Stretching type stress corrosion testing machine with microcrack detection function

Abstract

The invention provides a tensile stress corrosion testing machine with a microcrack detection function. The tensile stress corrosion testing machine comprises a weight lever tensile device, a fracture time monitoring system, a corrosion environment box and a data acquisition system, wherein infrared timing systems are attached to the two ends of the weight lever tensile device; the sample is arranged in the corrosion environment box and is connected with the weight lever stretching device through the sample clamp; the weight lever stretching device is used for carrying out dead-load stretching on the sample in the corrosion environment box, and the data acquisition system is used for automatically acquiring and recording crack initiation and expansion images of the sample in an experiment process. By applying the technical scheme, the problem that crack initiation and expansion cannot be combined with time at the same time in the prior art can be solved.

Description

Technical Field

[0001] The present invention relates to the technical field of stress corrosion testing of metal materials, and in particular to a tensile stress corrosion testing machine with a microcrack detection function. Background Art

[0002] When metal materials and their components are in long-term service in harsh environments such as corrosive environments and stress, their service reliability and stability are important factors affecting the operation safety and economy of metal parts. Metal materials such as stainless steel, iron and steel materials, and aluminum bronze are in long-term service in high-stress and water corrosion environments, and stress corrosion cracking (SCC) will occur under the interaction of corrosion and stress. For example, in December 1967, a suspension bridge in West Virginia, USA, suddenly collapsed, and the entire structure fell into the river. After investigation, it was found that due to the high-load and low-temperature environment of the bridge, stress corrosion occurred due to rusting of the main connection joints; in recent years, similar pipeline accidents have occurred frequently, attracting wide attention. On August 15, 2021, a natural gas transmission pipeline in Coolidge, Arizona, USA, had an accident. This pipeline with a diameter of 30 inches had serious problems with the tape winding method, resulting in stress corrosion cracking, and finally broke at the weld, causing the pipeline to rupture. A large amount of natural gas vapor leaked, was instantly ignited and caused a violent explosion. The shock wave and the raging fire generated by the explosion completely destroyed a farmhouse about 451 feet away. Tragically, 2 people in the farmhouse were unfortunately killed and 1 person was seriously injured. After a detailed investigation by the National Transportation Safety Board of the United States, it was determined that the improper tape winding was the culprit of this accident.

[0003] Worldwide, there have been many incidents of pipeline failures caused by stress corrosion cracking (SCC). In China, it has not been spared either. At 6:00 in the morning on January 25, 2021, near Jinbo Seaview in Youyi Sub-district, Jinpu New Area, Dalian City, Liaoning Province, a gas pipeline explosion accident broke the early morning tranquility. After investigation, the accident was caused by the lack of soil support at the bottom of the pipeline, resulting in partial suspension. At the same time, the anti-corrosion coating in the weld area of the pipeline cracking was missing, and the cathodic protection measures were not set according to the design requirements. The accumulated water continuously eroded the pipeline, and coupled with the additional vertical load above the pipeline, a strong tensile stress was formed at the pipeline leakage point, ultimately causing the pipeline girth weld to crack and leak. The leaked gas diffused everywhere through underground spaces such as underground cables, forming a highly dangerous explosive gas around. When encountering an open fire, it instantly triggered an explosion. This accident caused the tragic consequences of 3 deaths and 6 minor injuries.

[0004] At 4:24 on June 18, 2022, a serious explosion accident occurred in the area of the ethylene oxide refining tower of the ethylene glycol unit in the Chemical Engineering Department of Shanghai Petrochemical Co., Ltd. The direct cause of the accident was that during the long-term operation of the pipeline weld, under the continuous action of the corrosive medium chloride ion, cracks gradually initiated at the high-stress part of the inner wall weld toe. Over time, these cracks continuously corroded and fatigued and expanded along the circumferential and wall thickness directions. When the cracks on the weld expanded to the point where the effective load-bearing cross-section was insufficient, under the action of the axial stress, the weld was completely pulled apart, resulting in the pipeline rupture and further triggering an explosion. This accident caused 1 death, 1 injury, and the direct economic loss was approximately 9.7148 million yuan. The initiation time of SCC cracks in the actual service environment is generally relatively long. Under laboratory conditions, accelerated experiments usually need to be carried out, applying a certain tensile stress / strain to the specimen to promote the initiation of cracks.

[0005] The commonly used experimental method is the constant load tensile experiment, which can efficiently compare the SCC sensitivities of different materials or the same material under different corrosion environments and different stress conditions. Under laboratory conditions, accelerated experiments usually need to be carried out, applying a certain tensile stress / strain to the specimen to promote the initiation of cracks, and the operation convenience is not enough. Summary of the Invention

[0006] In view of this, the purpose of the present invention is to provide a tensile stress corrosion testing machine with a micro-crack detection function, to solve the problems that the prior art cannot combine the initiation and propagation of cracks with time, etc.

[0007] To achieve the above purpose, the present invention adopts the following technical solutions: A tensile stress corrosion testing machine with a micro-crack detection function includes a weight lever tensile device, a monitoring fracture time system, a corrosion environment chamber, a data acquisition system, and an infrared timing system attached to both ends of the weight lever tensile device; the specimen is arranged in the corrosion environment chamber, and the specimen is connected to the weight lever tensile device through a specimen fixture; the weight lever tensile device performs constant load tensile on the specimen in the corrosion environment chamber, and the data acquisition system automatically collects and records the images of the initiation and propagation of cracks on the specimen during the experiment.

[0008] In a preferred embodiment, the corrosion environment chamber includes a box body and a box cover; the top of the box cover is connected to the upper guide shaft through a connecting piece, and a water inlet and a water outlet are arranged in the box cover, and are communicated with the box body through the water inlet and the water outlet. The upper guide shaft performs a constant load tensile experiment on the specimen in the corrosion environment chamber; the weight lever tensile device performs constant load tensile on the specimen in the corrosion environment chamber, and the data acquisition system automatically collects and records the surface change images of the specimen.

[0009] In a preferred embodiment, the testing machine is fixed on the bottom plate, and the bottom plate is fixed to the ground through anchor bolts, forming an integrated structure that can be moved as a whole; guiding columns are evenly distributed on the bottom plate, and the guiding columns and the bottom plate are fixed by latches; a horizontally arranged fixing plate is connected to the guiding columns, which are, from bottom to top, the force sensor fixing plate, the corrosion environment box fixing plate, and the three-layer plate, and the fixing plate and the guiding columns are fixed by latches; the top of the box body is connected by a lifting steel cable, which sequentially passes through the central holes of the three-layer plate, the upper guiding shaft, and the fulcrum of the lever, and the box body of the corrosion environment box is lifted and lowered through a handwheel adjustment system; the handwheel adjustment system is composed of an adjustment handwheel, a connecting bearing, and a lower guiding bearing; the box cover is fixed on the box cover fixing plate through a flange and fastened with bolts, a hot water inlet and an outlet are connected to the box cover, and the outlet on the inner side of the box cover is connected to a stainless steel pipe that extends into the bottom of the kettle body, and the handwheel adjustment system is fixed on the fixing plate by bolts; an internal force elimination column is connected between the handwheel fixing plate and the box cover fixing plate, and both ends of the internal force elimination column are connected by threads.

[0010] In a preferred embodiment, the monitoring fracture time system includes an optoelectronic device, and the optoelectronic device controls the operation of the motor through distance; when the distance is less than or equal to a certain value d, the optoelectronic device will continuously close, and then start the motor to control the operation of the timing device, otherwise the optoelectronic device disconnects, stopping the timing device, and thus obtaining the fracture time of the specimen test.

[0011] In a preferred embodiment, the monitoring fracture time system includes an optoelectronic device, a motor, and a timing device. The optoelectronic device is an infrared diffuse reflection optoelectronic switch, and the wiring port is connected to the motor, and the motor controls the timing device; the maximum distance of the light ray emitted by the optoelectronic device is d. When the specimen is not fractured, the distance from the optoelectronic device to the upper top plate of the stress corrosion device <= d, and at this time the optoelectronic device is in a closed state, the motor starts, and the stopwatch is in a timing state; when the experiment is completed, the ray emitted by the optoelectronic device cannot recognize the upper top plate, and the distance from the optoelectronic device to the upper top plate > d, thereby causing the optoelectronic device to disconnect and the stopwatch to stop timing, and outputting the fracture time.

[0012] In a preferred embodiment, the image acquisition system includes an acquisition camera, a camera guide rail, and a camera support platform. The acquisition camera has an adjustment device to achieve 0° - 180° rotation adjustment, and the camera guide rail on the camera support platform realizes the freedom of left - right translation of the acquisition camera, and the position of the acquisition camera is adjusted according to requirements.

[0013] In a preferred embodiment, the principle of calculating displacement field data after image acquisition: Normalized least - square distance function:

[0014]

[0015] where f and g are the reference image and the deformed image respectively. This calculation formula is not sensitive to the overall brightness change of the image;

[0016] After obtaining the full-field displacement field distribution, the strain is obtained by using the SG filter; the relationship between displacement and strain is described by the first-order shape function, where u and v represent the lateral and vertical displacements.

[0017]

[0018] In a preferred embodiment, a limiting device is further included, and the limiting device includes a limiting rod and a limiting plate; the root of the limiting rod has a thread that mates with the top plate.

[0019] In a preferred embodiment, the limiting plate is fixed in cooperation with the upper guide shaft through the middle threaded hole therein; the limiting device restricts the position of the upper guide shaft when the experiment is completed, so that the upper guide shaft stops in time.

[0020] In a preferred embodiment, the constant load tensile device of the weight lever includes a lever, a weight pan, a roller, a deep groove ball bearing, a lifting ring, a roller support, a roller guide rail, an upper guide shaft, a force sensor, and a displacement sensor; the roller is fixed on the lifting ring through the roller support, and the lifting ring transmits the tensile force generated by the lever to the specimen through the upper guide shaft; the tensile force required for the test is provided by changing the mass on the weight pan, and the tensile force is measured by the force sensor.

[0021] Compared with the prior art, the present invention has the following beneficial effects: realizing the recording of the moment when cracks initiate in the stress corrosion experiment, and providing a new research direction for the research on the detection of microcracks in the stress corrosion experiment Brief Description of the Drawings

[0022] Figure 1 It is a schematic diagram of the overall structure of the preferred embodiment of the present invention;

[0023] Figure 2 It is a schematic top view of the preferred embodiment of the present invention;

[0024] Figure 3 It is a schematic right view of the preferred embodiment of the present invention;

[0025] Figure 4 It is a schematic diagram of the structure of the monitoring fracture time system of the preferred embodiment of the present invention;

[0026] Figure 5 It is a schematic diagram of the usage state of the monitoring fracture time system of the preferred embodiment of the present invention;

[0027] Figure 6 It is a schematic diagram of the structure of the data acquisition system of the preferred embodiment of the present invention;

[0028] Figure 7Schematic diagram of the partial structure of the weight lever stretching device according to the preferred embodiment of the present invention;

[0029] Figure 8 Schematic diagram of the working principle of the monitoring fracture time system according to the preferred embodiment of the present invention;

[0030] Figure 9 Diagram of the flat specimen (including notch) according to the preferred embodiment of the present invention;

[0031] Figure 10 Simulation result diagram of the stress distribution when the specimen according to the preferred embodiment of the present invention is subjected to tensile force;

[0032] Figure 11 Time-strain curve of the stress corrosion tensile test of the flat specimen according to the preferred embodiment of the present invention.

[0033] Reference numerals: 1 - lever, 2 - weight pan, 3 - roller, 4 - limit rod, 5 - limit plate, 6 - fixture, 7 - connecting bearing, 8 - adjusting handwheel, 9 - floor footing, 10 - adjusting guide bearing, 11 - force sensor, 12 - lower guide bearing, 13 - specimen, 14 - upper guide shaft, 15 - T-shaped bearing, 16 - bottom plate, 17 - column, 18 - top plate, 19 - second-layer plate, 20 - third-layer plate; 28 - upper top plate, 29 - light rays emitted by the optoelectronic device, 30 - optoelectronic device, 31 - control motor, 32 - specimen, 33 - lower top plate; 34 - specimen after fracture; 21 - acquisition camera, 22 - camera guide rail, 23 - camera support platform; 24 - lifting ring, 25 - deep groove ball bearing, 26 - roller support, 27 - roller guide rail; 35 - fulcrum shaft, 36 - bearing support, 37 - linear bearing, 38 - linear bearing of the upper guide shaft, 39 - adjusting nut; 40 - position where the corrosion environment chamber is located. Detailed implementation manners

[0034] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0035] It should be noted that the following detailed descriptions are all illustrative and are intended to provide further explanations for the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present application belongs.

[0036] It should be noted that the terms used herein are only for describing specific implementation manners and are not intended to limit the exemplary embodiments according to the present application; as used herein, unless the context clearly indicates otherwise, the singular forms are also intended to include the plural forms. In addition, it should be understood that when the terms "include" and / or "comprise" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or their combinations.

[0037] A tensile stress corrosion testing machine with crack detection function, refer to Figures 1-11 , including a weight lever stretching device, a monitoring fracture time system, a corrosion environment chamber, a data acquisition system, an infrared timing system attached to both ends of the weight lever stretching system, a tensile specimen is arranged in the corrosion environment chamber, and the specimen 13 is connected to the stretching device through a specimen fixture 6. The weight lever stretching device conducts constant load stretching on the test specimen in the corrosion environment chamber, and the data acquisition system automatically acquires and records images of crack initiation and propagation of the specimen during the experiment. Among them, the weight lever stretching device consists of an upper guiding shaft 14, a force sensor 11, a displacement sensor 41, etc.

[0038] For the tensile stress corrosion testing machine with crack detection function, the corrosion environment chamber is a structure with the box body and the box cover arranged up and down. The top of the box cover is connected to the upper guiding shaft through a connecting piece. An inlet and an outlet are arranged in the box cover and are communicated with the box body through the inlet and the outlet. The upper guiding shaft 14 conducts a stretching experiment on the specimen in the corrosion environment chamber; conducts constant load stretching on the specimen in the corrosion environment chamber through the weight lever stretching device, automatically acquires and records images of the specimen surface change through the data acquisition system, and outputs a time-strain curve through the displacement sensor 41.

[0039] For the tensile stress corrosion testing machine with crack detection, the entire testing machine is fixed on the bottom plate 16, and the bottom plate is fixed on the ground through the anchor feet 9 to form an integrally movable integrated structure; guiding columns 17 are evenly distributed on the bottom plate 16, and the guiding columns 17 are fixed to the bottom plate 16 through latches; a horizontally arranged fixing plate is connected to the guiding columns, which are respectively a force sensor fixing plate, a corrosion environment chamber fixing plate, and a three-layer plate 20 from bottom to top. The fixing plate and the guiding columns are fixed with latches; the top of the box body is connected through a lifting steel cable, which successively passes through the central holes of the three-layer plate, the upper guiding shaft 14, and the fulcrum of the lever, and the box body of the corrosion environment chamber is lifted and lowered through the handwheel adjustment system; the handwheel adjustment system is composed of an adjustment handwheel 8, a connecting bearing 7, and a lower guiding bearing 12; the box cover is fixed on the box cover fixing plate through a flange and fastened with bolts. A hot water inlet and an outlet are connected to the box cover, and the outlet inside the box cover is connected to a stainless steel pipe extending into the bottom of the kettle body. The handwheel adjustment system is fixed to the fixing plate with bolts; an internal force elimination column is connected between the handwheel fixing plate and the box cover fixing plate, and both ends of the internal force elimination column are connected with threads.

[0040] Figure 6 As the working principle diagram of the monitoring fracture time system, the core component of this system is the optoelectronic device 30, and the optoelectronic device 30 controls the operation of the motor through distance. When the distance is less than or equal to a certain value d, the optoelectronic device will continuously close, and then start the motor to control the operation of the timing device. Otherwise, the optoelectronic device disconnects, stopping the timing device, and thus obtaining the fracture time.

[0041] The monitoring fracture time system consists of three parts: a photoelectric device 30, a motor 31, and a timing device. The above-mentioned photoelectric device is an infrared diffuse reflection photoelectric switch, which is connected to the motor at the wiring port, and the motor controls the timing device. Figure 2 The maximum distance of the emitted light is d. When the specimen is not fractured, the distance from the photoelectric device to the upper plate 28 of the stress corrosion device ≤ d. At this time, the photoelectric device is in a closed state, the motor starts, and the stopwatch is in a timing state. Figure 3 This is a schematic diagram of the device when the experiment is completed. In the figure, the specimen is fractured. At this time, the ray emitted by the photoelectric device cannot recognize the upper plate 28. The distance from the photoelectric device 30 to the upper plate 28 > d, which causes the photoelectric device 30 to disconnect, the stopwatch stops timing, and the fracture time is output.

[0042] The image acquisition system is composed of three parts: an acquisition camera 21, a camera guide rail 22, and a camera support platform 23. The acquisition camera 21 has a self-rotation adjustment function, which can ensure rotation adjustment from 0° to 180°. The camera guide rail 22 on the camera support platform 23 can ensure the freedom of left and right translation of the acquisition camera 21. Users can adjust the position of the acquisition camera 21 according to their own needs to meet the optimal requirements for the experiment. The camera support platform 23 can support the entire image acquisition system.

[0043] The principle of calculating displacement field data after image acquisition: Normalized least square distance function:

[0044]

[0045] Among them, f and g are the reference image and the deformed image respectively. This calculation formula is not sensitive to the overall brightness change of the image, so it has good compatibility with speckle images under less ideal lighting conditions and can maximize the measurement accuracy.

[0046] After obtaining the full-field displacement field distribution, theoretically, the strain field can be obtained by using the method of displacement field difference. However, direct difference has a large error, and the SG filter method can be used to obtain the strain. This method can smooth and filter the displacement data to a certain extent and has an inhibitory effect on random noise. The relationship between displacement and strain is described by a first-order shape function, and u and v represent the horizontal and vertical displacements.

[0047]

[0048] In the post-processing, the three-dimensional digital image operation software Ncroo is used for the final result analysis. The specific operation steps of the above software are as follows:

[0049] 1). Set the reference image

[0050] 2). Set the current image

[0051] 3). Set the region of interest

[0052] 4). Set parameters

[0053] 5). DIC analysis

[0054] 6). Format displacement

[0055] 7). Calculate the strain of the microcracks occurring in the tensile stress corrosion test

[0056] 8). Plot a graph

[0057] 9). Output DIC data

[0058] The limiting device is composed of two parts: a limiting rod 4 and a limiting plate 5. The root of the limiting rod has a thread that mates with the top plate 18 and is fixed by it; the limiting plate 5 is fixed in cooperation with the upper guide shaft 14 through the middle threaded hole therein. The function of the limiting device is to limit the position of the upper guide shaft 14 when the experiment is completed, so that it stops in time to avoid the tipping of the entire device and accidents.

[0059] The weight lever system is composed of a lever 1, a weight pan 2, a roller 3, a deep groove ball bearing 25×4, a lifting ring 24, a roller support 26, and a roller guide 27. The roller 3 is fixed on the lifting ring 24 through the roller support 26. The function of the lifting ring 24 is to transmit the tensile force generated by the lever to the specimen 13 through the upper guide shaft 14. The weight lever system provides the tensile force required for the test for the entire system by changing the mass on the weight pan 2, and the tensile force can be measured by the force sensor 11.

[0060] KIC refers to the plane strain fracture toughness, which is a measure of the ability of a material to resist the unstable propagation of cracks. When the stress intensity factor KI at the crack tip reaches the KIC value of the material, the crack will start to propagate unstably, which may lead to brittle fracture of the material. It is an important parameter in linear elastic fracture mechanics. In engineering safety design, by measuring the KIC value of the material, it can help engineers judge the safety of the structure in the presence of cracks and prevent the occurrence of low-stress brittle fracture accidents. For example, in the aerospace field, when designing aircraft components, K IC is a key consideration factor.

[0061] The specimen 13 is in the case where the prefabricated crack is semi-circular arc-shaped, and the calculation method of its K IC value is an approximate calculation formula based on the theory of elasticity and fracture mechanics:

[0062]

[0063] Among them, Y is a dimensionless geometric factor related to the crack geometry and loading conditions, σ is the applied nominal stress, and a is the radius of the semi-circular arc crack.

[0064] In some simple loading cases, such as in a uniform tensile stress field, for a surface semi-elliptical crack (which can be approximately regarded as a semi-circular arc crack under certain conditions), the expression of Y can be obtained through empirical formulas or numerical calculations. The above formula is only an approximate calculation. In practice, the stress distribution around the crack is affected by various factors such as material properties and loading methods. To accurately calculate K IC It may be necessary to rely on numerical methods such as finite element analysis or more complex theoretical models.

Claims

1. A tensile stress corrosion testing machine with microcrack detection function, characterized in that: It includes a weight lever stretching device, a fracture time monitoring system, a corrosion environment chamber, and a data acquisition system. The infrared timing system is attached to both ends of the weight lever stretching device. The sample is set in the corrosion environment chamber, and the sample is connected to the weight lever stretching device through a sample clamp. The weight lever stretching device performs constant load and stretching on the sample in the corrosion environment chamber, and the data acquisition system automatically collects and records images of crack initiation and expansion of the sample during the experiment.

2. The tensile stress corrosion testing machine with microcrack detection function according to claim 1, characterized in that: The corrosion environment box includes a box body and a box cover; the top of the box cover is connected to the upper guide shaft through a connecting piece, a water inlet and a water outlet are arranged in the box cover, and the box cover is connected to the box body through the water inlet and the water outlet, and the upper guide shaft performs a constant load tensile test on the sample in the corrosion environment box; the sample in the corrosion environment box is subjected to constant load and tensile test through a weight lever tensile device, and an image of the sample surface change is automatically collected and recorded through a data acquisition system.

3. The tensile stress corrosion testing machine with microcrack detection function according to claim 2, characterized in that: The testing machine is fixed on the bottom plate, and the bottom plate is fixed to the ground by feet to form an integrated structure that can be moved as a whole; the guide pillars are evenly distributed on the bottom plate, and the guide pillars are fixed to the bottom plate by locks; the guide pillars are connected to a horizontally arranged fixed plate, which are a force sensor fixed plate, a corrosion environment box fixed plate, and a three-layer plate from bottom to top, and the fixed plate and the guide pillars are fixed by locks; the top of the box body is connected by a lifting steel cable, which passes through the center hole of the three-layer plate, the upper guide shaft and the fulcrum of the lever in turn, and the box body of the corrosion environment box is raised and lowered by a handwheel adjustment system; the handwheel adjustment system is composed of an adjustment handwheel, a connecting bearing, and a lower guide bearing; the box cover is fixed to the box cover fixing plate by a flange and fastened with bolts, the box cover is connected to a hot water inlet and outlet, the water outlet on the inner side of the box cover is connected to a stainless steel pipe and extends into the bottom of the kettle body, and the handwheel adjustment system is fixed to the fixing plate by bolts; an internal force elimination pillar is connected between the handwheel fixing plate and the box cover fixing plate, and both ends of the internal force elimination pillar are connected by threads.

4. The tensile stress corrosion testing machine with microcrack detection function according to claim 1, characterized in that: The fracture time monitoring system includes a photoelectric device, which controls the operation of the motor through the distance; when the distance is less than or equal to a certain value d, the photoelectric device will continue to close, thereby starting the motor to control the operation of the timing device, otherwise the photoelectric device will be disconnected, stopping the timing device, and then obtaining the fracture time of the sample test.

5. The tensile stress corrosion testing machine with microcrack detection function according to claim 4, characterized in that: The fracture time monitoring system includes a photoelectric device, a motor, and a timing device. The photoelectric device is an infrared diffuse reflection photoelectric switch, and the motor is connected to the wiring port, and the motor controls the timing device. The maximum distance of the light emitted by the photoelectric device is d. When the test piece is not broken, the distance from the photoelectric device to the upper top plate of the stress corrosion device is <= d. At this time, the photoelectric device is in a closed state, the motor is started, and the stopwatch is in a timing state. When the experiment is completed, the rays emitted by the photoelectric device cannot identify the upper top plate, and the distance from the photoelectric device to the upper top plate is > d, so the photoelectric device is disconnected, the stopwatch stops timing, and the fracture time is output.

6. The tensile stress corrosion testing machine with microcrack detection function according to claim 1, characterized in that: The image acquisition system includes an acquisition camera, a camera guide rail, and a camera support platform. The acquisition camera has an adjustment device to achieve 0°-180° rotation adjustment. The camera guide rail on the camera support platform realizes the freedom of left and right translation of the acquisition camera, and the position of the acquisition camera is adjusted according to needs.

7. The tensile stress corrosion testing machine with microcrack detection function according to claim 6, characterized in that: Principle of calculating displacement field data after image acquisition: Normalized least square distance function: Where f and g are the reference image and the deformed image respectively. This calculation formula is not sensitive to the overall brightness change of the image; After obtaining the full-field displacement field distribution, the strain field data is obtained by using the SG filter method. The first-order shape function is used to describe the relationship between displacement and strain, and u and v represent the lateral and vertical displacements.

8. The tensile stress corrosion testing machine with microcrack detection function according to claim 1, characterized in that: It also includes a limiting device, which includes a limiting rod and a limiting plate; the root of the limiting rod is provided with a thread that matches the top plate.

9. The tensile stress corrosion testing machine with microcrack detection function according to claim 8, characterized in that: The limit plate is fixed with the upper guide shaft through the middle threaded hole; the limit device limits the position of the upper guide shaft when the experiment is completed, so that the upper guide shaft stops in time.

10. The tensile stress corrosion testing machine with microcrack detection function according to claim 1, characterized in that: The weight lever constant load stretching device includes a lever, a weight plate, a roller, a deep groove ball bearing, a lifting ring, a roller support, a roller guide, an upper guide shaft, a force sensor, and a displacement sensor; the roller is fixed to the lifting ring through the roller support, and the lifting ring transmits the stretching force generated by the lever to the sample through the upper guide shaft; the weight lever stretching device provides the stretching force required for the test by changing the mass on the weight plate, and the stretching force is measured by the force sensor.

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