A positioning device for tensile testing of fiber composite materials under cryogenic conditions

Abstract

This invention discloses a positioning device for tensile testing of fiber-reinforced composite materials under cryogenic conditions, comprising a specimen positioning system, a refrigeration system, and a specimen perpendicularity adjustment system. The refrigeration system includes modules for overall environmental refrigeration, test zone refrigeration, and specimen refrigeration. The specimen positioning system includes a first tensile clamp and a second tensile clamp. The specimen perpendicularity adjustment system includes a suction cup, a multi-degree-of-freedom robotic arm, a gripper, an industrial camera, and an image processing system. The rotation angle calculation includes steps such as binarization processing, edge detection, and Hough transform. After determining the specimen angle and position, the internal telescopic shaft of the tensile clamp extends, pushing the gripper to move and clamp the specimen. Finally, different refrigeration methods are selected according to different experimental requirements to perform tensile testing of the fiber-reinforced composite materials. This invention, through a positioning device for tensile testing of fiber-reinforced composite materials under cryogenic conditions, achieves multi-stage refrigeration of the test environment and automatic specimen replacement and perpendicularity adjustment.

Description

Technical Field

[0001] This invention relates to the field of mechanical property testing technology for composite materials, and in particular to a positioning device for tensile testing of fiber composite materials under cryogenic conditions. Background Technology

[0002] Fiber composites are high-strength, high-modulus fibers with a carbon content exceeding 90%. They are made from acrylic and viscose fibers through high-temperature oxidation and carbonization. Their high-temperature resistance is the best among all synthetic fibers, making them excellent materials for manufacturing high-tech equipment in aerospace and other fields. Accurate characterization of the properties of fiber composites is crucial for their practical application. Tensile testing of fiber composites is a standard evaluation method for the properties of unidirectional fiber-wound composites. The test results are important parameters for product design and finite element analysis; therefore, tensile testing of fiber composites is necessary to provide accurate and effective data.

[0003] In the testing of fiber-reinforced composite materials, the perpendicularity of the specimen is crucial. Otherwise, the tensile force cannot be parallel to the fiber axis during tensile testing, resulting in internal shear forces and distorted test results. Traditional methods of manually clamping specimens and visually adjusting their perpendicularity are no longer sufficient. Furthermore, fiber-reinforced composite materials are widely used for storing cryogenic liquids such as liquid hydrogen and liquid nitrogen, but their tensile performance tests are mostly conducted at room temperature, failing to consider the impact of cryogenic temperatures on tensile properties. Therefore, constructing a multi-gradient cryogenic testing environment to test the low-temperature mechanical properties of fiber-reinforced composite materials has become a key research focus. Moreover, manually changing specimens and adjusting perpendicularity during testing in cryogenic environments can easily cause frostbite to operators, necessitating the development of a device for tensile testing and specimen positioning of fiber-reinforced composite materials in cryogenic environments. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of existing technologies by providing a positioning device for tensile testing of fiber composite materials in a cryogenic environment, enabling multi-stage cooling of the testing environment and automatic specimen replacement and verticality adjustment.

[0005] To solve the above-mentioned technical problems, the present invention provides the following technical solution:

[0006] A positioning device for tensile testing of fiber composite materials under cryogenic conditions is characterized by comprising a specimen positioning system and a cooling system. The specimen positioning system includes a first tensile clamp and a second tensile clamp arranged vertically opposite each other. The first tensile clamp is located at the upper end, and the second tensile clamp is located at the lower end. Both the first and second tensile clamps include a clamping groove, a clamp, a telescopic shaft, a spring, and a limiting baffle. The clamp, spring, and limiting baffle are arranged in pairs. The clamp is wedge-shaped and is disposed in the clamping groove. The contact surface between the clamp and the clamping groove is an inclined surface. The telescopic shaft abuts against the clamp and can push the clamp to slide up and down along the contact surface through telescopic movement. The distance between the clamps changes with sliding. One end of the spring is connected to the clamping groove, and the other end is connected to the clamp. One end of the limiting baffle is fixedly connected to the clamping groove, and the other end abuts against the side of the clamp. The cooling system is disposed around the specimen positioning system to reduce the temperature of the specimen and its surrounding environment.

[0007] Furthermore, the clamp of the first tensile fixture is equipped with a first sensor and a second sensor for positioning the specimen.

[0008] Furthermore, the refrigeration system includes an overall environmental refrigeration system, which includes a liquid nitrogen vaporization device, a liquid nitrogen reflux pipeline, a sealing shell, and an insulation layer. The sealing shell, liquid nitrogen reflux pipeline, and insulation layer are sequentially installed outside the specimen positioning system from the inside out. The liquid nitrogen vaporization device includes a vaporization nozzle and a liquid nitrogen storage tank. The vaporization nozzle is located inside the sealing shell. The liquid nitrogen storage tank is connected to the vaporization nozzle through a pressure supply device. The liquid nitrogen reflux pipeline is connected to the pressure supply device and the liquid nitrogen storage tank.

[0009] Furthermore, the insulation layer is composed of a multi-layered honeycomb vacuum gradient structure, with its density gradually decreasing from the inside to the outside.

[0010] Furthermore, it also includes a lead screw moving device, which includes a stepper motor, a lead screw, and a fixed block. The stepper motor is connected to and drives the lead screw to rotate. The fixed block is threaded onto the lead screw and has a protruding shaft. A limit switch is provided on the travel trajectory of the protruding shaft, and the limit switch can control the stepper motor's on / off state. The refrigeration system includes a test zone refrigeration system, which includes two semi-cylindrical housings. Each housing has an injection hole at the top and an outlet hole at the bottom for liquid nitrogen injection and reflux. Bolts are provided on the edges of the housings, and the two housings are sealed together by bolts. One end of the lead screw is movably connected to the housing, allowing the housing to open and close when the lead screw rotates. The specimen positioning system is located inside the housing.

[0011] Furthermore, the refrigeration system also includes a first specimen refrigeration system, which includes a double-helix refrigeration coil, a helium reservoir, a pressure reducing valve, and a pressure boosting valve. The outlet of the helium reservoir, the pressure reducing valve, the inlet of the double-helix refrigeration coil, the outlet of the double-helix refrigeration coil, the pressure boosting valve, and the inlet of the helium reservoir are sequentially connected to form a circulation loop. The lead screw is movably connected to the double-helix refrigeration coil. When the lead screw rotates, it can realize the movement of the double-helix refrigeration coil, and the specimen is inserted into the double-helix refrigeration coil.

[0012] Furthermore, the refrigeration system also includes a second specimen refrigeration system, which includes an adjustable liquid reservoir, a conduit, and a nitrogen reservoir. The adjustable liquid reservoir includes a support body, a left wall, and a right wall. The left and right walls are hinged to both sides of the support body, and a port is formed between the left and right walls. The nitrogen reservoir is connected to the adjustable liquid reservoir through a conduit. The specimen is embedded in the port through a sealing ring, and the lead screw is movably connected to the adjustable liquid reservoir.

[0013] Furthermore, the system includes a specimen perpendicularity adjustment system, which comprises a suction cup, a multi-degree-of-freedom robotic arm, a gripper, an industrial camera, and an image processing system. The gripper is located at the front end of the multi-degree-of-freedom robotic arm and can rotate freely 360°. The industrial camera is located next to the specimen positioning system. The image processing system is connected to both the industrial camera and the multi-degree-of-freedom robotic arm. The specimen image captured by the industrial camera is transmitted to the image processing system for specimen perpendicularity calculation, and the result is fed back to the multi-degree-of-freedom robotic arm to drive the specimen to rotate.

[0014] Furthermore, the specific steps of the image processing system in calculating the perpendicularity of the specimen are as follows:

[0015] S1: Image binarization processing based on adaptive neighborhood. The adaptive neighborhood is constructed for each seed pixel and consists of all 8-neighbor pixels that satisfy a certain similarity property with the seed pixel. Gray level is used as the basic pixel property. The adaptive neighborhood used for preprocessing is based on an additive or multiplicative tolerance range. All pixels that are 8-neighbors of the seed pixel and whose gray level is within the tolerance range belong to the adaptive neighborhood. f ( i , j The adaptive neighborhood of ) can be expressed as:

[0016] or

[0017] In the formula, f ( i , j ) represents the seed pixel. i and j Seed pixel coordinates,f ( k , l ) represents the seed pixel 8 - its 8 adjacent pixels. k and l For the neighboring pixel coordinates, T 1. T 2 is the parameter of the adaptive neighborhood, representing the maximum dissimilarity between neighboring pixels and the seed pixel;

[0018] The local adaptive threshold is determined based on the pixel value distribution of the pixel's neighboring blocks to establish the binarization threshold at that pixel location. The binarization threshold at each pixel location is determined by the distribution of its surrounding neighboring pixels. The local adaptive threshold is selected as the Gaussian weighted sum of the local neighboring blocks. This algorithm calculates the weighted distance between the surrounding pixels and the center pixel using a Gaussian function. The formula for calculating the Gaussian weight value is as follows:

[0019]

[0020] In the formula, x and y It is the distance between the surrounding pixels and the center pixel in the x and y directions, and σ is... x Standard deviation;

[0021] S2: Perform edge detection. Select the Canny operator to perform edge detection on the image. First, the image... f with operators G n Perform convolution:

[0022]

[0023] In the formula, n is the normal vector of the local edge. G n yes G The first directional derivative along the n-direction; * represents the convolution symbol.

[0024] Next, estimate the normal n of the local edge for each pixel:

[0025]

[0026] The edge is located G n With images f The location of the local maximum of convolution in the n-direction:

[0027]

[0028] The formula for calculating the edge position can be further obtained as follows:

[0029]

[0030] S3: Perform a Hough transform to transform the image from the Cartesian coordinate plane to the polar coordinate plane. Based on the geometric model relationships, we can obtain:

[0031]

[0032] In the formula, ρ The perpendicular distance from the origin to the line is... θ for ρ and x The angle between the positive directions of the axis;

[0033] S4: Solve for the rotation angle by statistically analyzing the peak values ​​of points in the polar coordinate plane, transforming the polar coordinates of the peak positions into the equation of a straight line in the Cartesian coordinate plane, thereby obtaining the tilt angle of the specimen. θ .

[0034] Compared with existing technologies, the beneficial effects of this invention are: 1. By constructing a multi-stage refrigeration system including an overall environmental refrigeration system, a zone refrigeration system, and a specimen refrigeration system, multi-stage gradient refrigeration of fiber composite materials under different required test temperatures can be achieved, meeting the mechanical property testing requirements and tensile performance indicators of fiber-reinforced composite materials at different temperatures. 2. The specimen positioning system can achieve automatic positioning and stretching of the specimen, preventing frostbite hazards to operators under manual operation. 3. The verticality adjustment system can overcome the defects of relying on visual adjustment of specimen verticality, ensuring that the verticality accuracy of the specimen meets the test requirements, making the test results more accurate. 4. The lead screw moving device can be dynamically connected to the zone refrigeration system, the first specimen refrigeration system, and the second specimen refrigeration system according to different experimental needs, realizing the function of a rapid switching device. Attached Figure Description

[0035] Figure 1 This is a schematic diagram of the structure of Embodiment 1 of the present invention;

[0036] Figure 2 This is a schematic diagram of the first tensioning fixture structure according to Embodiment 1 of the present invention;

[0037] Figure 3 This is a schematic diagram of the specimen clamping surface structure of the chuck in Embodiment 1 of the present invention;

[0038] Figure 4 This is a schematic diagram of the cross-sectional structure of the insulation layer in Embodiment 1 of the present invention;

[0039] Figure 5 This is a schematic diagram of the casing structure according to Embodiment 1 of the present invention;

[0040] Figure 6 This is a schematic diagram of the structure of Embodiment 2 of the present invention;

[0041] Figure 7This is a schematic diagram of the refrigeration system structure of the first specimen in Embodiment 2 of the present invention;

[0042] Figure 8 This is a schematic diagram of the structure of Embodiment 3 of the present invention;

[0043] Figure 9 This is a schematic diagram of the adjustable liquid storage head structure in Embodiment 3 of the present invention;

[0044] Figure 10 This is a schematic diagram of the specimen perpendicularity adjustment system in an embodiment of the present invention;

[0045] Wherein: 1-First tensile clamp, 2-Second tensile clamp, 3-Specimen, 4-Screw moving device, 11-Chuck groove, 12-Chuck, 13-Telescopic shaft, 14-Spring, 15-Limiting baffle, 16-First sensor, 17-Second sensor, 21-Liquid nitrogen reflux pipeline, 22-Sealed housing, 23-Insulation layer, 24-Vaporization nozzle, 25-Liquid nitrogen storage tank, 26-Pressure supply device, 27-Hinge, 31-Cover, 32-Injection hole, 33-Outflow hole 34- Bolt, 41- Stepper motor, 42- Lead screw, 43- Fixing block, 44- Protruding shaft, 45- Limit switch, 51- Double helix cooling coil, 52- Helium storage container, 53- Pressure reducing valve, 54- Pressure boosting valve, 61- Conduit, 62- Nitrogen storage container, 63- Support body, 64- Left wall, 65- Right wall, 66- Port, 67- Sealing ring, 71- Suction cup, 72- Multi-DOF robotic arm, 73- Gripper, 74- Industrial camera, 75- Vacuum pump, 76- Feeding tray. Detailed Implementation

[0046] To enhance understanding of the present invention, we will now describe it in further detail with reference to the accompanying drawings. These embodiments are for illustrative purposes only and do not constitute a limitation on the scope of protection of the present invention.

[0047] Example 1:

[0048] Figure 1-4This paper presents an embodiment of a positioning device for tensile testing of fiber composite materials under cryogenic conditions. The device includes a specimen positioning system, a lead screw moving device 4, and a refrigeration system. The specimen positioning system includes a first tensile clamp 1 and a second tensile clamp 2 arranged vertically opposite each other. The first tensile clamp 1 is located at the upper end, and the second tensile clamp 2 is located at the lower end. Both the first tensile clamp 1 and the second tensile clamp 2 include a clamp groove 11, a clamp 12, a telescopic shaft 13, a spring 14, and a limiting baffle 15. The clamp 12, spring 14, and limiting baffle 15 are arranged in pairs. The clamp 12 is wedge-shaped and is disposed within the clamp groove 11. The connection between the clamp 12 and the clamp groove 11... The contact surface is an inclined plane. The telescopic shaft 13 abuts against the clamp 12 and can push the clamp 12 to slide up and down along the contact surface through telescopic movement. The distance between the clamps 12 will change with sliding. One end of the spring 14 is connected to the clamp groove 11 and the other end is connected to the clamp 12. One end of the limiting baffle 15 is fixedly connected to the clamp groove 11 and the other end abuts against the side of the clamp 12. The clamp 12 of the first tension fixture 1 is provided with a first sensor 16 and a second sensor 17 for positioning the specimen 3. The cooling system is set around the specimen positioning system to reduce the temperature of the specimen 3 and its surrounding environment, including the overall environmental cooling system and the test area cooling system.

[0049] The lead screw moving device includes a stepper motor 41, a lead screw 42, and a fixed block 43. The stepper motor 41 is connected to and drives the lead screw 42 to rotate. The fixed block 43 is threadedly connected to the lead screw 42. A protruding shaft 44 is provided on the fixed block 43. A limit switch 45 is provided on the travel trajectory of the protruding shaft 44. The limit switch 45 can control the switching of the stepper motor 41.

[0050] The overall environmental refrigeration system includes a liquid nitrogen vaporization device, a liquid nitrogen reflux pipeline 21, a sealed shell 22, and an insulation layer 23. The sealed shell 22, liquid nitrogen reflux pipeline 21, and insulation layer 23 are sequentially installed outside the specimen positioning system and the screw moving device 4 from the inside out. The liquid nitrogen vaporization device includes a vaporization nozzle 24 and a liquid nitrogen storage tank 25. The vaporization nozzle 24 is located inside the sealed shell 22. The liquid nitrogen storage tank 25 is connected to the vaporization nozzle 24 via a pressure supply device 26. The liquid nitrogen reflux pipeline 21 is connected to the pressure supply device 26 and the liquid nitrogen storage tank 25. Figure 4As shown, the insulation layer 23 is composed of a multi-layered honeycomb vacuum gradient structure, with its density gradually decreasing from the inside to the outside, increasing heat reflection. The interior of the insulation layer is evacuated to a vacuum state. After the specimen 3 is clamped, the sealing shell 22 is rotated so that it can rotate around the hinge 27 to close. The target ambient temperature and change threshold are set, the vaporization nozzle 24 and the liquid nitrogen return pipeline 21 are opened, and the pressure supply device 26 is turned on to perform dual cooling through liquid nitrogen vaporization and radiation cooling. When the temperature sensor detects that the environment has reached the target temperature, the vaporization nozzle 24 is closed, the liquid nitrogen return pipeline 21 stops operating, and the test environment is kept in a heat preservation state; when the temperature is higher than the threshold temperature, the vaporization nozzle 24 is opened again, and the liquid nitrogen return pipeline 21 starts operating, achieving dynamic balance of the overall ambient temperature.

[0051] The test chamber cooling system includes two semi-cylindrical housings 31. Each housing 31 has an injection port 32 at the top and an outlet port 33 at the bottom for liquid nitrogen injection and reflux. Bolts 34 are attached to the edges of each housing 31, sealing the two housings together. One end of a lead screw 42 is movably connected to a housing 31. When the stepper motor 41 drives the lead screw 42 to rotate, the two housings 31 can move relative to each other, opening and closing. A specimen positioning system is located inside the housing 31. After the specimen 3 is clamped, the stepper motor 41 drives the lead screw 42 to rotate, causing the housing 31 to move and gradually close. When the limit switch 45 is activated, the housing 31 stops moving. At this time, the solenoid valve in the injection port 32 opens, injecting liquid nitrogen into the housing 31. The liquid nitrogen injection time is determined by the size of the port and the liquid nitrogen flow rate. At the end of the test, the solenoid valve in the outlet port 33 opens, liquid nitrogen flows out of the housing 31, the housing 31 opens, and the specimen 3 is removed, completing the test.

[0052] Example 2:

[0053] Figure 5 , 6This document presents a second embodiment of a positioning device for tensile testing of fiber composite materials under cryogenic conditions. The specimen positioning system and screw movement device are the same as in the first embodiment. The refrigeration system uses a first specimen refrigeration system, which includes a double-helix refrigeration coil 51, a helium reservoir 52, a pressure reducing valve 53, and a pressure boosting valve 54. The outlet of the helium reservoir 52, the pressure reducing valve 53, the inlet of the double-helix refrigeration coil 51, the outlet of the double-helix refrigeration coil 51, the pressure boosting valve 54, and the inlet of the helium reservoir 52 are sequentially connected to form a circulation loop. The screw 42 is movably connected to the double-helix refrigeration coil 51. When the screw 42 rotates, it moves the double-helix refrigeration coil 51, and the specimen 3 is inserted into the double-helix refrigeration coil 51. The testing method and steps of the first specimen refrigeration system differ slightly from those of the overall environmental refrigeration system and the test zone refrigeration system. First, the angle of the specimen 3 is determined, and verticality adjustment is performed. Then, the stepper motor 41 drives the lead screw 42 to rotate, which in turn moves the double-helix cooling coil 51. When the limit switch 45 is triggered, the specimen 3 is sent into the double-helix cooling coil 51, and then the specimen is positioned and clamped. The pressure reducing valve 53 and the pressure boosting valve 54 are opened, and liquid helium saturated vapor is injected to achieve refrigeration.

[0054] Example 3:

[0055] Figure 7 , 8 This paper presents a third embodiment of a positioning device for tensile testing of fiber composite materials under cryogenic conditions. The specimen positioning system and the screw moving device 4 are the same as in the first embodiment. The refrigeration system adopts a second specimen refrigeration system, which includes an adjustable liquid reservoir 6, a conduit 61, and a nitrogen reservoir 62. The adjustable liquid reservoir 6 includes a support body 63, a left wall 64, and a right wall 65. The left wall 64 and the right wall 65 are hinged to both sides of the support body 63, and a port 66 is formed between the left wall 64 and the right wall 65. The nitrogen reservoir 62 is connected to the adjustable liquid reservoir 6 through the conduit 61. The specimen 3 is embedded in the port 66 through a sealing ring 67. The screw 42 is movably connected to the adjustable liquid reservoir 6. The width of port 66 of the adjustable reservoir head 6 is adjustable and needs to be set in advance according to the width of the sample 3. The adjustable reservoir head 6 has an injection hole at the top and an outlet hole at the bottom, and the opening and closing of the holes is controlled by a solenoid valve. The injection hole and the outlet hole are connected to the nitrogen reservoir 62 through a conduit 61. At the beginning of the test, the injection hole is open and the outlet hole is closed. At the end of the test, the injection hole is closed and the outlet hole is open. The adjustable reservoir head 6 moves left and right through a lead screw 42 under the action of a stepper motor 41. A sealing ring 67 is fixed at port 66 of the adjustable reservoir head 6. The sealing ring 67 is in direct contact with the sample 3 and generates friction force between them. The measured tensile force in the test needs to be reduced by the friction force, and the friction force should be tested before the test.

[0056] Preferably, in embodiments one, two, and three above, a specimen verticality adjustment system is also provided. The specimen verticality adjustment system includes a suction cup 71, a multi-degree-of-freedom robotic arm 72, a gripper 73, an industrial camera 74, and an image processing system. The gripper 73 is located at the front end of the multi-degree-of-freedom robotic arm 72 and can achieve 360° free rotation. The industrial camera 74 is located next to the specimen positioning system. The image processing system is connected to both the industrial camera 74 and the multi-degree-of-freedom robotic arm 72. After the specimen photo taken by the industrial camera 74 is transmitted to the image processing system to solve the specimen verticality, the result is fed back to the multi-degree-of-freedom robotic arm 72 to drive the specimen 3 to rotate.

[0057] The specific method for specimen clamping and perpendicularity adjustment is as follows: Vacuum pump 75 is turned on, suction cup 71 picks up specimen 3 from feeding tray 76, clamping nozzle 73 holds the lower end of specimen 3, and sends it to industrial camera 74. Then, industrial camera 74 takes a picture and transmits it to image processing system to solve the current specimen perpendicularity. The specific steps are as follows:

[0058] S1: Image binarization based on adaptive neighborhood. The adaptive neighborhood is constructed for each seed pixel and consists of all 8-neighbor pixels that satisfy a certain similarity property to the seed pixel. Using grayscale level as the basic pixel property, the adaptive neighborhood used for preprocessing is based on an additive or multiplicative tolerance range. All pixels 8-neighbors of the seed pixel whose grayscale level falls within the tolerance range belong to the adaptive neighborhood. f ( i , j The adaptive neighborhood of ) can be expressed as:

[0059] or

[0060] In the formula, f ( i , j ) represents the seed pixel. i and j Seed pixel coordinates, f ( k , l ) represents the seed pixel 8 - its 8 adjacent pixels. k and l For the neighboring pixel coordinates, T 1. T 2 is the parameter of the adaptive neighborhood, representing the maximum dissimilarity between neighboring pixels and the seed pixel.

[0061] The local adaptive threshold determines the binarization threshold at a pixel location based on the pixel value distribution of its neighboring blocks. The binarization threshold at each pixel location is determined by the distribution of its surrounding neighboring pixels. The local adaptive threshold uses a Gaussian weighted sum of local neighboring blocks. This algorithm calculates the weighted sum based on the distance between surrounding pixels and the center pixel using a Gaussian function. The formula for calculating the Gaussian weight is as follows:

[0062]

[0063] In the formula, x and y It is the distance between the surrounding pixels and the center pixel in the x and y directions, and σ is... x The standard deviation.

[0064] S2: Perform edge detection. Select the Canny operator to perform edge detection on the image. First, the image... f with operators G n Perform convolution:

[0065]

[0066] In the formula, n is the normal vector of the local edge; G n yes G The first directional derivative along the n-direction; * is the convolution symbol.

[0067] Next, estimate the normal n of the local edge for each pixel:

[0068]

[0069] The edge is located G n With images f The location of the local maximum of convolution in the n-direction:

[0070]

[0071] The formula for calculating the edge position can be further obtained as follows:

[0072]

[0073] S3: Perform the Hough transform. Transform the image from the Cartesian coordinate plane to the polar coordinate plane. Based on the geometric model, we can obtain:

[0074]

[0075] In the formula, ρ The perpendicular distance from the origin to the line; θ for ρ and xThe angle between the positive directions of the axis.

[0076] S4: Solving for the rotation angle. Statistically determine the peak values ​​of points in the polar coordinate plane, transform the polar coordinates of the peak positions into the equation of a straight line in the Cartesian coordinate plane, thereby obtaining the tilt angle of the specimen. θ .

[0077] Then, the image processing system feeds back the tilt angle to the multi-degree-of-freedom robotic arm 72, and the gripper 73 rotates according to the feedback tilt angle to ensure that the specimen 3 is in a vertical state for stretching.

[0078] The clamp 73 vertically delivers the specimen 3 to the specimen positioning system area. The clamp 73 first moves the specimen 3 laterally, stopping when the first sensor 16 is triggered. Then it moves the specimen 3 longitudinally, stopping when the second sensor 17 is triggered, indicating that the fixture has been accurately positioned. Then, the telescopic shaft 13 in the first tension fixture 1 extends, pushing the clamp 12 downward. The distance between the two clamps 12 gradually decreases, clamping the upper end of the specimen 3. Then the clamp 73 releases, and the telescopic shaft 13 in the second tension fixture 2 extends, pushing the clamp 12 upward. The distance between the two clamps 12 gradually decreases, clamping the lower end of the specimen 3, thus completing the clamping of the specimen 3.

[0079] The above specific embodiments are only for illustrating the technical concept and structural features of the present invention, and are intended to enable those skilled in the art to implement them. However, the above content does not limit the scope of protection of the present invention. Any equivalent changes or modifications made in accordance with the spirit and essence of the present invention should fall within the scope of protection of the present invention.

Claims

1. A tensile testing positioning device for fiber composites in a cryogenic environment, characterized by: The test piece positioning system comprises a first stretching clamp (1) and a second stretching clamp (2) oppositely arranged in a vertical direction, the first stretching clamp (1) is located at an upper end, and the second stretching clamp (2) is located at a lower end, the first stretching clamp (1) and the second stretching clamp (2) each comprise a chuck groove (11), a chuck (12), an extension shaft (13), a spring (14) and a limiting baffle (15), the chuck (12), the spring (14) and the limiting baffle (15) are arranged in pairs, the chuck (12) is a wedge-shaped block, which is arranged in the chuck groove (11), a contact surface between the chuck (12) and the chuck groove (11) is an inclined surface, the extension shaft (13) abuts against the chuck (12) and can push the chuck (12) to slide up and down along the contact surface through extension and retraction, the distance between the chucks (12) changes with the sliding, one end of the spring (14) is connected to the chuck groove (11), and the other end is connected to the chuck (12), one end of the limiting baffle (15) is fixedly connected to the chuck groove (11), and the other end abuts against a side surface of the chuck (12), and the refrigeration system is arranged around the test piece positioning system and is used to reduce the temperature of the test piece (3) and the surrounding environment thereof. The refrigeration system comprises an overall environment refrigeration system, the overall environment refrigeration system comprises a liquid nitrogen vaporization device, a liquid nitrogen return pipeline (21), a sealed shell (22) and an adiabatic layer (23), the sealed shell (22), the liquid nitrogen return pipeline (21) and the adiabatic layer (23) are sequentially arranged outside the test piece positioning system from inside to outside, the liquid nitrogen vaporization device comprises a vaporization nozzle (24) and a liquid nitrogen storage tank (25), the vaporization nozzle (24) is arranged inside the sealed shell (22), the liquid nitrogen storage tank (25) is connected to the vaporization nozzle (24) through a pressure supply device (26), and the liquid nitrogen return pipeline (21) is connected to the pressure supply device (26) and the liquid nitrogen storage tank (25); The adiabatic layer (23) is composed of a plurality of honeycomb-shaped vacuum gradient structures, and the density gradually decreases from inside to outside. Also include the screw moving device (4), the screw moving device includes the step motor (41), the screw (42), the fixed block (43), the step motor (41) is connected and drives the screw (42) rotation, the fixed block (43) is threadedly connected on the screw (42), the fixed block (43) is provided with the protruding shaft (44), is provided with the travel switch (45) on the travel locus of the protruding shaft (44), the travel switch (45) can control the switch of step motor (41);The refrigeration system includes test interval refrigeration system, the test interval refrigeration system includes two half cylindrical shells (31), the shell (31) is provided with injection hole (32) above, is provided with efflux hole (33) below, is used for the injection and backflow of liquid nitrogen, the edge of the shell (31) is provided with bolt (34), two shell (31) are sealedly connected through bolt (34), one end of the screw (42) is movably connected in the shell (31), when the screw (42) rotates, the shell (31) can realize opening and closing, the test piece positioning system is arranged in the shell (31).

2. The positioning device for tensile testing of fiber composite materials in cryogenic environments according to claim 1, characterized in that The chuck (12) of the first stretching clamp (1) is provided with a first sensor (16) and a second sensor (17) for positioning the test piece (3).

3. The positioning device for tensile testing of fiber composite materials in cryogenic environments according to claim 1, characterized in that The refrigeration system further includes a first test piece refrigeration system, the first test piece refrigeration system includes a double-helix refrigeration ring (51), a helium storage device (52), a pressure reducing valve (53) and a pressure increasing valve (54), the outlet of the helium storage device (52), the pressure reducing valve (53), the inlet of the double-helix refrigeration ring (51), the outlet of the double-helix refrigeration ring (51), the pressure increasing valve (54) and the inlet of the helium storage device (52) are sequentially connected to form a circulation loop, the screw (42) is movably connected to the double-helix refrigeration ring (51), and the double-helix refrigeration ring (51) can be moved when the screw (42) rotates, and the test piece (3) is arranged in the double-helix refrigeration ring (51).

4. The positioning device for tensile testing of fiber composite materials in cryogenic environments according to claim 1, characterized in that The refrigeration system further includes a second test piece refrigeration system, the second test piece refrigeration system includes an adjustable liquid storage head (6), a conduit (61) and a nitrogen storage device (62), the adjustable liquid storage head (6) includes a support body (63), a left wall (64) and a right wall (65), the left wall (64) and the right wall (65) are hingedly connected to two sides of the support body (63), and a port (66) is formed between the left wall (64) and the right wall (65), the nitrogen storage device (62) is communicated with the adjustable liquid storage head (6) through the conduit (61), the test piece (3) is embedded on the port (66) through a sealing ring (67), and the screw (42) is movably connected to the adjustable liquid storage head (6).

5. The positioning device for tensile testing of fiber composite materials in cryogenic environments according to claim 1, characterized in that The test piece perpendicularity adjustment system comprises a suction disc (71), a multi-degree-of-freedom mechanical arm (72), a clamping nozzle (73), an industrial camera (74) and an image processing system, the clamping nozzle (73) is arranged at the front end of the multi-degree-of-freedom mechanical arm (72) and can rotate freely by 360 degrees, the industrial camera (74) is arranged beside the test piece positioning system, the image processing system is connected with the industrial camera (74) and the multi-degree-of-freedom mechanical arm (72) respectively, the test piece (3) photo taken by the industrial camera (74) is transmitted to the image processing system for solving the test piece perpendicularity, and then the result is fed back to the multi-degree-of-freedom mechanical arm (72) to drive the test piece (3) to rotate.

6. The tensile testing positioning device for fiber composite materials in cryogenic environments of claim 5, wherein: The specific steps for solving the perpendicularity of the test piece by the image processing system are as follows: S1: adaptive neighborhood based image binarization, the adaptive neighborhood is constructed for each seed pixel, which consists of all 8-adjacent pixels that satisfy a certain similarity property with the seed pixel, the gray level is taken as the basic pixel property, the adaptive neighborhood for pre-processing is based on an additive or multiplicative tolerance interval, all 8-adjacent pixels of the seed pixel and the gray level within the tolerance interval belong to the adaptive neighborhood, the pixel f ( i , j ) adaptive neighborhood can be expressed as: or wherein f ( i , j ) are the seed pixel, i and j are the seed pixel coordinates, f ( k , l ) are the 8-adjacent pixels of the seed pixel, k and l are the neighborhood pixel coordinates, T 1、 T 2 is a parameter of the adaptive neighborhood, representing the maximum dissimilarity of a neighboring pixel from the seed pixel; The local adaptive threshold determines the binarization threshold at the pixel position according to the pixel value distribution of the neighborhood block of the pixel, the binarization threshold at each pixel position is determined by the distribution of the surrounding neighborhood pixels, the local adaptive threshold selects the Gaussian weighted sum of the local neighborhood block, and the Gaussian weight value is calculated according to the distance between the surrounding pixels and the center pixel, and the Gaussian weight value calculation formula is as follows: wherein x and y are the distances of the surrounding pixels and the center pixel in the x and y directions, and σ is x the standard deviation of the distances. S2: edge detection, select Canny operator to detect the edge of the image, first convert the image f with the operator G n convolution: where n is the normal vector of the local edge, G n is G the first directional derivative in the n direction, * is the convolution symbol; The normal n of each pixel local edge is re-estimated: The edge is located at G n With the image f The convolution is at the position of the local maximum in the n direction: Further, the calculation formula of the edge position is as follows: S3: perform Hough transform to transform the image from the Cartesian coordinate plane to the polar coordinate plane, and according to the geometric model relationship, the following can be obtained: wherein ρ is the perpendicular distance from the origin to the line, θ is ρ is the angle between the positive direction of the axis and x the positive direction of the axis. S4: rotation angle solving, statistics of the peak value of the polar coordinate plane point, the peak value position polar coordinate is transformed into the straight line equation of the Cartesian coordinate plane, so as to obtain the inclination angle of the test piece θ .

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