A three-point bending test device for evaluating pinhole bending-torsion combined fretting damage

Abstract

The present application belongs to the technical field of material fatigue life test, and particularly relates to a three-point bending test device for evaluating pin-hole bending-torsion combined fretting damage, which comprises a top seat, a fretting block, cylindrical pins and a base. Two arc surfaces I are formed on the lower side of the fretting block. The upper side of the cylindrical pin is provided with arc surfaces II which are adapted to the arc surfaces I. The arc surfaces II of the cylindrical pin and the arc surfaces I of the fretting block are in contact to form a conformal contact pair. Two cylindrical pins are arranged on the upper surface of the base. The present application integrates three-point bending and conformal contact, and can complete three-point bending fretting fatigue test through a single test. The present application can simulate the bending stress borne by actual components during service, accurately reproduce the bending-torsion combined fretting fatigue phenomenon of the components, reduce the dispersion of the fatigue test results, and improve the rationality and scientificity of the test.

Description

Technical Field

[0001] This invention belongs to the field of material fatigue life testing technology, specifically relating to a three-point bend test device for evaluating pin hole bending and torsion combined fretting damage. Background Technology

[0002] Fretting fatigue refers to the micrometer-scale reciprocating sliding between two closely contacting metal surfaces under cyclic loading. This fretting can induce localized wear, oxidation, and crack initiation in the contact area, significantly reducing the fatigue life of materials or components. It is not simply wear or fatigue, but a complex failure mode resulting from the synergistic effects of wear, corrosion, and fatigue. Because it poses a fatal threat to almost all tightly connected structures subjected to vibration or alternating loads, it is a "killer" in engineering design. Its main impact is a significant reduction in fatigue life; compared to a smooth specimen without fretting, the crack initiation cycle can be shortened by more than an order of magnitude when fretting is present. This means that safe structures designed based on traditional fatigue strength may fail prematurely under fretting. Fretting cracks typically initiate at the edge of the contact area, are difficult to observe with the naked eye, and are also difficult to detect with conventional non-destructive testing, easily leading to catastrophic accidents. Most current fatigue fretting failure mechanisms target Hertzian contacts; however, the contact forms of the vast majority of engineering components are conformal contacts that do not satisfy Hertzian contact. Pin-hole fits are widely used in the fastening of various components due to their simple structure and high load-bearing efficiency. The size and radius of curvature of the pin-hole contact area are similar, which is a conformal contact.

[0003] Patent CN201555758U discloses a three-point fretting test auxiliary device, which combines a three-point bending support structure with a fretting loading component to achieve a point-contact fretting fatigue test device. However, the contact stress concentration is severe, and the fretting loading and three-point bending loading are independent of each other, making it difficult to achieve load coupling control. Patent CN106950107B discloses a three-point bending fatigue test device, which combines a rotatable clamping mechanism with a three-point bending loading structure to achieve a pure three-point bending fatigue test under negative stress ratio conditions. However, it can only achieve pure bending fatigue testing, and the indenter and the specimen are in line contact, resulting in severe local stress concentration. Patent CN110686989A discloses a material symmetrical loading three-point bending fatigue test device, which combines a support system, a loading system, and an auxiliary system to achieve a zero-crossing cyclic three-point bending fatigue test under symmetrical or pulsating loads. However, the contact form is point-line contact, resulting in large local stress concentration, and it can only study single bending fatigue failures.

[0004] Pin-hole fits are typical examples of conformal cylindrical-cylindrical contact, where fretting damage originates from the coupling effect of minute relative slippage at the conformal interface and contact stress. Existing patents for three-point bend fatigue testing devices all involve non-conformal contact, with research focusing primarily on characterizing single failure modes, and the contact pairs are mostly non-conformal point-line contacts. This design relies on Hertzian contact theory, leading to high local stress concentration, making the failure mechanism dependent on contact geometry rather than the intrinsic response of the material interface behavior, and failing to simulate the true contact area, stress field, and fretting slip mode of the pin hole. Therefore, combining three-point bend loading with conformal contact is a necessary prerequisite for simulating the real-world service conditions of pin hole fretting damage and is the core foundation for the engineering application value of such devices. Further improvements and refinements are needed to address the shortcomings of existing three-point bend testing devices and methods. Summary of the Invention

[0005] The purpose of this invention is to provide a three-point bending test device for evaluating pin hole bending-torsional fretting damage. It integrates three-point bending and conformal contact, and completes the three-point bending fretting fatigue test in a single test. It can simulate the bending stress that the actual component is subjected to during service, accurately reproduce the bending-torsional fretting fatigue phenomenon of the component, reduce the dispersion of fatigue test results, and improve the rationality and scientificity of the test.

[0006] The specific technical solution adopted by this invention is as follows: A three-point bend test device for evaluating pin hole bending and torsion combined fretting damage includes a top seat, a fretting block, a cylindrical pin, and a base. The micro-moving block is a plate-shaped structure, and two arc-shaped surfaces are provided on the lower side of the micro-moving block; The number of cylindrical pins is two, and the cylindrical pins are semi-cylindrical structural components. The upper side of the cylindrical pin is provided with an arc surface two that matches the arc surface one. The arc surface two of the cylindrical pin contacts the arc surface one of the micro-moving block to form a conformal contact pair. The two cylindrical pins are disposed on the upper surface of the base.

[0007] Furthermore, the lower end of the top seat is provided with a connecting boss.

[0008] Furthermore, the encirclement angle between the second arc surface and the first arc surface is less than 180°.

[0009] Furthermore, the center distance of the micro-movement block is 35mm, the width is 9mm, and the height is 6mm, and the encirclement angle between the second arc surface and the first arc surface is 106.26°.

[0010] Furthermore, the base includes a clamping end, an upper platform, and two small cavities; The upper side of the clamping end is fixedly connected to the lower side of the upper platform. Both small cavities are opened on the upper side of the upper platform. A rectangular boss is fixedly connected to the lower side of the cylindrical pin. The rectangular boss can be snapped into the interior of the small cavity.

[0011] Furthermore, the base also includes a large cavity, which is located on the upper side of the upper platform and between two small cavities. The depth of the large cavity is greater than the deflection of the micro-moving block under maximum load.

[0012] An experimental system for evaluating fretting damage caused by bending and torsion in pin holes includes a three-point bending test apparatus, a fatigue testing device, and a fretting wear detection system. The fatigue testing equipment is provided with an upper fixing groove and a lower fixing groove. The upper end of the top seat is clamped in the upper fixing groove of the fatigue testing equipment, and the lower end of the clamping end is clamped inside the lower fixing groove of the fatigue testing equipment. The micro-motion wear detection system includes a scanning electron microscope, a high-magnification digital microscope, and a computer, which are electrically connected.

[0013] An experimental method for evaluating combined bending and torsion fretting damage in pin holes, the experimental method comprising the following steps: The first step is to install the three-point bending test device, fatigue test equipment, and fretting wear detection system for pin hole fretting damage proposed in this invention into place; The second step is to set the test conditions, ensuring that the maximum peak value of the applied cyclic load does not exceed the material strength limit at the weakest point of the specimen. The third step is to start the experiment and stop it after exceeding the set maximum number of test cycles. The fourth step involves analyzing the morphology of the fretting wear marks using a high-magnification digital microscope and photographing them using a scanning electron microscope to analyze their fatigue failure mechanism.

[0014] The technical effects achieved by this invention are as follows: This invention provides a three-point bend test apparatus for evaluating pin hole bending-torsional combined fretting damage. The proposed three-point bend test apparatus simulates the normal load and fretting phenomena experienced during service. Utilizing a precision-machined conformal contact pair, it ensures a macroscopically uniform distribution of contact stress, thereby reducing interference from contact geometry singularities and allowing the research focus to return to the fretting damage at the contact interface itself. It directly simulates the real conformal contact forms found in high-value components such as aero-engine tenon-groove connections, precision bearing rings, and flange sealing surfaces, making the test data more engineering-guiding. Existing devices only characterize a single failure mode, while this apparatus, through the design of a curved conformal contact fixture and a three-point bend, generates composite stress in the conformal contact area—namely, normal pressure and tangential shear force—causing coupled radial expansion and contraction and tangential slippage on the conformal contact surface, i.e., composite fretting. In actual engineering, the components rarely experience pure tangential or radial fretting during service. Instead, they are more often subjected to multi-directional fretting coupling caused by temperature cycling, vibration, or eccentric loads. Therefore, this invention can improve the accuracy of assessment of bending-torsional composite fretting damage of conformal contact components and optimize the material selection and surface modification process of conformal contact components. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the fatigue testing equipment in this invention; Figure 2 This is a schematic diagram of the three-point bend test device in this invention; Figure 3 This is an exploded view of the three-point bend test device in this invention; Figure 4 This is a schematic diagram of the structure of the base of the present invention.

[0016] The attached diagram lists the components represented by each number as follows: 1. Top seat; 11. Connecting boss; 2. Micro-moving block; 21. Arc surface one; 3. Cylindrical pin; 31. Arc surface two; 32. Rectangular boss; 4. Base; 41. Clamping end; 42. Upper platform; 43. Small cavity; 44. Large cavity; 5. High-magnification digital microscope; 6. Fatigue testing equipment; 61. Upper fixing groove; 62. Lower fixing groove. Detailed Implementation

[0017] To make the objectives and advantages of this invention clearer, the invention will be specifically described below with reference to embodiments. It should be understood that the following text is merely used to describe one or more specific embodiments of the invention and does not strictly limit the scope of protection specifically claimed by the invention.

[0018] A fatigue testing device (6) is provided with an upper fixed groove (61) and a lower fixed groove (62). The fatigue testing device (6) is preferably an electromagnetic vibration fatigue testing machine.

[0019] Example 1: like Figures 1-4 As shown, a three-point bend test device for evaluating pin hole bending and torsion combined fretting damage includes a top seat 1, a fretting block 2, a cylindrical pin 3, and a base 4.

[0020] The top seat 1 has a columnar structure. The upper end of the top seat 1 is clamped in the upper fixing groove 61 of the fatigue testing equipment 6, and the lower end of the top seat 1 is provided with a connecting boss 11 for transmitting axial load. The micro-moving block 2 has a plate-like structure. Two arc-shaped surfaces 21 are opened on the lower side of the micro-moving block 2. The fatigue testing equipment 6 can drive the connecting boss 11 of the top seat 1 to abut against the upper surface of the micro-moving block 2 for transmitting load and forming conformal contact with the cylindrical pin 3. The cylindrical pin 3 is a semi-cylindrical structural component with a D-shaped cross-section. The upper side of the cylindrical pin 3 is provided with an arc-shaped surface 21 that is compatible with the arc-shaped surface 21. The arc-shaped surface 21 of the cylindrical pin 3 contacts the arc-shaped surface 21 of the micro-moving block 2 to support the micro-moving block 2 and form a conformal contact pair. There are two cylindrical pins 3, and the two cylindrical pins 3 are symmetrically arranged on the upper surface of the base 4 with the central axis of the base 4 as the center line; Specifically, the cylindrical pin 3 can be fixedly connected to the upper surface of the base 4 by snap-fit, plug-in or other means. In this technical solution, snap-fit ​​is preferred. The base 4 includes a clamping end 41, an upper platform 42, two small cavities 43 and a large cavity 44. The lower end of the clamping end 41 is clamped inside the lower fixing groove 62 of the fatigue testing equipment 6, and the fatigue testing equipment 6 provides support for the clamping end 41. The upper side of the clamping end 41 is fixedly connected to the lower side of the upper platform 42, and the clamping end 41 and the upper platform 42 are in vertical contact in a T-shaped structure. The large cavity 44 and the two small cavities 43 are all opened on the upper side of the upper platform 42, and the large cavity 44 is located between the two small cavities 43. A rectangular boss 32 is fixedly connected to the lower side of the cylindrical pin 3. The rectangular boss 32 can be snapped into the inside of the small cavity 43. The combination of the rectangular boss 32 and the small cavity 43 forms a limiting fit structure, which restricts the displacement of the cylindrical pin 3 while realizing the positioning of the cylindrical pin 3, so as to ensure that the arc surface 21 on the cylindrical pin 3 and the arc surface 21 on the micro-moving block 2 achieve conformal contact. In some embodiments, the depth of the large cavity 44 is greater than the deflection of the micro-moving block 2 under maximum load, providing space for the bending deformation of the micro-moving block (2) and reducing the phenomenon of the micro-moving block 2 embedding in the large cavity 44. During the test, the arc-shaped surface 31 on the cylindrical pin 3 contacts the arc-shaped surface 21 on the micro-moving block 2, forming a conformal contact pair, and a three-point bending fatigue test is achieved under the axial loading of the top seat 1.

[0021] It should be noted that the arc-shaped surface 31 on the cylindrical pin 3 and the arc-shaped surface 21 on the micro-moving block 2 are in a cylindrical surface fit, and the size of their contact area is half the cylindrical surface of the cylindrical pin 3, which is a typical cylindrical conformal contact. The micro-moving block 2 and the cylindrical pin 3 are subjected to a normal load from the top seat 1, causing a relative sliding with an amplitude of about tens of micrometers at the contact area between the micro-moving block 2 and the cylindrical pin 3. A composite stress is generated in the conformal contact area between the cylindrical pin 3 and the micro-moving block 2, namely normal pressure and tangential shear force. At this time, the conformal contact surface produces a coupled motion of radial expansion and contraction and tangential sliding, that is, a composite micro-movement.

[0022] This device is a three-point bending device, meaning it has two-point support formed by two cylindrical pins 3, and a single-point loading formed by the load on the top seat 1. The maximum stress of the three-point bending specimen under these conditions is... for: (1) in L The center distance of a simply supported beam. h Indicates the height of a simply supported beam. F This indicates the concentrated external load borne by a simply supported beam. b The width of the simply supported beam.

[0023] The center distance L of a simply supported beam directly determines the stress distribution. If the center distance is too small, the stress on the fretting block 2 will concentrate at the middle loading point, failing to fully reflect the bending characteristics. If the center distance is too large, the bending deformation of the fretting block 2 will be excessive, resulting in uneven stress transmission. Therefore, the stress calculation in the three-point bend experiment strictly depends on the size of the center distance, which directly affects the stress magnitude. The size of the center distance should be determined first, while the influence of other parameters on the results can be corrected through subsequent calibration. The relationship between the center distance of the fretting block 2 and the stress magnitude is nonlinear; simply reducing the center distance cannot reduce the stress. Therefore, the fixture optimization problem is a nonlinear multi-objective optimization problem.

[0024] Fixture optimization includes the following steps: Step 1: Optimization of the volume of micro-movement block 2: The micro-moving block 2 is simplified as a simply supported beam.

[0025] The corner of a simply supported beam for: (2) in: q Indicates the uniform load intensity. c Indicates the length of the segment under uniformly distributed load. L Indicates the center distance of a simply supported beam. E Indicates the elastic modulus of a material. γ Represents the dimensionless coefficient. I Represents the polar moment of inertia of the cross section. γ = c / L .

[0026] The polar moment of inertia of a simply supported beam is: (3) in: b Let be the width of the beam. h The height of a simply supported beam with a rectangular cross-section Relative displacement arc length of a simply supported beam for: (4) in: r radius of curvature From formulas (1) and (2): (5) The mathematical expression for this nonlinear optimization problem is: (6) in X 1 represents the maximum stress of a simply supported beam. X 2 represents the relative displacement arc length of the simply supported beam.

[0027] The second step is to optimize the arc-shaped contact surface of the micro-motion block 2: From the perspective of the characteristics of the contact area in conformal contact, when the contact arc surface of the micro-moving block 2 is a semi-circular arc, the wrap angle is 180°. Due to the excessively long contact area, the contact stress is uniform and relatively small, resulting in a slow wear process, indistinct local wear characteristics, and shallow wear depth. This makes it difficult to measure wear marks in actual experiments. Therefore, to better observe wear marks, the semi-circular arc should be changed to a short circular arc. At this point, the wrap angle is less than 180°, the contact area is shortened, and while still conformal contact, the pressure distribution is concentrated. The shorter wear area leads to more concentrated contact stress, and the superposition of local slippage and stress concentration makes the wear depth more significant, facilitating measurement. Therefore, to facilitate measurement after the experiment, the semi-circular arc should be changed to a short semi-circular arc. The micro-moving block 2 is simplified as a simply supported beam bending deformation, and the elastic deformation of the cylindrical pin 3 is ignored.

[0028] The formula for the rotation angle of a simply supported beam under a central load is: (7) in: F This indicates the concentrated external load borne by a simply supported beam. L Indicates the center distance of a simply supported beam. E 1 This represents the elastic modulus of a simply supported beam. I This represents the polar moment of inertia of a simply supported beam section.

[0029] Convert radians to degrees : (8) After nonlinear multi-objective optimization, the span, height, and width of the micro-motion block 2 were determined. Furthermore, the arc-shaped contact surface of the micro-motion block 2 was changed from a semi-circular arc to a short circular arc. The dimensions of the micro-motion block 2, cylindrical pin 3, and base in the initial three-point bend micro-motion device were all altered. The optimized three-point bend micro-motion device applied a compressive load to the top seat 1. At this point, the stress acting on the micro-motion block 2 did not exceed the bearing limit, and the micro-motion block 2 would not fracture. Therefore, these dimensions are the final determined dimensions of the three-point bend specimen.

[0030] Example 2: like Figures 1-4 As shown, an experimental system for evaluating fretting damage caused by bending and torsion in pin holes includes a fatigue testing device 6, a three-point bending testing device, and a fretting wear detection system. The fretting wear detection system includes a scanning electron microscope, a high-magnification digital microscope 5, and a computer. The scanning electron microscope, the high-magnification digital microscope 5, and the computer are electrically connected. The high-magnification digital microscope 5 can be installed on the fatigue testing equipment 6, which can observe fretting wear marks in real time and record the data in real time via the computer.

[0031] Among them, the high-magnification digital microscope 5 is preferably a laser confocal optical microscope.

[0032] Example 3: This embodiment discloses an experimental method for evaluating the combined bending and torsion fretting damage of pin holes, based on the above embodiments. According to the established three-point bending test device, the fretting damage mechanism of the combined bending and torsion of pin holes is evaluated by starting with different cyclic loads.

[0033] The working principle of this experimental method is as follows: The specimen is processed using the same processing parameters to ensure that the machining quality of each plane of the top seat 1, micro-movement block 2, cylindrical pin 3, and base 4 meets the national standard requirements for specimens. After each workpiece is processed, the test device is installed on the fatigue testing equipment 6 via the top seat 1 and base 4, and the same test environment, temperature, and conditions are configured. The fatigue load, load ratio, and fatigue test frequency are set, and fatigue tests are conducted. Under the given test conditions, the fatigue wear marks on the specimen are observed, including the location and type of fatigue wear marks. This simulates the fatigue failure problems and potential hazards of components in real service environments, providing guidance for the structural design, processing technology optimization, and selection of surface strengthening methods for components.

[0034] Specifically, the experimental method includes the following steps: The first step is to install the three-point bend test device for pin hole fretting damage, fatigue test equipment 6 and fretting wear detection system proposed in this invention. The resonance frequency of this test device can reach 100Hz, which can be adapted to electromagnetic vibration fatigue testing machine to shorten the test time. The second step is to set the test conditions. The maximum peak value of the applied cyclic load should not exceed the material strength limit of the weakest part of the specimen. To prevent the device from experiencing impact vibration, the load ratio should be greater than 0. The third step is to start the experiment and stop it after exceeding the set maximum number of test cycles. The fourth step involves analyzing the morphology of fretting wear marks using a laser confocal optical microscope and photographing the fretting wear marks using a scanning electron microscope to analyze their fatigue failure mechanism.

[0035] Example 4: The parameters in this embodiment are set as follows: uniformly distributed load intensity 500 N / mm, load application length 4 mm, short circular arc radius of curvature of micro-moving block 2 3 mm, elastic modulus 110000 MPa, external load 2000 N. The constraints are: ensuring the relative displacement arc length is approximately 25 μm, making it easier to observe morphological features; ensuring the maximum stress does not exceed 300 MPa to reduce the risk of micro-moving block 2 fracture; and limiting the dimensions of micro-moving block 2 to be greater than 5 mm and less than 50 mm in width and height, and greater than 10 mm and less than 100 mm in center distance. Furthermore, to better observe wear marks after the test, the semi-circular arc of the contact surface of micro-moving block 2 is changed to a short circular arc. Simplifying micro-moving block 2 as a simply supported beam, the method proposed in this invention calculates the center distance of micro-moving block 2 to be 35 mm, the width to be 9 mm, the height to be 6 mm, and the central angle of the short circular arc to be 106.26°, which is the final result.

[0036] The above description is merely a preferred embodiment of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention. Structures, devices, and operating methods not specifically described or explained in this invention are implemented according to conventional methods in the art unless otherwise specified or limited.

Claims

1. A three-point bending test apparatus for evaluating combined bending and torsion fretting damage in pin holes, characterized in that: Includes top seat (1), micro-moving block (2), cylindrical pin (3), and base (4); The micro-moving block (2) is a plate-shaped structure, and two arc-shaped surfaces (21) are opened on the lower side of the micro-moving block (2). There are two cylindrical pins (3). The cylindrical pins (3) are semi-cylindrical structural parts. The upper side of the cylindrical pins (3) is provided with an arc surface two (31) that is compatible with the arc surface one (21). The arc surface two (31) of the cylindrical pins (3) and the arc surface one (21) of the micro-moving block (2) are in contact to form a conformal contact pair. The two cylindrical pins (3) are disposed on the upper surface of the base (4).

2. The three-point bending test device for evaluating pin hole bending-torsional combined fretting damage according to claim 1, characterized in that: The lower end of the top seat (1) is provided with a connecting boss (11).

3. The three-point bending test device for evaluating combined bending and torsion fretting damage of pin holes according to claim 1, characterized in that: The enclosing angle between the second arc surface (31) and the first arc surface (21) is less than 180°.

4. The three-point bending test device for evaluating pin hole bending-torsional combined fretting damage according to claim 3, characterized in that: The center distance of the micro-movement block (2) is 35mm, the width is 9mm and the height is 6mm. The wrap angle between the arc surface two (31) and the arc surface one (21) is 106.26°.

5. The three-point bending test device for evaluating pin hole bending-torsional combined fretting damage according to claim 1, characterized in that: The base (4) includes a clamping end (41), an upper platform (42), and two small cavities (43). The upper side of the clamping end (41) and the lower side of the upper platform (42) are fixedly connected. Both small cavities (43) are opened on the upper side of the upper platform (42). A rectangular boss (32) is fixedly connected to the lower side of the cylindrical pin (3). The rectangular boss (32) can be snapped into the inside of the small cavity (43).

6. The three-point bending test apparatus for evaluating combined bending and torsion fretting damage of pin holes according to claim 5, characterized in that: The base (4) also includes a large cavity (44), which is located on the upper side of the upper platform (42) and between two small cavities (43). The depth of the large cavity (44) is greater than the deflection of the micro-moving block (2) under the maximum load.

7. An experimental system for evaluating combined bending and torsion fretting damage in pin holes, characterized in that: The three-point bending test apparatus, fatigue test equipment (6), and fretting wear detection system are included in any one of claims 5-6. The fatigue testing equipment (6) is provided with an upper fixing groove (61) and a lower fixing groove (62). The upper end of the top seat (1) is clamped in the upper fixing groove (61) of the fatigue testing equipment (6), and the lower end of the clamping end (41) is clamped inside the lower fixing groove (62) of the fatigue testing equipment (6). The micro-motion wear detection system includes a scanning electron microscope, a high-magnification digital microscope (5), and a computer, which are electrically connected.

8. An experimental method for evaluating fretting damage caused by bending and torsion in pin holes, employing the three-point bending test apparatus, fatigue testing equipment (6), and fretting wear detection system as described in claim 7, characterized in that: The experimental method Includes the following steps: The first step is to install the three-point bending test device, fatigue test equipment (6) and fretting wear detection system for pin hole fretting damage proposed in this invention into place; The second step is to set the test conditions, ensuring that the maximum peak value of the applied cyclic load does not exceed the material strength limit at the weakest point of the specimen. The third step is to start the experiment and stop it after exceeding the set maximum number of test cycles. The fourth step is to analyze the morphology of the fretting wear marks by using a high-power digital microscope (5) and to photograph the fretting wear marks by scanning electron microscope to analyze their fatigue failure mechanism.

9. The three-point bending test device for evaluating pin hole bending-torsional combined fretting damage according to claim 1, characterized in that: In the second step, the test conditions are set so that the load ratio is greater than 0.

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