Method for measuring crack growth rate and threshold value at extremely high temperature

Abstract

The invention discloses a method for measuring a crack growth rate and a threshold value at an extremely high temperature. Firstly, a test sample and a clamp are manufactured; then a high-temperature furnace is installed, a test sample and a thermocouple are installed, the test sample, a reference sample and a potential measurement system are connected through a high-temperature-resistant wire, and the reference sample is arranged outside the high-temperature furnace; the temperature of the high-temperature furnace is raised to 1000-1200 DEG C, heat preservation is conducted, and meanwhile water cooling is conducted to the cooling connecting rod; the low-cycle fatigue testing machine starts to load and test the crack of the sample through a potential measurement system; a K-drop method is adopted for testing, a potential measurement system monitors crack propagation in real time through a direct-current potential method, the test is ended when the crack propagation rate reaches 1 * 10 <-mm > / cycle, and a fatigue crack propagation threshold value is obtained through data processing. According to the invention, the problems of current conduction stability, load transmission accuracy and crack length measurement accuracy in the environment of 1000-1200 DEG C are solved.

Description

Technical Field

[0001] This invention belongs to the field of mechanical property testing technology for metallic materials, specifically a method for determining the crack propagation rate and threshold value at extremely high temperatures. Background Technology

[0002] High-temperature alloys are key materials for manufacturing hot-end components (such as turbine disks and blades) in high-end equipment like aero engines and gas turbines. With the increasing demands for efficiency and thrust-to-weight ratio in propulsion systems, the operating temperatures of these components have generally exceeded 1000℃, and are even approaching 1200℃. Under these extreme conditions, the damage tolerance of materials under cyclic loading—that is, their ability to resist the initiation and propagation of fatigue cracks—directly determines the service safety and lifespan of the components.

[0003] Fatigue crack propagation threshold value ΔK th ΔK is a core parameter characterizing a material's ability to prevent fatigue crack propagation and is the cornerstone of damage tolerance design and life prediction. Accurately obtaining ΔK of high-temperature alloys at actual service temperatures is crucial. th Data is of irreplaceable importance for optimizing material composition, improving manufacturing processes, ensuring structural reliability, and establishing scientific maintenance cycles. Currently, there are relatively mature standard methods for fatigue crack propagation testing at temperatures below 1000℃. The main methods for measuring crack length in test specimens include:

[0004] (1) Visual inspection method: The crack length is directly observed and recorded using a high-temperature microscope or industrial camera system. This method has simple equipment, but at ultra-high temperatures, the severe oxidation of the test sample surface, thermal radiation interference, and reduced clarity of the environmental chamber window make it difficult to identify the crack tip, resulting in large human error in the measurement results, and it basically fails at extremely high temperatures.

[0005] (2) Compliance method: Crack length is inferred by measuring crack opening displacement (COD). This method performs well under high loads and crack propagation rates. However, when measuring the threshold value ΔKth, the applied load level is very low, resulting in a very small change in COD. Under high-temperature conditions, the thermal expansion of the extension rod, the loosening of the clamps, and the noise of the measurement system itself are amplified dramatically, resulting in an extremely low signal-to-noise ratio for displacement measurement, large data dispersion, and unreliable accuracy.

[0006] (3) Direct Current Potentiometric Method (DCPD): A constant direct current is applied to the test specimen, and the crack length is indirectly calculated by monitoring the voltage change caused by the change in resistance due to crack propagation. This method has high calculation accuracy for standard test specimens such as C(T) and M(T), and is easy to automate and implement closed-loop control, making it the mainstream technology for medium and high temperature testing. However, when the temperature exceeds the critical point of 1000℃, conventional DCPD technology faces three major obstacles:

[0007] Temperature limit of the conductor system: Ordinary high-temperature resistant conductors will rapidly oxidize and become brittle in an oxidizing atmosphere at 1200℃, resulting in a sharp increase in resistance or even melting, making it impossible to stably conduct the large current required for measurement (usually 10-25A).

[0008] High-temperature insulation failure: Commonly used high-temperature insulation materials experience a sharp decline in insulation performance at this temperature, or react with conductors or test samples, causing current leakage, distortion and drift of potential measurement signals, or even short circuits.

[0009] Insufficient high-temperature performance of the fixture system: The metal fixtures experience a decrease in strength and creep at extremely high temperatures, and their coefficient of thermal expansion does not match that of the test specimen, which introduces additional assembly stress and affects the accuracy of load transfer and the coaxiality of the test.

[0010] In summary, existing technologies lack a complete testing scheme capable of long-term stable operation within the 1000℃ to 1200℃ range, while ensuring measurement accuracy and reliability. This severely restricts the research and safe application of next-generation ultra-high temperature materials. This invention aims to overcome this key technological bottleneck. Summary of the Invention

[0011] To address the problems in the prior art regarding testing the fatigue crack propagation threshold value of high-temperature alloys at temperatures above 1000℃, this invention provides a method for determining the crack propagation rate and threshold value at extremely high temperatures. This method achieves accurate and efficient testing, providing reliable data for damage tolerance design and performance evaluation of high-temperature alloys. The technical solution includes:

[0012] Step 1: Fabrication of test specimens and fixtures: Prepare high-temperature alloy materials into test specimens and reference specimens for crack propagation rate measurement, and fabricate cooling connecting rods and ceramic U-shaped fixtures.

[0013] Step 2: Install the high-temperature furnace, install the test sample and the thermocouple, and then use high-temperature resistant wires to connect the test sample, the reference sample and the potential measurement system, wherein the reference sample is placed outside the high-temperature furnace;

[0014] Step 3: Start the test: The high-temperature furnace is raised to 1000-1200℃ and held at that temperature, while water cooling is gradually introduced into the cooling connecting rod; the low-cycle fatigue testing machine is loaded and the crack in the specimen is tested through the potential measurement system; the K-method is used for the test, and the potential measurement system monitors the crack propagation in real time through the DC potential method. When the crack propagation rate reaches 1×10⁻⁶, the test is completed. -7 The test was terminated when the fatigue crack propagation threshold value ΔK was obtained through data processing. th .

[0015] The thickness of the test specimen and the reference specimen is 5 mm;

[0016] The test specimen and reference specimen include: a main body and two tensile parts, the two tensile parts being symmetrically arranged vertically and integrally fixed to the same side of the main body, the two tensile parts being separated by a tensile opening, the innermost side of the tensile opening being a 30-degree angled part.

[0017] Each of the upper and lower stretching sections has a stretching section mounting hole for connection to its respective ceramic U-shaped clamp pin; each stretching section has a stretching section wire connection hole on the side of the same side as the stretching opening, and a set of main body wire connection holes are symmetrically arranged on the upper and lower end faces of the main body; the stretching section wire connection holes are connected to the voltage interface, and the main body wire connection holes are connected to the current interface.

[0018] The high-temperature furnace is a split-type high-temperature furnace, which is divided into at least two independent heating resistance wire groups. Each heating resistance wire group is set separately along the plane or plane group where the load direction of the low-cycle fatigue testing machine is located. Each heating resistance wire group adopts an independent temperature control system to achieve effective isolation of the magnetic field in the furnace cavity.

[0019] 5. The method for determining the crack propagation rate and threshold value at extremely high temperatures according to claim 3, characterized in that, in step 2, the process of installing the test specimen is as follows:

[0020] Step 21: Install the cooling connecting rods and ceramic U-shaped clamps: First, install the two cooling connecting rods 3 onto the threaded holes of the upper and lower actuator cylinders of the low-cycle fatigue testing machine through the threaded screws. Then, install the ceramic U-shaped clamps onto the two cooling connecting rods 3 through the threaded holes. Then connect the water cooling inlet and water cooling outlet. The cooling connecting rods are equipped with cooling channels to prevent the ceramic U-shaped clamps from cracking.

[0021] Step 22: Install the test specimen: Insert the test specimen into the thin groove of the ceramic U-shaped clamp and connect the test specimen and the ceramic U-shaped clamp with a pin.

[0022] The ceramic U-shaped fixture is custom-made from alumina ceramic, and the fit accuracy between the ceramic U-shaped groove and the test sample pin hole reaches the H7 / g6 level; the installation coaxiality deviation of the entire fixture system is strictly limited to within ±0.8mm.

[0023] In step 2, the process of connecting the test sample, the reference sample, and the potential measurement system using high-temperature resistant wires is as follows:

[0024] The first current interface of the potential measurement system is sequentially connected to a wire connection hole in one main body of the reference sample, a wire connection hole in another main body of the reference sample, a wire connection hole in one main body of the test sample, a wire connection hole in another main body of the test sample, and the second current interface of the potential measurement system.

[0025] One tensile wire connection hole of the test specimen is connected to the first interface of the first group of voltages, and the other tensile wire connection hole of the test specimen is connected to the second interface of the first group of voltages; one tensile wire connection hole of the reference specimen is connected to the first interface of the second group of voltages, and the other tensile wire connection hole of the reference specimen is connected to the second interface of the second group of voltages.

[0026] The high-temperature resistant wire is a platinum-rhodium alloy wire; the high-temperature resistant wire is installed using wire mounting bolts, and the high-temperature resistant wire wrapped around the wire mounting bolt is fixed between the wire mounting bolt and the alumina ceramic gasket.

[0027] The potential measurement system is a DCPD test device manufactured by MTS;

[0028] During the test in step 3, the load ratio R should be greater than or equal to 0.1, and the normalized gradient of the stress intensity factor should be set to -0.08 mm. - ¹, In environments above 1000℃, the normalized gradient value is no greater than -0.08 mm. - ¹; The test waveform used was a sine wave with a frequency of 0-30Hz.

[0029] Step 3 includes the following steps:

[0030] Step 31: According to the test settings, move the high-temperature furnace to the working position via the slide rail; set the test temperature, heat the high-temperature furnace to the target temperature, and keep it at that temperature for a period of time to allow the test sample to reach a stable test temperature; at the same time, water cooling is introduced into the cooling connecting rod.

[0031] Step 32: Apply axial fatigue load to the test specimen on the low-cycle fatigue testing machine. The load range is determined according to the test requirements, and the test frequency is controlled between 0-30Hz.

[0032] Step 33: Using a potential measurement system, monitor the change in crack length in real time by measuring the change in voltage on the test specimen. Calculate the fatigue crack propagation rate based on the relationship between voltage and crack length. Continue the fatigue test until the crack propagation rate reaches 10... -7 The experiment ends when there are enough logarithmic data (mm / time).

[0033] Step 34: Select at least 5 evenly distributed values ​​at 10 -6 ~10 -7 For the data pairs of da / dN-ΔK between mm / times, with da / dN as the independent variable and ΔK as the dependent variable, a linear regression method was used to fit the data points. The crack propagation rate was calculated as 10 from the fitting results. -7 The ΔK value is measured in mm / cycle; this value is the fatigue crack propagation threshold ΔK. th .

[0034] The preparation of high-temperature alloy materials into test specimens and reference specimens for crack propagation rate measurement includes:

[0035] Step 11: Rough machining of the outer contour;

[0036] Step 12: Machining the stretching opening and wire connection hole, and finely machining the outer contour;

[0037] Step 13: Tap the wire connection hole;

[0038] Step 14: Remove burrs from the outer contour and clean the surface with a lint-free cloth and anhydrous ethanol.

[0039] The beneficial effects of this invention are as follows:

[0040] 1. A complete system solution is provided, encompassing test specimen preparation, clamping, wire connection, insulation treatment, and test control. Through the synergistic innovation of "platinum-rhodium alloy wires + alumina ceramic clamps + ceramic gaskets," the challenges of current conduction stability, load transfer accuracy, and crack length measurement accuracy in environments ranging from 1000℃ to 1200℃ are fundamentally solved.

[0041] 2. In the extremely high temperature range of 1000℃-1200℃, the method of this invention, by employing methods such as separate current control on both sides of the high-temperature furnace to suppress the magnetic field inside the furnace, achieves an online measurement resolution of crack length better than 0.005mm, and the final measured ΔK th The test repeatability coefficient of the value is less than 5%.

[0042] 3. This invention clarifies and successfully implements ΔK in the ultra-high temperature range of 1000℃-1200℃. thReliable testing methods provide an indispensable testing means for the performance evaluation and screening of next-generation high-temperature materials that will serve in more demanding environments in the future. They also provide important technical support for the performance evaluation and damage tolerance design of high-temperature alloy materials, and have significant technological leading value and industrial application prospects. Attached Figure Description

[0043] Figure 1 This is a schematic flowchart of an embodiment of a method for determining crack propagation rate and threshold value at extremely high temperatures according to the present invention.

[0044] Figure 2 This is a front view schematic diagram of a tensile testing machine connected with a high-temperature resistant wire during the test state in an embodiment of the present invention;

[0045] Figure 3 This is a front view schematic diagram of the test sample in an embodiment of the present invention;

[0046] Figure 4 These are front and side view schematic diagrams of the ceramic U-shaped clamp in an embodiment of the present invention;

[0047] Figure 5 This is a front view schematic diagram of the cooling connecting rod in an embodiment of the present invention;

[0048] Figure 6 This is a curve showing the fatigue crack propagation threshold value da / dN-ΔK of the high-temperature alloy in an embodiment of the present invention.

[0049] Wherein: 1-Low-cycle fatigue testing machine, 2-Split-type high-temperature furnace, 3-Cooling connecting rod, 4-Ceramic U-shaped clamp, 5-Test specimen, 6-Ceramic gasket, 7-High-temperature resistant wire, 8-DCPD device, 9-Reference specimen, 51-Main body, 52-Tension part, 521-Tension part mounting hole, 53-Tension opening, 54-Main body wire connection hole, 55-Tension part wire connection hole. Detailed Implementation

[0050] The present invention will be further described in detail below with reference to the accompanying drawings.

[0051] like Figures 1-5 The broad embodiments of the present invention shown include:

[0052] Step 1: Fabrication of test specimens and fixtures: Prepare high-temperature alloy materials into test specimens and reference specimens for crack propagation rate measurement, and fabricate cooling connecting rods and ceramic U-shaped fixtures.

[0053] Step 2: Install the high-temperature furnace, install the test sample, install the thermocouple, and then connect the test sample, reference sample, and potential measurement system (DCPD device 8) using high-temperature resistant wires. The reference sample is placed outside the high-temperature furnace. Figure 2 As shown;

[0054] Step 3: Start the test: The high-temperature furnace is raised to the test temperature (1000-1200℃) and held at that temperature, while water cooling is gradually introduced into the cooling connecting rod; the low-cycle fatigue testing machine is loaded and the crack in the specimen is tested through the potential measurement system; the K-method is used for the test, and the potential measurement system monitors the crack propagation in real time through the DC potential method. When the crack propagation rate reaches 1×10⁻⁶, the test is completed. -7 The test was terminated when the fatigue crack propagation threshold value ΔK was obtained through data processing. th .

[0055] Step 4: After the test, measure the test sample and perform data analysis.

[0056] The test specimen 5, with a flat rectangular outer contour, has the same shape as the reference specimen 9, such as... Figure 3 As shown, each includes: a main body 51 and two stretching parts 52, the two stretching parts 52 being symmetrically arranged vertically and integrally fixed to the same side of the main body 51. Figure 3 (Left and right in the middle), the two stretching parts 52 are separated by a stretching opening 53, and the innermost side of the stretching opening 53 is a 30-degree angled part;

[0057] Each of the upper and lower stretching parts 52 has a stretching part mounting hole 521 for connection with its respective ceramic U-shaped clamp 4 pin; each stretching part 52 has a stretching part wire connection hole 55 on the side of the same side as the stretching opening 53, and a set of main body wire connection holes 54 are symmetrically provided on the upper and lower end faces of the main body 51; the stretching part wire connection hole 55 is connected to the voltage interface, and the main body wire connection hole 54 is connected to the current interface.

[0058] In this embodiment, the test specimen 5 and the reference specimen 9 have the same dimensions and a thickness of 5 mm. If the thickness of the test specimen is too large, it will cause excessive load and damage to the ceramic fixture; if the thickness is too thin, the load will be too small and the test force value will be inaccurate. In addition, if the size is too small, it will affect the connection of the high-temperature resistant wire. Meanwhile, the external dimensions of the test specimen 5 are 60 mm * 62.5 mm * 5 mm; the distance between the tensile mounting hole 521 and the long side of the main body 51 is 50 mm; the distance between the two tensile wire connection holes 55 is 20 mm; and the distance between the main body wire connection hole 54 and the long side of the main body 51 is 25 mm.

[0059] In step 1, the high-temperature alloy material is prepared into a test specimen and a reference specimen for crack propagation rate measurement, including the following steps:

[0060] Step 11: Rough machining of the outer contour: Use a face milling cutter, carbide milling cutter and drill bit to machine the surface of the test specimen, the two holes (tension part mounting hole 521) and the four sides, leaving a ≥0.25mm allowance on each side; remove the markings of the opening position and the reference surface, deburr and machine the other side; clamp both sides with flat-jaw pliers, make the tension opening 53 perpendicular to the clamping surface, mill the largest surface, leaving a ≥0.25mm allowance.

[0061] Step 12: Machining the stretch opening and wire connection hole and fine machining the outer contour: Take the test sample and perform wire cutting to make the stretch opening (cutting the opening), then rotate the surface grinder, place the reference surface on the worktable, grind the other surface (grinding amount of about 0.1mm, direction perpendicular to the opening), flip it over and grind the other surface and four sides to the specified size.

[0062] Step 13, Tapping the wire connection hole: Tap the opening position of the wire connection hole to process an M3 internal thread, which will serve as the wire connection interface.

[0063] Step 14: Remove burrs from the outer contour and clean the surface with a lint-free cloth and anhydrous ethanol.

[0064] In step 1, the manufactured ceramic U-shaped fixture includes a fixture threaded part connected to the cooling connecting rod 3 at the top, a long rod part in the middle, and a sample connecting part at the bottom;

[0065] In step 2, a suitable high-temperature furnace is selected according to the test settings. Specifically, a split-type high-temperature furnace is used, which is divided into at least two independent heating resistance wire groups. Each heating resistance wire group is set separately along the plane or plane group where the load direction of the low-cycle fatigue testing machine is located. Each heating resistance wire group adopts an independent temperature control system to achieve effective isolation of the magnetic field in the furnace cavity and ensure that the DCPD measurement accuracy is better than 0.005mm. In this embodiment, two heating resistance wire groups are used. The heating resistance wire groups on both sides are connected in parallel and can be independently controlled by their respective British Continental 0.1-class temperature controllers, with a maximum temperature of 1200℃. The split-type high-temperature furnace isolates the current (usually with a peak value of tens of A) loops of the resistance wires on both sides through the parallel structure, effectively eliminating the alternating magnetic field generated by Ampere's law, thereby eliminating the electromagnetic induction interference caused by it and ensuring the accuracy of DCPD crack monitoring data.

[0066] In step 2, the process of installing the test specimen is as follows:

[0067] Step 21: Install the cooling connecting rod and ceramic U-shaped clamp: (e.g.) Figure 5 The two cooling connecting rods 3 shown are first installed onto the threaded holes of the upper and lower actuator cylinders of the low-cycle fatigue testing machine through threaded screws. The two cooling connecting rods 3 are then installed onto the ceramic U-shaped clamps through the threaded holes. The water cooling inlet and water cooling outlet are then connected. The cooling connecting rods are provided with cooling channels to prevent the ceramic U-shaped clamps from cracking.

[0068] Step 22: Install the test specimen: Insert the test specimen into the thin groove of the ceramic U-shaped clamp and connect the test specimen and the ceramic U-shaped clamp with a pin;

[0069] The ceramic U-shaped clamps used are custom-made from high-purity (≥99%), high-density alumina ceramics to ensure high strength and low creep at 1200℃;

[0070] The ceramic U-shaped clamp used has a precision-machined imperial thread on one end for connecting with the cooling connecting rod 3. The other end of the ceramic U-shaped clamp is designed as a U-shaped groove, which is connected to the test sample through a precision-machined high-temperature alloy pin. The fit accuracy between the U-shaped groove and the pin hole of the test sample reaches the H7 / g6 level. The installation coaxiality deviation of the entire clamp system is strictly limited to within ±0.8mm to ensure the straightness of the load transmission path.

[0071] The cooling connecting rod 3 is made of GH4169 material; the side of the cooling connecting rod is machined with a 1 / 4 thread (imperial) hole for connecting water cooling pipes to cool the device, effectively preventing direct cooling of the ceramic fixture and causing the ceramic fixture to crack.

[0072] In this embodiment, the low-cycle fatigue testing machine used is model MTS370.10, with a maximum test force of ±25KN, static force accuracy of ±0.5%, dynamic force accuracy of ±1%, and coaxiality of no more than 8%.

[0073] In step 2, the process of connecting the test sample, the reference sample, and the potential measurement system using high-temperature resistant wires is as follows:

[0074] The first current interface of the potential measurement system is sequentially connected to a wire connection hole in one main body of the reference sample, a wire connection hole in another main body of the reference sample, a wire connection hole in one main body of the test sample, a wire connection hole in another main body of the test sample, and the second current interface of the potential measurement system.

[0075] One of the tensile wire connection holes of the test specimen is connected to the first interface of the first group of voltages, and the other tensile wire connection hole of the test specimen is connected to the second interface of the first group of voltages; one of the tensile wire connection holes of the reference specimen is connected to the first interface of the second group of voltages, and the other tensile wire connection hole of the reference specimen is connected to the second interface of the second group of voltages.

[0076] Connect an external resistor;

[0077] The high-temperature resistant wires used are made of platinum-rhodium alloy wire with a diameter of 1.0 mm. Platinum-rhodium alloy still has excellent oxidation resistance, low volatility and stable resistance characteristics in an oxidizing atmosphere up to 1700℃, which is the key to ensuring long-term stable current transmission. The connection between the high-temperature resistant wires and the test specimen uses an alumina ceramic gasket as an insulation layer, which effectively prevents electrical short circuits caused by material sintering or deformation at extremely high temperatures. All high-temperature resistant wires are installed using wire mounting bolts (made of the same material as the test specimens), and the high-temperature resistant wires wrapped around the wire mounting bolts are fixed between the wire mounting bolts and the alumina ceramic gaskets.

[0078] Step 3 includes the following steps:

[0079] Step 31: According to the test settings, move the high-temperature furnace to the working position using the slide rail. Set the test temperature, heat the high-temperature furnace to the target temperature, and hold it at that temperature for a period of time to allow the test sample to reach a stable test temperature;

[0080] Step 32: Apply axial fatigue load (triangular wave, sine wave, etc.) to the test specimen on the low-cycle fatigue testing machine. The load range is determined according to the test requirements, and the test frequency is controlled between 0-30Hz.

[0081] Step 33: Using a potential measurement system, monitor the change in crack length in real time by measuring the change in voltage on the test specimen. Calculate the fatigue crack propagation rate based on the relationship between voltage and crack length. Continue the fatigue test until the crack propagation rate reaches 10... -7 The experiment ends when there are enough logarithmic data (mm / time).

[0082] Step 34: Select at least 5 evenly distributed values ​​at 10 -6 ~10 -7 For the data pairs of da / dN-ΔK between mm / time, with lg(da / dN) as the independent variable and lg(ΔK) as the dependent variable, a linear regression method was used to fit the data points. The crack propagation rate was calculated to be 10 from the fitting results. -7 The ΔK value is measured in mm / cycle; this value is the fatigue crack propagation threshold ΔK. th ,like Figure 6 As shown.

[0083] The potential measurement system used is a DCPD test device manufactured by MTS, which has current output and voltage acquisition functions, a maximum DC voltage of 5V, and a maximum amplifier gain of 5000 times.

[0084] In the potential measurement system, the test specimen and a reference specimen of the same geometry and material are connected in series in the DCPD measurement circuit to compensate for the influence of ambient temperature fluctuations on the overall resistance. This uses the reference specimen connected in series with the current circuit to eliminate current fluctuations. Before the crack monitoring officially begins, the DCPD system automatically performs a current direction switch under program control, and by calculating the average of two measurements, the steady-state thermoelectric potential in the circuit is effectively eliminated, significantly improving the measurement accuracy under low voltage signals. During actual testing, the current value in the DCPD measurement circuit needs to be adjusted to maintain the voltage value in the test specimen at 1±0.5V. This voltage value ensures the stability of the crack detection value during the test.

[0085] During the test in step 3, the load ratio R should be greater than or equal to 0.1, and the stress intensity factor normalization gradient (i.e., K decay rate) should be set to -0.08 mm. - ¹, In environments above 1000℃, the normalized gradient value should not exceed -0.08 mm. - ¹, This value is the minimum value specified in the standard; the test waveform is a sine wave with a frequency of 0-30Hz.

[0086] In the method of this invention, the high-temperature alloy fatigue crack propagation threshold test method has a detection temperature range of 1000℃-1200℃.

[0087] Example 1: The following example uses a test specimen made of high-temperature alloy with grade GH5188 to measure the fatigue crack propagation threshold value of high-temperature alloy materials at 1050℃.

[0088] (1) Preparation of specimens for fatigue crack propagation threshold value

[0089] 1) Determine the sample size, using a 5mm C(T) sample, as shown in the image. Figure 1 As shown.

[0090] 2) Retrieve the machining program corresponding to the sample and machining method from the machining center. Use a face milling cutter, a solid carbide end mill, and a drill bit to perform milling. The machining area includes one surface, two bearing holes, and four sides. Leave a finishing allowance of at least 0.25 mm on each machined surface. Remove the sample and mark both sides of the sample opening. Simultaneously, mark the machined surface as the reference surface. Remove sharp corners and burrs using a scraper, fine-tooth file, or fine sandpaper. After completion, machine the other surface. Clamp the sample on both sides using precision vises, ensuring the opening direction is perpendicular to the clamping surface. Use a face milling cutter to mill the last large surface, leaving a finishing allowance of at least 0.25 mm on the machined surface.

[0091] 3) Then take the sample to a wire EDM machine (medium wire) for cutting and opening.

[0092] 4) After cutting, transfer the sample to a surface grinder, place the reference surface on the worktable, and grind the other surface. The grinding amount is approximately 0.1 mm, depending on whether it is flat (the grinding direction should be perpendicular to the opening direction). After grinding, rotate the sample 180° and use the ground surface as a reference to grind the other side (the grinding direction must be perpendicular to the opening direction). Grind the other four sides until they meet the requirements of the drawing.

[0093] 5) Remove sharp corners and burrs from the outer contour using a scraper or fine sandpaper. Wipe the sample surface with lint-free wiping paper or a soft cotton cloth in the presence of anhydrous ethanol.

[0094] 6) Measure the sample thickness B, width W, and initial crack length a0.

[0095] (2) Install test specimens

[0096] 1) Adjust the upper and lower ceramic U-shaped clamps to center them to minimize the eccentricity of the loading system. The centerline deviation of the upper and lower load bars should be less than ±0.8mm. The sample should be located in the center of the ceramic U-shaped clamp, with a deviation not exceeding ±0.8mm. The dimensions of the ceramic U-shaped clamp are as follows: Figure 2 As shown.

[0097] 2) Wrap the wire around a wire mounting bolt made of the test material. Connect the wire to the sample using the prepared screws, ceramic washers, and mica sheets, in the following order: screws → wire wrapping → ceramic washers → sample. The purpose of using ceramic washers is to prevent direct contact between the wire and the sample, which could lead to inaccurate connection. The test sample and the reference sample are connected in series, and the current line is connected to the junction box.

[0098] 3) Turn on the DCPD dedicated station, select displacement control as the control mode, check the force, displacement and voltage protection, and turn on the current source switch.

[0099] 4) Install the sample on the U-shaped clamp and connect it with pins. After installing the sample, adjust the control mode to force control, input the initial force value, and change the current value so that the voltage value is about 1V and the current value does not exceed 25A. After applying the current, if the wires heat up, reduce the current value appropriately. Turn on the oscilloscope and observe the voltage waveform noise displayed on the oscilloscope. In order to eliminate the thermocouple effect, the direction of the primary current should be changed after powering on.

[0100] 5) Attach the thermocouples to the cracks on both sides of the sample, close the high-temperature furnace, and set the test temperature to begin heating. After the high-temperature test reaches the test temperature, maintain the temperature and observe the waveform on the oscilloscope. The test can only begin after the voltage value can be maintained stably for at least 10 minutes.

[0101] (3) Pre-existing cracks

[0102] 1) Open the template, select the DC potential method for measuring crack propagation rate test template, input the specimen size information, and select the calculation method corresponding to the position of the specimen connecting wire (Johnson formula is recommended).

[0103] 2) Input the length of the pre-crack, the pre-crack load ratio (usually 0.1), the number of pre-crack cycles, and the percentage of the measured loading level (usually 40%, maximum 50%). The test frequency range is 10~30Hz.

[0104] 3) Begin the pre-fatigue crack test.

[0105] (4) Fatigue crack propagation threshold test

[0106] 1) After the pre-crack is completed, select an appropriate initial test load based on the pre-crack parameters to ensure that the initial crack propagation rate is 10. -4 ~10 -5 mm / time, using the K-reduction method to test the crack propagation threshold value;

[0107] 2) Use a sine wave with a frequency of 20Hz. Select K control mode, with the initial maximum K value slightly larger than the final K value of the pre-crack, a load ratio of 0.1, a K normalization gradient of 0.08, and a measured load level percentage of 40%.

[0108] 3) When the crack propagation rate reaches 10 -7 The experiment ends when there are enough logarithmic data (mm / time).

[0109] (5) Results processing

[0110] 1) After the test, a crack inspection should be performed and recorded. The crack length and propagation angle on both the front and back surfaces of the specimen should be checked to ensure that the symmetry of the crack meets the relevant standard requirements. At any point during the test, the crack should deviate from the plane of symmetry by no more than ±20° at a distance of 0.1W or greater, and the difference in crack size between the front and back surfaces should not exceed 0.25B.

[0111] 2) Save the test data, export the corrected original test data, exit the test software, shut down the host, and then shut down the computer.

[0112] 3) Calculate the crack propagation threshold: Select at least 5 values ​​that are evenly distributed at 10 -6 ~10 -7 For the data pairs da / dN-ΔK between mm / times, with lg(da / dN) as the independent variable and lg(ΔK) as the dependent variable, a linear regression method was used to fit the data points, see [link to documentation]. Figure 4 The crack propagation rate relationship in the near-gate region was obtained, and the crack propagation rate was calculated to be 10 from the fitting results. -7The ΔK value is measured in mm / cycle; this value is the fatigue crack propagation threshold ΔK. th .

[0113] This invention relates to a device and method for testing the fatigue crack propagation threshold of high-temperature alloys based on the DC potential method. First, the material is processed to a specific size. Then, using a specific high-temperature conductor, the sample is energized at temperatures above 1000°C to accurately measure the crack length. A ceramic U-shaped clamp is used to connect the sample, and a low-cycle fatigue testing machine is used for loading. The da / dN-ΔK data is then obtained, and the fatigue crack propagation threshold ΔK is calculated. th .

Claims

1. A method for determining the crack propagation rate and threshold value at extremely high temperatures, characterized in that, include: Step 1: Fabrication of test specimens and fixtures: Prepare high-temperature alloy materials into test specimens and reference specimens for crack propagation rate measurement, and fabricate cooling connecting rods and ceramic U-shaped fixtures. Step 2: Install the high-temperature furnace, install the test sample and the thermocouple, and then use high-temperature resistant wires to connect the test sample, the reference sample and the potential measurement system, wherein the reference sample is placed outside the high-temperature furnace; Step 3: Start the test: The high-temperature furnace is raised to 1000-1200℃ and held at that temperature, while water cooling is gradually introduced into the cooling connecting rod; the low-cycle fatigue testing machine is loaded and the crack in the specimen is tested through the potential measurement system; the K-method is used for the test, and the potential measurement system monitors the crack propagation in real time through the DC potential method. When the crack propagation rate reaches 1×10⁻⁶, the test is completed. -7 The test was terminated when the fatigue crack propagation threshold value ΔK was obtained through data processing. th .

2. The method for determining the crack propagation rate and threshold value at extremely high temperatures according to claim 1, characterized in that, The thickness of the test specimen and the reference specimen is 5 mm.

3. The method for determining the crack propagation rate and threshold value at extremely high temperatures according to claim 2, characterized in that, The test specimen and reference specimen include: a main body and two tensile parts, the two tensile parts being symmetrically arranged vertically and integrally fixed to the same side of the main body, the two tensile parts being separated by a tensile opening, the innermost side of the tensile opening being a 30-degree angled part. Each of the upper and lower stretching sections has a stretching section mounting hole for connection to its respective ceramic U-shaped clamp pin; each stretching section has a stretching section wire connection hole on the side of the same side as the stretching opening, and a set of main body wire connection holes are symmetrically arranged on the upper and lower end faces of the main body; the stretching section wire connection holes are connected to the voltage interface, and the main body wire connection holes are connected to the current interface.

4. The method for determining the crack propagation rate and threshold value at extremely high temperatures according to claim 3, characterized in that, The high-temperature furnace is a split-type high-temperature furnace, which is divided into at least two independent heating resistance wire groups. Each heating resistance wire group is set separately along the plane or plane group where the load direction of the low-cycle fatigue testing machine is located. Each heating resistance wire group adopts an independent temperature control system to achieve effective isolation of the magnetic field in the furnace cavity.

5. The method for determining the crack propagation rate and threshold value at extremely high temperatures according to claim 3, characterized in that, In step 2, the process of installing the test specimen is as follows: Step 21: Install the cooling connecting rods and ceramic U-shaped clamps: First, install the two cooling connecting rods 3 onto the threaded holes of the upper and lower actuator cylinders of the low-cycle fatigue testing machine through the threaded screws. Then, install the ceramic U-shaped clamps onto the two cooling connecting rods 3 through the threaded holes. Then connect the water cooling inlet and water cooling outlet. The cooling connecting rods are equipped with cooling channels to prevent the ceramic U-shaped clamps from cracking. Step 22: Install the test specimen: Insert the test specimen into the thin groove of the ceramic U-shaped clamp and connect the test specimen and the ceramic U-shaped clamp with a pin.

6. The method for determining the crack propagation rate and threshold value at extremely high temperatures according to claim 5, characterized in that, The ceramic U-shaped fixture is custom-made from alumina ceramic, and the fit accuracy between the ceramic U-shaped groove and the test sample pin hole reaches the H7 / g6 level; the coaxiality deviation of the entire fixture system is within ±0.8mm.

7. The method for determining the crack propagation rate and threshold value at extremely high temperatures according to claim 3, characterized in that, In step 2, the process of connecting the test sample, the reference sample, and the potential measurement system using high-temperature resistant wires is as follows: The first current interface of the potential measurement system is sequentially connected to a wire connection hole in one main body of the reference sample, a wire connection hole in another main body of the reference sample, a wire connection hole in one main body of the test sample, a wire connection hole in another main body of the test sample, and the second current interface of the potential measurement system. One tensile wire connection hole of the test specimen is connected to the first interface of the first group of voltages, and the other tensile wire connection hole of the test specimen is connected to the second interface of the first group of voltages; one tensile wire connection hole of the reference specimen is connected to the first interface of the second group of voltages, and the other tensile wire connection hole of the reference specimen is connected to the second interface of the second group of voltages.

8. A method for determining the crack propagation rate and threshold value at extremely high temperatures according to claim 3 or 7, characterized in that, The high-temperature resistant wire is a platinum-rhodium alloy wire; the high-temperature resistant wire is installed using wire mounting bolts, and the high-temperature resistant wire wrapped around the wire mounting bolt is fixed between the wire mounting bolt and the alumina ceramic gasket. The potential measurement system is a DCPD test device manufactured by MTS; During the test in step 3, the load ratio R should be greater than or equal to 0.1, and the normalized gradient of the stress intensity factor should be set to -0.08 mm. - ¹, In environments above 1000℃, the normalized gradient value is no greater than -0.08 mm. - ¹; The test waveform used was a sine wave with a frequency of 0-30Hz.

9. A method for determining the crack propagation rate and threshold value at extremely high temperatures according to claim 1 or 3, characterized in that, Step 3 includes the following steps: Step 31: According to the test settings, move the high-temperature furnace to the working position via the slide rail; set the test temperature, heat the high-temperature furnace to the target temperature, and keep it at that temperature for a period of time to allow the test sample to reach a stable test temperature; at the same time, water cooling is introduced into the cooling connecting rod. Step 32: Apply axial fatigue load to the test specimen on the low-cycle fatigue testing machine. The load range is determined according to the test requirements, and the test frequency is controlled between 0-30Hz. Step 33: Using a potential measurement system, monitor the change in crack length in real time by measuring the change in voltage on the test specimen. Calculate the fatigue crack propagation rate based on the relationship between voltage and crack length. Continue the fatigue test until the crack propagation rate reaches 10... -7 The experiment ends when there are enough logarithmic data (mm / time). Step 34: Select at least 5 evenly distributed values ​​at 10 -6 ~10 -7 For the data pairs of da / dN-ΔK between mm / times, with da / dN as the independent variable and ΔK as the dependent variable, a linear regression method was used to fit the data points. The crack propagation rate was calculated as 10 from the fitting results. - 7 The ΔK value is measured in mm / cycle; this value is the fatigue crack propagation threshold ΔK. th .

10. The method for determining the crack propagation rate and threshold value at extremely high temperatures according to claim 3, characterized in that, The preparation of high-temperature alloy materials into test specimens and reference specimens for crack propagation rate measurement includes: Step 11: Rough machining of the outer contour; Step 12: Machining the stretching opening and wire connection hole, and finely machining the outer contour; Step 13: Tap the wire connection hole; Step 14: Remove burrs from the outer contour and clean the surface with a lint-free cloth and anhydrous ethanol.

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