack propagation test platform to more accurately simulate the combined effect of temperature change and mechanical load on high-temperature pressure-bearing equipment. Based on the stiffness evolution of the test material during loading, a relationship with crack length is established, and the results are compared and corrected in conjunction with real-time observations, which can accurately obtain the fatigue crack propagation rate;

[0035] (2) By constructing a standardized DIC preparation and acquisition procedure and operation analysis process, this invention ensures the accuracy and repeatability of the strain and crack tip plastic zone size calculation results, and realizes a reliable characterization of the thermomechanical fatigue crack propagation rate.

[0036] (3) This invention enriches the research methods for thermomechanical fatigue crack propagation behavior, provides experimental data support for damage tolerance design and structural integrity assessment of high-temperature pressure equipment, and promotes the engineering application of fracture mechanics theory under complex working conditions. Attached Figure Description

[0037] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments and descriptions of this application are used to explain this application and do not constitute an undue limitation of this application. In the drawings:

[0038] Figure 1 This is a flowchart of the thermomechanical fatigue crack propagation rate testing method based on stiffness and crack tip strain field according to an embodiment of the present invention.

[0039] Figure 2 The normalized crack length a / W and normalized stiffness E are given in this embodiment of the invention. FE,crk / E FE,uncrk The functional relationship is a / W = F(E) FE,crk / E FE,uncrk ) Schematic diagram;

[0040] Figure 3 The images shown are speckle preparation patterns and correction patterns according to embodiments of the present invention. (a) is a speckle preparation pattern taken under an optical microscope, (b) is a grayscale comparison result in DIC software, and (c) and (d) are images of the calibration plate before and after image distortion correction, respectively.

[0041] Figure 4The following are comparison diagrams before and after the use of thermal strain compensation for thermo-mechanical decoupling in an embodiment of the present invention. In the diagram, (a) is the nominal stress-thermal strain hysteresis curve obtained during the experiment, (b) is the nominal stress-total strain hysteresis curve, and (c) is the nominal stress-mechanical strain hysteresis curve.

[0042] Figure 5 In this embodiment of the invention, the cracking stiffness E is determined based on the hysteresis curve. crk A schematic diagram;

[0043] Figure 6 This is a partial enlarged view of the initial segment data unloading according to an embodiment of the present invention;

[0044] Figure 7 This is an aN curve plotted based on the stiffness method and observation method according to an embodiment of the present invention;

[0045] Figure 8 This is a diagram showing the dimensions of the crack tip plastic zone in an embodiment of the present invention;

[0046] Figure 9 This is a graph showing the relationship between the size of the crack tip plastic zone and the fatigue crack propagation rate in an embodiment of the present invention. Detailed Implementation

[0047] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0048] It should be noted that the steps shown in the flowchart in the accompanying drawings can be executed in a computer system such as a set of computer-executable instructions, and although a logical order is shown in the flowchart, in some cases the steps shown or described may be executed in a different order than that shown here.

[0049] This embodiment proposes a thermomechanical fatigue crack propagation rate testing method based on stiffness and crack tip strain field, such as... Figure 1 ,include:

[0050] Based on the full-size finite element model of the fatigue crack propagation specimen, the stiffness evolution characteristics under different crack lengths were calibrated, and the relationship function between normalized crack length and normalized stiffness was constructed.

[0051] DIC speckle patterns were prepared on the sample surface, and crack propagation and strain field analysis were observed.

[0052] Establish the elastic modulus-temperature reference curve of the uncracked specimen, determine the reference value of the uncracked stiffness, and determine the thermal strain reference required for thermo-mechanical decoupling through thermal strain testing and zero stress measurement.

[0053] Thermomechanical fatigue crack propagation tests were conducted, and the crack length was calculated based on the stiffness change results of the tests. The aN relationship curve was plotted in conjunction with the DIC synchronous observation results and compared and analyzed.

[0054] The crack propagation rate was calculated using the three-point moving average secant method. The size of the crack tip plastic zone was determined based on the DIC strain field analysis, and the crack propagation rate results were correlated to achieve the test of the thermomechanical fatigue crack propagation rate.

[0055] This embodiment reveals the quantitative relationship between fatigue crack propagation rate and crack tip plastic zone size by simulating thermomechanical fatigue loading on high-temperature components and combining it with crack tip strain field measurement methods. This provides experimental data support for damage tolerance design and structural integrity assessment of high-temperature pressure equipment, and promotes the engineering application of fracture mechanics theory under complex working conditions.

[0056] Furthermore, the relationship function between the normalized crack length and the normalized stiffness is:

[0057]

[0058] In the formula, a is the crack length, W is the net cross-sectional width at the notch root, and E... FE,crk and E FE,uncrk Let F represent the uncracking stiffness and the cracked stiffness, respectively, reflecting the normalized stiffness E. FE,crk / E FE,uncrk The mapping relationship between the normalized crack length a / W and the normalized crack length.

[0059] Specifically, a full-size finite element model of the fatigue crack propagation specimen is established based on its specific dimensions.

[0060] A constant-temperature tensile test is performed on a standard specimen of the same material as the test sample to obtain the material properties of the test sample, including elastic modulus and Poisson's ratio.

[0061] Elastic loading was applied to the finite element model to obtain the uncracked stiffness E of the crack-free model. FE,uncrk The crack stiffness E corresponding to a series of crack length models FE,crk Construct normalized crack length a / W and normalized stiffness E FE,crk / E FE,uncrk The relational function model.

[0062] Furthermore, DIC speckle patterns were prepared on the sample surface, and crack propagation and strain field analysis were observed, including:

[0063] Speckle patterns were prepared by adjusting the spray gun air pressure and spraying distance. The areal density was calculated and the spatial distribution randomness was analyzed based on digital image correlation (DIC) technology. Image distortion was corrected using a calibration plate.

[0064] Specifically, a high-temperature resistant matte paint was used, and speckle patterns were prepared by adjusting the spray gun air pressure and spraying distance. The areal density was calculated and the spatial distribution randomness was analyzed based on the grayscale extraction and comparison program in the DIC software, and a calibration plate was used to correct image distortion.

[0065] Furthermore, the thermal strain reference required for thermo-mechanical decoupling is determined through thermal strain testing and zero-stress measurement, including:

[0066] Thermocouple wires are welded to the gauge length of the specimen, and the fixture installation reference line and the high temperature extensometer positioning point are marked. The specimen and the high temperature extensometer are installed according to the installation reference line and the high temperature extensometer positioning point. The elastic modulus is tested at different temperatures. A reference curve of the elastic modulus of the uncracked specimen changing with temperature is established through a series of temperature control cycles, and the reference value of the uncracked stiffness is determined.

[0067] Based on the uncracked stiffness reference value, thermal strain testing and zero stress measurement are performed to obtain thermal strain data of the sample during temperature loading cycles, and to determine the thermal strain benchmark required for the thermo-mechanical decoupling.

[0068] The elastic modulus test requires completing at least a preset number of elastic loading and unloading cycles at several characteristic temperature points, and the temperature gradient of the sample in the direction perpendicular to the heating direction during the thermal strain test does not exceed a preset temperature range.

[0069] Only by obtaining the elastic modulus of the material at different temperatures and determining the stiffness change during temperature loading cycles can subsequent thermal strain tests and zero-stress measurements be performed to determine the thermal strain benchmark.

[0070] Specifically, thermocouple wires are welded to the gauge length of the specimen (the welding point is ≥5mm from the center of the specimen). At the same time, the installation reference line of the fatigue testing machine fixture and the contact positioning point of the high-temperature extensometer blade are marked.

[0071] After installing the specimen and a high-temperature extensometer, the elastic modulus was first tested at different temperatures. A reference curve of the elastic modulus of the uncracked specimen as a function of temperature was established through a series of temperature-controlled cycles, thereby determining the reference value E of the uncracked stiffness. ref Subsequently, thermal strain testing and zero-stress measurement were performed to obtain thermal strain data of the sample during temperature cycling, and to determine the thermal strain reference required for thermo-mechanical decoupling.

[0072] Elastic modulus testing needs to be conducted at multiple characteristic temperature points, with at least three elastic loading and unloading cycles performed at each temperature point. A baseline curve of the elastic modulus of the uncracked specimen versus temperature is established using the stress-strain response results at each temperature point, thereby determining the reference value E for the uncracked stiffness. ref .

[0073] During thermal strain testing and zero-stress measurement, the temperature gradient in the vertical heating direction of the specimen must be strictly controlled to not exceed 2% of the set maximum temperature. The specimen must be heated at the maximum temperature for at least 1 hour to eliminate the effect of speckle oxidation. At least 10 temperature measurement cycles must be performed to verify the stability of the thermal strain data and establish a reliable thermal strain benchmark.

[0074] Furthermore, thermomechanical fatigue crack propagation tests are conducted, and the crack length is calculated based on the stiffness change results of the tests, including:

[0075] Set the loading parameters, data acquisition frequency, and test termination conditions for the thermomechanical fatigue test, conduct the thermomechanical fatigue crack propagation test, and simultaneously acquire speckle images of the sample surface and observe the crack propagation length until the test termination conditions are met.

[0076] Based on the thermal strain data during the temperature loading cycle, the nominal total strain ε measured by the high-temperature extensometer is calculated using a thermal strain compensation method. total Perform thermo-mechanical decoupling to separate the mechanical strain component ε mec With thermal strain component ε th ;

[0077] Combined with mechanical strain component ε mec Using synchronous stress data, a nominal stress-mechanical strain hysteresis curve is plotted. Stiffness evolution data of the linear portion in the initial stage of unloading is extracted. The cracking stiffness is determined by piecewise linear fitting. The cracking stiffness is then substituted into the relationship function between the normalized crack length and the normalized stiffness to calculate the crack length.

[0078] Specifically, speckle images of the sample surface are acquired and crack propagation length is observed, including:

[0079] The magnification of the high-speed camera was preset and the scale was calibrated. The speckle image of the sample surface was acquired in real time through the high-temperature DIC system and the crack propagation length was observed simultaneously.

[0080] The reference stiffness E for the temperatures corresponding to the start and end points of the linear portion. ref Determine the uncracked stiffness E uncrk The relational expression is:

[0081]

[0082] In the formula, E ref (T1) and E ref (T2) represents the reference stiffness corresponding to the starting temperature T1 and the ending temperature T2, respectively.

[0083] The method for determining crack stiffness through piecewise linear fitting is as follows:

[0084]

[0085] In the formula, i is the current fitting order, n is the preset maximum number of fitting iterations, and E crk,i E is the local crack initiation stiffness of the segmented interval corresponding to the current fitting order. crk This refers to the crack stiffness.

[0086] Furthermore, based on the DIC synchronous observation results, the crack length calculation results were plotted into an aN curve for comparative analysis, and the crack propagation rate was calculated using the three-point moving average secant method.

[0087] Specifically, the three-point moving average secant method is as follows:

[0088]

[0089] In the formula, (a i N i () represents the current data point. The crack propagation rate corresponding to the data point, (a i+1 N i+1 ), (a i1 N i1 All of them are adjacent data points.

[0090] Furthermore, the size of the crack tip plastic zone is determined by setting DIC calculation parameters, including fixing the area of ​​the calculation region, the size of the subset, the correlation coefficient threshold, and the Gaussian low-pass filter.

[0091] Specifically, the strain field calculation at the crack tip must strictly maintain the consistency of parameters such as the area of ​​the calculation region, the size of the subset, the setting of the correlation coefficient, and the type of noise reduction filter, so as to ensure the accuracy of the strain calculation results and the size of the plastic zone at the crack tip.

[0092] To more clearly illustrate the technical solution of the present invention, specific embodiments are provided below for description:

[0093] This embodiment selects 316L austenitic stainless steel, a key structural material for petrochemical and nuclear power systems, as the research object.

[0094] Step S1: Based on the specific dimensions of the fatigue crack propagation specimen, establish a full-size finite element model of the specimen. Using data from the isothermal tensile test, obtain the material parameters of the finite element model to calibrate the evolution characteristics of stiffness under different crack lengths, and then establish the relationship between the normalized crack length a / W and the normalized stiffness E. FE,crk / E FE,uncrk Relational functions, such as Figure 2 As shown.

[0095] Normalized crack length a / W and normalized stiffness E FE,crk / E FE,uncrkThe relational function model is shown below:

[0096]

[0097] Where a is the crack length, W is the net cross-sectional width at the notch root, and E FE,crk and E FE,uncrk The stiffness before and after cracking are respectively determined.

[0098] Step S2: High-temperature resistant matte paint is used to prepare speckled patterns on the sample surface. The average particle size of the speckled patterns is controlled to be 40 μm and the maximum particle size is not more than 100 μm by adjusting the air pressure of the spray gun and the spraying distance. Figure 3 (a) shows the speckle pattern preparation image taken under an optical microscope. The areal density was calculated and the spatial distribution randomness was analyzed using the grayscale extraction and comparison program within the DIC software, such as... Figure 3 As shown in (b), the speckle features on the sample surface are accurately captured, meeting the requirements for areal density calculation and spatial randomness, thus ensuring the validity of the DIC measurement results. Subsequently, a calibration plate is used for image distortion correction, as shown... Figure 3 (c)- Figure 3 As shown in (d), it can be seen that when the calibration plate is translated and rotated, the positions of all grid nodes on the calibration plate are determined based on the three reference grid nodes, which verifies the reliability of the image distortion correction technology of the DIC acquisition system, thereby eliminating the image distortion problem caused by high temperature heat flow disturbance, sample surface reflection and lens vibration.

[0099] Step S3: Weld thermocouple wires to the gauge length of the specimen (welding point ≥ 5mm from the center of the specimen). At the same time, mark the installation reference line of the fatigue testing machine fixture and the contact positioning point of the high-temperature extensometer blade.

[0100] Step S4: Install the specimen and high-temperature extensometer. First, conduct elastic modulus tests at different temperature points within the test temperature range. At each temperature point, complete three elastic loading and unloading cycles using a stress control of 0–20 MPa. The data acquisition frequency is 10 Hz to capture details of elastic modulus changes. Establish a reference curve for the elastic modulus of the uncracked specimen as a function of temperature based on the stress-strain response results at each temperature point, thereby determining the reference value E for the uncracked stiffness. ref Subsequently, thermal strain testing and zero-stress measurement are performed. This process requires strict control of the temperature gradient in the direction perpendicular to the heating of the specimen, ensuring that it does not exceed 2% of the set maximum temperature. The specimen must be held at the maximum temperature for at least 1 hour to eliminate speckle oxidation effect, and the stability of the thermal strain data is verified through no less than 10 temperature measurement cycles, thereby establishing the thermal strain benchmark required for thermo-mechanical decoupling.

[0101] Step S5: Set the loading parameters, data acquisition frequency, and test termination conditions for the thermomechanical fatigue test. In this embodiment, two sets of phase angle loading tests were conducted: in-phase (0°) and out-of-phase (180°). The mechanical load control mode was strain control, with a strain amplitude of 0.6%, a strain ratio of -1, and a strain rate of 2 × 10⁻⁶. -4 s -1 The average temperature during the temperature cycling was 550℃, with a loading range of 475–625℃. The test loading waveform was a triangular wave, and one loading cycle took 120 seconds. Data acquisition considered the synergistic effect of mechanical response and temperature changes: when the mechanical strain fluctuation exceeded the set value by 5% or the temperature deviation exceeded ±5℃, a high-frequency sampling mode was triggered; during the parameter stabilization phase, the basic sampling frequency was maintained to ensure the stability of the PID control loop. When the mechanical strain loading error was ≥20% of the set value or the temperature deviation exceeded ±15℃, a safety protection mechanism was triggered, and the test was interrupted. In addition, a set of 550℃ isothermal tests was conducted as a reference group, with mechanical load loading parameters consistent with those of the thermomechanical fatigue test.

[0102] Step S6: Conduct a thermomechanical fatigue crack propagation test. The high-speed camera with a magnification of 50× used in the high-temperature DIC system is set up perpendicular to the sample surface to acquire speckle images of the sample surface in real time and simultaneously observe the crack propagation length (the optical observation result is the crack length on the sample surface). When the predetermined crack length is reached, the test is stopped.

[0103] Step S7: After the experiment, process the collected experimental data. Based on the thermal strain data obtained in step S4 during the temperature loading cycle, use the thermal strain compensation method to adjust the nominal total strain ε measured by the high-temperature extensometer. total Perform thermo-mechanical decoupling, such as Figure 4 (a)- Figure 4 As shown in (c), the mechanical strain component ε is obtained by decomposition. mec With thermal strain component ε th .

[0104] Step S8: Decompose the mechanical strain components ε mec By combining the synchronously acquired stress data, the nominal stress-mechanical strain hysteresis curve is plotted. The linear portion of the initial unloading phase of each loading cycle is extracted, such as... Figure 5 As shown, data from the 95%–75% strain range are uniformly extracted for stiffness calculation. The reference stiffness E is determined based on the temperatures corresponding to the starting and ending points of the extracted portion. ref Determine the uncracked stiffness E uncrk Uncracked stiffness E uncrk The relational expression is shown below:

[0105]

[0106] In the formula, E ref (T1) and E ref (T2) represents the reference stiffness corresponding to the starting temperature T1 and the ending temperature T2, respectively. Under the same phase (0°) condition, T1 = 617.5℃, T2 = 587.5℃; under the opposite phase (180°) condition, T1 = 482.5℃, T2 = 512.5℃.

[0107] The cracking stiffness E under loading cycle was determined by piecewise linear fitting. crk The relational expression is as follows:

[0108]

[0109] In the formula, i is the current fitting order, n is the preset maximum number of fitting iterations, and E crk,i This is the local crack initiation stiffness corresponding to the segmented interval of the current fitting order. This stiffness is obtained by linearly fitting the nominal stress and mechanical strain of the data points in the segmented interval, such as... Figure 6 As shown, each segment interval of the linear part in the initial stage of unloading is calculated one by one, and the number of data points in each segment interval is 5.

[0110] According to the above E uncrk and E crk Substitute the relationship function model determined in step S1 to solve for the crack length.

[0111] Step S9: Based on the surface crack length observation results from Step S6, plot the aN curve of the crack length calculation results from Step S8 for comparative analysis to verify the accuracy of the results. Figure 7 As shown, the crack length calculated by the stiffness method under different experimental conditions is in good agreement with the results of optical observation, effectively verifying the reliability of the crack length calculation method of this invention. Subsequently, the crack propagation rate (da / dN) was calculated based on the aN curve using the three-point moving average secant method.

[0112]

[0113] The crack propagation rate (da / dN) corresponding to any data point (a, N) is the average of the slopes of the two secant lines before and after that point.

[0114] Step S10: Based on the speckle image of the sample surface acquired in real time in step S6, calculate the strain field at the crack tip. The calculation area is 600×200 pixels, the subset size is 21×21 pixels, the correlation coefficient threshold is set to 0.95, and the noise reduction filter type is Gaussian low-pass filter. The size of the plastic zone at the crack tip is determined by linearly scanning the strain at the crack propagation front. For metallic materials, the region with a strain value greater than 0.2% can be considered the plastic zone, such as... Figure 8 As shown. The size of the plastic zone is correlated with the crack propagation rate in step S9, as follows. Figure 9 As shown, the vast majority of data points are concentrated within the 2x error band of the crack propagation rate, and the crack propagation rate and the size of the plastic zone at the crack tip show a significant linear positive correlation.

[0115] The above are merely preferred embodiments of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A method for testing the thermomechanical fatigue crack propagation rate based on stiffness and crack tip strain field, characterized in that, include: Based on the full-size finite element model of the fatigue crack propagation specimen, the stiffness evolution characteristics under different crack lengths were calibrated, and the relationship function between normalized crack length and normalized stiffness was constructed. DIC speckle patterns were prepared on the sample surface, and crack propagation and strain field analysis were observed. Establish the elastic modulus-temperature reference curve of the uncracked specimen, determine the reference value of the uncracked stiffness, and determine the thermal strain reference required for thermo-mechanical decoupling through thermal strain testing and zero stress measurement. Thermomechanical fatigue crack propagation tests were conducted, and the crack length was calculated based on the stiffness variation results from the tests. The crack length was then plotted using the results of simultaneous DIC observations. aN Comparative analysis of the relationship curves; The crack propagation rate was calculated using the three-point moving average secant method. The size of the crack tip plastic zone was determined based on the DIC strain field analysis, and the crack propagation rate results were correlated to achieve the test of the crack propagation rate of thermomechanical fatigue. The thermomechanical fatigue crack propagation test is conducted, and the crack length is calculated based on the stiffness change results of the test, including: Set the loading parameters, data acquisition frequency and test termination conditions for thermomechanical fatigue test, conduct thermomechanical fatigue crack propagation test, and simultaneously acquire speckle images of the sample surface and observe the crack propagation length until the test termination condition is reached. Based on the thermal strain data during the temperature loading cycle, a thermal strain compensation method is used to perform thermo-mechanical decoupling on the nominal total strain measured by the high-temperature extensometer, separating the mechanical strain component from the thermal strain component. Combining the mechanical strain components and synchronous stress data, a nominal stress-mechanical strain hysteresis curve is plotted. The stiffness evolution data of the linear portion in the initial stage of unloading is extracted. The cracking stiffness is determined by piecewise linear fitting. The cracking stiffness is then substituted into the relationship function between the normalized crack length and the normalized stiffness to calculate the crack length.

2. The method for testing the thermomechanical fatigue crack propagation rate based on stiffness and crack tip strain field according to claim 1, characterized in that, The relationship function between the normalized crack length and the normalized stiffness is as follows: ； In the formula, a The length of the crack. W The net cross-sectional width at the root of the notch. and Stiffness before and after cracking, respectively. F Represents a functional relationship; Among them, a full-size finite element model of the specimen is established based on the specific dimensions of the fatigue crack propagation specimen. A constant-temperature tensile test is performed on a standard specimen of the same material as the test sample to obtain the material properties of the test sample, including elastic modulus and Poisson's ratio. Elastic loading was applied to the full-size finite element model to obtain the uncracked stiffness of the crack-free model. Cracking stiffness corresponding to a series of crack length models Construct normalized crack length a / W With normalized stiffness / The relational function model.

3. The thermomechanical fatigue crack propagation rate testing method based on stiffness and crack tip strain field according to claim 1, characterized in that, DIC speckle patterns were prepared on the sample surface, and crack propagation and strain field analysis were observed, including: Speckle patterns were prepared by adjusting the spray gun air pressure and spraying distance. The areal density was calculated and the spatial distribution randomness was analyzed based on digital image correlation (DIC) technology. Image distortion was corrected using a calibration plate.

4. The thermomechanical fatigue crack propagation rate testing method based on stiffness and crack tip strain field according to claim 1, characterized in that, The thermal strain reference required for thermo-mechanical decoupling is determined through thermal strain testing and zero-stress measurement, including: Thermocouple wires are welded to the gauge length of the specimen, and the fixture installation reference line and the high temperature extensometer positioning point are marked. The specimen and the high temperature extensometer are installed according to the installation reference line and the high temperature extensometer positioning point. The elastic modulus is tested at different temperatures. A reference curve of the elastic modulus of the uncracked specimen changing with temperature is established through a series of temperature control cycles, and the reference value of the uncracked stiffness is determined. Based on the uncracked stiffness reference value, thermal strain testing and zero stress measurement are performed to obtain thermal strain data of the sample during temperature loading cycles, and to determine the thermal strain benchmark required for the thermo-mechanical decoupling. The elastic modulus test requires completing at least a preset number of elastic loading and unloading cycles at several characteristic temperature points, and the temperature gradient of the sample in the direction perpendicular to the heating direction during the thermal strain test does not exceed a preset temperature range.

5. The thermomechanical fatigue crack propagation rate testing method based on stiffness and crack tip strain field according to claim 1, characterized in that, Acquire speckle images of the sample surface and observe crack propagation length, including: The magnification of the high-speed camera was preset and the scale was calibrated. The speckle image of the sample surface was acquired in real time through the high-temperature DIC system and the crack propagation length was observed simultaneously.

6. The method for testing the thermomechanical fatigue crack propagation rate based on stiffness and crack tip strain field according to claim 1, characterized in that, The method for determining the crack stiffness by piecewise linear fitting is as follows: ； In the formula, i This is the current fitting order. n It is the preset maximum number of fits. It represents the local crack initiation stiffness of the segmented interval corresponding to the current fitting order. This refers to the crack stiffness.

7. The thermomechanical fatigue crack propagation rate testing method based on stiffness and crack tip strain field according to claim 1, characterized in that, The three-point moving average secant method is as follows: In the formula, ( , () represents the current data point. The crack propagation rate corresponding to the data point, ( , ), ( , All of them are adjacent data points.

8. The method for testing the thermomechanical fatigue crack propagation rate based on stiffness and crack tip strain field according to claim 1, characterized in that, The size of the crack tip plastic zone is determined by setting DIC calculation parameters, including fixed calculation area, subset size, correlation coefficient threshold, and Gaussian low-pass filtering.

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