A test piece and test tool for measuring fatigue performance of diffusion bonded test specimen

Abstract

The application discloses a test piece for measuring fatigue performance of diffusion joint samples, which comprises coaxially arranged first and second ring bodies, the inner diameter of the first ring body is larger than the inner diameter of the second ring body and smaller than the outer diameter of the second ring body; the top surface of the second ring body close to the outer ring is diffusion jointed with the bottom surface of the first ring body close to the inner ring to form a diffusion joint interface; wherein the first and second ring bodies are metal rings; by adopting the coaxial unequal-diameter ring structure, the loading force can be transmitted along the central axis during the fatigue test, the axial deviation caused by geometric asymmetry is effectively avoided, and the eccentric load phenomenon is eliminated.

Description

Technical Field

[0001] This invention belongs to the field of diffusion bonding fatigue testing technology, and particularly relates to a test piece and testing fixture for measuring the fatigue performance of diffusion bonding specimens. Background Technology

[0002] Diffusion bonding, as a solid-state joining process, is widely used in the manufacture of advanced structural materials such as high-temperature alloys, titanium alloys, and ceramic matrix composites, especially in aerospace propulsion systems and hot-end components, where it plays a crucial role in load-bearing and thermal management. During actual service, diffusion-bonded interfaces must withstand the coupled effects of alternating thermal stress and mechanical loads over long periods. Their interface microstructure (such as interdiffusion layers, residual stress distribution, microporosity, and second-phase precipitation) significantly influences fatigue crack initiation and propagation behavior. Therefore, obtaining the fatigue response characteristics of diffusion-bonded interfaces under cyclic shear or tensile-shear combined loads is a crucial foundation for evaluating the long-term reliability of bonded structures.

[0003] Currently, there is no unified standard for testing methods of fatigue performance at diffusion-bonded interfaces, and related studies mostly draw on the testing approaches of traditional welded or adhesive joints. For example, some studies use double-lapped shear specimens, apply in-plane shear loads on a high-frequency fatigue testing machine, and characterize the interface stiffness degradation behavior by recording the load-displacement hysteresis loop changes; other studies use improved fracture mechanics specimens, combined with digital image correlation technology to monitor the evolution of the strain field at the interface front, in order to identify the fatigue crack initiation location and propagation rate.

[0004] However, in existing technologies, the use of through holes at both ends and screw connections makes it difficult to guarantee geometric alignment stability under long-term cyclic loading. Furthermore, the accumulation of errors during part processing, assembly, and clamping easily leads to a misalignment between the loading axis and the normal of the diffusion connection interface. This axial misalignment prevents the applied load from being transmitted strictly along the central axis of the connection interface, introducing uncontrollable additional bending stress components into the interface region. This causes the actual stress state at the diffusion connection interface to deviate significantly from the target working condition, resulting in uneven stress distribution and localized stress concentration. Consequently, this interferes with the accurate identification of fatigue crack initiation locations and propagation paths, reducing the repeatability and physical accuracy of test results. Summary of the Invention

[0005] The purpose of this invention is to provide a test piece and testing fixture for measuring the fatigue performance of diffusion bonded specimens, so that the applied force is transmitted along the central axis during the fatigue test.

[0006] The present invention adopts the following technical solution: a test piece for measuring the fatigue performance of diffusion bonded specimens, comprising a first ring and a second ring arranged coaxially, wherein the inner diameter of the first ring is larger than the inner diameter of the second ring and smaller than the outer diameter of the second ring; The top surface of the second ring near the outer ring diffuses and connects with the bottom surface of the first ring near the inner ring, forming a diffusion connection interface; wherein, both the first and second rings are metal rings.

[0007] Another technical solution of the present invention: a test fixture for measuring the fatigue performance of a diffusion bonded specimen, the test fixture being used in conjunction with the above-mentioned test piece; The test fixture includes a loading rod, the diameter of which is slightly smaller than the inner diameter of the second ring of the test piece; The loading rod is axially spaced with a first loading ring and a second loading ring, which are used to cooperate to clamp the second ring body.

[0008] Another technical solution of the present invention: a test fixture for measuring the fatigue performance of a diffusion bonded specimen, the test fixture comprising the above-mentioned test piece; The test fixture includes a loading rod, the diameter of which is slightly smaller than the inner diameter of the second ring of the test piece; The loading rod is axially spaced with a first loading ring and a second loading ring, which are used to cooperate to clamp the second ring body.

[0009] The beneficial effects of this invention are: by using two coaxial ring structures with unequal diameters, this invention can ensure that the load on the test piece is transmitted along the central axis during fatigue testing, effectively avoiding axis offset caused by geometric asymmetry, thereby eliminating the off-center loading phenomenon. Attached Figure Description

[0010] Figure 1 This is a schematic diagram of the structure of a test piece for measuring the fatigue performance of a diffusion-bonded specimen, according to an embodiment of the present invention. Figure 2 This is a schematic diagram of a test fixture for measuring the fatigue performance of a diffusion-bonded specimen according to an embodiment of the present invention; Figure 3 This is a schematic diagram of the working state of the test specimen and test fixture in an embodiment of the present invention.

[0011] Wherein: 100. First ring body; 200. Second ring body; 300. Diffusion connection interface; 400. Loading rod; 500. First loading ring; 600. Second loading ring. Detailed Implementation

[0012] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0013] In the field of diffusion bonding fatigue testing, when evaluating the fatigue performance of diffusion bonding interfaces in advanced structural materials such as high-temperature alloys and titanium alloys, existing test specimen structures are generally based on the overall material configuration design, without geometric adaptation to the unique thin-layer interdiffusion region and gradient transition characteristics of the parent material at the diffusion bonding interface. Existing double-lapped shear specimens or improved fracture mechanics specimens have limitations in the controllability of the loading path and the interface normal / tangential stress components. Because the interface between the through-holes and screws at both ends cannot guarantee geometrical alignment stability under long-term cyclic loading, coupled with the accumulation of micron-level form and position errors during part processing, assembly, and clamping, the loading axis is easily offset from the normal of the diffusion bonding interface, resulting in axial offset. Under axial offset conditions, the applied tensile or compressive load cannot be strictly transmitted along the central axis of the bonding interface, thus introducing an uncontrollable off-center loading effect in the interface region, inducing additional bending stress components. This causes the actual stress state borne by the diffusion bonding interface to deviate significantly from the target working condition, resulting in uneven stress distribution and local stress concentration at the interface. This interferes with the identification of fatigue crack initiation locations and propagation paths, significantly reducing the repeatability and physical authenticity of the test results.

[0014] Existing test specimens also lack refined designs for micron-level connection interfaces in terms of clamping constraints, load introduction methods, and strain measurement markings in the interface area. The test specimen structures used in existing diffusion-bonded fatigue tests are generally based on a "dumbbell-shaped" configuration formed by two coaxial cylindrical or plate-shaped parts diffused together in the middle, and rely on the testing machine drive end to connect to the test specimen through through holes at both ends via screws to achieve loading.

[0015] Because the interface between the through holes at both ends and the screw connection cannot guarantee geometric alignment stability under long-term cyclic loading, coupled with the accumulation of micron-level form and position errors during part machining, assembly, and clamping, it is highly likely that the loading axis will shift from the normal direction of the diffusion connection interface, resulting in "axis offset". Under axis offset conditions, the applied tensile / compressive load cannot be strictly transmitted along the central axis of the connection interface, thus introducing an uncontrollable "off-center loading effect" in the interface region, inducing additional "bending stress components". After this bending stress is superimposed on the cyclic main load, the actual stress state borne by the diffusion connection interface deviates significantly from the target working condition (such as pure shear or axial cyclic load), which not only causes uneven stress distribution and local stress concentration at the interface, but also interferes with the identification of fatigue crack initiation location and propagation path, significantly reducing the repeatability, comparability, and physical authenticity of the test results.

[0016] In addition, the existing structure lacks an active constraint mechanism for the loading force flow path, and cannot achieve the triple coincidence of "force line - axis - interface normal" at the test piece body level. Therefore, it is difficult to support the pure force model required for high-precision interface constitutive behavior modeling and life prediction.

[0017] The aforementioned defects essentially stem from the fact that traditional test specimen structures fail to coordinate and adapt to the thinness, anisotropy, and micro-area load-bearing characteristics of the diffusion connection interface in three dimensions: geometric configuration, connection area arrangement, and loading introduction method. This results in the fatigue testing process being constantly limited by unexpected mechanical disturbances, thus restricting the ability to quantitatively analyze the essential fatigue mechanism of the interface.

[0018] See Figure 1 This embodiment provides a test specimen for measuring the fatigue performance of diffusion-bonded samples, aiming to solve the problems of off-center loading and additional bending stress caused by axial misalignment in existing material bonding performance tests. It includes a first ring 100 and a second ring 200 coaxially arranged. The inner diameter of the first ring 100 is larger than the inner diameter of the second ring 200 but smaller than its outer diameter. The top surface of the second ring 200 near the outer ring is diffusion-bonded to the bottom surface of the first ring 100 near the inner ring, forming a diffusion-bonded interface 300. Both the first ring 100 and the second ring 200 are metal rings.

[0019] The first ring 100 and the second ring 200 are coaxially arranged annular metal components, wherein the inner diameter of the first ring 100 is larger than the inner diameter of the second ring 200 but smaller than its outer diameter, forming an assembly relationship. A local area on the top surface of the outer ring of the second ring 200 and a local area on the bottom surface of the inner ring of the first ring 100 are metallurgically bonded through a diffusion bonding process, forming a stable and continuous diffusion bonding interface 300.

[0020] By employing two coaxial rings of unequal diameters, the applied force can be transmitted along the central axis during fatigue testing, effectively avoiding axial offset caused by geometric asymmetry and thus eliminating off-center loading. The top surface of the second ring 200, near the outer ring, and the bottom surface of the first ring 100, near the inner ring, are locally diffused together, forming a stable and symmetrical connection interface. This ensures uniform load distribution at the interface and reduces the risk of stress concentration.

[0021] The first ring 100 refers to the annular metal component that forms the external fixed end of the test piece. It serves as the base or stationary end of the entire device and is used to be clamped and fixed by the testing machine fixtures during fatigue testing. The inner diameter of the first ring 100 is set to be larger than the inner diameter of the second ring 200 and smaller than the outer diameter of the second ring 200. In this mating relationship, the first ring 100 not only provides structural support but also ensures the coincidence of their central axes through its relative position to the second ring 200. The material of the first ring 100 can be a nickel-based high-temperature alloy, titanium alloy, or other metal materials suitable for diffusion bonding processes. Its specific shape and height can be set according to the clamping space and load requirements of the actual testing equipment.

[0022] The second ring 200 refers to the annular metal component constituting the internal loading end of the test piece, coaxially positioned below the inner cavity of the first ring 100. The outer diameter of the second ring 200 matches the inner diameter of the first ring 100, allowing the top edge region of the second ring 200 to contact the bottom edge region of the first ring 100. The second ring 200 is also made of metal, and its material can be the same as or different from that of the first ring 100 to simulate the actual working conditions of diffusion bonding of dissimilar materials. The structural parameters of the second ring 200, such as outer diameter, inner diameter, and height, can be set according to actual conditions. For example, it can be a thin-walled cylindrical structure or a thick-walled annular structure, as long as it meets the requirements of coaxial nesting and connection with the first ring 100.

[0023] The diffusion bonding interface 300 refers to the metallurgical bonding region formed between the first ring body 100 and the second ring body 200 through a solid-state diffusion bonding process. This interface is located on the contact surface between the top surface of the second ring body 200 near the outer ring and the bottom surface of the first ring body 100 near the inner ring. Because the inner diameter of the first ring body 100 is smaller than the outer diameter of the second ring body 200, the contact surface is not the entire end face, but is limited to the annular overlapping region. The formation of the diffusion bonding interface 300 relies on atomic interdiffusion under high temperature and pressure, eliminating macroscopic gaps and achieving continuous bonding at the lattice level. Functionally, this interface plays a crucial role in transferring cyclic loads. Its location is intentionally designed in the overlapping region near the edges of the inner and outer rings, rather than the central region. This layout helps optimize the stress transfer path, making the load more evenly distributed along the circumferential direction of the interface.

[0024] This invention constructs a coaxial nested double-layer ring structure consisting of a first ring 100 and a second ring 200. By restricting the radial dimensional relationship between the two (i.e., the inner diameter of the first ring is between the inner and outer diameters of the second ring), a unique and defined annular overlapping area is forcibly formed. This geometric configuration naturally locks the diffusion connection interface 300 at a specific radius away from the central axis, while utilizing the symmetry of the outer ring wrapping the inner ring to eliminate the eccentricity risk common in traditional docking structures.

[0025] During fatigue testing, the outer periphery of the first ring 100 is fixedly clamped to the loading system of the testing machine, while the inner ring of the second ring 200 is clamped and loaded via a loading rod 400 that runs along the central axis. Figure 3As shown, the loading rod 400 passes through the inner cavity of the second ring 200, on which two spaced-apart first loading rings 500 and second loading rings 600 are mounted, located above and below the second ring 200 respectively. By synchronously adjusting the preload of the two loading rings, a uniform tensile load is applied to the axial end face of the second ring 200. Since the first ring 100 and the second ring 200 are strictly coaxial, and the loading path is transmitted entirely along the central axis, there is no eccentricity, torsion, or additional bending moment generated during the entire cyclic loading process. The diffusion connection interface 300 only bears uniform pure shear or axial cyclic load, significantly improving the purity of the load state and the reliability of the test data. This structural design fundamentally avoids the secondary stress problems caused by misalignment of holes or uneven force in traditional through-hole screw connections, achieving high-precision and highly repeatable fatigue performance evaluation.

[0026] During fatigue testing, the first ring 100, as the stationary end, is firmly clamped by an external fixture, while the second ring 200, as the movable end, has its inner hole passing through the loading rod 400. When the loading rod 400 applies a tensile or compressive load axially, the force flow first acts on the second ring 200, and then is transmitted to the first ring 100 through the diffusion connection interface 300. Since the first ring 100 and the second ring 200 are strictly coaxial, and the connection interface 300 is symmetrically distributed in a ring shape, the load decomposition vector at the interface is mainly canceled out symmetrically along the axial and radial directions, preventing the generation of unexpected bending moments or torques. This structural design allows the diffusion connection interface 300 to primarily bear pure axial cyclic stress or shear stress, avoiding additional bending stress caused by clamping errors or structural asymmetry, thus truly reflecting the fatigue characteristics of the material interface.

[0027] During testing, the first ring 100 is fixed, and the second ring 200 is clamped by two spaced first loading rings 500 and second loading rings 600 on the loading rod 400 to achieve tensile loading. This clamping method does not rely on through-hole screw connection, thus avoiding the additional bending stress and torsional load introduced by traditional connection structure.

[0028] The two rings are arranged coaxially, and combined with the non-destructive clamping mechanism of the second ring 200, the diffusion connection interface mainly bears uniform pure shear or axial cyclic load, which significantly improves the purity of the load state and the repeatability of the test results.

[0029] In one embodiment, to address the issue of buckling instability caused by insufficient structural stability of the second ring 200, the geometric parameters of the second ring 200 are further optimized based on the aforementioned structure. The second ring 200 is a hollow structure with a wall thickness to outer diameter ratio of 1:(3~5), i.e., when the outer diameter D and wall thickness t have 3≤D / t≤5, preferably 4:1. Simultaneously, the height H of the second ring 200 should exceed the length h of the diffusion connection interface 300, preferably H≥2h. This dimensional ratio design ensures that under tensile loads, the second ring 200 primarily undergoes axial elongation deformation rather than lateral buckling, effectively improving its structural stability under multiple cyclic loading. Furthermore, this structure facilitates a precise transition fit with the inner cavity of the first ring 100, reducing assembly gaps before diffusion connection and lowering the initial defect density caused by fretting wear, thereby improving the consistency of connection quality. This embodiment is particularly suitable for fatigue testing scenarios of high-strength but stress-concentration-sensitive material systems such as high-temperature alloys or titanium alloys.

[0030] The second ring 200 is designed as a hollow structure. This structural form not only helps reduce the overall weight of the test piece, but more importantly, it optimizes its mechanical response characteristics under cyclic loading by rationally configuring its geometric dimensions. The ratio of the wall thickness to the outer diameter of the second ring 200 is limited to a range of 1:3 to 1:5. This ratio defines the balance point between the radial stiffness and buckling resistance of the second ring 200. Specifically, if the wall thickness is too thin (i.e., the ratio is less than 1:5), the second ring 200 is prone to lateral buckling instability under high-amplitude axial cyclic loads, leading to distorted test data; if the wall thickness is too thick (i.e., the ratio is greater than 1:3), excessive stiffness may lead to significant stress concentration effects, or unnecessary cost increases and difficulties in controlling the heat-affected zone due to material redundancy. Therefore, the ratio range of 1:3 to 1:5 is the preferred range determined based on the mechanical properties and fatigue testing requirements of common aerospace structural materials such as high-temperature alloys and titanium alloys. Of course, the ratio can also be set according to the actual situation, such as 1:3.2, 1:4 or 1:4.8, depending on the yield strength, elastic modulus and specific load conditions of the actual test material.

[0031] The second ring 200, being a hollow structure, has a clearly defined fit and connection with the aforementioned first ring 100. The specific wall thickness and outer diameter ratio helps reduce assembly clearance and lower the initial defect density caused by fretting wear. During fatigue testing, the second ring 200 engages with the loading rod 400 through its inner hole, and axial loads are applied by the first loading ring 500 and the second loading ring 600. At this time, the defined wall thickness and outer diameter ratio ensures that the second ring 200 undergoes uniform axial elongation under stress, effectively suppressing unintended bending or torsional deformation caused by insufficient local stiffness, thereby ensuring the purity of the stress state at the diffusion connection interface 300.

[0032] Specifically, when the testing machine applies an axial cyclic tensile load to the second ring 200, the load is transmitted to the end face of the second ring 200 through the loading rod 400. Because the second ring 200 adopts a hollow thin-walled structure design with a wall thickness to outer diameter ratio of 1:3 to 5, its cross-sectional moment of inertia to cross-sectional area ratio is optimized. Under load, the tube wall of the second ring 200 can undergo uniform elastic and plastic deformation, effectively transmitting the axial force to the diffusion connection interface 300 connected to the first ring 100. This geometric proportion limits the lateral deflection of the second ring 200 under compression half-cycle or with slight eccentricity, preventing Euler buckling and ensuring that the diffusion connection interface 300 is always under a preset shear or tensile-shear combined stress state, avoiding interference with test results due to additional bending moments introduced by specimen instability.

[0033] To optimize the mechanical load-bearing capacity and fatigue life consistency of the diffusion connection interface, this embodiment limits the location and contact area of ​​the connection region. The diffusion connection interface 300 is located on the bottom surface of the first ring 100 near the inner ring, and is connected to the corresponding position on the top surface of the outer ring of the second ring 200. This axial positioning avoids the connection area being too close to the middle of the width of the first ring 100, which would lead to an excessively wide heat-affected zone or excessively high residual stress accumulation. At the same time, the area of ​​the diffusion connection interface 300 accounts for 40% to 60% of the top surface area of ​​the second ring 200, optimizing the matching relationship between connection strength and stress distribution, and improving the consistency and reliability of interface fatigue life. This ensures sufficient effective load-bearing area to transmit high-amplitude cyclic loads, while preventing uneven interface stress distribution or increased risk of local debonding due to excessive contact area. Experiments show that this configuration can achieve an interface fatigue life dispersion coefficient of less than 8% while ensuring that the connection strength is not less than 85% of the base material, which is significantly better than conventional full-end-face connection structures. This implementation is particularly suitable for research scenarios that require statistical analysis of fatigue SN curves.

[0034] The diffusion bonding interface 300 refers to the metallurgical bonding area formed by the diffusion bonding process between the bottom surface of the first ring 100 near the inner ring and the top surface of the second ring 200 near the outer ring. The specific area of ​​the diffusion bonding interface 300 can be set according to actual testing requirements and material properties. For example, it can be 40%, 50%, or 60% of the total area of ​​the top surface of the second ring 200.

[0035] In the overall technical solution, the diffusion connection interface 300 serves as the key path for load transfer, and its area ratio directly determines the stress distribution and load-bearing capacity at the interface. The diffusion connection interface 300 works in conjunction with the first ring body 100 and the second ring body 200 to control the relative size of the connection area, so that the cyclic load can be applied evenly to the joint surface. This avoids premature failure due to local stress concentration caused by an excessively small connection area, or excessive expansion of the heat-affected zone and excessive accumulation of residual stress due to an excessively large connection area.

[0036] Specifically, by limiting the area of ​​the diffusion connection interface 300 to within 40%–60% of the top surface area of ​​the second ring 200, an optimized match between connection strength and stress distribution is achieved. Within this range, the diffusion connection interface 300 possesses sufficient effective load-bearing area to transfer high-amplitude cyclic loads, ensuring that the connection strength is not less than 85% of the base material strength, while effectively avoiding the risk of uneven interface stress distribution or local debonding that may result from full-end-face connections. This area configuration allows the interface to primarily bear uniform pure shear or axial cyclic loads during fatigue testing, reducing additional bending stress caused by improper geometric configuration. This significantly improves the consistency of interface fatigue life and keeps the fatigue life dispersion coefficient at a low level, making it particularly suitable for test scenarios requiring statistical analysis such as constructing SN curves.

[0037] To further improve the bonding quality and uniformity of the diffusion connection interface, this embodiment introduces surface treatment and intermediate layer technology. The connecting portions of the first ring 100 and the second ring 200 are both mirror-polished before assembly, with a surface roughness Ra ≤ 0.05 μm, to reduce interface oxide and contaminant residues. Based on this, the connecting portion on the top surface of the second ring 200 has a metal intermediate layer, and diffusion connection is achieved with the first ring 100 through this metal intermediate layer.

[0038] Specifically, a nanoscale metal interlayer (preferably a titanium interlayer) is formed on the top surface of the second ring 200 using magnetron sputtering, with a thickness controlled between 0.5 μm and 2 μm. This enhances atomic diffusion capability, lowers the threshold for diffusion bonding processes, and improves the uniformity of interfacial bonding quality and the consistency of mechanical response.

[0039] During the diffusion bonding process, the titanium layer acts as an active element, promoting the cross-interface diffusion of matrix metal atoms, reducing the required bonding temperature by 50°C to 100°C, and decreasing the pressure required. The resulting bonding interface has a dense structure, free of obvious pores or cracks, with a gentle microhardness gradient, exhibiting excellent consistency in mechanical response. This method is suitable for high-reliability bonding tests of difficult-to-weld materials such as Ni-based superalloys and TiAl intermetallic compounds.

[0040] The metal interlayer refers to a thin film structure disposed on the surface of the connection area near the outer ring on the top surface of the second ring 200. Its material can be a metal with high diffusion activity, such as titanium, nickel, copper, or silver, or a metal that can form a good solid solution with the base metal. In the overall technical solution, the metal interlayer functions as a diffusion promoter and interface purifier, used to build an atomic-level bridge between the bottom surface of the first ring 100 and the top surface of the second ring 200. In specific implementations, the metal interlayer can be prepared on the surface of the connection portion of the second ring 200 using processes such as magnetron sputtering, physical vapor deposition, or electroplating. Its thickness can be set according to actual conditions, for example, it can be any value between 0.5 μm and 2 μm.

[0041] This metal interlayer has a close working relationship with other current technical features: one side is bonded to the matrix material of the second ring 200, while the other side, during the high-temperature and high-pressure diffusion bonding process with the first ring 100, acts as an active element source to promote interdiffusion of matrix metal atoms across the interface. Through this cooperation, the metal interlayer can effectively reduce the activation energy required for diffusion bonding, allowing the bonding process to be completed under relatively low temperature and pressure conditions. It also helps to remove oxide residues at the interface and enhances the wettability of the interface. A dense diffusion bonding interface 300 with no obvious pores or cracks and a gentle microhardness gradient is formed between the first ring 100 and the second ring 200, thereby ensuring uniform load transfer at this interface.

[0042] Specifically, during the test piece preparation stage, the connecting part on the top surface of the second ring 200 is first mirror polished, and then a metal intermediate layer of a predetermined thickness is deposited on its surface. Next, the bottom surface of the first ring 100 is attached to the surface with the metal intermediate layer, and a certain temperature and pressure are applied to achieve diffusion bonding. During this process, the metal intermediate layer melts or undergoes a solid-phase reaction with the substrate, fills the microscopic voids and accelerates atomic migration, and finally cools and solidifies to form a high-strength metallurgical bonding interface.

[0043] Because a metal intermediate layer is set on the top surface of the second ring 200, its high diffusion activity reduces the connection threshold and improves the consistency of the interface microstructure, thereby obtaining a diffusion connection interface with higher bonding quality and more uniform mechanical response. It is particularly suitable for fatigue performance testing of high-temperature alloys or intermetallic compound materials with extremely high requirements for connection reliability.

[0044] To eliminate potential external alignment errors introduced during the clamping process, this embodiment adds a mechanical positioning feature to the first ring body 100 structure. Preferably, the outer wall of the first ring body 100 has a radially outward protruding positioning boss, which is used to match the groove in the testing machine fixture. The cross-section of the positioning boss is rectangular or polygonal arc-shaped, and the height is 1mm to 2mm. The positioning boss is embedded in the corresponding matching groove in the testing machine fixture, forming a radial limiting fit. Through this mechanical limiting method, the coaxiality of the first ring body 100 and the loading system is forcibly maintained, and the coaxial deviation is controlled within ±0.02mm, further suppressing alignment errors caused by external clamping and improving the convenience and positioning accuracy of the testing operation. This design significantly reduces the difficulty of manual clamping and improves the repeatability of positioning between multiple tests, making it particularly suitable for automated fatigue testing platforms or large-batch sample screening scenarios.

[0045] A positioning boss refers to an annular or segmented protrusion on the outer circumferential surface of the first ring body 100, with a radial dimension larger than the basic outer diameter of the first ring body 100. The function of this positioning boss is to serve as a mechanical centering reference, achieving radial positioning of the first ring body 100 in the clamping state through interference fit, transition fit, or clearance fit with a pre-set groove within the testing machine fixture. In the coaxial structure formed by the first ring body 100 and the second ring body 200, the positioning boss is integrally formed with the first ring body 100 or fixed to the outer wall of the first ring body 100 by welding, threaded connection, or other methods. Its axial position can be set according to actual clamping requirements; for example, it can be located in the middle of the outer wall of the first ring body 100 or near the end face of the first ring body 100. This application embodiment does not impose any special limitations on this. When the first ring body 100 is installed into the testing machine fixture, the positioning boss is embedded in the fixture groove, limiting the radial displacement and angular deflection of the first ring body 100 relative to the center of the fixture, thereby forcibly ensuring that the central axis of the first ring body 100 coincides with the central axis of the testing machine loading system. This fit eliminates coaxiality deviations caused by manual clamping operation errors or insufficient precision of the fixture itself, ensuring that the diffusion connection interface 300 only bears the load transmitted along the axial direction in subsequent fatigue tests, avoiding additional bending moments caused by eccentric loading.

[0046] During the test piece installation phase, the operator places the first ring 100 with the positioning boss into the corresponding groove of the testing machine fixture. Due to the geometric matching between the positioning boss and the groove, the first ring 100 is automatically guided to the center position of the fixture and completes radial locking. Subsequently, the loading rod 400 passes through the inner hole of the second ring 200 and applies an axial load to the second ring 200 through the first loading ring 500 and the second loading ring 600. During this process, since the first ring 100 has achieved high-precision coaxial positioning through the positioning boss, the loading force flow from the loading rod 400 through the second ring 200 and the diffusion connection interface 300 to the first ring 100 always remains on the same central axis. It will not generate lateral component force or torsional moment due to the attitude deviation of the first ring 100, thus ensuring the accuracy and repeatability of fatigue test data.

[0047] This invention also discloses a testing fixture for measuring the fatigue properties of diffusion-bonded specimens, which is used in conjunction with the aforementioned test piece. For example... Figure 2 As shown, the test fixture includes a loading rod 400, the diameter of which is slightly smaller than the inner diameter of the second ring body 200 of the test piece; a first loading ring 500 and a second loading ring 600 are axially spaced on the loading rod 400, and the first loading ring 500 and the second loading ring 600 are used to cooperate to clamp the second ring body 200.

[0048] To avoid localized damage to the second ring 200 caused by the clamping loading rod 400 and to precisely control the clamping force, this embodiment optimizes the fit between the loading rod 400 and the second ring 200. The diameter of the loading rod 400 is slightly smaller than the inner diameter of the second ring 200, with a uniform annular gap of 0.1mm to 0.3mm maintained between them to ensure that the loading rod 400 does not directly contact the inner wall of the second ring 200 or generate frictional constraints. The first loading ring 500 and the second loading ring 600 are located at the upper and lower end faces of the second ring 200, respectively. By adjusting the preload through the upper and lower loading rings, non-invasive clamping of the second ring 200 is achieved, avoiding thread damage to the test piece and reducing secondary bending moments caused by constraints.

[0049] When the loading ring and loading rod 400 are threadedly connected, the loading rod 400 is a screw, and both the first loading ring 500 and the second loading ring 600 are bolts. Tightening is performed synchronously using a torque wrench to a set torque, ensuring the clamping force is evenly applied to the end face of the second ring 200 without introducing bias pressure. This design avoids stress concentration problems caused by directly machining threads onto the test piece, and also allows the second ring 200 to freely adapt to axial deformation during tension, minimizing secondary bending moments caused by clamping constraints. This implementation significantly improves the fidelity of test results, and is particularly suitable for low-amplitude, high-sensitivity fatigue response measurements.

[0050] When the loading ring and loading rod 400 are connected by other means, such as welding or bolting along the radial direction of the loading ring, it is necessary to ensure that the radial connection between the loading ring and the loading rod 400 is uniformly stressed, so as to avoid uneven stress on the second ring body 200 due to uneven stress on the connection between the two.

[0051] The loading rod 400 is a slender rod-shaped component used to transmit axial loads. Its function is to act as the core of the force transmission in the entire testing fixture, introducing the tensile or compressive load generated by the testing machine into the test piece. The diameter of the loading rod 400 is configured to be slightly smaller than the inner diameter of the second ring 200. This means there is a small annular gap between the loading rod 400 and the inner wall of the second ring 200, which can be set according to actual conditions, for example, from 0.1 mm to 0.3 mm. Through this dimensional fit, the loading rod 400 does not directly contact or create frictional constraints with the inner wall of the second ring 200 when passing through its cavity, thus ensuring that the load is transmitted only through the clamping assembly at the end, avoiding additional bending moments or torsional loads caused by lateral contact. The loading rod 400 can be made of high-strength alloy steel, and its surface can be hardened to improve wear resistance. The specific material and heat treatment process can be selected according to the actual test load and ambient temperature.

[0052] The first loading ring 500 and the second loading ring 600 are annular limiting components sleeved on the loading rod 400 and capable of moving or being fixed axially. The first loading ring 500 and the second loading ring 600 are axially spaced on the loading rod 400, and the distance between them can be adjusted according to the axial height of the second ring body 200 to ensure that they can respectively abut against the upper and lower end faces or adjacent areas of the second ring body 200. In the system linkage, the first loading ring 500 and the second loading ring 600 cooperate. When the loading rod 400 is subjected to tension, one loading ring (such as the first loading ring 500) acts as a force fulcrum against the upper end face of the second ring body 200, and the other loading ring (such as the second loading ring 600) acts as an auxiliary limiting or pre-tightening component, or vice versa, depending on the specific loading direction design. When a compressive load needs to be applied, the relative positional relationship between the two can also be adjusted accordingly to achieve bidirectional clamping of the second ring body 200. This fitting method allows the second ring 200 to be suspended and clamped between the two loading rings. The load is applied evenly to the end face of the second ring 200 through the loading rings, and then transferred to the diffusion connection interface 300, achieving non-invasive and non-destructive clamping. The first loading ring 500 and the second loading ring 600 can be in the form of bolts, nuts, snap rings, or adjusting rings screwed onto the loading rod 400, or they can be shoulder structures welded and fixed to specific positions on the loading rod 400.

[0053] Specifically, the testing fixture, used in conjunction with the aforementioned test piece, inserts the loading rod 400 into the inner hole of the second ring 200 within the test piece. Since the diameter of the loading rod 400 is slightly smaller than the inner diameter of the second ring 200, the rod remains in non-contact with the inner wall of the ring. Subsequently, the axial positions of the first loading ring 500 and the second loading ring 600 on the loading rod 400 are adjusted so that they are positioned above and below the second ring 200, respectively (or at different positions on the same side to create clamping force, depending on the loading requirements). By synchronously adjusting the preload of the first loading ring 500 and the second loading ring 600, or by directly utilizing the reciprocating motion of the testing machine, the two loading rings work together to clamp the second ring 200. During this process, external force is transmitted along the central axis of the loading rod 400, evenly distributed to the end face of the second ring 200 via the first loading ring 500 and the second loading ring 600, driving the second ring 200 to displace relative to the fixed first ring 100, thereby causing the diffusion connection interface 300 to bear pure axial tensile or compressive loads or shear loads. By eliminating the eccentricity and internal wall friction interference caused by traditional screw connections, this process ensures the purity of the interface stress state.

[0054] Through the above technical solution, a non-contact gap exists between the loading rod 400 and the inner wall of the second ring 200, and the end face of the second ring 200 is clamped by the cooperation of the first loading ring 500 and the second loading ring 600. Therefore, the stress concentration problem caused by machining threaded holes on the test piece body is avoided, and the additional bending stress and torsional load introduced by misalignment of clamping are eliminated. As a result, the diffusion connection interface 300 only bears pure axial or shear cyclic load, which significantly improves the fidelity, repeatability and measurement capability of fatigue test data for low amplitude and high sensitivity fatigue response.

[0055] The loading rod 400 is a screw, meaning that its outer surface is continuously or intermittently provided with a helical thread structure along the axial direction. As the core transmission component for transmitting tensile or compressive loads, the loading rod 400's screw structure provides a precise axial displacement adjustment reference. This screw can form a helical pair with the corresponding internal threads in the inner holes of the first loading ring 500 and the second loading ring 600. By rotating the first loading ring 500 and / or the second loading ring 600, the rotational motion is converted into linear displacement along the axis of the loading rod 400 using the thread helix angle principle, thereby changing the relative or absolute position of the two loading rings on the loading rod 400, and thus applying or releasing the preload force on the clamped object. The screw pitch, thread angle, and thread accuracy can be set according to the force control accuracy and adjustment sensitivity required for actual testing. For example, it can be a metric coarse thread for quick clamping, or a fine thread for fine-tuning control.

[0056] Both the first loading ring 500 and the second loading ring 600 are bolts, meaning these two components are annular fasteners with a central through hole and an internal thread on the inner wall of the through hole that matches the loading rod 400. In this invention, the first loading ring 500 and the second loading ring 600 are located on opposite axial sides (e.g., top and bottom surfaces) of the clamped second ring body 200. When the first loading ring 500 and the second loading ring 600 are screwed into the loading rod 400, their end faces can directly contact the end face of the second ring body 200 or indirectly contact it through a pad. As the bolts are screwed in, they apply axial clamping force to the second ring body 200, forming a stable clamping state. The outer diameter, height, and end face shape of the bolts can be adjusted according to the size of the second ring body 200 and the required force-bearing area. For example, the head of a hexagonal head bolt can be modified into an annular shape, or a specially designed cylindrical nut structure can be used, as long as it has internal threads and can provide a flat force-bearing end face.

[0057] This invention achieves high versatility and interchangeability of the clamping components of the test fixture due to the use of a standard threaded fit structure of screws and bolts, reducing manufacturing costs and maintenance difficulty. Because the bolted connection allows for precise control of the preload through torque, it can effectively ensure the uniformity and symmetry of the clamping force, avoiding eccentric bending moments caused by unilateral force. At the same time, this non-welded or non-interference fit connection method allows the second ring to have a small adaptive adjustment space along the axial direction when it expands under heat or stress, reducing secondary stress caused by mismatch in thermal expansion coefficients or excessive constraint, thereby improving the authenticity and reliability of fatigue test data.

[0058] The present invention also discloses a test fixture for measuring the fatigue performance of a diffusion bonded specimen. The test fixture includes the aforementioned test specimen. The test fixture includes a loading rod 400, the diameter of which is slightly smaller than the inner diameter of the second ring 200 of the test specimen. A first loading ring 500 and a second loading ring 600 are axially spaced on the loading rod 400. The first loading ring 500 and the second loading ring 600 are used to cooperate to clamp the second ring 200.

[0059] The test fixture is an integrated mechanical testing system. Its core lies in integrating a dedicated diffusion-connection test piece with a matching loading mechanism to form a complete test unit. This test fixture not only includes the mechanical components that perform the load application function, but also directly contains the test piece itself, which is the object under test, thereby ensuring the coaxiality between the components and the consistency of force flow transmission at the system level.

[0060] The test specimen refers to the component used to measure the fatigue performance of diffusion-bonded specimens as defined in any of the above embodiments. In this test fixture, the test specimen serves as the core object to be clamped and loaded, and its geometric configuration determines the transmission path of the loading force. Depending on the actual testing requirements, the test specimen can be a form with only a basic double-ring structure, or it can be an optimized form with further defined features such as wall thickness ratio, connection area ratio, metal interlayer, or positioning bosses.

[0061] The loading rod 400 is a key transmission component in the testing fixture used to introduce axial loads. It is a rod-shaped member extending along the central axis. The diameter of the loading rod 400 is slightly smaller than the inner diameter of the second ring 200 in the test piece. This means that a small annular gap remains between the loading rod 400 and the central hole of the second ring 200. This gap prevents the loading rod 400 from rigidly contacting or frictionally constraining the inner wall of the second ring 200 when no clamping force is applied. This allows the second ring 200 to freely adapt to axial displacement during deformation under stress, avoiding the introduction of additional radial stress or secondary bending moments due to interference fit or rough contact. The material of the loading rod 400 can be set according to actual conditions; it can be high-strength alloy steel or titanium alloy, as long as its stiffness is sufficient to maintain straightness under test loads. The fit between the loading rod 400 and the second ring 200 forms the internal guiding mechanism of the testing fixture, ensuring that the load is transmitted along the central axis.

[0062] The first loading ring 500 and the second loading ring 600 are clamping actuators disposed on the loading rod 400, and are spaced apart along the axial direction of the loading rod 400. The first loading ring 500 and the second loading ring 600 are annular components sleeved around the outer periphery of the loading rod 400 and capable of axial movement or fixation. Their inner diameter matches the outer diameter of the loading rod 400, while their outer diameter is larger than the outer diameter of the second ring body 200 or at least covers the end face area of ​​the second ring body 200. In the working state of the test fixture, the first loading ring 500 and the second loading ring 600 are located above and below (or on both sides) the second ring body 200, respectively. By adjusting their relative position or preload on the loading rod 400, a uniform clamping force is applied to the axial end face of the second ring body 200. This fit achieves non-invasive clamping of the second ring body 200, meaning that there is no need to machine threaded holes or through holes on the test piece body; instead, the two loading rings lock the second ring body 200, allowing it to withstand tensile or compressive loads as the loading rod 400 moves. The specific structural forms of the first loading ring 500 and the second loading ring 600 can be set according to the actual situation. For example, they can be nuts that are threadedly connected to the loading rod 400, or they can be retaining rings that are fixed to the loading rod 400 by pins or keyways.

[0063] It should be understood that the technical features in the above embodiments can be combined arbitrarily without conflict. For example, the first ring 100 with positioning bosses can be combined with the surface treatment process of the titanium intermediate layer and applied to the same test piece; the geometric proportion control of the second ring 200 can also be implemented in conjunction with the design of its inner ring clamping gap. These combinations can synergistically enhance the coaxiality, connection quality and loading purity of the test piece, and are within the scope of protection of this invention.

Claims

1. A test specimen for measuring the fatigue properties of diffusion-bonded specimens, characterized in that, It includes a first ring body (100) and a second ring body (200) arranged coaxially, wherein the inner diameter of the first ring body (100) is larger than the inner diameter of the second ring body (200) and smaller than the outer diameter of the second ring body (200); The top surface of the second ring (200) near the outer ring is diffusely connected to the bottom surface of the first ring (100) near the inner ring, forming a diffusion connection interface (300); wherein, the first ring (100) and the second ring (200) are both metal rings.

2. The test specimen for measuring the fatigue properties of diffusion-bonded specimens as described in claim 1, characterized in that, The second ring (200) has a hollow structure, and its wall thickness to outer diameter ratio is 1:(3~5).

3. A test specimen for measuring the fatigue properties of diffusion-bonded specimens as described in claim 1, characterized in that, The area of ​​the diffusion connection interface (300) accounts for 40% to 60% of the top surface area of ​​the second ring (200).

4. A test specimen for measuring the fatigue properties of diffusion-bonded specimens as described in claim 1, characterized in that, The connecting portion on the top surface of the second ring (200) has a metal intermediate layer, and is diffusely connected to the first ring (100) through the metal intermediate layer.

5. A test specimen for measuring the fatigue properties of diffusion-bonded specimens as described in claim 1, characterized in that, The outer wall of the first ring body (100) is provided with a positioning boss that protrudes radially outward, and the positioning boss is used to match the groove in the fixture of the testing machine.

6. A testing fixture for measuring the fatigue properties of diffusion-bonded specimens, characterized in that, The testing fixture is used in conjunction with the test specimen as described in any one of claims 1-5; The test fixture includes a loading rod (400), the diameter of which is slightly smaller than the inner diameter of the second ring (200) of the test piece; The loading rod (400) is axially spaced with a first loading ring (500) and a second loading ring (600), which are used to cooperate to clamp the second ring body (200).

7. The test fixture for measuring the fatigue properties of diffusion-bonded specimens as described in claim 6, characterized in that, The loading rod (400) is a screw rod, and the first loading ring (500) and the second loading ring (600) are both bolts.

8. A testing fixture for measuring the fatigue properties of diffusion-bonded specimens, characterized in that, The testing fixture includes the test piece as described in any one of claims 1-5; The test fixture includes a loading rod (400), the diameter of which is slightly smaller than the inner diameter of the second ring (200) of the test piece; The loading rod (400) is axially spaced with a first loading ring (500) and a second loading ring (600), which are used to cooperate to clamp the second ring body (200).

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