Thermal mechanical fatigue crack growth rate testing method based on rigidity and crack tip strain field

Abstract

The invention discloses a thermal mechanical fatigue crack growth rate testing method based on rigidity and a crack tip strain field. The method comprises the following steps: firstly, establishing a relation function between normalized crack length and normalized rigidity through finite element calculation; secondly, performing DIC speckle preparation for observing crack propagation and analyzing a strain field; then carrying out a thermal mechanical fatigue test, calculating the crack length based on a rigidity change result of the test, drawing an a-N relation curve in combination with a DIC synchronous observation result, and carrying out comparative analysis; and finally, calculating the crack growth rate through a three-point moving average secant method, and analyzing and determining the size of a crack tip plastic zone according to a DIC strain field, so as to accurately test the crack growth rate. The fatigue crack growth behavior under the thermal-mechanical coupling effect can be effectively tested, the thermal mechanical fatigue crack growth rate is accurately obtained by combining a crack tip strain field measurement method, and test data support is provided for damage tolerance design and structural integrity evaluation of high-temperature pressure-bearing equipment.

Description

Technical Field

[0001] The present invention relates to the technical field of fracture mechanics performance parameter testing, in particular to a thermomechanical fatigue crack growth rate testing method based on stiffness and crack tip strain field. Background Art

[0002] With the continuous innovation of material manufacturing level and equipment processing technology, key equipment in the petrochemical and nuclear power fields are gradually developing in the direction of multi-parameter coordinated control, large-scale integrated manufacturing, long life and high reliability, in order to meet the sustainability and stability requirements under extreme working conditions. As the core pressure-bearing components of high-temperature and high-pressure systems, the inner wall of the hydrogenation reactor and cracking furnace tubes in the petrochemical field, as well as the reactor loop pipes and steam generator heat transfer tubes in the nuclear power system, these equipment are subjected to mechanical cyclic loads (internal pressure fluctuations, fluid shock) during long-term service, and they also need to deal with periodic thermal loads caused by temperature changes. The cumulative effect of fatigue damage caused by the coupling of this thermal-mechanical cyclic load will significantly accelerate the crack initiation and fracture failure process of the components, and is a key factor restricting the service life of process equipment.

[0003] Currently, there is no standardized system for testing thermomechanical fatigue crack growth rate. Traditional fatigue crack growth rate testing methods do not consider the impact of temperature fluctuations on material properties and equipment accuracy, and the measurement results under thermomechanical fatigue testing conditions have significant errors. Therefore, it is urgent to establish an accurate and reliable thermomechanical fatigue crack growth rate characterization method that systematically considers the influence of thermal-mechanical coupling effects on fatigue crack growth behavior and seeks fatigue crack growth rate characterization parameters suitable for thermomechanical load spectra to provide experimental data support for damage tolerance design and structural integrity assessment of high-temperature pressure-bearing equipment. Summary of the Invention

[0004] In order to solve the technical problems existing in the above-mentioned prior art, the present invention proposes a thermomechanical fatigue crack growth rate test method based on stiffness and crack tip strain field, which realizes dynamic observation of crack length and real-time capture of crack tip strain field, and makes up for the shortcomings of traditional fatigue crack growth rate test methods under the action of thermo-mechanical coupling field.

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

[0006] Based on the full-scale finite element model of fatigue crack growth specimen, the stiffness evolution characteristics under different crack lengths are calibrated, and the relationship function between normalized crack length and normalized stiffness is constructed;

[0007] DIC speckle preparation was performed on the sample surface to observe crack growth and strain field analysis;

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

[0009] Conduct thermomechanical fatigue crack growth tests, calculate crack length based on the stiffness change results of the tests, and draw aN relationship curves based on DIC synchronous observation results for comparative analysis;

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

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

[0012]

[0013] Where a is the crack length, W is the net cross-sectional width at the root of the notch, and E FE,crk and E FE,uncrk are the uncracked stiffness and cracked stiffness respectively, and F represents the functional relationship.

[0014] Preferably, DIC speckle preparation is performed on the sample surface to observe crack propagation and strain field analysis, including:

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

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

[0017] Welding thermocouple wires at the gauge length section of the specimen and marking a fixture installation reference line and a high-temperature extensometer positioning point, installing the specimen and the high-temperature extensometer according to the installation reference line and the high-temperature extensometer positioning point, performing elastic modulus tests at different temperatures, establishing a reference curve of the elastic modulus of the uncracked specimen versus temperature through a series of temperature control cycles, and determining the uncracked stiffness reference value;

[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 the temperature loading cycle, and to determine the thermal strain benchmark required for the thermal-mechanical decoupling;

[0019] The elastic modulus test needs to complete no less than a preset number of elastic loading and unloading cycles at several characteristic temperature points, and the temperature gradient of the sample in the vertical heating direction during the thermal strain test does not exceed a preset temperature range.

[0020] Preferably, performing the thermomechanical fatigue crack growth test and calculating the crack length based on the stiffness change result of the test comprises:

[0021] Setting the loading parameters, data acquisition frequency, and test termination conditions of the thermomechanical fatigue test, conducting a thermomechanical fatigue crack growth test, and simultaneously acquiring a speckle image of the sample surface and observing the crack growth length until the test termination conditions are reached;

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

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

[0024] Preferably, collecting a speckle image of the sample surface and observing the crack propagation length includes:

[0025] The magnification of the high-speed camera is pre-set and the scale is calibrated. The high-temperature DIC system is used to collect speckle images of the sample surface in real time and simultaneously observe the crack extension length.

[0026] Preferably, the method for determining the cracking stiffness by piecewise linear fitting is:

[0027]

[0028] Where i is the current fitting order, n is the preset maximum fitting number, E crk,i is the local crack opening stiffness of the segmented interval corresponding to the current fitting order, E crk is the cracking stiffness.

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

[0030]

[0031] In the formula, (a i , N i ) is the current data point, is the crack growth rate corresponding to the data point, (a i+1 , N i+1 )、(a i1 , N i1 ) are adjacent data points.

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

[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 growth test platform to more accurately simulate the effects of temperature changes and mechanical loads on high-temperature pressure-bearing equipment. Based on the stiffness evolution of the test material during loading, a relationship is established between the stiffness and the crack length. By combining real-time observations for comparison and correction, the fatigue crack growth rate can be accurately obtained.

[0035] (2) The present invention ensures the accuracy and repeatability of the strain and crack tip plastic zone size calculation results by constructing a standardized DIC preparation and acquisition procedure and operation analysis process, thereby achieving reliable characterization of the thermomechanical fatigue crack growth rate;

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

[0037] The accompanying drawings, which constitute part of this application, are intended to provide a further understanding of this application. The exemplary embodiments and descriptions of this application are intended to explain this application and do not constitute an improper limitation on this application. In the accompanying drawings:

[0038] Figure 1 This is a flow chart of a method for testing thermomechanical fatigue crack growth rate 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 the normalized stiffness E of the embodiment of the present invention are FE,crk / E FE,uncrk The functional relationship a / W=F(E FE,crk / E FE,uncrk ) Schematic diagram;

[0040] Figure 3 Figures 1 and 2 show the speckle preparation pattern and correction images of an embodiment of the present invention. (a) shows the speckle preparation pattern photographed under an optical microscope, (b) shows the grayscale contrast result in DIC software, and (c) and (d) show the calibration plate image before and after distortion correction, respectively.

[0041] Figure 4Before and after comparison of thermal-mechanical decoupling using thermal strain compensation according to an embodiment of the present invention, wherein (a) is the nominal stress-thermal strain hysteresis curve obtained during the test, (b) is the nominal stress-total strain hysteresis curve, and (c) is the nominal stress-mechanical strain hysteresis curve;

[0042] Figure 5 The cracking stiffness E is determined according to the hysteresis curve in the embodiment of the present invention. crk Schematic diagram of;

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

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

[0046] Figure 9 This is a correlation diagram between the crack tip plastic zone size and the fatigue crack growth rate according to an embodiment of the present invention. DETAILED DESCRIPTION

[0047] It should be noted that, in the absence of conflict, the embodiments and features of the embodiments in this application can be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

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

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

[0050] Based on the full-scale finite element model of fatigue crack growth specimen, the stiffness evolution characteristics under different crack lengths are calibrated, and the relationship function between normalized crack length and normalized stiffness is constructed;

[0051] DIC speckle preparation was performed on the sample surface to observe crack growth and strain field analysis;

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

[0053] Conduct thermomechanical fatigue crack growth tests, calculate crack length based on the stiffness change results of the tests, and draw aN relationship curves based on DIC synchronous observation results for comparative analysis;

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

[0055] This example simulates thermomechanical fatigue loading on high-temperature components and combines it with the crack tip strain field measurement method to reveal the quantitative relationship between fatigue crack growth rate and the size of the plastic zone at the crack tip. This provides experimental data support for the damage tolerance design and structural integrity assessment of high-temperature pressure-bearing equipment, and promotes the engineering application of fracture mechanics theory under complex working conditions.

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

[0057]

[0058] Where a is the crack length, W is the net cross-sectional width at the root of the notch, and E FE,crk and E FE,uncrk are the uncracked stiffness and cracked stiffness respectively, and F represents the functional relationship, which reflects the normalized stiffness E FE,crk / E FE,uncrk and the mapping relationship between the normalized crack length a / W.

[0059] Specifically, according to the specific size of the fatigue crack growth specimen, a full-size finite element model of the specimen is established.

[0060] Conduct a constant temperature tensile test on a standard specimen of the same material as the test specimen to obtain the material properties of the test specimen, including elastic modulus and Poisson's ratio;

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

[0062] Furthermore, DIC speckle preparation was performed on the sample surface to observe crack propagation and strain field analysis, including:

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

[0064] Specifically, high-temperature resistant matte paint was used to prepare speckles by adjusting the spray gun air pressure and spraying distance. The surface density calculation and spatial distribution randomness analysis were performed based on the grayscale extraction and contrast program in the DIC software, and a calibration plate was used to correct image distortion.

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

[0066] Welding thermocouple wires at the gauge length section of the specimen and marking a fixture installation reference line and a high-temperature extensometer positioning point, installing the specimen and the high-temperature extensometer according to the installation reference line and the high-temperature extensometer positioning point, performing elastic modulus tests at different temperatures, establishing a reference curve of the elastic modulus of the uncracked specimen versus temperature through a series of temperature control cycles, and determining the uncracked stiffness reference value;

[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 the temperature loading cycle and determine the thermal strain benchmark required for the thermal-mechanical decoupling;

[0068] The elastic modulus test needs to complete no less than a preset number of elastic loading and unloading cycles at several characteristic temperature points, and the temperature gradient of the sample in the vertical heating direction during the thermal strain test does not exceed a preset temperature range.

[0069] By obtaining the elastic modulus of the material at different temperatures and determining the stiffness change during the temperature loading cycle, subsequent thermal strain testing and zero stress measurement can be performed to determine the thermal strain benchmark.

[0070] Specifically, weld the thermocouple wires at the gauge length of the specimen (the welding point is ≥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.

[0071] Install the specimen and high temperature extensometer, first conduct elastic modulus tests at different temperatures, and establish a reference curve of elastic modulus of uncracked specimens versus temperature through a series of temperature control cycles, so as to determine the reference value of uncracked stiffness E. ref Subsequently, thermal strain testing and zero stress measurement were performed to obtain the thermal strain data of the sample during the temperature cycle process and determine the thermal strain benchmark required for thermal-mechanical decoupling.

[0072] The elastic modulus test needs to be carried out at multiple characteristic temperature points, and at least three elastic loading and unloading cycles are completed at each temperature point. The reference curve of the elastic modulus of the uncracked specimen changing with temperature is established through the stress-strain response results of each temperature point, so as to determine the reference value of the uncracked stiffness E. ref .

[0073] During the thermal strain test and zero stress measurement, the temperature gradient of the specimen in the vertical heating direction is strictly controlled to not exceed 2% of the set maximum temperature. The specimen needs to be heated at the maximum temperature for at least 1 hour to eliminate the influence of speckle oxidation, and no less than 10 temperature measurement cycles are carried out to verify the stability of the thermal strain data and establish a reliable thermal strain benchmark.

[0074] Furthermore, a thermomechanical fatigue crack growth test is performed, and the crack length is calculated based on the stiffness change results of the test, including:

[0075] Setting the loading parameters, data acquisition frequency, and test termination conditions of the thermomechanical fatigue test, conducting a thermomechanical fatigue crack growth test, and simultaneously acquiring a speckle image of the sample surface and observing the crack growth length until the test termination conditions are reached;

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

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

[0078] Specifically, the speckle image of the sample surface is collected and the crack extension length is observed, including:

[0079] The magnification of the high-speed camera is pre-set and the scale is calibrated. The high-temperature DIC system is used to collect speckle images on the sample surface in real time and simultaneously observe the crack extension length.

[0080] Reference stiffness E at the starting and ending points of the linear part corresponding to the temperature ref Determine the uncracked stiffness E uncrk The relational expression is:

[0081]

[0082] Where, E ref (T1) and E ref (T2) are the reference stiffness corresponding to the starting point temperature T1 and the ending point temperature T2 respectively.

[0083] The method for determining the cracking stiffness by piecewise linear fitting is:

[0084]

[0085] Where i is the current fitting order, n is the preset maximum fitting number, E crk,i is the local crack opening stiffness of the segmented interval corresponding to the current fitting order, E crk is the cracking stiffness.

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

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

[0088]

[0089] In the formula, (a i , N i ) is the current data point, is the crack growth rate corresponding to the data point, (a i+1 , N i+1 )、(a i1 , N i1 ) are adjacent data points.

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

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

[0092] In order to more clearly express the technical solution of the present invention, the following specific embodiments are provided to introduce the solution:

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

[0094] Step S1: According to the specific size of the fatigue crack growth specimen, a full-scale finite element model of the specimen is established. According to the constant temperature tensile test data, the material parameters of the finite element model are obtained to calibrate the evolution characteristics of the stiffness at different crack lengths, and then establish the normalized crack length a / W and the normalized stiffness E FE,crk / E FE,uncrk The relationship function, such as Figure 2 shown.

[0095] Normalized crack length a / W and normalized stiffness E FE,crk / E FE,uncrkThe relationship function model is as follows:

[0096]

[0097] Where a is the crack length, W is the net cross-sectional width at the root of the notch, and E FE,crk and E FE,uncrk Uncracked stiffness and cracked stiffness, respectively.

[0098] Step S2: using high-temperature resistant matte paint to prepare speckles on the surface of the sample, and adjusting the spray gun air pressure and spraying distance to control the average particle size of the speckles to be 40 μm and the maximum particle size not exceeding 100 μm. Figure 3 (a) is a speckle preparation pattern taken under an optical microscope. The grayscale extraction and contrast program in the DIC software is used to calculate the surface density and analyze the spatial distribution randomness. Figure 3 As shown in (b), the speckle features on the sample surface are accurately captured and meet the surface density calculation requirements and spatial distribution randomness, thus ensuring the validity of the DIC measurement results. The calibration plate is then used to correct the image distortion, as shown in 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, verifying the reliability of the image distortion correction technology of the DIC acquisition system, thereby eliminating image distortion problems caused by high-temperature heat flow disturbances, sample surface reflections, and lens vibrations.

[0099] Step S3: Weld the thermocouple wires at the gauge length of the specimen (the welding point is ≥5 mm from the specimen center). 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, perform elastic modulus tests at different temperature points within the test temperature range. At each temperature point, perform three elastic loading and unloading cycles using a 0-20 MPa stress control cycle. The data acquisition frequency is 10 Hz to capture the details of the elastic modulus change. The stress-strain response results at each temperature point are used to establish a reference curve of the elastic modulus of the uncracked specimen versus temperature, thereby determining the uncracked stiffness reference value E. ref Thermal strain testing and zero-stress measurements are then performed. During this process, the temperature gradient perpendicular to the heating direction of the specimen must be strictly controlled to ensure it does not exceed 2% of the set maximum temperature. The specimen must be maintained at the maximum temperature for at least one hour to eliminate the speckle oxidation effect. The stability of the thermal strain data is verified through no fewer than ten temperature measurement cycles, thereby establishing the thermal strain benchmark required for thermal-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 phase angle loading tests were performed: in-phase (0°) and anti-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 -4 s -1 The average temperature of the temperature cycle is 550°C, the loading range is 475-625°C, the test loading waveform is a triangular wave, and it takes 120 seconds to complete a loading cycle. Data acquisition takes into account the synergistic effect of mechanical response and temperature changes: when the mechanical strain fluctuation amplitude exceeds 5% of the set value or the temperature deviation exceeds ±5°C, the high-frequency sampling mode is triggered; in the parameter stabilization stage, the basic sampling frequency is maintained to ensure the stability of the PID control loop. When the mechanical strain loading error is ≥20% of the set value or the temperature deviation exceeds ±15°C, the safety protection mechanism is triggered and the test is interrupted. In addition, a set of 550°C isothermal tests were conducted as a reference group, and its mechanical load loading parameters were consistent with the thermomechanical fatigue test.

[0102] Step S6: Conduct a thermomechanical fatigue crack growth test. A high-speed camera with a magnification of 50×, used in conjunction with the high-temperature DIC system, is mounted perpendicular to the sample surface. This camera is used to capture speckle patterns on the sample surface in real time and simultaneously observe the crack growth length (the optical observation result is the crack length on the sample surface). The test is terminated when the predetermined crack length is reached.

[0103] Step S7: After the test is completed, the collected test data is processed, and the nominal total strain ε measured by the high temperature extensometer is compensated using the thermal strain compensation method based on the thermal strain data in the temperature loading cycle obtained in step S4. total Perform thermo-mechanical decoupling, such as Figure 4 (a)- Figure 4 As shown in (c), the mechanical strain component ε is decomposed mec and thermal strain component ε th .

[0104] Step S8: Decompose the mechanical strain component ε mec , combined with the stress data collected simultaneously, the nominal stress-mechanical strain hysteresis curve is drawn. The linear part of the initial stage of unloading of each loading cycle is extracted, such as Figure 5 As shown in the figure, the data of the 95% to 75% strain range are uniformly extracted for stiffness calculation. According to the reference stiffness E corresponding to the temperature of the starting and ending points of the extracted part ref Determine the uncracked stiffness E uncrk , uncracked stiffness E uncrk The relational expression is as follows:

[0105]

[0106] Where, E ref (T1) and E ref (T2) are the reference stiffness corresponding to the starting point temperature T1 and the ending point temperature T2, respectively. Under the in-phase (0°) condition, T1 = 617.5°C, T2 = 587.5°C; under the anti-phase (180°) condition, T1 = 482.5°C, T2 = 512.5.

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

[0108]

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

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

[0111] Step S9: Based on the surface crack length observation result of step S6, the crack length calculation result of step S8 is plotted and analyzed by aN curve to verify the accuracy of the result. Figure 7 As shown in the figure, it can be seen that the crack length calculated by the stiffness method under different test conditions is in good agreement with the results of optical observation, which effectively verifies the reliability of the crack length calculation method of the present invention. The crack growth rate (da / dN) is then calculated based on the aN curve using the three-point moving average secant method.

[0112]

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

[0114] Step S10: Calculate the crack tip strain field based on the speckle image of the sample surface acquired in real time in step S6. 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 filtering. The size of the crack tip plastic zone is determined by linearly scanning the strain at the crack propagation front. For metal materials, the area with a strain value greater than 0.2% can be considered as the plastic zone, such as Figure 8 The plastic zone size is correlated with the crack growth rate in step S9, as shown in Figure 9 As shown in the figure, the vast majority of data points are concentrated within the error band of 2 times the crack growth rate, and the crack growth rate and the size of the crack tip plastic zone show a relatively significant linear positive correlation.

[0115] The above are merely preferred embodiments of the present application, but the scope of protection of the present application is not limited thereto. Any changes or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in this application should be included in the scope of protection of the present application. Therefore, the scope of protection of the present application should be based on the scope of protection of the claims.

Claims

1. A thermomechanical fatigue crack growth rate test method based on stiffness and crack tip strain field, characterized in that: include: Based on the full-scale finite element model of fatigue crack growth specimen, the stiffness evolution characteristics under different crack lengths are calibrated, and the relationship function between normalized crack length and normalized stiffness is constructed; DIC speckle preparation was performed on the sample surface to observe crack growth and strain field analysis; Establish the elastic modulus-temperature reference curve of the uncracked specimen, determine the uncracked stiffness reference value, and determine the thermal strain reference required for thermal-mechanical decoupling through thermal strain testing and zero stress measurement; Conduct thermomechanical fatigue crack growth tests, calculate crack length based on the stiffness change results of the tests, and draw aN relationship curves based on DIC synchronous observation results for comparative analysis; The crack growth rate is calculated using the three-point moving average secant method. The size of the crack tip plastic zone is determined based on the DIC strain field analysis, and the crack growth rate results are correlated to achieve the test of the thermomechanical fatigue crack growth rate.

2. The thermomechanical fatigue crack growth rate testing method 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: Where a is the crack length, W is the net cross-sectional width at the root of the notch, and E FE,crk and E FE,uncrk are the uncracked stiffness and cracked stiffness respectively, and F represents the functional relationship.

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

4. The thermomechanical fatigue crack growth rate testing method based on stiffness and crack tip strain field according to claim 1, characterized in that: Determine the thermal strain benchmark required for thermo-mechanical decoupling through thermal strain testing and zero stress measurements, including: Welding thermocouple wires at the gauge length section of the specimen and marking a fixture installation reference line and a high-temperature extensometer positioning point, installing the specimen and the high-temperature extensometer according to the installation reference line and the high-temperature extensometer positioning point, performing elastic modulus tests at different temperatures, establishing a reference curve of the elastic modulus of the uncracked specimen versus temperature through a series of temperature control cycles, and determining the uncracked stiffness reference value; 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 the temperature loading cycle, and to determine the thermal strain benchmark required for the thermal-mechanical decoupling; The elastic modulus test needs to complete no less than a preset number of elastic loading and unloading cycles at several characteristic temperature points, and the temperature gradient of the sample in the vertical heating direction during the thermal strain test does not exceed a preset temperature range.

5. The thermomechanical fatigue crack growth rate testing method based on stiffness and crack tip strain field according to claim 4, characterized in that: The thermomechanical fatigue crack growth test is performed, and the crack length is calculated based on the stiffness change result of the test, including: Setting the loading parameters, data acquisition frequency, and test termination conditions of the thermomechanical fatigue test, conducting a thermomechanical fatigue crack growth test, and simultaneously acquiring a speckle image of the sample surface and observing the crack growth length until the test termination conditions are reached; Based on the thermal strain data during the temperature loading cycle, a thermal strain compensation method is used to perform thermal-mechanical decoupling on the nominal total strain measured by the high-temperature extensometer, thereby separating the mechanical strain component from the thermal strain component; Combining the mechanical strain component and synchronous stress data, a nominal stress-mechanical strain hysteresis curve is drawn, and the stiffness evolution data of the linear part in the initial unloading stage is extracted. The cracking stiffness is determined by piecewise linear fitting, and the cracking stiffness is substituted into the relationship function between the normalized crack length and the normalized stiffness to calculate the crack length.

6. The thermomechanical fatigue crack growth rate testing method based on stiffness and crack tip strain field according to claim 5, characterized in that: Collect speckle images on the sample surface and observe the crack extension length, including: The magnification of the high-speed camera is pre-set and the scale is calibrated. The high-temperature DIC system is used to collect speckle images on the sample surface in real time and simultaneously observe the crack extension length.

7. The thermomechanical fatigue crack growth rate testing method based on stiffness and crack tip strain field according to claim 5, characterized in that: The method for determining the cracking stiffness by piecewise linear fitting is: Where i is the current fitting order, n is the preset maximum fitting number, E crk,i is the local crack opening stiffness of the segmented interval corresponding to the current fitting order, E crk is the cracking stiffness.

8. The thermomechanical fatigue crack growth 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: In the formula, (a i , N i ) is the current data point, is the crack growth rate corresponding to the data point, (a i+1 , N i+1 )、(a i1 , N i1 ) are adjacent data points.

9. The thermomechanical fatigue crack growth rate testing method 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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