Test tool and test method for low-cycle fatigue crack propagation of double-rib stiffened plate

Abstract

The invention discloses a test tool and a test method for low-cycle fatigue crack propagation of a double-rib stiffened plate. The test tool comprises a loading end tool assembly and a fixed end tool assembly which are symmetrically arranged in the length direction of the double-rib stiffened plate. According to the invention, the loading end band plate connecting piece, the loading end side rib connecting piece and the loading end middle rib connecting piece are utilized to fix the band plate at the upper end of the double-rib stiffened plate and the double-rib plate, so that the upper end of the double-rib stiffened plate can bear a stable reciprocating load in a fatigue test process; meanwhile, the fixed end band plate connecting piece, the fixed end side rib connecting piece and the fixed end middle rib connecting piece act synergistically to provide firm support for the lower end of the double-rib stiffened plate, test deviation caused by uneven stress is avoided, the defect that a single strip-shaped open slot cannot support double-rib cooperative stress in the prior art is overcome, and the test efficiency is improved. And the cooperative load state of the double ribs and the band plate in the actual working condition is accurately simulated, so that the accuracy and the reliability of test data are ensured.

Description

Technical Field

[0001] This invention relates to the field of structural testing and mechanical testing technology, and in particular to a test fixture and test method for low-cycle fatigue crack propagation in double-stiffened plates. Background Technology

[0002] Stiffened plates are typical supporting structural components installed on the hull of ships, and are an important part of the hull, possessing advantages such as high rigidity, light weight, and high strength. The load-bearing capacity of the hull is inseparable from the load-bearing capacity of the stiffened plates. The buckling load and ultimate strength of the stiffened plates are important reference indicators for hull structural design; premature buckling or fatigue cracking of the structure threatens the navigation safety of the hull. Fatigue crack initiation life is one of the main means of evaluating the properties of metallic materials in fracture mechanics. Therefore, it is essential to conduct experimental research on the initiation and propagation laws of fatigue cracks in stiffened plates under axial compressive loads.

[0003] In existing technologies, a notch is pre-fabricated in the middle of a single-stiffener plate to induce crack initiation. During testing, the stiffener plate is clamped at both ends using two clamping assemblies and mounted on a fatigue testing machine. Through the coordinated operation of the main screw and the auxiliary screw, the base plate and the stiffener plate are simultaneously loaded to simulate the common stress state of various parts of the component under actual working conditions. Subsequently, a longitudinal cyclic load is applied, and the speckle image is monitored using a DIC system until crack initiation is detected, thereby analyzing the critical strain and initiation life. In the above technology, the single strip-shaped opening and the matching auxiliary clamping system are geometrically insufficient to accommodate and effectively support the double-stiffener plate, thus affecting the testing results of the double-stiffener plate. Therefore, it is necessary to design a test fixture for low-cycle fatigue crack propagation of double-stiffener plates. Summary of the Invention

[0004] In order to overcome the shortcomings of the prior art, the purpose of this invention is to provide a test fixture and test method for low-cycle fatigue crack propagation in double-stiffened plates.

[0005] The technical solution adopted in this invention is as follows: a test fixture for low-cycle fatigue crack propagation in a double-stiffened plate, comprising a loading end fixture assembly and a fixed end fixture assembly symmetrically arranged along the length of the double-stiffened plate. The loading end fixture assembly is used to connect to the loading head of a fatigue testing machine to transfer reciprocating loads; the fixed end fixture assembly is used to rigidly fix to the test bench to provide stable support; the loading end fixture assembly includes a loading end main fixture, a loading end strip plate connector, a loading end side rib connector, and a loading end intermediate rib connector; the upper end of the double-stiffened plate has a strip plate connector... The plate is located between the main loading end fixture and the loading end plate connector. The main loading end fixture and the loading end plate connector are connected by bolts to fix the plate at the upper end of the double-reinforced plate. There are two loading end side rib connectors, located on both sides of the loading end intermediate rib connector. The loading end side rib connectors and the loading end intermediate rib connectors are respectively bolted to the loading end plate connector. The two loading end side rib connectors are connected to the loading end intermediate rib connector by bolts to fix the rib plate at the upper end of the double-reinforced plate. The fixed-end tooling assembly includes a fixed-end main tooling, a fixed-end strip plate connector, a fixed-end side rib connector, and a fixed-end intermediate rib connector. The strip plate at the lower end of the double-reinforced plate is located between the fixed-end main tooling and the fixed-end strip plate connector. The fixed-end main tooling is connected to the fixed-end strip plate connector by bolts to fix the strip plate at the lower end of the double-reinforced plate. There are two fixed-end side rib connectors, located on both sides of the fixed-end intermediate rib connector. The fixed-end side rib connectors and the fixed-end intermediate rib connector are respectively bolted to the fixed-end strip plate connector. The two fixed-end side rib connectors are connected to the fixed-end intermediate rib connector by bolts to fix the rib plate at the lower end of the double-reinforced plate.

[0006] As a further description of the above technical solution: The main tooling of the loading end includes a first base plate and a first vertical plate. The first base plate is connected to the loading head of the fatigue testing machine by bolts. The first vertical plate is fixedly connected to the middle position of the lower end of the first base plate. The first vertical plate is bolted to the loading end plate connector.

[0007] As a further description of the above technical solution: The lower end of the first base plate is fixedly connected to a first rib and a second rib. There are two of each of the first and second ribs, and both the first and second ribs are fixedly connected to the first upright plate.

[0008] As a further description of the above technical solution: The fixed end main fixture includes a second base plate and a second vertical plate. The lower end of the second base plate is connected to the test bench by bolts. The second vertical plate is fixedly connected to the middle position of the upper end of the second base plate. The second vertical plate is bolted to the fixed end plate connector.

[0009] As a further description of the above technical solution: The upper end of the second base plate is fixedly connected with a third rib and a fourth rib. There are two of each of the third and fourth ribs, and both the third and fourth ribs are fixedly connected to the second upright plate.

[0010] As a further description of the above technical solution: Both the loading end plate connector and the fixed end plate connector are L-shaped. The surface of the horizontal section of the loading end plate connector has a first slot-shaped through hole, and the surface of the vertical section of the loading end plate connector has two first strip-shaped notches. The surface of the horizontal section of the fixed end plate connector has a third slot-shaped through hole, and the surface of the vertical section of the fixed end plate connector has two second strip-shaped notches. The first and second strip-shaped notches correspond to the stiffeners of the double-stiffened plate.

[0011] As a further description of the above technical solution: Both the loading end side rib connector and the fixed end side rib connector are L-shaped. The surface of the horizontal section of the loading end side rib connector is provided with a second slot-shaped through hole. The surface of the horizontal section of the fixed end side rib connector is provided with a fourth slot-shaped through hole. Both the loading end intermediate rib connector and the fixed end intermediate rib connector are U-shaped.

[0012] A test method for low-cycle fatigue crack propagation in double-stiffened plates, based on the aforementioned test and measurement device for low-cycle fatigue crack propagation in double-stiffened plates, includes the following steps: S1: Apply a speckled spray coating to the surface of the double-stiffened plate; S2: Install the industrial camera at the preset shooting position and connect the industrial camera to the test computer terminal; S3: The two ends of the double-stiffened plate are clamped and fixed using the loading end tooling assembly and the fixed end tooling assembly, and then placed on the fatigue testing machine. S4: Apply longitudinal load to the double-stiffened plate using a fatigue testing machine to conduct cyclic tests, and simultaneously use an industrial camera to acquire images to obtain speckle photographs until fatigue cracks appear on the double-stiffened plate. S5: Use a test computer terminal to process and analyze speckle photographs to obtain the critical strain and initiation life at the time of fatigue crack initiation in double-stiffened plates.

[0013] The present invention has the following beneficial effects: 1. This invention, by setting up a loading end plate connector, a loading end side rib connector, a loading end intermediate rib connector, a fixed end plate connector, a fixed end side rib connector, and a fixed end intermediate rib connector, allows for the fixation of the upper plate and double-ribbed plate of the double-ribbed plate during use. This ensures that the upper end of the double-ribbed plate can withstand stable reciprocating loads during fatigue testing. Simultaneously, the fixed end plate connector, fixed end side rib connector, and fixed end intermediate rib connector work together to provide solid support for the lower end of the double-ribbed plate, avoiding test deviations caused by uneven stress. This design overcomes the deficiency of existing technologies where a single strip-shaped opening slot cannot support the coordinated stress of the double ribs, accurately simulating the coordinated load state of the double ribs and plate in actual working conditions, thereby ensuring the accuracy and reliability of the test data.

[0014] 2. The main fixture of the loading end of this invention consists of a first base plate and a first vertical plate, and the main fixture of the fixing end consists of a second base plate and a second vertical plate. This allows the load to be effectively transferred to the strip plate and stiffener plate of the double-reinforced plate through bolt connection, simulating the stress state in actual working conditions. At the same time, with the cooperation of the first rib plate, the second rib plate, the third rib plate and the fourth rib plate, the rigidity of the main fixture is further enhanced, avoiding the deformation of the device caused by local stress concentration and extending the service life of the test device. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the overall structure of the present invention from a first-view perspective; Figure 2 This is a schematic diagram of the overall structure of the invention from a second perspective; Figure 3 This is a schematic diagram of the structure of the double-stiffened plate of the present invention; Figure 4 This is a first-view structural diagram of the fixed-end tooling assembly after disassembly according to the present invention; Figure 5 For the present invention Figure 4 Enlarged view of section A in the middle; Figure 6 This is a second-view structural diagram of the fixed-end tooling assembly after disassembly according to the present invention; Figure 7 This is a first-view structural diagram of the loading end tooling component after disassembly according to the present invention; Figure 8 For the present invention Figure 7 Enlarged view of section B; Figure 9 This is a second-view structural diagram of the loading end tooling component after it has been disassembled according to the present invention.

[0016] Legend: 1. Loading end tooling assembly; 101. Loading end main tooling; 1011. First base plate; 1012. First upright plate; 102. Loading end plate connector; 1021. First slotted through hole; 103. Loading end side rib connector; 1031. Second slotted through hole; 104. Loading end intermediate rib connector; 2. Fixed end tooling assembly; 201. Fixed end main tooling; 2011. Second base plate; 2012. Second upright plate; 202. Fixed end plate connector; 2021. Third slotted through hole; 203. Fixed end side rib connector; 2031. Fourth slotted through hole; 204. Fixed end intermediate rib connector; 3. Double-ribbed stiffening plate; 4. First rib; 5. Second rib; 6. Third rib; 7. Fourth rib; 8. First strip notch; 9. Second strip notch. Detailed Implementation

[0017] Reference Figure 1-9 The present invention provides a test fixture for low-cycle fatigue crack propagation in a double-stiffened plate, comprising a loading end fixture assembly 1 and a fixed end fixture assembly 2 symmetrically arranged along the length of the double-stiffened plate 3. The loading end fixture assembly 1 is used to connect to the loading head of a fatigue testing machine to transfer reciprocating loads; the fixed end fixture assembly 2 is used to rigidly fix to the test bench to provide stable support; the loading end fixture assembly 1 includes a loading end main fixture 101, a loading end plate connector 102, a loading end side rib connector 103, and a loading end... Intermediate rib connector 104; the strip plate at the upper end of the double-ribbed stiffened plate 3 is located between the loading end main fixture 101 and the loading end strip plate connector 102. The loading end main fixture 101 and the loading end strip plate connector 102 are fixed to the strip plate at the upper end of the double-ribbed stiffened plate 3 by bolt connection; there are two loading end side rib connectors 103, which are located on both sides of the loading end intermediate rib connector 104. The loading end side rib connectors 103 and the loading end intermediate rib connector 104 are respectively connected to the loading end strip plate connector. 102 bolt connection, the two loading end side rib connectors 103 are connected to the loading end intermediate rib connector 104 by bolts to fix the upper rib plate of the double-ribbed stiffened plate 3; the fixed end tooling assembly 2 includes a fixed end main tooling 201, a fixed end plate connector 202, a fixed end side rib connector 203 and a fixed end intermediate rib connector 204, the plate at the lower end of the double-ribbed stiffened plate 3 is located between the fixed end main tooling 201 and the fixed end plate connector 202, the fixed end main tooling 201 is connected by bolts to the loading end intermediate rib connector 104. The bolts are connected to the fixed end plate connector 202 to fix the plate at the lower end of the double-reinforced plate 3. There are two fixed end side rib connectors 203, which are located on both sides of the fixed end intermediate rib connector 204. The fixed end side rib connectors 203 and the fixed end intermediate rib connectors 204 are respectively bolted to the fixed end plate connector 202. The two fixed end side rib connectors 203 are connected to the fixed end intermediate rib connector 204 by bolts to fix the rib plate at the lower end of the double-reinforced plate 3.

[0018] In use, the double-ribbed stiffener 3 is first sprayed with a speckled coating. Then, both ends of the double-ribbed stiffener 3 are fixed. During the fixing process, the lower end of the double-ribbed stiffener 3 is inserted between the fixed-end main body fixture 201 and the fixed-end plate connector 202, and then connected using bolts. At this point, the plate at the lower end of the double-ribbed stiffener 3 is fixed by the cooperation of the fixed-end plate connector 202 and the fixed-end main body fixture 201. During the fixing of the lower plate of the double-ribbed stiffener 3, the fixed-end intermediate rib connector 204 is connected to the fixed-end plate connector 202, and then the bolts are... After passing the bolts through the fixed-end side rib connector 203 and the fixed-end intermediate rib connector 204, tighten the bolts to fix the lower end of the double-ribbed plate 3. Then, pass the bolts through the fixed-end main fixture 201, the fixed-end plate connector 202, and the fixed-end side rib connector 203 to fix the fixed-end plate connector 202 and the fixed-end side rib connector 203 to the fixed-end main fixture 201. After fixing the lower end of the double-ribbed plate 3, insert the upper end of the double-ribbed plate 3 into the loading-end main fixture 101 and the loading-end plate connector 102. Then, bolts are used for connection. At this point, the upper plate of the double-reinforced plate 3 is fixed with the cooperation of the loading end main fixture 101 and the loading end plate connector 102. During the fixing of the upper plate of the double-reinforced plate 3, the loading end intermediate rib connector 104 is connected to the loading end plate connector 102. Then, bolts are passed through the loading end side rib connector 103 and the loading end intermediate rib connector 104, and then the bolts are tightened. This fixes the upper double-reinforced plate of the double-reinforced plate 3. Finally, bolts are passed through the loading end main fixture 101 and the loading end plate connector 102. Connecting component 102 and loading end side rib connector 103, the loading end plate connector 102 and loading end side rib connector 103 can then be fixed on the loading end main fixture 101. After fixing the double-ribbed stiffener 3, the double-ribbed stiffener 3 is placed vertically, so that the loading end main fixture 101 is connected to the loading head of the fatigue testing machine, and the fixed end main fixture 201 is connected to the test bench. After assembly, the fatigue testing machine applies longitudinal load to the double-ribbed stiffener 3 for cyclic testing, and simultaneously uses an industrial camera to acquire images to obtain speckle photographs until fatigue cracks appear on the double-ribbed stiffener 3. The captured speckle photographs are processed and analyzed using a test computer terminal to obtain the corresponding test data. (It should be noted that the fatigue testing machine, industrial camera, and supporting test computer terminal are all existing technology equipment on the market and are not shown in the figure.)

[0019] The loading end main fixture 101 includes a first base plate 1011 and a first vertical plate 1012. The first base plate 1011 is connected to the loading head of the fatigue testing machine by bolts. The first vertical plate 1012 is fixedly connected to the middle position of the lower end of the first base plate 1011. The first vertical plate 1012 is bolted to the loading end plate connector 102. The lower end of the first base plate 1011 is fixedly connected to a first rib 4 and a second rib 5. There are two of each of the first rib 4 and the second rib 5. Both the first rib 4 and the second rib 5 are fixedly connected to the first vertical plate 1012. During operation, the first base plate 1011 is tightly connected to the loading head of the fatigue testing machine by bolts to ensure that the load can be smoothly transferred to the upper part of the double-stiffened plate 3. At the same time, the first vertical plate 1012, as the main load-bearing component, has a rigid design that effectively avoids deformation problems caused by external loads. Then, with the cooperation of the first rib 4 and the second rib 5, the overall stability of the loading end main fixture 101 is further enhanced, so that the device has higher fatigue resistance when subjected to reciprocating loads.

[0020] The fixed-end main fixture 201 includes a second base plate 2011 and a second vertical plate 2012. The lower end of the second base plate 2011 is connected to the test bench by bolts, and the second vertical plate 2012 is fixedly connected to the middle of the upper end of the second base plate 2011. The second vertical plate 2012 is bolted to the fixed-end plate connector 202. A third rib 6 and a fourth rib 7 are fixedly connected to the upper end of the second base plate 2011. There are two of each of the third rib 6 and the fourth rib 7. Both the third rib 6 and the fourth rib 7 are fixedly connected to the second vertical plate 2012. During operation, the second base plate 2011 is tightly connected to the test bench by bolts, ensuring that the fixed-end main fixture 201 can provide stable support for the double-stiffened plate 3. As the core load-bearing component, the second vertical plate 2012 is structurally designed to fully consider rigidity requirements and effectively prevent deformation caused by external loads. Meanwhile, the synergistic effect of the third rib 6 and the fourth rib 7 further enhances the overall stability of the fixed end main fixture 201, making it exhibit stronger fatigue resistance when subjected to complex stress, thereby ensuring the reliability of the device during the test.

[0021] Both the loading-end plate connector 102 and the fixed-end plate connector 202 are L-shaped. The horizontal section of the loading-end plate connector 102 has a first slotted through-hole 1021, and the vertical section has two first strip-shaped notches 8. The horizontal section of the fixed-end plate connector 202 has a third slotted through-hole 2021, and the vertical section has two second strip-shaped notches 9. The first and second strip-shaped notches 8 correspond to the stiffeners of the double-stiffened plate 3. During operation, the L-shaped design of the loading-end plate connector 102 and the fixed-end plate connector 202 better adapts to the structural characteristics of the double-stiffened plate 3. The first slotted through-hole 1021 of the loading-end plate connector 102 and the third slotted through-hole 2021 of the fixed-end plate connector 202 provide greater adjustment space during installation, ensuring that the plates of the double-stiffened plate 3 can be firmly clamped. The design of the first strip notch 8 and the second strip notch 9 facilitates the insertion of the double stiffener plate 3 into the loading end plate connector 102 and the fixing end plate connector 202, thereby facilitating the fixing of the plate of the double stiffener plate 3.

[0022] Both the loading end side rib connector 103 and the fixed end side rib connector 203 are L-shaped. The surface of the horizontal section of the loading end side rib connector 103 has a second slot-shaped through hole 1031; the surface of the horizontal section of the fixed end side rib connector 203 has a fourth slot-shaped through hole 2031; both the loading end intermediate rib connector 104 and the fixed end intermediate rib connector 204 are U-shaped. During operation, the L-shaped design of the loading end side rib connector 103 and the fixed end side rib connector 203, combined with the slot-shaped through holes on their surfaces, provides a flexible adjustment range during installation. At the same time, since both the loading end intermediate rib connector 104 and the fixed end intermediate rib connector 204 are U-shaped, the loading end side rib connector 103 and the fixed end side rib connector 203 can fit tightly with the rib portion of the double-ribbed stiffener 3, ensuring that the rib of the double-ribbed stiffener 3 is subjected to a uniform clamping force during fatigue testing.

[0023] A test method for low-cycle fatigue crack propagation in double-stiffened plates, based on the aforementioned test and measurement device for low-cycle fatigue crack propagation in double-stiffened plates, includes the following steps: S1: Apply a speckled spray coating to the surface of the double-stiffened plate 3; S2: Install the industrial camera at the preset shooting position and connect the industrial camera to the test computer terminal; S3: The two ends of the double-stiffened plate 3 are clamped and fixed using the loading end tooling assembly 1 and the fixed end tooling assembly 2, and then placed on the fatigue testing machine. S4: Apply longitudinal load to the double-stiffened plate 3 using a fatigue testing machine to carry out cyclic testing, and simultaneously use an industrial camera to acquire images to obtain speckle photographs until fatigue cracks appear on the double-stiffened plate 3. S5: The speckle photographs were processed and analyzed using a test computer terminal to obtain the critical strain and initiation life of fatigue crack initiation in the double-stiffened plate 3.

[0024] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A test fixture for low-cycle fatigue crack propagation in double-stiffened plates, characterized in that: The system includes a loading end fixture assembly (1) and a fixed end fixture assembly (2) symmetrically arranged along the length of the double-reinforced plate (3). The loading end fixture assembly (1) is used to connect with the loading head of the fatigue testing machine to transmit reciprocating loads. The fixed end fixture assembly (2) is used to rigidly fix the system to the test bench to provide stable support. The loading end fixture assembly (1) includes a loading end main fixture (101), a loading end strip plate connector (102), a loading end side rib connector (103), and a loading end intermediate rib connector (104). The strip plate at the upper end of the double-reinforced plate (3) is located between the loading end main fixture (101) and the loading end strip plate connector (104). 2) Between the loading end main tooling (101) and the loading end plate connector (102), the plate at the upper end of the double-reinforced plate (3) is fixed by bolt connection; there are two loading end side rib connectors (103), which are located on both sides of the loading end middle rib connector (104). The loading end side rib connectors (103) and the loading end middle rib connectors (104) are bolted to the loading end plate connector (102) respectively. The two loading end side rib connectors (103) are connected to the loading end middle rib connectors (104) by bolts to fix the rib plate at the upper end of the double-reinforced plate (3); The fixed-end tooling assembly (2) includes a fixed-end main tooling (201), a fixed-end strip plate connector (202), a fixed-end side rib connector (203), and a fixed-end intermediate rib connector (204). The strip plate at the lower end of the double-ribbed reinforced plate (3) is located between the fixed-end main tooling (201) and the fixed-end strip plate connector (202). The fixed-end main tooling (201) is connected to the fixed-end strip plate connector (202) by bolts to realize the lower end of the double-ribbed reinforced plate (3). The end plate is fixed. There are two fixed end side rib connectors (203) and they are located on both sides of the fixed end middle rib connector (204). The fixed end side rib connectors (203) and the fixed end middle rib connectors (204) are bolted to the fixed end plate connectors (202). The two fixed end side rib connectors (203) are connected to the fixed end middle rib connectors (204) by bolts to fix the ribs at the lower end of the double rib reinforced plate (3).

2. The test fixture for low-cycle fatigue crack propagation in a double-stiffened plate according to claim 1, characterized in that: The loading end main fixture (101) includes a first base plate (1011) and a first upright plate (1012). The first base plate (1011) is connected to the loading head of the fatigue testing machine by bolts. The first upright plate (1012) is fixedly connected to the middle position of the lower end of the first base plate (1011). The first upright plate (1012) is bolted to the loading end plate connector (102).

3. The test fixture for low-cycle fatigue crack propagation in a double-stiffened plate according to claim 2, characterized in that: The lower end of the first base plate (1011) is fixedly connected with a first rib plate (4) and a second rib plate (5). There are two of each of the first rib plate (4) and the second rib plate (5). Both the first rib plate (4) and the second rib plate (5) are fixedly connected to the first upright plate (1012).

4. The test fixture for low-cycle fatigue crack propagation in a double-stiffened plate according to claim 1, characterized in that: The fixed end main fixture (201) includes a second base plate (2011) and a second upright plate (2012). The lower end of the second base plate (2011) is connected to the test bench by bolts. The second upright plate (2012) is fixedly connected to the middle position of the upper end of the second base plate (2011). The second upright plate (2012) is bolted to the fixed end plate connector (202).

5. The test fixture for low-cycle fatigue crack propagation in a double-stiffened plate according to claim 4, characterized in that: The upper end of the second base plate (2011) is fixedly connected with a third rib (6) and a fourth rib (7). There are two of each of the third rib (6) and the fourth rib (7). Both the third rib (6) and the fourth rib (7) are fixedly connected to the second upright plate (2012).

6. The experimental fixture for low-cycle fatigue crack propagation in a double-stiffened plate according to claim 1, characterized in that: Both the loading end plate connector (102) and the fixed end plate connector (202) are L-shaped. The surface of the horizontal section of the loading end plate connector (102) is provided with a first slot-shaped through hole (1021), and the surface of the vertical section of the loading end plate connector (102) is provided with two first strip-shaped notches (8). The surface of the horizontal section of the fixed end plate connector (202) is provided with a third slot-shaped through hole (2021), and the surface of the vertical section of the fixed end plate connector (202) is provided with two second strip-shaped notches (9). The first strip-shaped notches (8) and the second strip-shaped notches (9) correspond to the stiffeners of the double-stiffened plate (3).

7. The test fixture for low-cycle fatigue crack propagation in a double-stiffened plate according to claim 1, characterized in that: Both the loading end side rib connector (103) and the fixed end side rib connector (203) are L-shaped. The surface of the horizontal section of the loading end side rib connector (103) is provided with a second slot-shaped through hole (1031). The surface of the horizontal section of the fixed end side rib connector (203) is provided with a fourth slot-shaped through hole (2031). Both the loading end intermediate rib connector (104) and the fixed end intermediate rib connector (204) are U-shaped.

8. A test method for low-cycle fatigue crack propagation in a double-stiffened plate, characterized in that, Based on the experimental fixture for low-cycle fatigue crack propagation in double-stiffened plates according to any one of claims 1 to 7, the experimental measurement method includes the following steps: S1: Spraying a speckled coating onto the surface of the double-stiffened plate (3); S2: Install the industrial camera at the preset shooting position and connect the industrial camera to the test computer terminal; S3: The two ends of the double-stiffened plate (3) are clamped and fixed using the loading end tooling assembly (1) and the fixed end tooling assembly (2), and then placed on the fatigue testing machine. S4: Apply longitudinal load to the double-stiffened plate (3) using a fatigue testing machine to carry out cyclic testing, and at the same time use an industrial camera to acquire images to obtain speckle photographs until fatigue cracks appear on the double-stiffened plate (3). S5: Use the test computer terminal to process and analyze the speckle photos to obtain the critical strain and initiation life of fatigue crack initiation in the double-stiffened plate (3).

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