A device and method for testing the post-repair strength and fatigue of a curved honeycomb sandwich structure

Abstract

The application provides a curved honeycomb sandwich structure post-repair strength and fatigue test device and method, comprising the following steps: step 1: an upper force loading rod and a lower force loading rod are respectively clamped between upper and lower hydraulic clamps of a fatigue testing machine; step 2: a lever of a test system distributes static and fatigue pressure applied to both sides of a test piece; step 3: loading is performed according to test requirements, and a connecting rod, a force applying double ear and a support and the like load transmission and conversion are converted into a moment acting on two force conversion plates on both sides of the test piece by moving the relative positions of the upper force loading rod and the lower force loading rod; and step 4: the moment is converted into out-of-plane tension and compression on a preset rectangular area on both sides of the test piece by each force conversion plate, and is applied to a surface of the test piece, so that single-channel multi-point loading test is realized.

Description

Technical Field

[0001] This invention belongs to the field of aerospace structural performance design and verification, specifically relating to a device and method for testing the strength and fatigue of a curved honeycomb sandwich structure after repair. Background Technology

[0002] Composite sandwich structures possess advantages such as lightweight and high bending resistance, and are widely used in aircraft structures, including radar radomes, fairings, tail fin leading and trailing edges, and control surfaces. However, these structures were previously often considered secondary structures that did not bear primary loads, and fatigue analysis was rarely conducted. With the increasingly widespread application of composite sandwich structures in aircraft load-bearing and functional structures, there is a growing focus on the fatigue characteristics of these structures, especially after damage and repair, and a growing demand for life assessment.

[0003] Existing research and analytical verification methods on composite material fatigue mainly focus on laminated composite materials and their structures, and are mostly applied to components and test specimens. Methods for verifying the strength and fatigue performance of samples under complex operating conditions are rarely seen. The fatigue damage modes of sandwich composite materials and their structures are unique and complex, and related fatigue research literature is even scarcer than that for laminated structures, requiring specialized research. Summary of the Invention

[0004] Existing testing methods for honeycomb sandwich composite structures have the following main drawbacks: Poor specificity: Existing composite material testing methods are mostly designed for flat plate specimens or standard test pieces. However, the stress state of curved honeycomb sandwich structures is significantly different from that of flat plate structures. Directly applying flat plate testing methods cannot truly reflect the actual stress state and failure mode of the repair area.

[0005] Separate evaluation: Static strength testing and fatigue testing are usually conducted separately, requiring two different sets of test specimens and fixtures. This is costly, time-consuming, and makes it impossible to examine the correlation between static strength decay and fatigue damage propagation. Supporting design difficulties: The lack of a universal testing method for curved structure repairs leads to a lack of reliable data support for the design and verification of repair schemes, making it difficult to accurately assess the long-term service safety and lifespan after repair.

[0006] The main purpose of this invention is to overcome the shortcomings of existing testing techniques, provide a reliable, boundary-oriented, and efficient method for strength and fatigue testing, improve the verification accuracy of the strength and fatigue performance of honeycomb sandwich composite structures after repair, achieve the application and coupling accuracy of complex loads, and improve the efficiency of test verification.

[0007] In a first aspect, this application provides a testing device for the strength and fatigue testing of a curved honeycomb sandwich structure after repair, comprising an upper loading rod, a lever, two force-applying lugs, a bracket, a lower loading rod, a mounting base plate, a rubber loading block, a force conversion plate, a positioning plate, and a connecting rod, wherein: The upper and lower loading rods are the clamps of the static testing machine. The lever connects the upper loading rod and the connecting rod. The upper loading rod is tightly connected to the center of the lever via a pin. Both ends of the lever are connected to the connecting rod via pins, transferring the force load to the connecting rod. The connecting rod is used to adjust the direction of the force load according to the shape of the test piece. Both ends of the connecting rod are connected to the lever and the force-applying lugs respectively via holes. The installation angle between the connecting rod and the force-applying lugs is 90°. After the angle adjustment is confirmed, positioning is achieved using positioning plates to ensure that the installation angle does not change during loading. The positioning piece is an F-shaped structure, with its three corners tightly connected to the lever, connecting rod, and force-applying lugs via bolts. The force-applying lugs are T-shaped cross-section structures, with the upper end connected to the connecting rod structure via a pin and the lower end connected to the force conversion plate structure via bolts. The force conversion plate is a rectangular steel plate of uniform thickness, with one side fixedly connected to the force-applying lugs via bolts, the other side connected to the rubber loading block via bolts, and the middle position connected to the bracket via a pin. One side of each rubber loading block is bonded to the test piece with epoxy resin, and the other side is connected to the force conversion plate via bolts.

[0008] Secondly, this application provides a method for testing the strength and fatigue of a curved honeycomb sandwich structure after repair, which is implemented using the aforementioned device for testing the strength and fatigue of a curved honeycomb sandwich structure after repair. The method includes: Step 1: The upper and lower force loading rods are clamped between the upper and lower hydraulic chucks of the fatigue testing machine, respectively; Step 2: The levers of the testing system distribute the applied static and fatigue pressures to both sides of the specimen; Step 3: Load according to the test requirements. By moving the relative positions of the upper and lower loading rods, the loads of the connecting rod, the force-applying lugs, and the support are converted into torques that act on the two force conversion plates on both sides of the specimen. Step 4: The torque is converted into out-of-plane tensile and compressive forces on the pre-defined rectangular areas on both sides of the specimen through each force conversion plate, and applied to the surface of the specimen to realize a single-channel multi-point loading test.

[0009] In summary, this invention provides a method for testing the strength and fatigue performance of a repaired curved honeycomb sandwich structure. This method can be used to verify the strength and fatigue performance of a repaired honeycomb sandwich composite structure with complex curvature. The method is required to achieve load conversion and loading for structures with different curved surface shapes, requiring only adaptation adjustments based on the structural shape and repair area location without altering the overall structure. Different test load magnitudes and adjacent load distributions can be adjusted using a simple lever principle, thereby improving the accuracy and efficiency of verifying the strength and fatigue performance of the repaired honeycomb sandwich structure and avoiding errors in structural performance due to insufficient verification at the component level. Attached Figure Description

[0010] Figure 1 A method for testing the strength and fatigue of curved honeycomb sandwich structures after repair; Figure 2 This is a schematic diagram showing the loading points and loading directions of the test specimen; Figure 3 This is a diagram of a lever; Figure 4 This is a schematic diagram of the linkage; Figure 5 This is a schematic diagram of the positioning piece; Figure 6 A diagram illustrating the application of force to both ears; Figure 7 Schematic diagram of the force conversion plate; Figure 8 This is a schematic diagram of the support frame; Figure 9 This is a schematic diagram illustrating the effect. Figure 10 The load-strain curve is for a maximum static loading force of 4kN. Specific implementation methods Example 1 like Figure 1 As shown, this invention provides a strength and fatigue testing device for a curved honeycomb sandwich structure after repair, comprising an upper loading rod, a lever, two force-applying lugs, a bracket, a lower loading rod, a mounting base plate, a rubber loading block, a force conversion plate, a positioning plate, and a connecting rod, wherein: The upper and lower loading rods are generally the clamps of a static testing machine; The lever is used to connect the upper loading rod and the connecting rod. The upper loading rod is tightly connected to the center of the lever by a φ=26mm pin to ensure that there is no relative rotation that would cause uneven force distribution. Each end of the lever is connected to the connecting rod by a φ=18mm pin to transfer the force load to the connecting rod. The function of the connecting rod is to adjust the direction of the force load according to the shape of the test piece. The two ends of the connecting rod are connected to the lever and the force application lugs through a hole with a diameter of φ=18mm. The installation angle between the connecting rod and the force application lugs is 90°. After the angle adjustment is confirmed, the positioning plate is used to ensure that the installation angle will not change during the loading process. The positioning piece is an F-shaped structure, with its three corner points tightly connected to the lever, connecting rod, and force-applying lugs by a bolt with a diameter of φ=12.4mm. The force-applying double lugs are a T-shaped cross-section structure. The upper end is connected to the connecting rod structure by a φ=30mm pin, and the lower end is connected to the force conversion plate structure by eight φ=10mm bolts. The force conversion plate is a rectangular steel plate of equal thickness. One side is fixedly connected to the force-applying double ears by 8 φ=10mm bolts, and the other side is connected to the rubber loading block by 6 sets (3 in each set) of φ=18mm bolts. The middle position is connected to the bracket by a pin. One side of each rubber loading block is bonded to the test piece with epoxy resin, and the other side is connected to the force conversion plate with bolts.

[0011] Specifically, holes are drilled at both ends of the test piece, which is then fixed to the base plate of the tooling fixture using bolts. The contact surface between the base plate and the test piece is designed according to the product profile.

[0012] It should be noted that the connection between the four supports and the force conversion plate can protect the test specimen during the loading process, preventing the fatigue testing device from collapsing and damaging the test specimen due to the detachment of any connecting cone pin.

[0013] It should be noted that if the test piece suddenly fails during the strength or fatigue test, it will be unable to effectively support the loading fixture. The presence of the bracket can also ensure that all fixture mechanisms will not slip out or fall and cause damage to the test personnel or other equipment.

[0014] It should be noted that the test plan is not limited by changes in product shape and size. All components are connected by pins and can be freely combined.

[0015] It should be noted that the loading area can be adjusted by changing the size of the rubber loading block.

[0016] It should be noted that the material of the test device can be selected from aluminum alloy, steel or other metal materials according to the value of the design load. The device can also be used to conduct structural fatigue tests under high temperature or low temperature conditions.

[0017] Example 2 like Figure 1 As shown, this invention provides a method for testing the strength and fatigue of a curved honeycomb sandwich structure after repair, comprising: Step 1: The upper and lower force loading rods are clamped between the upper and lower hydraulic chucks of the fatigue testing machine, respectively; Specifically, step 1 includes: Step 11: Determine the appropriate fatigue testing machine based on the size of the test piece or the test conditions, and determine the displacement range of the upper and lower loading rods to ensure that the effective load can be applied during the loading process; Step 12: Install the test piece and test device as a whole according to the drawings. After installation, check the installation angle and other data of the connecting rod and the force-applying lugs. Step 13: Considering factors such as the weight of the test piece and the test device after overall installation, use a crane to install the upper and lower loading rods of the test device and the test machine; Step 14: After the rubber loading block is pasted onto the test piece, it needs to be cured for a certain period of time. It should not be moved directly to avoid test errors caused by poor adhesion.

[0018] Step 15: After the test piece is installed and inspected, a steel wire rope can be connected to the left and right sides of the test device to further enhance safety protection; Step 2: The levers of the testing system distribute the applied static and fatigue pressures to both sides of the specimen; Specifically, step 2 includes: Step 21: According to the design requirements, control the testing machine to first load 20%~30% of the value, and check the stability of the testing device and whether there is any looseness in the connections; Step 22: Use tools such as a right-angle ruler to measure whether there is a change in the installation angle between the connecting rod and the force-applying lugs; Step 23: Check whether the rubber loading block and the test piece are firmly bonded and whether there are any gaps.

[0019] Step 3: After completing Step 2, load the load according to the test requirements. By moving the relative positions of the upper and lower load rods, the loads of the connecting rod, the force-applying lugs, and the support are converted into torques that act on the two force conversion plates on both sides of the specimen. Specifically, step 3 includes: Step 31: During the loading process, observe the data changes of each measurement point in real time according to the measurement requirements, and check whether the data changes are stable. If there is any abnormality, the test can be stopped immediately. Step 32: During the loading process, observe whether each connection point in the test device is stable to ensure that the test loading meets the requirements; Step 4: The torque is converted into out-of-plane tensile and compressive forces on the pre-defined rectangular areas on both sides of the specimen through each force conversion plate, and applied to the specimen surface, such as... Figure 2 As shown, this enables a single-channel multi-point loading test.

[0020] Specifically, step 4 includes: Step 41: The rectangular area of ​​the rubber loading block can be adjusted according to verification requirements and location; specifically, for example... Figure 2 As shown, there are six 60mm×60mm rectangular areas preset, namely L1, L2, L3, L4, L5, L6 and R1, R2, R3, R4, R5, R6; Step 42: The testing device can perform local loading strength and fatigue tests on any area of ​​any curved honeycomb sandwich structure, realizing the assessment of the strength and fatigue performance of the repair area. Only the surface of the rubber loading block and the test piece need to be adjusted. Step 43: Using this test apparatus, the load-bearing capacity of the maintenance area can be characterized by the calculation of the test design load and the torque equivalence using a local structure. This avoids the need for test verification using full-size test pieces, reduces the test difficulty and cost while meeting the test verification accuracy requirements, and improves the test efficiency.

[0021] Step 44: Based on the actual load conditions of the curved surface structure, adjust the length of the lever and the angle between the connecting rod and the force-applying lugs using the lever design principle, thereby adjusting the load distribution of the two rows of loading points to achieve the evaluation of the test piece under different test conditions and load levels.

[0022] Example 3 like Figure 7 As shown, a test specimen with damage and repair was manufactured from the leading edge structure of a certain type of airfoil. Following the aforementioned test method, a test loading system was designed, manufactured, and constructed according to the specimen profile and repair area. Static loading was first performed. Preliminary experimental results showed that the desired loading method could be achieved. Uniaxial strain gauges were attached next to the repair area to record strain changes at these locations. The test was conducted on an MTS810±250kN fatigue testing machine. A static test with a maximum loading force of 4kN was first performed, and strain data at eight measuring points were collected. Figure 8 As shown, all strains in the figure are compressive values. At the loading points on the left and right sides of the top row, the strain on the left is greater than that on the right, which is due to the asymmetry of the specimen configuration on the left and right sides. The strain curves are initially nonlinear, but tend to be linear as the load increases.

[0023] Following the static test, a fatigue test was conducted under the same conditions with a maximum loading force of 6 kN, a fatigue stress ratio of R=10, and a loading frequency of 1 Hz. In the early stages of the fatigue test, with the alternating load cycles, the overall displacement range of the loading fixture and the relative rotational displacement between components were small, indicating stable loading. In the later stages of the test, the rigid body displacement of the loading fixture system gradually increased, indicating that the cyclic deformation of the test piece was gradually increasing. When the number of fatigue cycles reached 10... 5When the rigid body rotation between each tooling component reaches its limit position, it indicates that the specimen deformation is too large, and the specimen is considered to have reached its damage limit. During the entire fatigue test, no obvious damage was observed visually on the surface of the repair area of ​​the specimen. This may be because the development of damage to the internal core material or the core-panel interface led to a gradual decrease in the stiffness of the specimen, causing the structure to eventually fail.

[0024] In summary, this technical solution can be used to systematically conduct strength and fatigue tests on curved honeycomb sandwich structures after repair. A single set of testing fixtures can simultaneously meet the requirements for strength and fatigue testing verification, resulting in high testing efficiency, low cost, and the ability to meet active loading requirements under multiple working conditions and at multiple locations.

[0025] The above detailed embodiments are a description of the present invention. It should not be considered that the specific embodiments of the present invention are limited to these descriptions. For those skilled in the art, several simple deductions and substitutions can be made without departing from the concept of the present invention, and all of these should be considered to fall within the protection scope of the present invention.

Claims

1. A testing device for the strength and fatigue testing of a curved honeycomb sandwich structure after repair, characterized in that, Includes an upper loading rod, lever, force-applying lugs, bracket, lower loading rod, mounting base plate, rubber loading block, force conversion plate, positioning plate, and connecting rod, wherein: The upper and lower loading rods are the clamps of the static testing machine. The lever connects the upper loading rod and the connecting rod. The upper loading rod is tightly connected to the center of the lever via a pin. Both ends of the lever are connected to the connecting rod via pins, transferring the force load to the connecting rod. The connecting rod is used to adjust the direction of the force load according to the shape of the test piece. Both ends of the connecting rod are connected to the lever and the force-applying lugs respectively via holes. The installation angle between the connecting rod and the force-applying lugs is 90°. After the angle adjustment is confirmed, positioning is achieved using positioning plates to ensure that the installation angle does not change during loading. The positioning piece is an F-shaped structure, with its three corners tightly connected to the lever, connecting rod, and force-applying lugs via bolts. The force-applying lugs are T-shaped cross-section structures, with the upper end connected to the connecting rod structure via a pin and the lower end connected to the force conversion plate structure via bolts. The force conversion plate is a rectangular steel plate of uniform thickness, with one side fixedly connected to the force-applying lugs via bolts, the other side connected to the rubber loading block via bolts, and the middle position connected to the bracket via a pin. One side of each rubber loading block is bonded to the test piece with epoxy resin, and the other side is connected to the force conversion plate via bolts.

2. A method for testing the strength and fatigue of a curved honeycomb sandwich structure after repair, characterized in that, The strength and fatigue testing device for a curved honeycomb sandwich structure after repair, as described in claim 1, is used to achieve this, and the method includes: Step 1: The upper and lower force loading rods are clamped between the upper and lower hydraulic chucks of the fatigue testing machine, respectively; Step 2: The levers of the testing system distribute the applied static and fatigue pressures to both sides of the specimen; Step 3: Load according to the test requirements. By moving the relative positions of the upper and lower loading rods, the loads of the connecting rod, the force-applying lugs, and the support are converted into torques that act on the two force conversion plates on both sides of the specimen. Step 4: The torque is converted into out-of-plane tensile and compressive forces on the pre-defined rectangular areas on both sides of the specimen through each force conversion plate, and applied to the surface of the specimen to realize a single-channel multi-point loading test.

3. The method according to claim 2, characterized in that, Step 1 includes: Step 11: Determine the appropriate fatigue testing machine based on the size of the test specimen or the test conditions, and determine the displacement range of the upper and lower loading rods to ensure that the effective load can be applied during the loading process; Step 12: Install the test piece and test device as a whole according to the drawings. After installation, check the installation angle data of the connecting rod and the force-applying lugs. Step 13: Use the overhead crane to install the upper and lower loading rods of the testing device and testing machine; Step 14: After the rubber loading block is attached to the test piece, it is allowed to cure for a certain period of time; Step 15: After the test piece is installed and inspected, connect a steel wire rope to each side of the test device to further enhance safety protection.

4. The method according to claim 2, characterized in that, Step 2 includes: Step 21: According to the design requirements, control the testing machine to first load 20%~30% of the value, and check the stability of the testing device and whether there is any looseness in the connections; Step 22: Measure whether there is a change in the installation angle between the connecting rod and the force-applying lugs; Step 23: Check whether the rubber loading block and the test piece are firmly bonded and whether there are any gaps.

5. The method according to claim 2, characterized in that, Step 3 includes: Step 31: During the loading process, observe the data changes of each measurement point in real time according to the measurement requirements, and check whether the data changes are stable. If there is any abnormality, the test can be stopped immediately. Step 32: During the loading process, observe whether each connection point in the test device is stable to ensure that the test loading meets the requirements.

6. The method according to claim 2, characterized in that, Step 4 includes: Step 41: The area of ​​the rectangular rubber loading block can be adjusted according to verification requirements and location; Step 42: The testing device performs local loading strength and fatigue tests on any area of ​​any curved honeycomb sandwich structure to assess the strength and fatigue performance of the repair area. Only the surface of the rubber loading block and the test piece needs to be adjusted. Step 43: Using a curved honeycomb sandwich structure repair strength and fatigue testing device, the load-bearing capacity of the repaired area is characterized by calculating the test design load and torque equivalence, and by using a local structure.

7. The method according to claim 6, characterized in that, The preset rectangular areas are 6 x 60mm rectangles.

8. The method according to claim 6, characterized in that, Following step 43, the method further includes: Step 44: Based on the actual load conditions of the curved surface structure, adjust the length of the lever and the angle between the connecting rod and the force-applying lugs using the lever design principle, thereby adjusting the load distribution of the two rows of loading points to achieve the evaluation of the test piece under different test conditions and load levels.

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