A method of testing the rate of thermal mechanical fatigue crack growth

Abstract

The application discloses a kind of thermomechanical fatigue crack propagation rate test methods, the present application utilizes DCPD method to sample electric potential is calibrated: first, calibrate crack length a and the corresponding relation curve a-U of electric potential U, multiple crack lengths need to be calibrated;Second, calibrate the corresponding relation curve U-t of electric potential U and time t under temperature cycle of each crack length;Finally, through thermomechanical fatigue (TMF) experiment, the real-time corresponding relation curve U-t of electric potential U and time t in crack propagation process is obtained, time t is converted into cycle number N, is substituted into a-U and obtains a-N relationship curve. Meanwhile, another group of a-N curve is obtained by using long-focus microscope visual method. The crack propagation data obtained by visual method is used to correct and modify the a-N curve data obtained by DCPD measurement, so as to realize accurate and efficient measurement of thermomechanical fatigue crack propagation rate.

Description

Technical Field

[0001] This invention belongs to the field of crack propagation testing, specifically relating to a method for testing the crack propagation rate of thermomechanical fatigue. Background Technology

[0002] With the continuous advancement of science and technology in recent years, the aerospace field has developed rapidly, becoming a vital pillar of the national economy. As one of the most crucial core hot-end components of aero-engines, turbine disks endure enormous centrifugal and thermal loads during long-term service in harsh environments such as high temperature, high pressure, high speed, and alternating loads. Due to the thermal gradients existing in different parts of the turbine disk, it bears both mechanical fatigue loads and thermal cyclic stress and strain caused by these gradients. Therefore, fatigue damage and failure caused by the coupling effect of thermal and mechanical cyclic loads is a key factor limiting the service life of aero-engine turbine disks. Thus, conducting fatigue crack propagation performance tests on high-temperature alloys used in turbine disks is essential and of great significance for damage tolerance design and fatigue life prediction of high-performance aero-engine turbine disks.

[0003] Currently, methods such as visual inspection, crack opening displacement (COD) method, four-point DC potential drop method (DCPD) method, and compliance method can be used to measure crack propagation rate in isothermal fatigue crack propagation tests. However, there is no standardized measurement method for thermomechanical fatigue crack propagation rate testing. Under thermomechanical fatigue testing conditions, the limitations of the experimental equipment prevent the use of the crack opening displacement method and compliance method. Temperature fluctuations during the experiment cause changes in thermoelectric potential, and the potential method cannot be directly applied to thermomechanical crack propagation testing. The visual inspection method is currently a feasible measurement method, but it has a large measurement error.

[0004] The visual method involves observing the crack length on the sample surface in real time using a telephoto microscope. When using a telephoto microscope, focusing is the first step. Because the field of view is relatively small, a point light source is needed to assist focusing. Adjust the focus to concentrate the light spot at the crack tip, turn off the point light source, and turn on the camera and its accompanying software. Due to diffuse light scattering causing low contrast on the material surface, an external light source is added to enhance the crack observation effect. Observe the monitor and further adjust the focus until the image is as clear as possible. Note that during crack observation, the telephoto microscope lens must be kept perpendicular to the sample surface; otherwise, the measurement accuracy will be affected.

[0005] DCPD can be used to measure crack length in any conductive material. Its basic principle is to pass a stable current through the sample and measure the change in potential at the crack surface during crack propagation. As the crack length increases, the resistance increases due to the reduction in the original cross-sectional area of ​​the sample surface. This change in resistance causes a change in potential at the measurement point across the crack surface, establishing a relationship between a reference voltage and a reference crack length. The crack length can be calculated by detecting the change in potential. This method is convenient and fast, capable of measuring the average crack length across the thickness direction. However, it suffers from a significant thermoelectric effect during potential measurement and is prone to errors. Conventional isothermal fatigue crack propagation tests can correct for this by measuring the reverse voltage generated when the current reverses. However, the temperature cycling in thermomechanical fatigue crack propagation tests cannot eliminate the thermoelectric potential using the above methods. Therefore, this invention uses a combination of visual inspection and DCPD to measure the thermomechanical fatigue crack propagation rate. Summary of the Invention

[0006] The purpose of this invention is to provide a thermomechanical fatigue crack propagation rate testing method based on four-point DC potential drop (DCPD) combined with visual inspection. This method comprehensively utilizes both approaches to address the fatigue crack propagation problem in metallic materials under coupled thermal and mechanical loads. Numerical analysis is employed to optimize the accuracy of the crack propagation test results, thereby improving efficiency.

[0007] To achieve the above-mentioned technical objectives, the technical solution adopted by the present invention is as follows:

[0008] A method for testing the thermomechanical fatigue crack propagation rate includes the following steps:

[0009] Step S1: Using the crack propagation sample as the specimen, the potential characteristics of the specimen under different crack lengths and different external temperatures are calibrated using the four-point DC potential drop (DCPD) method.

[0010] Step S2: Perform thermomechanical fatigue (TMF) experiments on the sample, and link the potential characteristics obtained in step S1 to the potential-time relationship of crack propagation in the TMF experiment to obtain the crack propagation rate relationship based on the DCPD method.

[0011] Step S3: Use a long-focus microscope to record crack data during the thermomechanical fatigue (TMF) test of the sample in step S2, and obtain the crack propagation rate relationship based on visual inspection.

[0012] Step S4: In the data analysis software, select suitable cyclic node data for the crack propagation rate relationship curve based on the DCPD method obtained in step S2, and correct it using the crack propagation rate relationship curve based on the visual method in step S3 to obtain the corrected crack propagation rate relationship curve based on the DCPD method, thereby obtaining the accurate thermomechanical fatigue crack propagation rate.

[0013] To optimize the above technical solution, the specific measures also include:

[0014] Furthermore, the crack initiation point is located in the center of the specimen. Four thermocouples are fixedly welded to the upper and lower sides of the specimen along the crack propagation direction on both sides. The thermocouples are connected to the controller and used to monitor and provide feedback on the specimen surface temperature. DCPD wires are fixedly welded to the specimen surface and connected to the DCPD device. The DCPD device is used to record the potential calibration data of the specimen at different crack lengths and different external temperatures during the thermomechanical fatigue (TMF) test. The specimen is clamped at the center of the induction heating coil, which is connected to the high-frequency induction furnace. The high-frequency induction furnace is connected to the controller, which controls the temperature of the high-frequency induction furnace. A cooler is also installed at the center of the specimen heating and is connected to the controller. The controller controls the cooler to cool the specimen, thereby regulating the specimen temperature to meet the test requirements. The two ends of the specimen are fixed to the clamping ends of the thermomechanical fatigue testing machine. The controller is connected to the thermomechanical fatigue testing machine and controls the mechanical load of the thermomechanical fatigue testing machine.

[0015] Furthermore, the thermocouple is a type K thermocouple, and the welding position of the thermocouple is about 0.5 mm above and below the crack propagation direction on both sides of the sample.

[0016] Furthermore, the DCPD wires are symmetrically arranged at the four solder points of the sample with the crack in the sample as the center, and are 2 mm and 4 mm away from the crack, respectively.

[0017] Furthermore, in step S2, the thermomechanical fatigue (TMF) experiment used the DCPD method to record the potential of crack lengths of 1 mm, 3 mm, 5 mm, and 7 mm in the samples under temperature cycles of 300℃, 400℃, 500℃, 600℃, and 350–650℃. The relationship between crack length *a* and potential *U* at different temperatures was established, obtaining the relationship between the potential *U* of a single crack length under temperature cycles and the corresponding time *t*, thus obtaining the real-time curve of potential *U* versus time *t* during crack propagation. U is in mV and t is in s. The time t is converted to the number of cycles N, t = N * 100. The number of cycles N is substituted into the potential calibration to obtain U ~ 0.35a + 1.85, where a is in mm. The crack length and the number of cycles from a to N are obtained. The potential U-time t relationship is converted into the fatigue crack propagation rate relationship affected by thermomechanical coupling, and the crack propagation rate relationship curve based on the DCPD method is obtained.

[0018] Furthermore, in step S3, the lens of the long-focal-length microscope is positioned perpendicular to the sample surface above the sample, maintaining the observation direction perpendicular to the crack propagation direction. During the thermomechanical fatigue (TMF) experiment in step S2, the long-focal-length microscope is used visually. The focal length and scale of the long-focal-length microscope are adjusted, and the crack length of the sample is recorded at regular intervals. Finally, a crack propagation rate relationship curve based on visual estimation is plotted. Where a is the crack length and N is the cycle period.

[0019] Furthermore, in step S3, the certain number of cycles is 10.

[0020] Furthermore, in step S4, crack length data at the peak, trough, and median nodes are selected from step S2, with the peak being... trough The median is In the data analysis software Origin, the crack propagation rate relationship curve based on the visual method in step S3 is compared. When the crack propagation rate curve of a certain node among the peak, trough and median nodes has the highest fitting degree with the crack propagation rate relationship curve based on the visual method, the modified crack propagation rate relationship curve based on the DCPD method is adopted, that is, the thermomechanical fatigue crack propagation rate curve.

[0021] Compared with the prior art, the present invention has the following advantages:

[0022] For the problem of measuring thermomechanical fatigue crack propagation, the synchronous change of thermoelectric potential caused by the change of thermal and mechanical loads during the experiment makes DCPD unsuitable for testing the crack propagation rate of thermomechanical fatigue. This invention combines DCPD with visual inspection, using crack propagation data obtained by visual inspection to correct the a-N curve data obtained by DCPD measurement. This method combines the accuracy advantage of DCPD in measuring crack propagation while solving the problem of inaccurate crack length measurement due to potential fluctuations during thermomechanical fatigue testing. This method can obtain a more accurate thermomechanical fatigue crack propagation rate and improves the efficiency of crack propagation testing, providing convenience for crack propagation rate measurement under thermo-mechanical coupling environment. Attached Figure Description

[0023] Figure 1 This is a technical roadmap for the present invention;

[0024] Figure 2 This is a schematic diagram of DCPD potential connection;

[0025] Figure 3 The diagram shows a system diagram for thermomechanical fatigue crack propagation testing, where a is a schematic diagram of the overall system and b is a partial schematic diagram of the specimen.

[0026] Figure 4 The result of the corrected thermo-mechanical fatigue crack propagation rate curve. Detailed Implementation

[0027] The embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0028] This invention discloses a method for testing the thermomechanical fatigue crack propagation rate based on a four-point DC potential drop (DCPD) combined with visual inspection, such as... Figure 1 As shown, firstly, the potential of the sample was calibrated using the DCPD method, and the relationship between crack length *a* and potential *U* was determined as the curve *U* = 0.35a + 1.85. Four crack lengths of 1 mm, 3 mm, 5 mm, and 7 mm were calibrated. Secondly, the relationship between potential *U* and time *t* for a single crack length under temperature cycling at 300℃, 400℃, 500℃, 600℃, and 350–650℃ was determined as the curve *U* = t*. Four crack lengths were calibrated. Finally, thermomechanical fatigue (TMF) tests were conducted to obtain the real-time relationship curve U~t of the potential U during crack propagation, where time t was converted to the number of cycles N, t = N*100. Substituting this into a~U yielded the a~N relationship curve (the specific curve depends on the selected cycle period nodes). Simultaneously, another set of a~N curves was obtained visually using a long-focal-length microscope. Finally, based on the crack propagation data obtained through visual inspection, the accurate a-N curve was obtained by correcting the data using DCPD measurement. The specific steps are as follows:

[0029] Step 1: Press Figure 2 The DCPD wires and thermocouple wires are welded at the indicated locations. Before the test, the center of the specimen is positioned at the crack initiation point. The thermocouple connection positions are marked on the specimen surface. To improve alignment accuracy, a centerline is also drawn on the specimen surface. Four K-type thermocouples are connected to the specimen at approximately 0.5 mm above and below the crack propagation direction on both sides, according to the pre-drawn thermocouple welding positions. The thermocouple wires are connected to the controller for monitoring and feedback of the specimen surface temperature. The DCPD wires are symmetrically positioned at the four welding points on the specimen, centered on the crack, at distances of 2 mm and 4 mm from the crack, respectively. The DCPD wires are connected to the DCPD device for potential calibration of various groups of specimens with different crack lengths and ambient temperatures in the thermomechanical fatigue (TMF) test.

[0030] Step 2: After the thermocouple wires and DCPD wires are welded, the sample is clamped in the center of the induction heating coil. The induction heating coil is connected to the high-frequency induction furnace, and the high-frequency induction furnace is connected to the controller. The controller can control the temperature of the high-frequency induction furnace. A cooler is set at the center of the sample heating position. The cooler is connected to the controller. The controller cools the sample to regulate the sample temperature to meet the test requirements. The two ends of the sample are fixed to the clamping ends of the thermomechanical fatigue testing machine. The controller is connected to the thermomechanical fatigue testing machine and controls the mechanical load of the thermomechanical fatigue testing machine.

[0031] In the visual inspection method, the lens of a long-focal-length microscope is positioned perpendicular to the sample surface and above the sample. The observation direction of the long-focal-length microscope is perpendicular to the crack propagation direction to record the crack length of the sample. Figure 3 As shown in Figure b, a complete thermomechanical fatigue crack propagation test was performed. The potential change process was converted into a crack propagation process using the calibration results obtained in step 1.

[0032] When initiating the thermomechanical fatigue (TMF) test, the DCPD method was used to record the potential of crack lengths of 1 mm, 3 mm, 5 mm, and 7 mm in the specimens under temperature cycles of 300℃, 400℃, 500℃, 600℃, and 350–650℃. The relationship between crack length *a* and potential *U* at different temperatures was established, yielding the relationship between the potential *U* of a single crack length under temperature cycles and the corresponding time *t*. The real-time curve of potential *U* versus time *t* during crack propagation was obtained. Convert time t to the number of cycles N, t = N * 100. Substitute the number of cycles N into the U ~ 0.35a + 1.85 obtained from the potential calibration to obtain the a ~ N (crack length and number of cycles) relationship curve. Convert the potential U-time t relationship into the fatigue crack propagation rate relationship affected by thermomechanical coupling to obtain the crack propagation rate relationship curve based on the DCPD method.

[0033] Step 3: As Figure 3 The system comprises a thermomechanical fatigue testing system, a high-frequency induction furnace, a cooler, a long-focal-length microscope, a DCPD device, and a controller. The thermomechanical fatigue testing machine is used to hold the specimen and apply cyclic loads. The controller and cooler regulate the temperature load on the specimen. The controller also controls the mechanical load applied to the thermomechanical fatigue testing machine. The DCPD device is connected to the test specimen to collect potential data. Using a long-focal-length microscope, a visual method is employed. By adjusting the focal length and scale of the microscope, the crack length of the specimen is recorded at regular intervals. Finally, a crack propagation rate curve based on the visual method is plotted. Where a is the crack length and N is the cycle period.

[0034] Step 4: To address potential fluctuations during the measurement of thermal fatigue crack propagation, such as... Figure 4The image shows the curve and local area of ​​the potential U changing with time t during the thermo-mechanical fatigue crack propagation experiment. The "visual observation value" curve represents the visual observation result of the crack propagation of the sample obtained in step 3. During the data analysis, a series of crack length data at different cycle nodes (peaks, troughs, and medians) in step 2 were selected. The peak is... trough The median is Compare the observed crack propagation curve from step 3 with the corrected thermo-mechanical fatigue crack propagation curve based on the degree of fit, such as... Figure 4 The "DCPD correction value" curve shows that the crack propagation rate at the median of the thermomechanical fatigue cycle is closest to the visually observed data. Therefore, it serves as a correction for the DCPD method to measure the crack propagation rate, thereby achieving accurate and efficient measurement of the thermomechanical fatigue crack propagation rate.

[0035] The above are preferred embodiments of this application. However, those skilled in the art, once they understand the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of this application.

[0036] Obviously, those skilled in the art can make various modifications and variations to this application without departing from the spirit and scope of this application. Therefore, if such modifications and variations fall within the scope of the claims of this application and their equivalents, this application also intends to include such modifications and variations.

Claims

1. A method for testing the thermomechanical fatigue crack propagation rate, characterized in that, Includes the following steps: Step S1: Using the crack propagation sample as the specimen, the potential characteristics of the specimen under different crack lengths and different external temperatures are calibrated using the four-point DC potential drop (DCPD) method. Step S2: Perform thermomechanical fatigue (TMF) experiments on the sample, and link the potential characteristics obtained in step S1 to the potential-time relationship of crack propagation in the TMF experiment to obtain the crack propagation rate relationship based on the DCPD method. Step S3: Use a long-focus microscope to record crack data during the thermomechanical fatigue (TMF) test of the sample in step S2, and obtain the crack propagation rate relationship based on visual inspection. Step S4: In the data analysis software, for the crack propagation rate relationship curve based on the DCPD method obtained in step S2, select suitable cyclic node data, and correct it using the crack propagation rate relationship curve based on the visual method in step S3, to obtain the corrected crack propagation rate relationship curve based on the DCPD method, thereby obtaining the accurate thermomechanical fatigue crack propagation rate; specifically: Crack length data are selected from the peak, trough, and median nodes in step S2, with the peak being... The trough is The median is In the data analysis software Origin, the crack propagation rate relationship curve based on visual estimation in step S3 is compared with the crack propagation rate relationship curve based on visual estimation. When the crack propagation rate curve of a certain node among the peak, trough and median nodes has the highest fitting degree with the crack propagation rate relationship curve based on visual estimation, the modified crack propagation rate relationship curve based on DCPD method is adopted, that is, the thermomechanical fatigue crack propagation rate curve.

2. The method for testing the thermomechanical fatigue crack propagation rate according to claim 1, characterized in that, In step S1, the crack initiation point is located at the center of the sample. Four thermocouples are fixedly welded to the upper and lower sides of the crack propagation direction on both sides of the sample. The thermocouples are connected to the controller and used to monitor and provide feedback on the sample surface temperature. DCPD wires are fixedly welded to the sample surface and connected to the DCPD device. The DCPD device is used to record the potential calibration data of the sample at different crack lengths and different external temperatures during the thermomechanical fatigue (TMF) test. The sample is clamped at the center of the induction heating coil. The induction heating coil is connected to the high-frequency induction furnace. The high-frequency induction furnace is connected to the controller, which controls the temperature of the high-frequency induction furnace. A cooler is also provided at the center of the sample heating and is connected to the controller. The controller controls the cooler to cool the sample, thereby regulating the sample temperature to meet the test requirements. The two ends of the sample are fixed to the clamping ends of the thermomechanical fatigue testing machine. The controller is connected to the thermomechanical fatigue testing machine and controls the mechanical load of the thermomechanical fatigue testing machine.

3. The method for testing the thermomechanical fatigue crack propagation rate according to claim 2, characterized in that, The thermocouple is a type K thermocouple, and the welding position of the thermocouple is 0.5 mm above and below the crack propagation direction on both sides of the sample.

4. The method for testing the thermomechanical fatigue crack propagation rate according to claim 2, characterized in that, The DCPD wires are symmetrically arranged at the four solder points of the sample with the crack in the sample as the center, and are 2 mm and 4 mm away from the crack, respectively.

5. The method for testing the thermomechanical fatigue crack propagation rate according to claim 1, characterized in that, In step S2, the thermomechanical fatigue (TMF) experiment uses the DCPD method to record the potential of crack lengths of 1 mm, 3 mm, 5 mm, and 7 mm in the specimens under temperature cycles of 300℃, 400℃, 500℃, 600℃, and 350~650℃, respectively, to establish the crack length of a single crack at different temperatures. The relationship between potential U and time t for a single crack length under temperature cycling is obtained, resulting in a real-time curve showing the relationship between potential U and time t during crack propagation. U is in mv, t is in s. Convert time t to the number of cycles N, t=N 100, substituting the cycle number N into the potential calibration, yields... , The unit is mm, and we get The relationship curve between crack length and cycle number is obtained by converting the potential U-time t relationship into a fatigue crack propagation rate relationship affected by thermomechanical coupling, thus obtaining a crack propagation rate relationship curve based on the DCPD method.

6. The method for testing the thermomechanical fatigue crack propagation rate according to claim 1, characterized in that, In step S3, the lens of the telephoto microscope is positioned perpendicular to the sample surface and above the sample, maintaining the observation direction perpendicular to the crack propagation direction. During the thermomechanical fatigue (TMF) experiment in step S2, the telephoto microscope is used visually. The focal length and scale of the telephoto microscope are adjusted, and the crack length of the sample is recorded at regular intervals. Finally, a crack propagation rate curve based on visual estimation is plotted. ,in Where is the crack length and N is the cycle period.

7. The method for testing the thermomechanical fatigue crack propagation rate according to claim 6, characterized in that, In step S3, the certain number of cycles is 10.

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