CN115078139A - In-situ three-dimensional imaging fatigue testing machine and testing method in extremely low temperature environment - Google Patents
In-situ three-dimensional imaging fatigue testing machine and testing method in extremely low temperature environment Download PDFInfo
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- G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N3/32—Investigating strength properties of solid materials by application of mechanical stress by applying repeated or pulsating forces
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- G—PHYSICS
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- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N23/00—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00
- G01N23/02—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00 by transmitting the radiation through the material
- G01N23/04—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00 by transmitting the radiation through the material and forming images of the material
- G01N23/046—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00 by transmitting the radiation through the material and forming images of the material using tomography, e.g. computed tomography [CT]
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- G01N3/18—Performing tests at high or low temperatures
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- G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
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- G01N2203/006—Crack, flaws, fracture or rupture
- G01N2203/0062—Crack or flaws
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Abstract
A low-temperature cavity is placed on a base, a boss with a through hole is arranged on the base and serves as a lower clamp, the through hole in the lower portion of a stretching rod serves as the lower clamp, a sample clamp is fixed between the upper clamp and the lower clamp and located in the low-temperature cavity, the lower end of a screw rod of a servo electric cylinder is connected to the upper end of the stretching rod through a load sensor, the servo electric cylinder is installed on a supporting seat, the supporting seat is fixed on a cover plate, and a shell is arranged between the cover plate and the base and buckled on a sealing groove of the cover plate and the base. The low-temperature chamber is as follows: the cavity is internally provided with a heat insulation layer, the vacuumizing joint is communicated with an interlayer formed by two acrylic plates, and the low-temperature liquid inlet is communicated with the cavity of the cavity. The testing machine facilitates sample installation, and is compact in structure, light in weight and high in measurement precision.
Description
Technical Field
The invention relates to a fatigue test device for material mechanics test, in particular to an in-situ three-dimensional imaging fatigue test machine in an extreme low-temperature environment and a test method.
Background
The requirements of modern industrial extreme low-temperature high-performance structural materials such as deep space exploration, application superconductivity and gas industry are increasingly urgent, and external space vehicles, cryostats, liquid oxygen/nitrogen/hydrogen/helium transportation and storage and the like all put higher requirements on the performance of extreme low-temperature environment materials. Taking the aerospace field as an example, a carrier rocket is used as the only transport carrier of a human exploration space, and the recyclable carrier rocket is developed according to a plan at home and abroad, so that the recyclable carrier rocket is different from the traditional disposable carrier rocket, and the extreme service environment and the cycle characteristic of the service load become key consideration factors in the design of the recyclable carrier rocket. Liquid fuel (liquid hydrogen 20K, liquid oxygen 90K) storage tanks and the like are used as key structural components of the carrier rocket and need to reliably operate in an extremely low-temperature environment. In the face of risks brought to high-performance structural members by extreme multi-field environments such as fatigue loads and ultralow temperatures, deep and systematic research on extreme low-temperature fatigue performance, damage mechanisms and the like of key structural materials is urgently needed.
At present, the fatigue failure mechanism and damage evolution law of the extremely low-temperature material are mostly researched by adopting an off-line research method, namely, the material is firstly placed in an ultralow-temperature environment, mechanical loading is carried out after the internal temperature of the material is stable, and then the material is taken out to reconstruct the internal damage appearance of the loaded material through X-ray imaging in a room-temperature environment. However, the off-line imaging can only obtain the imaging results of the initial state and the fractured state of the material, and cannot obtain the damage evolution state inside the material in real time, so that the fatigue damage evolution process of the material in an extremely low temperature environment cannot be obtained.
There are a few in-situ loading test mechanisms equipped with low temperature environment, such as patent application No. 202110018265.4, application date 2021.01.07, entitled "in-situ loading micro CT characterization system and characterization method under ultra-low temperature environment". However, the mechanical loading structure described in the above patent has a large size, a sample is not easy to clamp, and the adopted traditional column mechanical loading mechanism is easy to shield X-rays around the sample, and is not easy to be compatible with the existing X-ray imaging platform for performing 180 ° or 360 ° rotational X-ray imaging. In addition, the limitation between the low-temperature heat preservation structure and the cooling medium can only carry out the in-situ test at the temperature of more than 77K (-196 ℃).
Therefore, at present, no in-situ imaging fatigue testing machine equipped with an extreme low-temperature environment can be used for carrying out in-situ imaging research on fatigue damage behaviors of materials such as liquid hydrogen (20K) fuel storage tanks or other service materials in the extreme low-temperature environment (4.5K-273K).
Disclosure of Invention
The invention aims to provide an in-situ three-dimensional imaging fatigue testing machine for an extreme low-temperature environment, which is convenient to load and unload a sample, compact in structure and high in precision, and aims to obtain a unit image of internal damage of a material and microstructure change information such as stress strain, structure and phase change by observing the damage evolution behavior of the material under the extreme low-temperature environment and mechanical coupling loading in situ in real time.
The purpose of the invention is realized as follows: the in-situ three-dimensional imaging fatigue testing machine for the extreme low-temperature environment comprises a CT imaging system adopting a synchrotron radiation light source, and the structure of a low-temperature chamber is as follows: a cylindrical inner PMMA cover and an outer PMMA cover are arranged between a round bottom plate and a top plate, an interlayer is formed between the inner PMMA cover and the outer PMMA cover, an inner cavity is arranged in the inner PMMA cover, a low-temperature liquid inlet and a vacuumizing joint are arranged on the side surface of the bottom plate, the vacuumizing joint is communicated with the interlayer, a reserved joint for introducing low-temperature liquid and a low-temperature gas outlet communicated with the inner cavity are arranged on the side surface of the top plate, a columnar boss is arranged in the middle of the round base, a through hole for a dumbbell-shaped sample to be sleeved is arranged on the upper portion of the boss, the boss of the base is sleeved in a hole in the middle of the bottom plate from bottom to top through a sealing structure, an O-shaped sealing ring in a sealing groove in the base forms sealing between the upper surface of the base and the lower surface of the bottom plate, a thermocouple is arranged on the boss, a stretching rod extends into the hole in the middle of the top plate from top to bottom through the sealing structure, and a heat insulation layer is arranged on the inner surfaces of the top plate and the bottom plate, the lower part of the stretching rod is provided with a through hole for the dumbbell-shaped sample to be sleeved in; the bellows is sleeved on the stretching rod, the lower end of the bellows is screwed on the top plate, and the lower end of a screw rod of the servo electric cylinder is connected with the upper end of the stretching rod through a load sensor; the servo electric cylinder is installed on the supporting seat, the supporting seat is fixed on the cover plate, the cylindrical shell made of PMMA is arranged between the cover plate and the base, and the shell is buckled on the sealing grooves of the cover plate and the base.
The base is installed on an X-ray imaging rotary objective table, and the upper end of the corrugated pipe is sealed and fixed on the stretching rod.
Two lifting cabin doors are symmetrically arranged on the left and right of the shell, and two lifting handles are respectively arranged on the outer PMMA cover of the low-temperature chamber at positions corresponding to the two lifting cabin doors; the shell is provided with a sample taking and placing window.
The device is also provided with a load displacement control system; the load sensor and the servo electric cylinder are respectively connected with a load displacement control system; the thermocouple is connected with a temperature control system.
The invention further aims to provide an in-situ three-dimensional imaging fatigue test method in an extremely low-temperature environment.
Another object of the invention is achieved by: a test method of an in-situ three-dimensional imaging fatigue tester in an extremely low temperature environment comprises the following steps:
1) firstly, placing an extreme low-temperature environment in-situ testing machine on an X-ray imaging rotary objective table, adjusting the installation position according to the height of an X-ray, and accurately positioning the sample testing position through a laser positioning system to ensure that the X-ray passes through the interested area of a sample;
2) after the position of the testing machine is adjusted, opening two cabin doors on the outer side of the cavity, lifting up the lifting handle of the sample cavity, lifting up the low-temperature cavity together, exposing the internal sample, replacing the clamp and replacing the sample according to the shape and size of the tested sample, dropping down the lifting handle after the sample is fixed, dropping down the low-temperature cavity along with the lifting handle, placing the sample in the cavity, and then closing the lifting cabin door;
3) after the steps are completed, starting a vacuumizing device, and vacuumizing an interlayer formed by the inner PMMA cover and the outer PMMA cover in the low-temperature cavity through a vacuumizing joint, so that the first stage is completed;
4) connecting each pipeline, opening a temperature control system and a load displacement control system, and determining a stress loading condition in an experiment according to the experiment purpose and the experiment material performance so as to determine a loading amplitude, namely a sample stress level;
5) liquid nitrogen and liquid helium are introduced from a low-temperature liquid inlet through a temperature control system, the flow proportion of the two low-temperature liquids is controlled through an electromagnetic valve to adjust the temperature of the cavity, and a thermocouple is connected with a temperature sensor, so that the ambient temperature change of the sample is monitored in real time, and a fatigue test is started after the environmental chamber reaches the test temperature;
6) turning on a synchrotron radiation light source and a CT imaging system, turning on a load displacement control system, controlling the load and displacement of the sample, feeding data back to the system by a displacement-load sensor, starting a fatigue test after relevant parameters are stable, and stopping the fatigue test after the sample is broken;
7) reconstructing the three-dimensional appearance in the material, and capturing the crack initiation and expansion process in the in-situ fatigue test process;
8) after the test is finished, the test apparatus is disassembled in the reverse procedure to that before the test.
Compared with the prior art, the invention has the following characteristics and advantages:
1. the invention relates to an in-situ imaging fatigue test device under the coupling action of an extreme low-temperature environment and mechanical loading. The testing machine has compact structure, the weight is within 20 Kg, good compatibility with an advanced light source test platform can be realized, common brand micron focus and nanometer focus CT equipment can be compatible, the requirements of a sample platform on weight and size are met, and the sample platform can rotate 180 degrees or 360 degrees along with an X-ray imaging rotating platform.
2. The sample environment chamber of the extreme low-temperature control device adopts a negative pressure sealing technology, namely a double-layer PMMA cover with a vacuum interlayer can effectively isolate heat exchange with the outside, so that the temperature in the cavity is kept in a stable state, and simultaneously, the formation of steam condensation outside the cavity can be prevented, and the X-ray imaging is hindered; the optimized cavity is made of high-specific-strength acrylic materials, and the designed cylindrical supporting structure, namely the acrylic shell, has small and uniform influence on X-ray penetration and does not influence the post-imaging data processing.
3. According to the invention, through liquid nitrogen/liquid helium mixed refrigeration and combined adoption of a soaking type cooling mode and a spraying type cooling mode, in-situ fatigue loading of a sample in an environment of 4.5-273K can be realized, the fatigue fracture behavior of the material in an extreme low temperature environment is dynamically observed in real time, and the damage evolution rule of the interior of the structural material under the coupling loading action of extreme low temperature and fatigue load is revealed.
4. According to the invention, the upper end seal of the corrugated pipe is fixed on the stretching rod, so that the upper part of the vacuum interlayer sample cavity can be sealed, and the sample cavity moves along the axial direction of the upper end stretching rod, so that the sample cavity can be manually lifted and dropped. When the sample is lifted, the sample is mounted and dismounted; when the test sample is lowered, the sample cavity forms a closed environment, low-temperature steam cooling is realized, the step of clamping the sample can be greatly simplified, and the test efficiency is improved.
5. The invention is provided with an axial displacement loading device, takes a servo electric cylinder as a driving mechanism, is provided with a high-precision load sensor and an electric cylinder servo, and carries out axial fatigue loading on a sample, the maximum loading frequency is 20 Hz, tensile stress can be applied to the sample in the three-dimensional imaging analysis process, the peak load reaches 5kN, so that microcracks generated in the sample are opened, further clearer three-dimensional crack appearance and microstructure characteristics are obtained, and the influence and mechanism of stress change on the crack expansion process are explored.
6. The invention can also independently carry out in-situ tensile experiment under extreme low temperature environment, and can carry out imaging and microstructure characterization on the samples under different stress levels by controlling the servo electric cylinder to apply axial tensile stress (the maximum loading force reaches 5 kN) to the samples.
Drawings
FIG. 1 is a schematic diagram of an in-situ three-dimensional imaging fatigue tester system of the present invention that can be used in extremely low temperature environments.
Fig. 2 is a perspective view of the fatigue testing machine shown in fig. 1.
Fig. 3 is a schematic view of the process of clamping the sample shown in fig. 2.
Fig. 4 is a schematic diagram of the cooling process.
Fig. 5 is a cross-sectional view of fig. 2.
Fig. 6 is an exploded view of fig. 2.
Fig. 7 is a cross-sectional view of the low temperature chamber and housing shown in fig. 2.
Fig. 8 is a cross-sectional view of the cryogenic chamber of fig. 7 (not including a base and stretch rod coupled thereto).
Detailed Description
In fig. 1: 1. electric cylinder (servo electric cylinder), 2, load sensor, 3, bellows, 4, low-temperature gas outlet, 5, (cavity) cover plate, 6, (support) shell, 7, (lifting) handle, 8, vacuumizing joint, 9, lifting cabin door, 10, (cavity) base, 11, low-temperature liquid inlet, 12, sample taking and placing window, 13, low-temperature chamber (low-temperature cavity), 14, reserved joint, 15, stretching rod, 16, supporting seat, 17, sample (sample), 18, thermocouple, 19, temperature control system, 20, low-temperature liquid tank, 21, rotary stage (prior art), 22, vacuumizing device, 23, load displacement control system.
In fig. 3: 6. The device comprises a support shell, 10-1 parts of a seal groove, 13-1 parts of a heat insulation layer, 13-2 parts of a (low-temperature cavity) inner PMMA cover, 13-3 parts of a (low-temperature cavity) outer PMMA cover, 15 parts of a stretching rod, 17 parts of a sample, 18 parts of a thermocouple.
In fig. 5: 1-1, (electric cylinder) screw (ball screw), 2, sensor, 15, stretching rod, 16, and support seat.
In fig. 8: 4. the low-temperature gas vacuum pump comprises a low-temperature gas outlet 8, a vacuumizing joint 11, a low-temperature liquid inlet 13-1, a heat insulation layer 13-2, a PMMA (polymethyl methacrylate) cover on the inner layer of a low-temperature cavity 13-3, a PMMA cover on the outer layer of the low-temperature cavity 14, a reserved joint 13-4, a bottom plate 13-5, a top plate 13-6 and an inner cavity.
Fig. 1, fig. 2, fig. 6 show a CT imaging system using a synchrotron radiation source, wherein the cryo-chamber 13 is configured as follows: a cylindrical inner PMMA cover 13-2 and an outer PMMA cover 13-3 are arranged between a round bottom plate 13-4 and a top plate 13-5, an interlayer is formed between the inner PMMA cover and the outer PMMA cover, an inner cavity 13-6 is arranged inside the inner PMMA cover, a low-temperature liquid inlet 11 and a vacuumizing joint 8 are arranged on the side surface of the bottom plate, the low-temperature liquid inlet is communicated with the inner cavity 13-6, the vacuumizing joint is communicated with the interlayer, the side surface of the top plate is provided with a reserved joint 14 communicated with the interlayer and used for introducing low-temperature liquid and a low-temperature gas outlet 4 communicated with the inner cavity, the middle part of a round base 10 is provided with a columnar boss, the upper part of the boss is provided with a through hole for sleeving a dumbbell-shaped sample 17, the boss of the base is sleeved in the hole in the middle part of the bottom plate from bottom to top through a sealing structure, and an O-shaped sealing ring in a sealing groove 10-1 on the base forms sealing between the upper surface of the base and the lower surface of the bottom plate, the boss is provided with a thermocouple 18, a stretching rod 15 extends into a hole in the middle of the top plate from top to bottom through a sealing structure, the inner and outer surfaces of the top plate and the bottom plate are respectively coated with a heat insulation layer 13-1, and the lower part of the stretching rod is provided with a through hole for a dumbbell-shaped sample to be sleeved in; the bellows 3 is sleeved on the stretching rod 15, the lower end of the bellows is screwed on the top plate 13-5, and the lower end of the screw rod 1-1 of the servo electric cylinder 1 is connected with the upper end of the stretching rod 15 through the load sensor 2; the servo electric cylinder 1 is arranged on a supporting seat 16, the supporting seat is fixed on a cover plate 5, a cylindrical shell 6 made of PMMA is arranged between the cover plate and a base, and the shell is buckled on a sealing accommodating groove of the cover plate and a sealing accommodating groove of the base 10.
Referring to fig. 8, the top plate has a thickness greater than the maximum upward travel of the stretch rod. After the sample on the base was loaded, before the test, the top surface of the boss of the base was flush with the top surface of the heat insulating layer on the top surface of the bottom plate, and the bottom surface of the stretching rod was flush with the bottom surface of the heat insulating layer on the bottom surface of the top plate (see fig. 7). The base 10 is arranged on an X-ray imaging rotary objective table 21, and the upper end of the corrugated pipe is sealed and fixed on a stretching rod; two lifting cabin doors 9 are symmetrically arranged on the left and right of the outer shell 6, and two lifting handles 7 are respectively arranged on the outer PMMA cover 13-3 of the low-temperature chamber 13 corresponding to the positions of the two lifting cabin doors; a sample taking and placing window 12 is arranged on the shell; there is also a load displacement control system 23; the load sensor 2 and the servo electric cylinder 1 are respectively connected with a load displacement control system; the thermocouple 18 is connected to a temperature control system 19. Fig. 1 and 2 show that a vacuum-pumping device 22 (e.g., a vacuum pump) is connected to the vacuum-pumping connection 8, and a cryogenic liquid tank is connected to the cryogenic liquid inlet 11 via a flow control mechanism (e.g., a metering valve).
Fig. 7 shows: the low temperature environment chamber is for having the double-deck PMMA cover of vacuum intermediate layer, and the sample is placed to the inlayer, and the intermediate layer evacuation for isolated cavity center carries out the heat exchange with the external world, and it is stable to maintain test temperature, prevents simultaneously that cavity surface condensation from causing the influence to X ray formation of image.
Fig. 8 shows: the left lower pipeline is a low-temperature liquid inlet 11 and is communicated into the inner chamber; the pipeline at the upper right side is a low-temperature gas recovery outlet 4 which is directly communicated with the inner chamber and used for collecting nitrogen and helium; the lower right side is provided with a vacuumizing joint 8 which is directly communicated with the cavity interlayer.
Fig. 2 shows a structure 5 and a structure 10 respectively as a cover plate and a base of a low temperature environment table, wherein a middle column-shaped boss (the upper part of the base 10 is provided with a through hole perpendicular to the axial direction, the upper part of the through hole is provided with an opening extending to the top surface of the boss, and the through hole and the opening can accommodate a dumbbell-shaped sample to be placed in) is used as a lower clamp for sample assembly, the cover plate is provided with two through holes, one of the through holes is used for passing through a low temperature gas pipeline to discharge residual gas, and the other through hole is reserved as a pipe joint of a reserved joint to pass through.
The low-temperature chamber 13 is a double-layer PMMA cover with a vacuum interlayer, a hole 11 is formed in the lower left of the chamber and used for being connected with a low-temperature cooling liquid pipeline, the pipeline is directly introduced into the chamber and can directly cool a sample, the contact part of the through hole and the pipeline is subjected to sealing treatment, a cover plate on the upper right of the chamber is provided with a low-temperature gas outlet joint 4, redundant gas in the chamber is discharged from the upper part of an end cover through a gas pipeline, and key parts are subjected to sealing treatment; the outer side of the interlayer at the right lower part of the cavity is connected with a vacuum joint 8, and the interlayer is vacuumized during the test, so that the temperature of the cavity is kept constant in a heat insulation way.
And (3) carrying out an extreme low-temperature in-situ three-dimensional imaging test based on an advanced light source.
The in-situ three-dimensional imaging fatigue test in the extremely low temperature environment can be carried out in two stages, namely a sample clamping stage and an in-situ fatigue test stage.
1) Firstly, placing an extreme low-temperature environment in-situ testing machine on an X-ray imaging rotary objective table 21, adjusting the installation position according to the height of an X-ray, and accurately positioning the sample testing position through a laser positioning system to ensure that the X-ray passes through the interested area of a sample;
2) after the position of the testing machine is adjusted, two cabin doors on the outer side of the cavity are opened, the lifting handle 7 of the sample cavity is lifted, the low-temperature cavity 13 is lifted together, the internal sample 17 is exposed at the moment, the clamp is replaced according to the shape and the size of the tested sample, the sample is replaced, the lifting handle falls after the sample is fixed, the low-temperature cavity falls along with the lifting handle, the sample is placed in the cavity, and the lifting cabin door 9 is closed immediately;
3) after the steps are completed, the vacuumizing device 22 is started, and the interlayer formed by the inner PMMA cover and the outer PMMA cover in the low-temperature cavity is vacuumized through the vacuumizing joint 8, so that the first stage is completed;
4) connecting each pipeline, opening a temperature control system 19 and a load displacement control system 23, and determining a stress loading condition in an experiment according to the experiment purpose and the experiment material performance so as to determine a loading amplitude, namely a sample stress level;
5) liquid nitrogen and liquid helium are introduced from a cryogenic liquid inlet through a temperature control system 19, the flow proportion of the two cryogenic liquids is controlled through an electromagnetic valve to adjust the temperature of the cavity, a thermocouple is connected with a temperature sensor, so that the ambient temperature change of the sample is monitored in real time, and a fatigue test is started after the environmental chamber reaches a test temperature;
6) and (3) turning on the synchrotron radiation light source and the CT imaging system, turning on a load displacement control system, controlling the load and displacement of the sample, feeding data back to the system by a displacement-load sensor, starting a fatigue test after the relevant parameters are stable, and stopping the fatigue test after the sample breaks.
7) And 3, reconstructing the three-dimensional appearance in the material, and capturing the crack initiation and expansion process in the in-situ fatigue test process.
8) After the test is finished, the test apparatus is disassembled in the reverse procedure to that before the test.
The main body of the testing machine consists of a low-temperature environment system and a mechanical loading system, can be used for in-situ observation of the damage evolution behavior of the material under the extremely low-temperature environment (4.5K-273K) and mechanical coupling loading in real time, and is an in-situ fatigue test loading device with low weight, compact structure and high precision. The extreme low temperature environment cavity of the testing machine adopts a negative pressure sealing technology, and the designed double-layer PMMA cover with the vacuum interlayer can effectively isolate heat exchange with the outside. The combined refrigeration of liquid helium and liquid nitrogen is adopted, so that the in-situ fatigue test of the material under the low-temperature environment of 4.5K-273K can be realized. The testing machine is integrally compatible with synchrotron radiation light sources, such as CT imaging systems of imaging line stations of Beijing synchrotron radiation devices (BSRF), Shanghai synchrotron radiation light sources (SSRF) and the like, and can also be used for common brand laboratory CT equipment of GE, YXLON, ZEISS and the like, in-situ three-dimensional imaging is carried out on damage evolution behaviors of materials in a fatigue loading process in an extreme low-temperature environment, applied loads, displacements, temperatures and the like can be quantitatively measured and displayed in real time, so that three-dimensional images of internal damages of the materials and microstructure change information of stress strain, texture, phase change and the like can be obtained, and the damage characteristics of the internal damages of the materials such as crack initiation and expansion in the fatigue damage process of the materials in the extreme low-temperature environment can be clearly and accurately reflected.
Claims (5)
1. The in-situ three-dimensional imaging fatigue testing machine for the extreme low-temperature environment comprises a CT imaging system adopting a synchrotron radiation light source, and is characterized in that a low-temperature chamber (13) has a structure as follows: a cylindrical inner PMMA cover (13-2) and an outer PMMA cover (13-3) are arranged between a round bottom plate (13-4) and a top plate (13-5), an interlayer is formed between the inner PMMA cover and the outer PMMA cover, an inner cavity (13-6) is arranged inside the inner PMMA cover, a low-temperature liquid inlet (11) and a vacuumizing joint (8) are arranged on the side surface of the bottom plate, the low-temperature liquid inlet is communicated with the inner cavity (13-6), the vacuumizing joint is communicated with the interlayer, a reserved joint (14) for introducing low-temperature liquid and a low-temperature gas outlet (4) communicated with the inner cavity are arranged on the side surface of the top plate, a columnar boss is arranged in the middle of the round base (10), a through hole for sleeving a dumbbell-shaped sample (17) is arranged in the upper part of the boss, the boss of the base is sleeved in the hole in the middle of the bottom plate from bottom to top through a sealing structure, and an O-shaped sealing ring in a sealing groove (10-1) on the base forms a sealing structure between the upper surface of the base and the lower surface of the bottom plate A thermocouple (18) is arranged on the boss, a stretching rod (15) extends into the hole in the middle of the top plate from top to bottom through a sealing structure, heat insulation layers (13-1) are laid on the inner surfaces of the top plate and the bottom plate, and a through hole for a dumbbell-shaped sample to be sleeved in is formed in the lower portion of the stretching rod; the bellows (3) is sleeved on the stretching rod (15), the lower end of the bellows is screwed on the top plate (13-5), and the lower end of a screw rod (1-1) of the servo electric cylinder (1) is connected with the upper end of the stretching rod (15) through a load sensor (2); the servo electric cylinder (1) is arranged on a supporting seat (16), the supporting seat is fixed on a cover plate (5), a cylindrical shell (6) made of PMMA is arranged between the cover plate and a base, and the shell is buckled on sealing grooves of the cover plate and the base (10).
2. The in-situ three-dimensional imaging fatigue tester in extreme low temperature environment as claimed in claim 1, wherein the base (10) is mounted on an X-ray imaging rotary stage (21), and the upper end of the corrugated pipe is sealed and fixed on a stretching rod.
3. The in-situ three-dimensional imaging fatigue testing machine for the extreme low temperature environment is characterized in that two lifting doors (9) are symmetrically arranged on the left and right of the outer shell (6), and two lifting handles (7) are respectively arranged on the outer PMMA cover (13-3) of the low temperature chamber (13) at the positions corresponding to the two lifting doors; the shell is provided with a sample taking and placing window (12).
4. The in-situ three-dimensional imaging fatigue testing machine for extreme low temperature environment of claim 1, further comprising a load displacement control system (23); the load sensor (2) and the servo electric cylinder (1) are respectively connected with a load displacement control system; the thermocouple (18) is connected with a temperature control system (19).
5. A test method using the test machine according to any one of claims 1 to 4, characterized in that it comprises the following steps:
1) firstly, an extreme low-temperature environment in-situ testing machine is placed on an X-ray imaging rotary objective table (21), the installation position is adjusted according to the height of an X-ray, a sample testing position is accurately positioned through a laser positioning system, and the X-ray is ensured to penetrate through a sample interesting area;
2) after the position of the testing machine is adjusted, two cabin doors on the outer side of the cavity are opened, a lifting handle (7) of the sample cavity is lifted, a low-temperature cavity (13) is lifted together, an internal sample (17) is exposed at the moment, a clamp is replaced according to the shape and the size of the tested sample, the sample is replaced, the lifting handle falls after the sample is fixed, the low-temperature cavity falls along with the low-temperature cavity, the sample is placed in the cavity, and the lifting cabin door (9) is closed immediately;
3) after the steps are completed, starting a vacuumizing device (22), and vacuumizing the interlayer formed by the inner PMMA cover and the outer PMMA cover in the low-temperature cavity through a vacuumizing joint (8), so that the first stage is completed;
4) connecting each pipeline, opening a temperature control system (19) and a load displacement control system (23), and determining a stress loading condition in an experiment according to the experiment purpose and the experiment material performance so as to determine a loading amplitude, namely a sample stress level;
5) liquid nitrogen and liquid helium are introduced from a low-temperature liquid inlet through a temperature control system (19), the flow proportion of the two low-temperature liquids is controlled through an electromagnetic valve to adjust the temperature of the cavity, and a thermocouple is connected with a temperature sensor, so that the ambient temperature change of the sample is monitored in real time, and a fatigue test is started after the environmental chamber reaches a test temperature;
6) turning on a synchrotron radiation light source and a CT imaging system, turning on a load displacement control system, controlling the load and displacement of the sample, feeding data back to the system by a displacement-load sensor, starting a fatigue test after relevant parameters are stable, and stopping the fatigue test after the sample is broken;
7) reconstructing the three-dimensional appearance in the material, and capturing the crack initiation and expansion process in the in-situ fatigue test process;
8) after the test is finished, the test apparatus is disassembled in the reverse procedure to that before the test.
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