High-temperature random ratchet wheel test clamp and method for small sample

Abstract

The invention discloses a high-temperature random ratchet wheel test clamp and method for a small sample, and relates to the technical field of material mechanical property measurement. The fatigue machine comprises a fatigue machine main body; one end of the upper heat exchange head is connected with an upper connecting rod of the fatigue machine main body, the other end of the upper heat exchange head is connected with an upper clamp head through threads, and the upper clamp head is used for fixing one end of a flat sample; one end of the lower heat exchange head is connected with a lower connecting rod of the fatigue machine main body, the other end of the lower heat exchange head is in threaded connection with a lower clamp head, and the lower clamp head is used for performing predetermined load excitation loading on the flat plate sample; and the high-temperature environment chamber is fixed on the stand column of the fatigue machine main body. According to the invention, under the condition of uniaxial loading of the structure, a high-temperature environment and a stress oscillation excitation effect can simultaneously appear in a test area; the clamp disclosed by the invention can be used for clamping a micro flat plate sample with the thickness range of 3.5-5mm, and is beneficial to saving sample materials; the clamp is simple in structure and easy to operate, and the convenience of sample replacement can be improved.

Description

Technical Field

[0001] The invention relates to the technical field of material mechanical property measurement, in particular to a high-temperature random ratchet test fixture and method for a small sample. Background Art

[0002] The fourth-generation sodium-cooled fast reactor nuclear power plant has become one of the fourth-generation nuclear energy systems with the broadest development prospects and commercial uses, relying on its high utilization rate of uranium resources and its technical advantages in processing long-lived actinide nuclides. During the operation of the sodium-cooled fast reactor nuclear power plant, due to the uneven mixing of the coolant and hot agent near the central column, a thermal oscillation environment is inevitably formed on the surface of the core cover. The increase in temperature will cause changes in the material properties of the structure and thermal deformation. The uneven temperature difference on the outer surface of the structure will cause random thermal stress, which will then lead to changes in the shape, strength and stiffness of the structure, directly affecting the dynamic strength of the structure. Therefore, the key structural components of nuclear power not only suffer from high temperature shock during service, but also thermal transients and coolant stratification will produce typical mechanical reactions. Under the random ratchet interaction of high temperature and non-zero average stress, the failure mechanism of the material is still unclear.

[0003] At present, it is known that key nuclear power structural components will be affected by thermal oscillations, including complex structures with prominent thin-wall effects such as T-tubes, elbows, valves and core covers. The main research focus of thermal oscillation phenomena is on numerical simulation and proportional water jet tests, and the life and deformation mechanism of nuclear power plants under service conditions are not very clear. However, the existing thermal engine fatigue test method of loading a horizontal stress curve on a standard specimen cannot simulate the complex stress problem of high-frequency fluctuations under a thermal oscillation environment. It is urgent to design a test method to study the ratchet problem of non-zero average random stress of materials at high temperatures, and to invent a tiny sample fixture with random stress clamping conditions, which has the ability to clamp for a long time at high temperatures. Based on this, the present invention proposes a high-temperature random ratchet test fixture and method for small specimens, which is of great significance for solving problems related to fatigue induced by thermal oscillations.

[0004] The above information disclosed in this background technology is only used to increase the understanding of the background technology of the present invention and therefore, it may include information that does not constitute the prior art known to ordinary technicians in this field. Summary of the invention

[0005] The purpose of the present invention is to address the technical defects involved in the background technology and provide a high-temperature random ratchet test fixture and method for small specimens, which simulates the fluctuating stress response of the material under thermal oscillation excitation conditions from the perspective of uniaxial testing. The test conditions can simultaneously simulate the performance of the material under temperature environment and internal excitation loads.

[0006] In order to solve the problem of fatigue induced by thermal oscillation, a random stress field of the structure is obtained based on the Fluent fluid heat transfer simulation and the thermal-solid coupling model. The present invention provides a high-temperature random ratchet test fixture for a small sample, comprising:

[0007] Fatigue machine body;

[0008] An upper heat exchange head, one end of which is connected to an upper connecting rod of the fatigue machine body, and the other end of which is connected to an upper fixture head through a thread, and the upper fixture head is used to fix one end of the flat plate specimen;

[0009] A lower heat exchange head, one end of which is connected to a lower connecting rod of the fatigue machine body, and the other end of which is connected to a lower fixture head through a thread, and the lower fixture head is used to perform a predetermined load excitation loading on the flat plate specimen;

[0010] High temperature environment chamber, which is fixed on the column of the fatigue machine body and is used to heat the flat sample. The high temperature environment chamber is provided with openings adapted to the upper heat exchange head and the lower heat exchange head. The water cooling system cools down the heat exchange head and the fatigue machine connecting rod sensor through the circulation system. The coolant is used to keep the upper heat exchange head at a constant temperature through the water inlet 1 and the water outlet 1, and the lower heat exchange head at a constant temperature through the water inlet 2 and the water outlet 2.

[0011] Water cooling system: The water cooling system cools down the heat exchange head and fatigue machine connecting rod sensor through a circulation system. The coolant is used to keep the upper heat exchange head at a constant temperature through water inlet 1 and water outlet 1, and to keep the lower heat exchange head at a constant temperature through water inlet 2 and water outlet 2;

[0012] Video Extensometer, Video Extensometer DIC device measures the strain of the straight section of the flat specimen through the observation port of the high temperature environment chamber.

[0013] Preferably, one end of the upper clamp head is provided with a threaded extension rod connected to the upper heat exchange head, and the other end of the upper clamp head has a semi-arc corner for reducing stress concentration, and a sample groove is opened inside one end of the semi-arc corner of the upper clamp head, and the sample groove placement surface is a plane.

[0014] Preferably, a baffle covering the sample slot is installed on the upper clamp head, and a raised knurling is provided on the inner side of the baffle to match the sample slot to increase the friction between the baffle and the flat sample when clamping, and the thickness of the knurling is smaller than the depth of the sample slot.

[0015] Preferably, the baffle is symmetrically provided with through hole one and through hole two, and the baffle is mounted on the upper fixture head by countersunk bolts matched with through hole one and through hole two.

[0016] Preferably, the upper fixture head is symmetrically provided with threaded through holes connected to the sample slots, the threaded through holes are V-shaped on the upper fixture head, and push rod bolts are connected to the threaded through holes. A gasket is provided on the top of the sample slot, and the gasket is supported by two groups of push rod bolts, and the clamping force is converted from two-point contact to surface contact.

[0017] Preferably, a fastening ring is installed on the threaded extension rod, and both the upper clamp head and the fastening ring are provided with a wrench.

[0018] The present invention also provides a test method of a high-temperature random ratchet test fixture for a small sample, comprising the following steps:

[0019] Step 1: Establish a finite element model of the stress field of thermal-solid coupling;

[0020] Step 1.1: Calculate the thermal oscillation temperature field on the surface of the structure

[0021] According to the actual working environment, the coaxial jet model is selected. On the other hand, considering that the core coolant of the fourth-generation sodium-cooled fast reactor is liquid sodium, the fluid material parameters are set to liquid sodium. In the Fluent heat conduction analysis process, the core cover plate components affected by the thermal oscillation effect are simplified to a simple circular cover plate, and the transient temperature field of the surface is calculated by large eddy simulation. The control equations of fluid fluctuation simulation mainly include eddy viscosity, analytical scale and gradient diffusion;

[0022] Eddy viscosity:

[0023] Analysis scale:

[0024] Gradient Diffusion:

[0025] Step 1.2: Determine the thermal-solid coupling heat transfer parameters, including the distance between the coaxial jet starting point and the simplified plate, the jet velocity, and the thickness of the plate.

[0026] After the temperature field calculation is performed, the temperature field calculation results are imported into the structural model as body loads. The maximum shear strain and the maximum normal strain that cause damage to the material are perpendicular to each other. In order to reduce the diversity of the test, the work of the thermo-solid coupling stress field mainly converts the simulated multi-axial stress field into uniaxial stress through Hooke's law using the existing uniaxial research results. The thermo-solid coupling stress field mainly considers the heat transfer coefficient, thermal expansion coefficient, and specific heat capacity of the material.

[0027] Step 2: Determine the power spectral density function of the random load and check whether the spectrum PSD conforms to the thermal oscillation characteristics;

[0028] Step 3: Export the load, eliminate the stress noise, and use the autoregressive model to check the load stability;

[0029] Step 3.1: Load spectrum optimization

[0030] In the Ncode module of ANSYS software, the Butterworth filter is used to optimize the acquired load spectrum, including preprocessing the abnormal phenomena such as burrs and low-frequency drift of the calculated stress spectrum signal; the conversion formula for converting the acquired stress spectrum signal from three-dimensional to one-dimensional signal is:

[0031]

[0032] Step 3.2: According to the definition (X i -X i-1 )(X i -X i+1 )>0, the peak-valley value extraction is realized by programming in MATLAB. In the generated load spectrum, some load peaks are too small to be effective for damage, so it is necessary to remove redundant signals; in order to facilitate the loading of the fatigue machine and improve the cycle efficiency, the non-peak-valley value points in the data are removed; the stress field of thermal oscillation is periodic, and its periodicity is to select the fluctuating 6s as the periodic load. In order to adapt to the cyclic load of the fatigue machine, the extracted peak-valley load is subjected to trajectory interpolation calculation to obtain the linear load spectrum for fatigue test;

[0033] Step 3.3: Use the AR process in the autoregressive model to check the stability of the load spectrum. The calculation formula is:

[0034] X t =αX t-1 +ε t , t∈Z, {ε t}~WN(0,σ 2 )

[0035] In the formula, when |α|>1, the sequence is in an unstable state; when |α|=1, the sequence oscillation becomes larger and larger, showing an explosive type; when |α|<1, the time series can tend to a stable state and can be used for fatigue cycle loading;

[0036] Step 4: Fixture and specimen loading and specimen heating;

[0037] Step 4.1: Place a section of the polished flat specimen in the specimen slot of the upper fixture head, press the knurled side of the baffle (6f) into the specimen slot, and then screw the countersunk bolts into the corresponding through holes 1 and 2 respectively, using the friction of the knurling to limit the up and down movement of the flat specimen and prevent the flat specimen from falling off during the test;

[0038] Step 4.2: Then screw the two push rod bolts through the threaded through holes. Here, ensure that the push rod bolts are in a tightened state so that the flat specimen will not be affected by sliding during the unloading process;

[0039] Step 4.3: Insert the fastening ring into the threaded extension rod, and use two wrenches to clamp the wrenches respectively, and further lock the V-shaped push rod bolts to form a self-locking function to prevent the push rod bolts from loosening;

[0040] Step 4.4: After the upper fixture head and the flat specimen are installed, debug and install the fatigue test system, connect the upper heat exchange head to the upper connecting rod of the fatigue machine body, connect the lower heat exchange head to the lower connecting rod of the fatigue machine body, turn on the water cooling system, and connect the water cooling pipeline to the heat exchange head accordingly to ensure that the heat dissipation system starts working;

[0041] Step 4.5: Install the assembled upper fixture head on the upper connecting rod through threaded connection, with the front of the upper fixture head facing perpendicular to the paper surface;

[0042] Step 4.6: Install the threaded extension rod of the lower fixture head on the lower connecting rod first, complete the centering and debugging of the upper and lower fixtures, and use the fatigue machine main control system to fine-tune the lower connecting rod to slowly rise until the specimen slot of the lower fixture head is at the same height as the flat specimen, and place the flat specimen in the specimen slot. The installation steps of the lower fixture head are the same as those of the upper fixture head. At this time, the installation of the fixture and the specimen is completed;

[0043] Step 4.7: Raise the temperature of the high temperature environment chamber to 520°C according to the test requirements and maintain it at this temperature for 30 minutes;

[0044] Step 4.8: While waiting for the temperature to rise, further install the video extensometer. An observation port is reserved in front of the high temperature environment chamber. The observation port will not be affected during the high temperature test, so it is convenient to place the video extensometer to measure the strain of the specimen.

[0045] Step 5: Input test parameters: input stress amplitude coefficient, data loading frequency, stress amplitude, number of cycles and filter corner frequency into the system before loading, and implement fatigue loading after the insulation is completed;

[0046] Step 6: Calculate the ratchet strain at high temperature based on the maximum and minimum strains. The calculation formula is:

[0047]

[0048] Compared with the prior art, the present invention has the following beneficial effects:

[0049] The present invention enables the structure to be subjected to uniaxial loading, and the test area can simultaneously experience the effects of a high temperature environment and stress oscillation excitation; the fixture of the present invention can clamp a micro-flat sample with a thickness range of 3.5-5 mm, which is beneficial to saving sample materials; the fixture of the present invention has a simple structure and is easy to operate, and can improve the convenience of replacing samples.

[0050] The above summary is for illustrative purposes only and is not intended to be limiting in any way. In addition to the illustrative aspects, embodiments and features described above, further aspects, embodiments and features of the present invention will be readily apparent by reference to the accompanying drawings and the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 It is a schematic diagram of the main structure of the high temperature random ratchet test device of the present invention.

[0052] Figure 2 (a)-(d) are schematic diagrams of the structure of the clamp head of the present invention; wherein Figure 2 (a) Main view with a cross-sectional view of the sample tank; Figure 2 (b) is a side view; Figure 2 (c) is a cross-sectional view; Figure 2 (d) is an oblique view with a partial cross-sectional view of the ejector bolt.

[0053] Figure 3 (a)-(b) are schematic diagrams of the structure of the baffle of the present invention; Figure 3 (a) is the front view of the baffle; Figure 3 (b) is an oblique view.

[0054] Figure 4 It is the structural schematic diagram of the flat plate specimen;

[0055] Figure 5 This is the assembly drawing of the fixture after the upper and lower assemblies are completed and the schematic diagram of the water cooling system;

[0056] Figure 6 The figure is a simplified flow chart of the test method of the present invention.

[0057] In the figure:

[0058] 1. Fatigue machine body;

[0059] 2. High temperature environment chamber; 2a. Opening; 2b. Observation port;

[0060] 3. Upper connecting rod;

[0061] 4. Lower fixture head;

[0062] 5. Upper heat exchange head; 5a. Water inlet 1; 5b. Water outlet 1;

[0063] 6, upper fixture head; 6a, fastening ring; 6b, push rod bolt; 6c, spanner; 6d, gasket; 6e, specimen slot; 6f, square baffle; 6f1, through hole 1; 6f2, through hole 2; 6f3, knurling; 6g, countersunk bolt; 6h, threaded extension rod; 6i, threaded through hole;

[0064] 7. Lower heat exchange head; 7a. Water inlet 2; 7b. Water outlet 2;

[0065] 8. Flat specimen;

[0066] 9. Lower connecting rod;

[0067] 10. Video extensometer;

[0068] 11. Circulatory system;

[0069] 12. Water cooling system. DETAILED DESCRIPTION

[0070] The present invention is mainly aimed at the loading of ratchets with high-temperature random non-zero average stress suitable for flat specimens, and designs a fixture for a small flat specimen. The fixture consists of two parts, the upper and lower parts. The technical scheme in the embodiment of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiment of the present invention. Obviously, the described embodiment is only a part of the embodiment of the present invention, not all of the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0071] Embodiment 1

[0072] A high temperature random ratchet test fixture for small specimens, comprising:

[0073] Fatigue machine body 1;

[0074] An upper heat exchange head 5, one end of which is connected to an upper connecting rod 3 of a fatigue machine body 1, and the other end of which is connected to an upper clamp head 6 by a thread, and the upper clamp head 6 is used to fix one end of a flat sample 8;

[0075] A lower heat exchange head 7, one end of which is connected to a lower connecting rod 9 of the fatigue machine body 1, and the other end of which is connected to a lower fixture head 4 through a thread, and the lower fixture head 4 is used to perform a predetermined load excitation loading on the flat plate sample 8;

[0076] A high temperature environment chamber 2 is fixed on the column of the fatigue machine body 1 and is used to heat the flat sample 8. The high temperature environment chamber 2 is provided with an opening 2a adapted to the upper heat exchange head 5 and the lower heat exchange head 7;

[0077] The water cooling system 12 cools down the heat exchange head and the fatigue machine connecting rod sensor through the circulation system 11. The coolant keeps the upper heat exchange head 5 at a constant temperature through the water inlet 1 5a and the water outlet 1 5b, and keeps the lower heat exchange head 7 at a constant temperature through the water inlet 2 7a and the water outlet 2 7b;

[0078] The video extensometer 10 , the video extensometer 10 DIC device measures the strain of the straight segment of the flat sample 8 through the observation port 2 b of the high temperature environment chamber 2 .

[0079] One end of the upper clamp head 6 is provided with a threaded extension rod 6h connected to the upper heat exchange head 5, and the other end of the upper clamp head 6 has a semi-arc corner for reducing stress concentration, and a sample groove 6e is opened inside one end of the semi-arc corner of the upper clamp head 6, and the placement surface of the sample groove 6e is a plane.

[0080] A baffle 6f covering the sample groove 6e is installed on the upper clamp head 6, and a raised knurling 6f3 is provided on the inner side of the baffle 6f to match the sample groove 6e, which is used to increase the friction force when the baffle 6f and the flat sample 8 are clamped. The thickness of the knurling 6f3 is smaller than the depth of the sample groove 6e.

[0081] The baffle plate 6f is symmetrically provided with a through hole 1 6f1 and a through hole 1 6f2. The baffle plate 6f is mounted on the upper fixture head 6 via countersunk bolts 6g matched with the through hole 1 6f1 and the through hole 2 6f2.

[0082] The upper clamp head 6 is symmetrically provided with threaded through holes 6i connected to the sample slot 6e. The threaded through holes 6i are V-shaped on the upper clamp head 6. A push rod bolt 6b is connected in the threaded through holes 6i. A gasket 6d is provided on the top of the sample slot 6e. The gasket 6d is supported by two groups of push rod bolts 6b, and the clamping force is converted from two-point contact to surface contact.

[0083] A fastening ring 6a is installed on the threaded extension rod 6h, and a wrench 6c is provided on the upper clamp head 6 and the fastening ring 6a.

[0084] Embodiment 2

[0085] A test method for a high temperature random ratchet test fixture for a small specimen comprises the following steps:

[0086] Step 1: Establish a finite element model of the stress field of thermal-solid coupling;

[0087] Step 1.1: Calculate the thermal oscillation temperature field on the surface of the structure

[0088] According to the actual working environment, the coaxial jet model is selected. On the other hand, considering that the core coolant of the fourth-generation sodium-cooled fast reactor is liquid sodium, the fluid material parameters are set to liquid sodium. In the Fluent heat conduction analysis process, the core cover plate components affected by the thermal oscillation effect are simplified to a simple circular cover plate, and the transient temperature field of the surface is calculated by large eddy simulation. The control equations of fluid fluctuation simulation mainly include eddy viscosity, analytical scale and gradient diffusion;

[0089] Eddy viscosity:

[0090] Analysis scale:

[0091] Gradient Diffusion:

[0092] Step 1.2: Determine the thermal-solid coupling heat transfer parameters, including the distance between the coaxial jet starting point and the simplified plate, the jet velocity, and the thickness of the plate.

[0093] After the temperature field calculation is performed, the temperature field calculation results are imported into the structural model as body loads. The maximum shear strain and the maximum normal strain that cause damage to the material are perpendicular to each other. In order to reduce the diversity of the test, the work of the thermo-solid coupling stress field mainly converts the simulated multi-axial stress field into uniaxial stress through Hooke's law using the existing uniaxial research results. The thermo-solid coupling stress field mainly considers the heat transfer coefficient, thermal expansion coefficient, and specific heat capacity of the material.

[0094] Step 2: Determine the power spectral density function of the random load and check whether the spectrum PSD conforms to the thermal oscillation characteristics;

[0095] Step 3: Export the load, eliminate the stress noise, and use the autoregressive model to check the load stability;

[0096] Step 3.1: Load spectrum optimization

[0097] In the Ncode module of ANSYS software, the Butterworth filter is used to optimize the acquired load spectrum, including preprocessing the abnormal phenomena such as burrs and low-frequency drift of the calculated stress spectrum signal; the conversion formula for converting the acquired stress spectrum signal from three-dimensional to one-dimensional signal is:

[0098]

[0099] Step 3.2: According to the definition (X i -X i-1 )(X i -X i+1)>0, the peak-valley value extraction is realized by programming in MATLAB. In the generated load spectrum, some load peaks are too small to be effective for damage, so it is necessary to remove redundant signals; in order to facilitate the loading of the fatigue machine and improve the cycle efficiency, the non-peak-valley value points in the data are removed; the stress field of thermal oscillation is periodic, and its periodicity is to select the fluctuating 6s as the periodic load. In order to adapt to the cyclic load of the fatigue machine, the extracted peak-valley load is subjected to trajectory interpolation calculation to obtain the linear load spectrum for fatigue test;

[0100] Step 3.3: Use the AR1 process in the autoregressive model to check the stationarity of the load spectrum. The calculation formula is:

[0101] X t =aX t-1 +ε t , t∈Z,[ε t}~WN(0,σ 2 )

[0102] In the formula, when |α|>1, the sequence is in an unstable state; when |α|=1, the sequence oscillation becomes larger and larger, showing an explosive type; when |α|<1, the time series can tend to a stable state and can be used for fatigue cycle loading;

[0103] Step 4: Fixture and specimen loading and specimen heating;

[0104] Step 4.1: Place a section of the polished flat sample 8 in the sample slot 6e of the upper fixture head 6. The flat sample 8 is processed into a shape such as Figure 4 As shown, the baffle 6f with the knurling 6f3 is pressed into the sample groove 6e, and then the countersunk bolts 6g are screwed into the corresponding through hole 1 6f1 and through hole 2 6f2 respectively, and the friction force of the knurling 6f3 is used to limit the up and down movement of the flat sample 8, and prevent the flat sample 8 from falling off during the test;

[0105] Step 4.2: Then screw two specially made push rod bolts 6b through the threaded through holes 6i, as shown in FIG. Figure 2 As shown in (d), the top rod bolt 6b is ensured to be in a tightened state, and the flat sample 8 will not be affected by sliding during the unloading process;

[0106] Step 4.3: Insert the fastening ring 6a into the threaded extension rod 6h, as shown in Figure 2 As shown in a, two wrenches are used to clamp the wrench opening 6c respectively, and the V-shaped mounted top rod bolt 6b is further locked. Since the two top rod bolts 6b are in different directions, self-locking can be formed, thereby preventing the top rod bolt 6b from loosening;

[0107] Step 4.4: After the upper fixture head 6 and the flat sample 8 are installed, the fatigue test system is debugged and installed. The fatigue machine body 1 uses a 10-ton mechanical fatigue testing machine, and the hydraulic chuck is disassembled for installing the fixture. The present invention does not limit the model of the fatigue machine body 1;

[0108] Before installation, raise the crossbeam of the fatigue machine body 1 to reserve enough space for high temperature environment and fixture installation. Figure 1 As shown, the high temperature environment chamber 2 is fixed to the column of the fatigue machine body 1 by a bracket to ensure that the high temperature environment chamber 2 is fixed during the test. The present invention does not limit the model and size of the high temperature environment chamber 2, and it can reach the high temperature condition of 500-550°C required for the test. At the same time, since the fixture needs to pass through the high temperature environment chamber 2, it is necessary to reserve openings 2a with a diameter of 50mm above and below the high temperature environment chamber 2, and the high temperature environment chamber 2 has good temperature insulation and temperature control accuracy.

[0109] Connect the large diameter end of the upper heat exchange head 5 to the upper connecting rod 3 of the fatigue machine body 1 through threads, and similarly connect the lower heat exchange head 7 to the lower connecting rod 9 of the fatigue machine body 1 through threads, turn on the water cooling system, and connect the water cooling pipeline to the heat exchange head accordingly to ensure that the heat dissipation system starts working;

[0110] Step 4.5: Install the assembled upper clamp head 6 on the upper connecting rod through threaded connection, with the front side of the upper clamp head 6 facing perpendicular to the paper surface;

[0111] Step 4.6: The structure of the lower fixture head 4 is similar to that of the upper fixture head 6. The threaded extension rod 6h of the lower fixture head 4 is first installed on the lower connecting rod 9 to complete the centering and debugging of the upper and lower fixtures. The lower connecting rod 9 is fine-tuned to rise slowly through the fatigue machine body 1 control system until the sample slot 6e of the lower fixture head 4 is at the same height as the flat sample 8. The flat sample 8 is placed in the sample slot 6e. The installation steps of the lower fixture head 4 are the same as those of the upper fixture head 6. At this time, the installation of the fixture and the sample is completed; note that when installing the upper fixture head 6 and the lower fixture head 4, the fastening ring 6a is pre-installed on the threaded extension rod 6h first, and it is not tightened to leave a certain space. After the top rod bolt 6b is installed, it is tightened.

[0112] Step 4.7: Raise the temperature of high temperature environment chamber 2 to 520°C according to the test requirements and maintain it at this temperature for 30 minutes;

[0113] Step 4.8: While waiting for the temperature to rise, a video extensometer 10 is further installed. An observation port 2b is reserved in front of the high temperature environment chamber 2. The observation port 2b will not be affected during the high temperature test, so that the video extensometer 10 can be placed to measure the strain of the sample.

[0114] Step 5: Input test parameters: input stress amplitude coefficient, data loading frequency, stress amplitude, number of cycles and filter corner frequency into the system before loading, and implement fatigue loading after the insulation is completed;

[0115] Step 6: Calculate the ratchet strain at high temperature based on the maximum and minimum strains. The calculation formula is:

[0116]

[0117] In the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0118] In the present invention, unless otherwise clearly specified and limited, a first feature being "above" or "below" a second feature may mean that the first and second features are in direct contact, or the first and second features are in indirect contact through an intermediate medium. Moreover, a first feature being "above", "above" or "above" a second feature may mean that the first feature is directly above or obliquely above the second feature, or simply means that the first feature is higher in level than the second feature. A first feature being "below", "below" or "below" a second feature may mean that the first feature is directly below or obliquely below the second feature, or simply means that the first feature is lower in level than the second feature.

[0119] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example" or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, unless they are contradictory.

[0120] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations of the present invention. A person skilled in the art may change, modify, replace and vary the above embodiments within the scope of the present invention.

Claims

1. A high temperature random ratchet test fixture for small specimens, characterized in that: include: Fatigue machine body (1); An upper heat exchange head (5), one end of which is connected to an upper connecting rod (3) of a fatigue machine body (1), and the other end of which is connected to an upper clamp head (6) via a thread, and the upper clamp head (6) is used to fix one end of a flat plate sample (8); A lower heat exchange head (7), one end of which is connected to a lower connecting rod (9) of a fatigue machine body (1), and the other end of which is connected to a lower fixture head (4) via a thread, and the lower fixture head (4) is used to perform a predetermined load excitation loading on a flat plate sample (8); A high temperature environment chamber (2), the high temperature environment chamber (2) is fixed on a column of the fatigue machine body (1), and is used to heat the flat plate sample (8). The high temperature environment chamber (2) is provided with an opening (2a) that is compatible with the upper heat exchange head (5) and the lower heat exchange head (7); A water cooling system (12), the water cooling system (12) cools down the heat exchange head and the fatigue machine connecting rod sensor through the circulation system (11), the coolant is used to maintain a constant temperature for the upper heat exchange head (5) through the water inlet 1 (5a) and the water outlet 1 (5b), and is used to maintain a constant temperature for the lower heat exchange head (7) through the water inlet 2 (7a) and the water outlet 2 (7b); A video extensometer (10), a DIC device of the video extensometer (10) measures the strain of a straight section of a flat sample (8) through an observation port (2b) of a high temperature environment chamber (2).

2. A small sample high temperature random ratchet test fixture according to claim 1, characterized in that: One end of the upper clamp head (6) is provided with a threaded extension rod (6h) connected to the upper heat exchange head (5), and the other end of the upper clamp head (6) has a semi-arc corner for reducing stress concentration, and a sample groove (6e) is opened inside one end of the semi-arc corner of the upper clamp head (6), and the placement surface of the sample groove (6e) is a plane.

3. A small sample high temperature random ratchet test fixture according to claim 2, characterized in that: The upper clamp head (6) is provided with a baffle (6f) covering the sample groove (6e), and the inner side of the baffle (6f) is provided with a raised knurling (6f3) which matches the sample groove (6e) and is used to increase the friction force when the baffle (6f) and the flat sample (8) are clamped, and the thickness of the knurling (6f3) is smaller than the depth of the sample groove (6e).

4. A small sample high temperature random ratchet test fixture according to claim 3, characterized in that: The baffle plate (6f) is symmetrically provided with a through hole one (6f1) and a through hole two (6f2). The baffle plate (6f) is mounted on the upper fixture head (6) via countersunk bolts (6g) that match the through hole one (6f1) and the through hole two (6f2).

5. A small sample high temperature random ratchet test fixture according to claim 2, characterized in that: The upper fixture head (6) is symmetrically provided with threaded through holes (6i) connected to the sample groove (6e). The threaded through holes (6i) are V-shaped on the upper fixture head (6). A push rod bolt (6b) is connected in the threaded through holes (6i). A gasket (6d) is provided on the top of the sample groove (6e). The gasket (6d) is supported by two groups of push rod bolts (6b), and the clamping force is converted from two-point contact to surface contact.

6. A small sample high temperature random ratchet test fixture according to claim 2, characterized in that: A fastening ring (6a) is installed on the threaded extension rod (6h), and a wrench (6c) is provided on the upper clamp head (6) and the fastening ring (6a).

7. A test method for a small sample high temperature random ratchet test fixture as described in any one of claims 1 to 6, characterized in that: The steps include: Step 1: Establish a finite element model of the stress field of thermal-solid coupling; Step 1.1: Calculate the thermal oscillation temperature field on the surface of the structure and determine the coaxial jet model. The control equations of fluid fluctuation simulation mainly include eddy viscosity, analytical scale and gradient diffusion; Step 1.2: Determine the thermal-solid coupling heat transfer parameters, including the distance between the coaxial jet starting point and the simplified plate, the jet velocity and the thickness of the plate, and import the temperature field calculation results into the structural model as body loads; Step 2: Determine the power spectral density function of the random load and check whether the spectrum PSD conforms to the thermal oscillation characteristics; Step 3: Export the load, eliminate the stress noise, and use the autoregressive model to check the load stability; Step 3.1: Optimize the acquired load spectrum, including preprocessing the burrs and low-frequency drift anomalies of the calculated stress spectrum signal; Step 3.2: According to the definition (X i -X i-1 )(X i -X i+1 )>0, the peak-valley value extraction is realized by programming in MATLAB, and the extracted peak-valley load is subjected to trajectory interpolation calculation to obtain the linear load spectrum for fatigue test; Step 3.3: Use the autoregressive model to check the stability of the load spectrum. The calculation formula is: X t =aX t-1 +e t ,t∈Z,{ε t }~WN(0,σ 2 ) In the formula, when |α|>1, the sequence is in an unstable state; when |α|=1, the sequence oscillation becomes larger and larger, showing an explosive type; when |α|<1, the time series can tend to a stable state and can be used for fatigue cycle loading; Step 4: Fixture and specimen loading and specimen heating; Step 4.1: Place a section of the polished flat sample (8) in the sample slot (6e) of the upper fixture head (6), press the baffle (6f) with the knurling (6f3) into the sample slot (6e), and then screw the countersunk bolts (6g) into the corresponding through hole 1 (6f1) and through hole 2 (6f2), respectively, and use the friction force of the knurling (6f3) to limit the up and down movement of the flat sample (8), and prevent the flat sample (8) from falling off during the test; Step 4.2: Then screw the two push rod bolts (6b) through the threaded through holes (6i). Here, ensure that the push rod bolts (6b) are in a tightened state so that the flat sample (8) will not be affected by sliding during the unloading process; Step 4.3: Insert the fastening ring (6a) into the threaded extension rod (6h), and use two wrenches to clamp the wrench openings (6c) respectively, and further lock the V-shaped push rod bolt (6b) to form a self-locking function to prevent the push rod bolt (6b) from loosening; Step 4.4: After the upper fixture head (6) and the flat specimen (8) are installed, the fatigue test system is debugged and installed. The upper heat exchange head (5) is connected to the upper connecting rod (3) of the fatigue machine body (1), and the lower heat exchange head (7) is connected to the lower connecting rod (9) of the fatigue machine body (1). The water cooling system is turned on, and the water cooling pipeline is connected to the heat exchange head accordingly to ensure that the heat dissipation system starts to work. Step 4.5: Install the assembled upper clamp head (6) on the upper connecting rod through threaded connection, with the front of the upper clamp head (6) facing perpendicular to the paper surface; Step 4.6: Install the threaded extension rod (6h) of the lower fixture head (4) on the lower connecting rod (9) to complete the centering and debugging of the upper and lower fixtures. Fine-tune the lower connecting rod (9) through the fatigue machine body (1) control system to slowly rise until the sample slot (6e) of the lower fixture head (4) and the flat sample (8) are at the same height. Place the flat sample (8) in the sample slot (6e). The installation steps of the lower fixture head (4) are the same as those of the upper fixture head (6). At this time, the installation of the fixture and the sample is completed. Step 4.7: Raise the temperature of the high temperature environment chamber (2) to 520°C according to the test requirements and maintain it at this temperature for 30 minutes; Step 4.8: While waiting for the temperature to rise, a video extensometer (10) is further installed. An observation port (2b) is reserved in front of the high temperature environment chamber (2). The observation port (2b) will not be affected during the high temperature test, so that the video extensometer (10) can be placed to measure the strain of the sample. Step 5: Input test parameters: input stress amplitude coefficient, data loading frequency, stress amplitude, number of cycles and filter corner frequency into the system before loading, and implement fatigue loading after the insulation is completed; Step 6: Calculate the ratchet strain at high temperature based on the maximum and minimum strains. The calculation formula is:

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