A high-temperature high-low cycle composite fatigue test method applied to single-crystal superalloys

Abstract

The present invention relates to a high-temperature high-low cycle composite fatigue test method applied to single-crystal superalloys, belonging to the technical field of mechanical engineering. It solves the problems in the prior art that the test results are inaccurate due to the inability to synchronously heat two design failure points, the test process is too cumbersome and easily damages the specimen. The present invention synchronously heats two symmetric design fracture positions through two parts of induction heating coils, and reserves an installation space for installing a frequency modulation block, enabling high-temperature high-low cycle composite fatigue tests on material specimens such as single-crystal superalloys with abnormal yield phenomena. The synchronous heating ensures accurate test results; the infrared sensor and laser displacement sensor used are both non-contact measurements, with simple installation, stable use, and no impact on the specimen during use, reducing the test cost.

Description

Technical Field

[0001] The present invention relates to the technical field of mechanical engineering, and particularly relates to a high-temperature high-low cycle composite fatigue test method applied to single-crystal superalloys. Background Art

[0002] When an aero-engine is operating, its turbine rotor blades are simultaneously subjected to the combined action of low-cycle fatigue caused by centrifugal load, temperature load, etc., and high-cycle fatigue load caused by weak aerodynamic disturbances and system vibrations. Therefore, for aero-engine materials, carrying out high-low cycle composite fatigue tests helps to explore the failure mechanism under actual service conditions.

[0003] Nickel-based single-crystal superalloys are widely used in aero-engine turbine blades due to their excellent properties. However, nickel-based single-crystal superalloys containing a high volume fraction of γ' exhibit complex yield behavior, that is, within a certain range, as the temperature increases, its yield strength increases to a certain peak and then drops sharply. Specifically, the change of its yield strength with temperature at high temperature can be divided into three stages: from room temperature to about 600 °C, its yield strength basically remains unchanged or slightly decreases; from 600 to 760 °C, its yield strength anomalously increases with the increase of temperature; above 760 °C, its yield strength drops sharply. The change of its yield strength is also reflected in its fatigue strength, and its fatigue strength also reaches the maximum near 760 °C.

[0004] Currently, there are relatively few test devices and test methods for biaxial high-low cycle composite fatigue. In the existing technology for high-low cycle composite fatigue tests of plate-shaped specimens, the fixture design scheme mainly connects the fixture with the clamping section of the specimen through bolts to transmit low-cycle loads; the excitation rod of the vibrator is connected to the middle boss of the specimen to provide high-cycle loads. However, on the one hand, since a frequency modulation block needs to be installed in the middle part of the specimen, the traditional induction coil cannot heat two symmetric design failure points on the specimen simultaneously, which will cause the temperatures of the two symmetric design failure points to be different; when the test temperature is 750 °C, due to the anomalous yield phenomenon of nickel-based single-crystal superalloys, at this time, the fatigue strength of the design failure point that has not been heated to 750 °C is lower than that of the design failure point that has been heated to 750 °C due to the lower temperature, so it will fracture first during the test, resulting in inaccurate test results; on the other hand, during the high-temperature test process, thermocouples and strain gauges are always used separately to monitor the temperature and stress, and the test process is too cumbersome and easy to damage the specimen. Therefore, the traditional test scheme cannot carry out high-temperature high-low cycle composite fatigue tests on materials with anomalous yield phenomena.

[0005] In summary, there are problems in the existing technology such as inaccurate test results caused by the inability to synchronously heat two design failure points, and the test process is too cumbersome and easy to damage the specimen. Summary of the Invention

[0006] In view of the above problems, the present invention provides a high-temperature high-low cycle composite fatigue test method for single-crystal superalloys, which solves the problems of inaccurate test results caused by the inability to synchronously heat two design failure points in the prior art, as well as the overly cumbersome test process and the easy damage to the specimens.

[0007] The present invention provides a high-temperature high-low cycle composite fatigue test method for single-crystal superalloys, comprising the following steps:

[0008] Step S1. Design the shape and dimensions of the plate specimen 1 and the frequency modulation block 2, and process to obtain the plate specimen 1 and the frequency modulation block 2; wherein, the two ends and the middle of the plate specimen 1 are respectively the clamping sections 101 and the bosses 102, and the two ends of the bosses 102 are two designed fracture positions 103; the material of the plate specimen 1 is a single-crystal superalloy;

[0009] Step S2. Conduct strain-displacement calibration at room temperature, paste strain gauges on the designed fracture positions 103 of the plate specimen 1, and use a laser displacement sensor to measure the amplitude of the frequency modulation block 2 under vibration excitation to obtain the strain-displacement calibration relationship;

[0010] Step S3. Calibrate the infrared temperature sensor using a thermocouple;

[0011] Step S4. Assemble the test equipment, install two fixtures, and adjust the position of the induction heating coil 3 so that its central axis coincides with the central axes of the two fixtures; pass the plate specimen 1 through the induction heating coil 3 and install it on the two fixtures;

[0012] The induction heating coil 3 includes two parts of coils with the same spiral direction, which are respectively used to heat the two designed fracture positions 103; one side between the two parts of coils is connected by a copper pipe, and the other side has an installation space; through the installation space between the two parts of coils, the frequency modulation block 2 is installed on the boss 102 of the plate specimen 1;

[0013] Step S5. Conduct a high-temperature high-low cycle composite fatigue test.

[0014] Furthermore, step S3 specifically includes:

[0015] Select a thermocouple according to the test temperature and the properties of the single-crystal superalloy, use a spot welder to weld the thermocouple to the designed fracture position 103, use the induction heating coil 3 to heat the designed fracture position 103, and use the thermocouple to measure its temperature; when the temperature measured by the thermocouple reaches the test temperature, use the infrared temperature sensor to measure the temperature of the designed fracture position 103, and by adjusting the emissivity of the infrared temperature sensor, make the temperature measured by the infrared temperature sensor the same as the temperature measured by the thermocouple.

[0016] Furthermore, step S5 specifically includes:

[0017] Turn on the high-frequency induction heating equipment, axial fatigue testing machine and vibrator; among them, the high-frequency induction heating equipment is used to start the induction heating coil 3 to synchronously heat the two designed fracture positions 103 to a test temperature of over 600°C; the axial fatigue testing machine is connected to two fixtures and is used to apply low-cycle loads to the plate specimen 1; the vibrator is used to apply high-cycle loads to the plate specimen 1 through the frequency modulation block 2;

[0018] During the test, a calibrated infrared temperature sensor is used to monitor the temperature of the two designed fracture positions 103, a laser displacement sensor is used to monitor the amplitude of the frequency modulation block 2 at high temperature, and the strain and stress corresponding to each displacement are obtained through the strain-displacement calibration relationship.

[0019] Furthermore, the top view cross-section of the induction heating coil 3 is a straight-sided ellipse, the number of turns of the two parts of the induction heating coil 3 is the same, and the spiral directions are the same; the spiral direction of the copper tube is the same as that of the two parts of the induction heating coil 3.

[0020] Compared with the prior art, the present invention has at least the following beneficial effects:

[0021] (1) The high-temperature high-low cycle composite fatigue test method for single-crystal superalloys of the present invention can carry out high-temperature high-low cycle composite fatigue tests on material specimens such as single-crystal superalloys with abnormal yield phenomena by synchronously heating two symmetrically designed fracture positions with two parts of induction heating coils and reserving an installation space for installing the frequency modulation block. The synchronous heating ensures accurate test results.

[0022] (2) The high-temperature high-low cycle composite fatigue test method for single-crystal superalloys of the present invention uses non-contact measurements for both the infrared sensor and the laser displacement sensor, which are simple to install, stable in use, and do not affect the specimen during use, reducing the test cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The drawings are only for the purpose of showing specific embodiments and are not considered to limit the present invention.

[0024] Figure 1 is a flowchart of the high-temperature high-low cycle composite fatigue test method for single-crystal superalloys disclosed by the present invention;

[0025] Figure 2 is an assembly schematic diagram of the plate specimen, induction heating coil and frequency modulation block disclosed by the present invention;

[0026] Figure 3 is a schematic diagram of the induction heating coil disclosed by the present invention;

[0027] Figure 4Schematic diagram of the plate specimen disclosed in the present invention;

[0028] Figure 5 Strain-displacement calibration relationship diagram disclosed in the present invention.

[0029] Reference numerals:

[0030] 1 - Plate specimen; 2 - Frequency modulation block; 3 - Induction heating coil; 101 - Clamping section; 102 - Boss; 103 - Designed fracture position. Detailed implementation manners

[0031] In order to more clearly understand the above objects, features and advantages of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific implementation manners. It should be noted that, without conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other. In addition, the present invention can also be implemented in other ways different from those described herein. Therefore, the protection scope of the present invention is not limited by the specific embodiments disclosed below.

[0032] The present invention discloses a high-temperature high-low cycle composite fatigue test method applied to single-crystal superalloys, as Figure 1 shown, and includes the following steps:

[0033] Step S1. Design and process the plate specimen 1 and the frequency modulation block 2 according to the material properties.

[0034] Specifically, design the shape and size of the plate specimen 1 and the frequency modulation block 2, query the density and elastic modulus of the material at the test temperature, and accordingly select the length and thickness of the plate specimen 1; verify whether the resonance frequency of the composite structure composed of the plate specimen 1 and the frequency modulation block 2 is within the frequency range of 200 - 2000 Hz that the exciter can provide through finite element software. The specific value is related to the performance of the exciter, and the resonance frequency is made to meet the requirements through design.

[0035] The plate specimen 1 is a symmetric structure in the three directions of length, width and height. The symmetric structure is easy to obtain a better vibration mode and a larger amplitude. The two ends and the middle of the plate specimen 1 are respectively the clamping section 101 and the boss 102. The clamping section 101 is double-wedge-shaped and its size matches the fixture; the boss 102 is used to install the frequency modulation block 2, and the frequency modulation block 2 does not contact the designed fracture position 103 to eliminate the influence of contact stress on the fatigue performance of the specimen; both ends of the boss 102 are two designed fracture positions 103; a straight section is provided between the boss 102 and the clamping section 101. The material of the plate specimen 1 is a single-crystal superalloy.

[0036] Process to obtain the plate specimen 1 and the frequency modulation block 2.

[0037] Step S2. Obtain the strain-displacement calibration relationship.

[0038] Specifically, at room temperature, strain-displacement calibration is carried out. The strain gauge is pasted at the designed fracture position 103 of the plate-shaped specimen 1, and the direction of the strain gauge grid wire should be parallel to the specimen axis. The laser displacement sensor is aligned with the frequency modulation block 2 so that the laser emitted by the laser displacement sensor is located in the first-order bending plane of the combined structure of the plate-shaped specimen 1 and the frequency modulation block 2. The amplitude of the frequency modulation block 2 under the vibration excitation of the shaker is measured by the laser displacement sensor. By adjusting the output power of the shaker and recording the amplitudes of the frequency modulation block 2 corresponding to different strains, the strain-displacement calibration relationship is obtained.

[0039] Step S3. Calibrate the infrared temperature sensor using a thermocouple.

[0040] Specifically, select a thermocouple according to the test temperature and the properties of the single-crystal superalloy. Use a spot welder to weld the thermocouple to the designed fracture position 103. Use the induction heating coil 3 to heat the designed fracture position 103, and measure its temperature using the thermocouple. When the temperature measured by the thermocouple reaches the test temperature, use the infrared temperature sensor to measure the temperature of the designed fracture position 103. By adjusting the emissivity of the infrared temperature sensor, make the temperature measured by the infrared temperature sensor the same as the temperature measured by the thermocouple.

[0041] Step S4. Assemble the test equipment.

[0042] Specifically, install the two fixtures and adjust the position of the induction heating coil 3 so that its central axis coincides with the central axes of the two fixtures. Pass the plate-shaped specimen 1 through the induction heating coil 3 and install it on the two fixtures. Install the frequency modulation block 2 and align the infrared temperature sensor and the laser displacement sensor with the predetermined positions.

[0043] The induction heating coil 3 includes two parts of coils with the same spiral direction, which are respectively used to heat the two designed fracture positions 103. One side between the two parts of coils is connected by a copper pipe, and the other side has an installation space. The frequency modulation block 2 is installed on the boss 102 of the plate-shaped specimen 1 through the installation space between the two parts of coils.

[0044] Step S5. Conduct a high-temperature high-low cycle composite fatigue test.

[0045] Specifically, turn on the high-frequency induction heating equipment, the axial fatigue testing machine and the shaker. Among them, the high-frequency induction heating equipment is used to start the induction heating coil 3 to synchronously heat the two designed fracture positions 103 to a test temperature above 600 °C. The axial fatigue testing machine is connected to the two fixtures and is used to transmit low-cycle loads to the plate-shaped specimen 1. The shaker is used to transmit high-cycle loads to the plate-shaped specimen 1 through the frequency modulation block 2.

[0046] It should be noted that since the two parts of the induction heating coil 3 are connected by copper pipes, only one high-frequency induction heating device is needed to synchronously heat the two designed fracture positions 103. The middle positions of the two parts of the coil respectively correspond to the two designed fracture positions 103 of the plate specimen 1, which are the places with the highest temperature.

[0047] During the test, a calibrated infrared temperature sensor was used to monitor the temperatures of the two designed fracture positions 103, and a laser displacement sensor was used to monitor the amplitude of the frequency modulation block 2 at high temperatures. The strain and stress corresponding to each displacement were obtained through the strain-displacement calibration relationship.

[0048] Compared with the prior art, the high-temperature high-low cycle composite fatigue test method for single-crystal superalloys of the present invention can carry out high-temperature high-low cycle composite fatigue tests on material specimens such as single-crystal superalloys with abnormal yield phenomena by synchronously heating two symmetric designed fracture positions 103 with two parts of the induction heating coil 3 and reserving an installation space for installing the frequency modulation block. The synchronous heating ensures the accuracy of the test results. The infrared sensor and laser displacement sensor used are both non-contact measurements, with simple installation, stable use, and no impact on the specimen during use, reducing the test cost.

[0049] In order to illustrate the effectiveness of the method proposed by the present invention, the above technical solutions of the present invention will be described in detail through a specific embodiment as follows:

[0050] Example 1

[0051] The purpose of this example is to carry out a high-temperature high-low cycle composite fatigue test on a nickel-based single-crystal superalloy. The material used in this example has an abnormal yield effect. From room temperature to about 600 °C, its yield strength basically remains unchanged or slightly decreases; from 600 to 760 °C, its yield strength abnormally increases with the increase of temperature; above 760 °C, its yield strength drops sharply. The test temperature is set at 760 °C, the low-cycle load requirement reaches 700 MPa, and the transverse high-cycle load requirement reaches 120 MPa. The specific test is carried out according to the following plan:

[0052] Design the plate specimen 1 and the frequency modulation block 2 according to the material properties. Figure 4 As shown in the structural schematic diagram of the plate specimen 1, the double-wedge clamping section 101 is convenient for installation, and the two ends of the boss 102 are two designed fracture positions 103; there is also a straight section between the boss 102 and the clamping section 101. Optionally, the width of the straight section is 5 mm, the thickness is 1.2 mm, and the length is 25 mm.

[0053] Obtain the strain-displacement calibration relationship. Paste the strain gauge on Figure 4The designed fracture position 103 of the middle plate-shaped specimen 1 is used to record the strain at this position. A laser displacement sensor is aligned with the frequency modulation block 2 to record the amplitude at this position. A vibration load is applied at room temperature, the output power of the exciter is adjusted to obtain 6 different strains, and the amplitude corresponding to each strain is recorded to obtain the strain-displacement calibration relationship as Figure 5 shown.

[0054] Calibrate the infrared temperature sensor. Select a k-type thermocouple for temperature calibration, fix the thermocouple at the designed fracture position 103 using a spot welder, select a current of 60A during spot welding, and measure the temperature at this position using an infrared temperature sensor at the same time. Start the induction heating coil 3 as Figure 3 shown. When the temperature measured by the thermocouple reaches the test temperature of 760 °C, adjust the emissivity of the infrared temperature sensor. When the emissivity is 0.78, the temperature measured by the infrared temperature sensor is the same as the temperature measured by the thermocouple.

[0055] Assemble the test equipment. During the installation process, first place the induction heating coil 3 at the predetermined position so that the central axis of the induction heating coil 3 coincides with the central axis of the fixture. Then, pass the plate-shaped specimen 1 through the induction heating coil 3 and install it on two fixtures. Finally, install the frequency modulation block 2, as Figure 2 shown.

[0056] After the above preparations are completed, carry out the high-temperature high-low cycle composite fatigue test.

[0057] As described above, only the preferred specific implementation manners of the present invention are provided, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A high-temperature, high-low-cycle composite fatigue test method for single-crystal high-temperature alloys, characterized in that: The steps include: Step S1. Designing the shape and size of the plate-shaped specimen (1) and the frequency modulation block (2), and processing the plate-shaped specimen (1) and the frequency modulation block (2); wherein the two ends and the middle of the plate-shaped specimen (1) are respectively a clamping section (101) and a boss (102), and the two ends of the boss (102) are two designed fracture positions (103); the material of the plate-shaped specimen (1) is a single crystal high-temperature alloy; Step S2. Strain-displacement calibration is performed at room temperature. A strain gauge is attached to the designed fracture position (103) of the plate specimen (1). The amplitude of the frequency modulation block (2) under vibration excitation is measured using a laser displacement sensor to obtain a strain-displacement calibration relationship. Step S3. Calibrate the infrared temperature sensor using a thermocouple; Step S4. Assemble the test equipment, install the two fixtures, and adjust the position of the induction heating coil (3) so that its central axis coincides with the central axes of the two fixtures; pass the plate-shaped specimen (1) through the induction heating coil (3) and install it on the two fixtures; The induction heating coil (3) includes two coils with the same spiral direction, each used to heat two designed fracture positions (103); one side of the two coils is connected by a copper tube, and the other side has an installation space; the frequency modulation block (2) is installed on the boss (102) of the plate-shaped specimen (1) through the installation space between the two coils; Step S5: Conduct high-temperature and high-low cycle composite fatigue tests.

2. The high-temperature, high-low-cycle composite fatigue test method for single crystal high-temperature alloys according to claim 1, characterized in that: Step S3 specifically includes: A thermocouple is selected according to the test temperature and the properties of the single crystal high temperature alloy, the thermocouple is welded to the designed fracture position (103) using a spot welder, the designed fracture position (103) is heated using an induction heating coil (3), and its temperature is measured using a thermocouple; when the temperature measured by the thermocouple reaches the test temperature, the temperature of the designed fracture position (103) is measured using an infrared temperature sensor, and the emissivity of the infrared temperature sensor is adjusted so that the temperature measured by the infrared temperature sensor is the same as the temperature measured by the thermocouple.

3. The high-temperature, high-low-cycle composite fatigue test method for single-crystal high-temperature alloys according to claim 2, characterized in that: Step S5 specifically includes: The high-frequency induction heating device, the axial fatigue testing machine, and the exciter are turned on; wherein the high-frequency induction heating device is used to start the induction heating coil (3) to simultaneously heat the two designed fracture positions (103) to a high-temperature test temperature of more than 600° C.; the axial fatigue testing machine is connected to the two fixtures and is used to transmit a low-frequency load to the plate-shaped specimen (1); and the exciter is used to transmit a high-frequency load to the plate-shaped specimen (1) through the frequency modulation block (2); During the test, a calibrated infrared temperature sensor was used to monitor the temperature of the two designed fracture positions (103), and a laser displacement sensor was used to monitor the amplitude of the frequency modulation block (2) at high temperature. The strain and stress corresponding to each displacement were obtained through the strain-displacement calibration relationship.

4. The high-temperature high-low cycle composite fatigue test method for single crystal high-temperature alloys according to claim 3, characterized in that: The top view cross section of the induction heating coil (3) is a straight-sided ellipse. The two coil parts of the induction heating coil (3) have the same number of turns and the same spiral direction. The spiral direction of the copper tube is the same as that of the two coil parts of the induction heating coil (3).

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