A double-layer closed ultra-low temperature mechanical testing device and testing method

Abstract

The present invention proposes a double-layer enclosed ultra-low temperature mechanical testing device, belonging to the field of measurement and testing technology, comprising a liquid nitrogen tank, liquid nitrogen, a specimen fixture, a load-bearing ground rail, a pulley assembly and a fixed bracket assembly, a steel wire rope, a dynamometer, an actuator, a CCD lens, a lens fixing bracket, a quartz glass cover, a quartz glass cover plate, a double air knife, a nitrogen bottle, a liquid nitrogen filling bottle, a digital image correlation measurement system, and a loading control system; the test piece is fixed horizontally in the liquid nitrogen; one side of the specimen fixture is connected to the dynamometer and the actuator; the airflow of the double air knife flows directly above the specimen fixture; the quartz glass cover is arranged directly above the quartz glass cover plate; the CCD lens is arranged directly above the test piece and connected to the digital image correlation measurement system; the digital image correlation measurement system is used to post-process the collected data. The present invention also proposes a test method, which solves the problem that the existing technology cannot realize the application of digital image correlation methods in low temperature environments.

Description

Technical Field

[0001] The invention belongs to the technical field of measurement and testing, and in particular relates to a double-layer closed ultra-low temperature mechanical testing device and a testing method. Background Art

[0002] In the aerospace industry, studying the cryogenic mechanical properties of materials and structures is of great significance. However, due to the complexity and particularity of cryogenic environments, it is often very difficult to accurately obtain the mechanical properties of materials and structures in cryogenic environments.

[0003] Digital image correlation (DIC) is an advanced optical, non-contact deformation measurement technique based on image acquisition. It is currently widely used to study the mechanical properties of materials and structures in both normal and high-temperature environments. However, in low-temperature environments, this optical measurement method can result in inaccurate results or even failure due to interfering factors such as frost, fogging, and airflow disturbances.

[0004] Existing technologies cannot realize deformation measurement and mechanical property research using digital image correlation methods in ultra-low temperature environments. It is very difficult to accurately obtain the mechanical properties of materials and structures in ultra-low temperature environments, and improvements are necessary. Summary of the Invention

[0005] The present invention provides a double-layer enclosed ultra-low temperature mechanical testing device and testing method, aiming to solve the problem in the prior art of inaccurate measurement results or even method failure when applying digital image correlation methods in low temperature environments.

[0006] The object of the present invention is achieved through the following technical solutions:

[0007] A double-layer closed ultra-low temperature mechanical testing device, comprising a liquid nitrogen tank, liquid nitrogen, a specimen fixture, a load-bearing ground rail, a pulley assembly and a fixed bracket assembly, a steel wire rope, a dynamometer, an actuator, a CCD lens, a lens fixing bracket, a quartz glass cover, a quartz glass cover plate, a double air knife, a nitrogen bottle, a liquid nitrogen filling bottle, a digital image correlation measurement system, and a loading control system; the load-bearing ground rail is fixed to the ground, and the upper surface is arranged horizontally; the liquid nitrogen tank is a box-shaped structure with an open top for containing liquid nitrogen, and the liquid nitrogen tank is horizontally fixed to the load-bearing ground rail; the specimen fixture respectively clamps the two ends of the test piece and fixes it horizontally in the liquid nitrogen in the liquid nitrogen tank; one side of the specimen fixture is aligned with the liquid nitrogen tank The side wall is connected and fixed, and the other side is connected to the dynamometer and the actuator in sequence after the wire rope passes through the pulley assembly and the fixed bracket assembly; the loading control system is used to control the actuator to apply load; the liquid nitrogen filling bottle is connected to the liquid nitrogen tank through a pipeline, and the loading control system is also used to control the liquid nitrogen filling bottle to add liquid nitrogen into the liquid nitrogen tank; the double air knife includes an upper air knife and a lower air knife arranged in parallel in the horizontal direction. The two sets of air knives are independently connected to the nitrogen bottles and are stacked above one side of the liquid nitrogen tank. The generated nitrogen air flow flows directly above the position of the specimen fixture; the quartz glass cover is an integrated structure made of quartz glass material with a symmetrical cross-section, and the projection surface is parallel to the opening of the liquid nitrogen tank. The shapes and sizes of the openings are consistent; the quartz glass cover plate includes a top cover, side plates and a support plate; the top cover is provided with a prefabricated hole for installing a steel wire rope; the top cover and the side plates form a ventilation slot; the lower air knife is provided on one side of the ventilation slot and is connected and fixed to the quartz glass cover plate; the quartz glass cover is provided directly above the quartz glass cover plate; the CCD lens is provided directly above the test piece, located between the quartz glass cover and the quartz glass cover plate, and is suspended and fixed by a lens fixing bracket; the CCD lens is connected to a digital image correlation measurement system, and the image acquisition is controlled by the digital image correlation measurement system; the cantilever part of the lens fixing bracket passes through the top of the quartz glass cover and is fixed to the external bracket; the quartz glass The cover is a split structure made of quartz glass, including an air knife side half cover and a loading side half cover; a notch is provided on the top mating surface of the air knife side half cover and the loading side half cover for installing a lens fixing bracket, and a prefabricated hole is also provided on the top of the loading side half cover for installing a wire rope. An open groove is also provided at the bottom of the quartz glass cover, and the upper air knife is set in the open groove of the air knife side half cover in the quartz glass cover and is connected and fixed to the quartz glass cover; the digital image correlation measurement system is used to post-process the image data collected by the CCD lens to obtain test results; the loading control system is also used to control the liquid nitrogen filling bottle to add liquid nitrogen to the liquid nitrogen tank to maintain a stable liquid level in the tank.

[0008] As a further optimization, the dynamometer and actuator are arranged vertically.

[0009] As a further optimization, the double air knives are connected and fixed by low-temperature glue or iron wire.

[0010] As a further optimization, the liquid nitrogen tank is made of Q345 steel.

[0011] As a further optimization, a blind hole is machined inside the side wall of the liquid nitrogen tank, and the specimen fixture is a double-ear fixture, one side of which is connected to the blind hole on the side wall of the liquid nitrogen tank through a wire rope and an adapter joint.

[0012] As a further optimization, the pulley assembly and the fixed bracket assembly use three sets of fixed pulleys.

[0013] Based on the same technical concept, the present invention also proposes a cryogenic mechanical testing method, which uses the above-mentioned double-layer enclosed cryogenic mechanical testing device and includes the following steps:

[0014] S1. Create a random speckle pattern on the surface of the test piece;

[0015] S2. Fix the liquid nitrogen tank on the load-bearing ground rail and install the specimen fixture on the side wall of the liquid nitrogen tank; fix the test piece with the specimen fixture and connect the dynamometer and the actuator in sequence through steel wire ropes;

[0016] S3. Add liquid nitrogen into the liquid nitrogen tank. After the liquid nitrogen level reaches a predetermined depth, install the quartz glass cover plate on the liquid nitrogen tank.

[0017] S4. Align the CCD lens with the test piece and adjust the position, fix it with the lens fixing bracket and connect it to the digital image correlation measurement system;

[0018] S5. Install the quartz glass cover onto the quartz glass cover plate, assembling the two parts into one; the CCD lens is set inside the quartz glass cover, and the lens fixing bracket passes through the top of the quartz glass cover and is connected and fixed to the external bracket;

[0019] S6. Fix the upper air knife and the lower air knife of the double air knife in the grooves of the quartz glass cover and the quartz glass cover respectively, and connect them to the nitrogen bottle;

[0020] S7. Open the nitrogen bottle and use a double air knife to output a continuous nitrogen flow between the liquid nitrogen surface and the quartz glass cover plate, and between the quartz glass cover plate and the CCD lens;

[0021] S8. Use the loading control system to control the actuator to apply the test predetermined load;

[0022] S9, controlling the CCD lens to collect the test image through the digital image correlation measurement system to realize the collection of the test image data;

[0023] S10. Post-process the image data using a digital image correlation measurement system to obtain test results.

[0024] As a further optimization, in step S3, the predetermined depth of the liquid nitrogen level in the liquid nitrogen tank is 0.3 m to 0.5 m.

[0025] The beneficial technical effects achieved by the present invention are:

[0026] Liquid nitrogen is used to simulate ultra-low temperature environments. Through a double-layer enclosed spatial structure design and the use of transparent quartz glass windows and air knife components, the effects of airflow disturbances caused by temperature differences on optical image acquisition are eliminated to the greatest extent possible. At the same time, interference factors such as mist emitted by liquid nitrogen and low-temperature frost are avoided. This creates favorable conditions for high-precision measurement using digital image correlation methods, helps to obtain more accurate experimental results, and thus accurately determines the mechanical properties of materials in ultra-low temperature environments. The technical solution described in the present invention realizes the use of digital image correlation methods for deformation measurement and mechanical property research in ultra-low temperature environments, solving the problem of inaccurate measurement results or even method failure when applying digital image correlation methods in low-temperature environments in the existing technology, and has outstanding substantive characteristics and significant progress. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a schematic diagram of the structure of a double-layer enclosed ultra-low temperature mechanical testing device according to one specific embodiment of the present invention;

[0028] Figure 2 This is a schematic structural diagram of a quartz glass cover plate according to one specific embodiment of the present invention;

[0029] Figure 3 This is a schematic diagram of the structure of a quartz glass cover according to one specific embodiment of the present invention;

[0030] Figure numerals: 1, liquid nitrogen tank; 2, liquid nitrogen; 3, specimen fixture; 4, test piece; 5, load-bearing ground rail; 6, pulley assembly and fixed bracket assembly; 7, wire rope; 8, dynamometer; 9, actuator; 10, CCD lens; 11, lens fixing bracket; 12, quartz glass cover; 13, quartz glass cover plate; 14, double air knife; 15, nitrogen bottle; 16, liquid nitrogen filling bottle; 121, air knife side half cover; 122, loading side half cover; 131, top cover; 132, side plate; 133, support plate. DETAILED DESCRIPTION

[0031] The technical solution of the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments. It is apparent that the embodiments described are only a portion of the embodiments of the present invention, not all of them. All other embodiments derived by those skilled in the art based on the embodiments of the present invention without inventive effort are also within the scope of protection claimed by the present invention.

[0032] like Figures 1 to 3As shown, a specific embodiment of a double-layer enclosed ultra-low temperature mechanical testing device is suitable for digital image correlation measurement methods. Liquid nitrogen is used to simulate an ultra-low temperature environment, a transparent quartz glass component is used to provide a visual environment for the digital image correlation measurement method, and an air knife is used to provide a stable airflow direction for the field of view space of optical image acquisition, thereby creating favorable conditions for high-precision measurement using the digital image correlation method, making it possible to use the digital image correlation method to perform deformation measurement and mechanical property research in an ultra-low temperature environment.

[0033] The double-layer enclosed cryogenic mechanical testing apparatus in this embodiment includes a liquid nitrogen tank 1, liquid nitrogen 2, a specimen fixture 3, a load-bearing ground rail 5, a pulley assembly and fixed bracket assembly 6, a steel wire rope 7, a dynamometer 8, an actuator 9, a CCD lens 10, a lens fixing bracket 11, a quartz glass cover 12, a quartz glass cover plate 13, a double air knife 14, a nitrogen bottle 15, a liquid nitrogen filling bottle 16, a digital image correlation measurement system (not shown), and a loading control system (not shown). A random speckle pattern must be pre-fabricated on the surface of the test piece 4 to enable digital image correlation measurement.

[0034] In this specific embodiment, the load-bearing ground rail 5 is fixed on the ground, with the upper surface arranged horizontally, serving as the base of the ultra-low temperature mechanics testing device, for supporting the liquid nitrogen tank 1 and liquid nitrogen 2, while isolating the liquid nitrogen tank 1 from the ground to reduce temperature loss during the test.

[0035] In this specific embodiment, the liquid nitrogen tank 1 is a box-shaped structure with an open top, made of Q345 steel, used to contain liquid nitrogen 2 and simulate an ultra-low temperature environment for the test piece 4. An insulation layer or interlayer can be added according to actual needs. The liquid nitrogen tank 1 is fixed to the load-bearing ground rail 5 by bolts. Depending on actual conditions, it can also be fixed by other means such as welding, blocks, etc. without affecting the use effect. Blind holes are machined inside the side walls of the liquid nitrogen tank 1 for installing the test piece fixture 3 and the pulley assembly and fixed bracket assembly 6. The test piece fixture 3 clamps the two ends of the test piece 4 respectively, so that it is fixed horizontally in the liquid nitrogen 2 in the liquid nitrogen tank 1. In this specific embodiment, the specimen fixture 3 is a two-ear fixture. One side of the specimen fixture 3 is connected to a blind hole in the side wall of the liquid nitrogen tank 1 via a steel wire rope and an adapter joint. It can also be fixed by a pull rod or other means. The other side of the specimen fixture 3 is connected to a dynamometer 8 and an actuator 9 in sequence via a steel wire rope 7. The load is applied via the actuator 9, and the actual load data is measured by the dynamometer 8. The pulley assembly and fixed bracket assembly 6 are used to change the direction of the force application and maintain the load magnitude. In this specific embodiment, the dynamometer 8 and actuator 9 are arranged vertically to avoid the influence of gravity on the test results. Other arrangements can also be used, which only differ in the subsequent data processing, but there is no substantial difference. In this specific embodiment, the load application of the actuator 9 is controlled by the loading control system.

[0036] In this specific embodiment, the pulley assembly and the fixed bracket assembly 6 use three sets of fixed pulleys. According to actual needs, other numbers of fixed pulleys or combinations of fixed pulleys and movable pulleys can also be used, which should be regarded as adaptive changes made by those skilled in the art on the basis of the technical solution described in this specific embodiment.

[0037] The depth of liquid nitrogen 2 added to liquid nitrogen tank 1 is determined by the installation height of specimen fixture 3 within the tank 1, aiming to just cover the upper surface of specimen 4. The preferred range is 0.3 to 0.5 meters. In this embodiment, the depth of liquid nitrogen 2 added to liquid nitrogen tank 1 is 0.5 meters. Liquid nitrogen filling bottle 16 is connected to liquid nitrogen tank 1 via a pipeline and, under the control of the loading control system, adds liquid nitrogen 2 to liquid nitrogen tank 1.

[0038] In this embodiment, the dual air knives 14 comprise two sets of horizontally parallel air knives: an upper air knife and a lower air knife. Each of the upper and lower air knives is independently connected to a nitrogen cylinder 15 to generate a fixed-direction nitrogen flow. The dual air knives 14 are stacked and positioned above one side of the liquid nitrogen tank 1. The generated nitrogen flow passes directly above the specimen fixture 3.

[0039] In this specific embodiment, the quartz glass cover plate 13 is an integrated structure made of quartz glass. Its cross-section is symmetrical, and its projected surface aligns with the shape and dimensions of the opening of the liquid nitrogen tank 1. It covers the liquid nitrogen tank 1 and serves as a ventilation slot for the lower air knife. The quartz glass cover plate 13 comprises a top cover 131, side panels 132, and a support plate 133. The top cover 131 is provided with a prefabricated hole for mounting the steel wire rope 7. The top cover 131 and side panels 132 form a ventilation slot, providing space for the fixed-flow nitrogen gas generated by the lower air knife. The support plate 133 is load-bearing and provides support for the quartz glass cover 12. The number and location of the prefabricated holes are determined by the dimensions of the steel wire rope 7 and installation requirements. The specific dimensions of the top cover 131 and side panels 132 are determined by the airflow generated by the dual air knife 14. The lower air knife in the dual air knife 14 is positioned on one side of the ventilation slot formed by the top cover 131 and side panels 132 and is secured to the quartz glass cover plate 13 using low-temperature glue or wire.

[0040] In this specific embodiment, the quartz glass cover 12 is disposed directly above the quartz glass cover plate 13 and is used to accommodate the CCD lens 10 and serve as a ventilation slot for the upper air knife.

[0041] In this specific embodiment, a CCD lens 10 is positioned directly above the test piece 4 and is connected to a digital image correlation measurement system via a cable or wireless connection. The digital image correlation measurement system controls the CCD lens 10 to capture images. The CCD lens 10 is positioned between a quartz glass cover 12 and a quartz glass cover plate 13 and is suspended and fixed by a lens fixing bracket 11. The cantilever portion of the lens fixing bracket 11 extends through the top of the quartz glass cover 12 and is fixed to an external bracket.

[0042] The quartz glass cover 12 is a split-piece structure made of quartz glass, comprising an air knife-side half cover 121 and a loading-side half cover 122. A semicircular notch is positioned at the same position on the top mating surface of each of the air knife-side half cover 121 and the loading-side half cover 122 for mounting the lens mounting bracket 11. The top of the loading-side half cover 122 also features a prefabricated hole for mounting the wire rope 7. The number and location of these prefabricated holes are determined by the dimensions of the wire rope 7 and its installation requirements. The bottoms of each of the air knife-side half cover 121 and the loading-side half cover 122 feature an open slot. The shape and size of these slots are determined by the airflow generated by the top cover 131 and side panels 132 of the quartz glass cover plate 13, as well as the upper air knife in the dual air knife 14.

[0043] The upper air knife in the double air knife 14 is arranged in the opening groove of the air knife side half cover 121 in the quartz glass cover 12, and is connected and fixed to the quartz glass cover 12 by low-temperature glue or iron wire.

[0044] In this specific embodiment, dual air knives 14 continuously deliver nitrogen gas flows between the liquid nitrogen 2 surface and the quartz glass cover plate 13, and between the quartz glass cover plate 13 and the CCD lens 10. A loading control system then controls the actuator 9 to apply a load, while a digital image correlation measurement system controls the CCD lens 10 to capture images, thereby acquiring test image data. After acquisition, the digital image correlation measurement system performs post-processing of the image data to obtain test results. During the test, the loading control system controls the liquid nitrogen refill bottle 16 to add liquid nitrogen 2 to the liquid nitrogen tank 1, maintaining a stable liquid level.

[0045] Based on the same technical concept, this specific embodiment also includes a cryogenic mechanical testing method, which uses the above-mentioned double-layer enclosed cryogenic mechanical testing device and includes the following steps:

[0046] S1. Create a random speckle pattern on the surface of the test piece 4;

[0047] S2. Fix the liquid nitrogen tank 1 to the load-bearing ground rail 5 with bolts, and install the specimen fixture 3 on the side wall of the liquid nitrogen tank 1; fix the test specimen 4 with the specimen fixture 3, and connect the dynamometer 8 and the actuator 9 in sequence through the wire rope 7;

[0048] S3, adding liquid nitrogen 2 into the liquid nitrogen tank 1, and after the liquid nitrogen 2 reaches a predetermined depth, installing the quartz glass cover plate 13 on the liquid nitrogen tank 1;

[0049] S4, aligning the CCD lens 10 with the test piece 4 and adjusting the position, fixing it with the lens fixing bracket 11 and connecting it to the digital image correlation measurement system;

[0050] S5. Install the quartz glass cover 12 onto the quartz glass cover plate 13, and assemble the air knife side half cover 121 and the loading side half cover 122 into one piece; the CCD lens 10 is disposed inside the quartz glass cover 12, and the cantilever portion of the lens fixing bracket 11 passes through the reserved hole at the top of the quartz glass cover 12 and is connected and fixed to the external bracket;

[0051] S6. Fix the upper air knife and the lower air knife of the double air knife 14 to the grooves of the quartz glass cover 13 and the quartz glass cover 12 respectively by low-temperature glue or iron wire, and connect them to the nitrogen bottle 15;

[0052] S7, open the nitrogen bottle 15, and use the double air knife 14 to output a continuous nitrogen flow between the liquid nitrogen 2 and the quartz glass cover plate 13, and between the quartz glass cover plate 13 and the CCD lens 10;

[0053] S8, using the loading control system to control the actuator 9 to apply the test predetermined load;

[0054] S9, controlling the CCD lens 10 to collect the test image through the digital image correlation measurement system to achieve the collection of test image data;

[0055] S10. Post-process the image data using a digital image correlation measurement system to obtain test results.

[0056] The beneficial technical effects achieved by this specific embodiment are:

[0057] Because there is a continuous and stable nitrogen flow in the field of view of the CCD lens to collect images, on the one hand, interference factors such as airflow disturbances caused by excessive temperature differences are eliminated, and on the other hand, frost and fogging caused by the presence of water vapor are eliminated. This solves the problem of inaccurate measurement results or even method failure caused by frost and fogging when using optical measurement methods in low-temperature environments.

[0058] Liquid nitrogen is used to simulate ultra-low temperature environments. Through the double-layer closed space structure design and the use of transparent quartz glass windows and air knife components, the influence of airflow disturbances caused by temperature differences on optical image acquisition is eliminated to the greatest extent. At the same time, interfering factors such as mist emitted by liquid nitrogen and low-temperature frost are avoided, creating favorable conditions for high-precision measurement of digital image correlation methods, helping to obtain more accurate experimental results, and thus accurately determine the mechanical properties of materials in ultra-low temperature environments.

[0059] The double-layer enclosed ultra-low temperature mechanical testing device in this specific embodiment is also applicable to the traditional strain gauge electrical measurement method, and can realize the comparison of the measurement results of the optical measurement method and the electrical measurement method, providing a new testing means for deformation measurement and mechanical property research.

Claims

1. A method for ultra-low temperature mechanical testing, characterized in that: A double-layer enclosed ultra-low temperature mechanical testing device is used, which includes a liquid nitrogen tank (1), liquid nitrogen (2), a specimen fixture (3), a load-bearing ground rail (5), a pulley assembly and a fixed bracket assembly (6), a steel wire rope (7), a dynamometer (8), an actuator (9), a CCD lens (10), a lens fixing bracket (11), a quartz glass cover (12), a quartz glass cover plate (13), a double air knife (14), a nitrogen bottle (15), a liquid nitrogen filling bottle (16), a digital image correlation measurement system, and a loading control system; The load-bearing ground rail (5) is fixed on the ground, and the upper surface is arranged horizontally; the liquid nitrogen tank (1) is a box-shaped structure with an open top, used for accommodating liquid nitrogen (2), and the liquid nitrogen tank (1) is horizontally fixed on the load-bearing ground rail (5); the specimen fixture (3) respectively clamps the two ends of the test piece so that it is horizontally fixed in the liquid nitrogen (2) in the liquid nitrogen tank (1); one side of the specimen fixture (3) is connected and fixed to the side wall of the liquid nitrogen tank (1), and the other side is connected to the dynamometer (8) and the actuator (9) in sequence after passing through the pulley assembly and the fixed bracket assembly (6) through a steel wire rope (7); the loading control system is used to control the actuator (9) to apply a load; the liquid nitrogen filling bottle (16) is connected to the liquid nitrogen tank (1) through a pipeline, and the loading control system is also used to control the liquid nitrogen filling bottle (16) to add liquid nitrogen (2) into the liquid nitrogen tank (1); The double air knife (14) comprises an upper air knife and a lower air knife arranged in parallel in a horizontal direction, and the two sets of air knives are independently connected to nitrogen bottles (15) and are stacked above one side of the liquid nitrogen tank (1), and the generated nitrogen gas flow flows directly above the position of the specimen fixture (3); The quartz glass cover plate (13) is an integrated structure, made of quartz glass material, has a symmetrical cross-section, and a projection surface that is consistent with the shape and size of the opening of the liquid nitrogen tank (1); the quartz glass cover plate (13) includes a top cover (131), a side plate (132) and a support plate (133); the top cover (131) is provided with a prefabricated hole for installing the steel wire rope (7); the top cover (131) and the side plate (132) form a ventilation slot; the lower air knife is provided on one side of the ventilation slot and is connected and fixed to the quartz glass cover plate (13); The quartz glass cover (12) is arranged directly above the quartz glass cover plate (13); the CCD lens (10) is arranged directly above the test piece, located between the quartz glass cover (12) and the quartz glass cover plate (13), and is suspended and fixed by a lens fixing bracket (11); the CCD lens (10) is connected to a digital image correlation measurement system, and the digital image correlation measurement system is used to control the acquisition of images; the cantilever portion of the lens fixing bracket (11) passes through the top of the quartz glass cover (12) and is fixed to an external bracket; The quartz glass cover (12) is a split structure, made of quartz glass material, and includes an air knife side half cover (121) and a loading side half cover (122); a notch is provided on the top joint surface of the air knife side half cover (121) and the loading side half cover (122) for installing the lens fixing bracket (11); a prefabricated hole is also provided on the top of the loading side half cover (122) for installing the wire rope (7); an open groove is also provided at the bottom of the quartz glass cover (12); the upper air knife is provided in the open groove of the air knife side half cover (121) in the quartz glass cover (12) and is connected and fixed to the quartz glass cover (12); The digital image correlation measurement system is used to post-process image data collected by the CCD lens (10) to obtain test results; the loading control system is also used to control the liquid nitrogen filling bottle (16) to add liquid nitrogen (2) into the liquid nitrogen tank (1) to maintain the liquid level in the tank at a stable height; The ultra-low temperature mechanical testing method comprises the following steps: S1. Create a random speckle pattern on the surface of the test piece; S2. Fix the liquid nitrogen tank (1) on the load-bearing ground rail (5) and install the test piece fixture (3) on the side wall of the liquid nitrogen tank (1); fix the test piece through the test piece fixture (3), and connect the dynamometer (8) and the actuator (9) in sequence through the wire rope (7); S3, adding liquid nitrogen (2) into the liquid nitrogen tank (1), and after the liquid level of the liquid nitrogen (2) reaches a predetermined depth, installing the quartz glass cover plate (13) on the liquid nitrogen tank (1); S4, aligning the CCD lens (10) with the test piece and adjusting the position, fixing it with the lens fixing bracket (11) and connecting it to the digital image correlation measurement system; S5. Install the quartz glass cover (12) onto the quartz glass cover plate (13), and assemble the two parts into one body; the CCD lens (10) is arranged inside the quartz glass cover (12), and the lens fixing bracket (11) passes through the top of the quartz glass cover (12) and is connected and fixed to the external bracket; S6. Fix the upper air knife and the lower air knife of the double air knife (14) in the grooves of the quartz glass cover (13) and the quartz glass cover (12), respectively, and connect them to the nitrogen bottle (15); S7. Open the nitrogen bottle (15) and use the double air knife (14) to output a continuous nitrogen gas flow between the liquid nitrogen (2) surface and the quartz glass cover plate (13), and between the quartz glass cover plate (13) and the CCD lens (10); S8, using a loading control system to control the actuator (9) to apply a predetermined test load; S9, controlling the CCD lens (10) to collect the test image through the digital image correlation measurement system to achieve the collection of test image data; S10. Post-process the image data using a digital image correlation measurement system to obtain test results.

2. The ultra-low temperature mechanical testing method according to claim 1, characterized in that: In step S3, the liquid nitrogen (2) is added to the liquid nitrogen tank (1) to a predetermined liquid level depth of 0.3m to 0.5m.

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