A fixture and method for implementing a high temperature fatigue crack growth test of a CT specimen under negative stress ratio

Abstract

The application discloses a kind of to realize CT sample negative stress ratio high-temperature fatigue crack propagation test fixture and method, including testing machine hydraulic rod, fixture adapter, testing machine pull rod, test fixture, center top rod and pin shaft, wherein: testing machine hydraulic rod is a pair, and is set up towards each other;Test fixture is a pair respectively with two testing machine hydraulic rod end tightly;Two test fixtures are used for placing CT test piece between;Fixture adapter is a pair, respectively with two test fixture near testing machine hydraulic rod one end connection;Testing machine pull rod is a pair, respectively with two fixture adapter connection;Center top rod is inserted in the through-hole of test fixture, one end is tightly combined with testing machine hydraulic rod end, and the other end is tightly combined with CT test piece;Pin shaft is transversely inserted into the pin hole of test fixture.The application can meet the demand of stable clamping under high-temperature state CT sample tension-compression fatigue load.

Description

Technical Field

[0001] This invention belongs to the technical field of fatigue performance testing equipment for metallic materials, and specifically relates to a uniaxial tensile-compression fatigue testing fixture for metallic materials. Background Technology

[0002] Disc-shaped components are common critical parts in aero-gas turbine engines. These components operate at high speeds, and their fracture typically results in non-containment damage, often with catastrophic consequences. Damage tolerance design requires components such as discs to have sufficient crack propagation life to improve their safety and reliability throughout the engine's lifespan. Crack propagation prediction analysis techniques are crucial, but this requires establishing a crack propagation model that can accurately and effectively describe the crack propagation behavior of materials. In many cases, the stress level in critical areas of the disc is high, leading to a small-scale yielding state and stress redistribution. This results in negative stress ratio fatigue cycles at stress concentration points, affecting the fatigue crack propagation behavior and process at these locations. Therefore, to study the crack propagation behavior of different materials under tensile-compressive cycles with negative stress ratios, a set of test fixtures needs to be designed to meet the tensile-compressive stress ratio loading test conditions.

[0003] The People's Republic of China National Standard GB / T6398-2017 describes the Standard Compact Tensile Specimen (CT specimen), which can be connected by a pin to apply axial tensile-tensile fatigue loads and study the crack propagation behavior of materials. The CT specimen has a longer effective crack propagation segment, requires less loading force from the testing machine, and is compact, thus saving material. However, with the pin connection, the gap between the pin, the specimen, and the fixture makes it impossible to stably apply tensile-compressive loads.

[0004] Chinese invention patent CN 108548716A discloses a fixture and test method for high-temperature tensile and compressive fatigue testing of round bar specimens. It solves the problem of stable clamping of round bar specimens of various sizes and enables uniaxial tensile and tensile / compressive fatigue testing under high-temperature conditions. However, the fixture is only suitable for round bar specimens and cannot be applied to CT specimens.

[0005] Chinese invention patent CN 111595695A discloses a gap elimination device and method suitable for low-temperature tensile and compressive fatigue testing. It solves the gap problem in low-temperature tensile and compressive fatigue testing, and the gap elimination effect improves as the test temperature decreases, providing a solution for tensile and compressive fatigue testing in ultra-low temperature environments. However, its connection method is a tapered clamp connection, which is not suitable for CT specimens and cannot achieve stable clamping at high temperatures.

[0006] Chinese invention patent CN 115420591A discloses a high-temperature tensile and compressive fatigue testing fixture for rod-shaped specimens and its usage method. It solves the problem that existing testing fixtures are prone to recurring thread gaps during high-temperature tensile and compressive fatigue testing, leading to inaccurate results. However, it is not suitable for high-temperature fatigue crack propagation testing of CT specimens.

[0007] Chinese invention patent CN 109724873A discloses a CT specimen crack propagation test fixture adapted to tensile and compressive loads. It solves the problem of preventing gaps between the fixture and the specimen during alternating tensile and compressive load loading while maintaining a constant loading axis. However, the fixture is relatively complex with many parts, and the gap-eliminating scheme uses fastening screws and wedge blocks for tightening. Under high temperatures, the fastening screws may not provide sufficient preload due to temperature changes. Therefore, it does not solve the reliability problem of tensile-compression crack propagation tests at high temperatures.

[0008] In summary, the existing test fixture design cannot reliably be used for crack propagation tests of CT specimens under tensile-compressive loads at high temperatures. It is necessary to design a new fixture to meet the requirement of stable clamping of CT specimens under tensile-compressive fatigue loads at high temperatures. The specific requirements include: (1) no gaps are generated between the fixture and the specimen under high temperatures; (2) the tensile-compressive loading axis remains unchanged; (3) it can be manufactured from a tension-tension fixture at low cost without changing the function of the tension-tension fixture itself; and (4) it is easy and reliable to assemble. Summary of the Invention

[0009] The purpose of this invention is to provide a fixture for conducting high-temperature fatigue crack propagation tests on CT specimens with negative stress ratios, in order to meet the requirement of stable clamping of CT specimens under tensile-compressive fatigue loads at high temperatures.

[0010] To achieve the above objectives, the present invention adopts the following technical solution: A fixture for performing high-temperature fatigue crack propagation tests on CT specimens with negative stress ratios includes a testing machine hydraulic rod, a fixture adapter, a testing machine pull rod, a testing fixture, a central push rod, and a pin, wherein: The hydraulic rods of the testing machine are a pair, arranged facing each other vertically. The test fixtures are a pair, arranged facing each other vertically, and are respectively in close contact with the ends of the hydraulic rods of the two testing machines; the space between the two test fixtures is used to place the CT test piece; a through hole is vertically opened in the center of the test fixture, and pin holes are opened on both sides of the test fixture; The fixture adapters are a pair, each connected to one end of the hydraulic rod of the testing machine near one of the two test fixtures; The testing machine has a pair of tie rods, which are respectively connected to two clamp adapters; The central push rod is inserted into the through hole of the test fixture, with one end tightly fitted to the end of the hydraulic rod of the testing machine and the other end tightly fitted to the CT test piece; The pin is inserted laterally into the pin hole of the test fixture.

[0011] The inner wall of the fixture adapter is provided with internal threads, and the outer wall of the test fixture is provided with external threads. The fixture adapter and the test fixture are connected by threads.

[0012] The inner wall of the testing machine pull rod is provided with internal threads, and the outer wall of the clamp adapter is provided with external threads. The testing machine pull rod and the clamp adapter are connected by threads.

[0013] The end of the hydraulic rod of the testing machine is tightly fitted with the test fixture and the central push rod, and is pressed tightly to ensure that there is no gap between the test fixture, the pin, the CT test piece and the central push rod, and becomes an integral part of the testing machine tie rod.

[0014] The central push rod is in contact with the hydraulic rod of the testing machine and the CT test piece, and is kept in a tight state by the hydraulic rod of the testing machine to transmit the compressive load.

[0015] A method for using a fixture to perform a high-temperature fatigue crack propagation test on a CT specimen with negative stress ratio includes the following steps: (1) First, install the clamp adapter on the test machine pull rod, ensuring that the clamp adapter fits the test machine pull rod; (2) Install the test fixture on the fixture adapter and screw it to the bottom of the fixture adapter to ensure that the upper and lower test fixtures are in the same direction. Insert the center push rod into the center hole of the test fixture. (3) Place the CT sample between two test fixtures, align the upper and lower ends of the two test fixtures with the pin holes of the test fixtures respectively, and insert the pin into the pin holes. (4) Apply pressure by manually pressing the hydraulic rod of the testing machine; at this time, the gap of the CT sample disappears and tightens as the pressure rises.

[0016] Beneficial effects: This invention enables alternating tensile and compressive loading while maintaining a constant loading axis, and ensures no gaps form between the fixture and the specimen even at high temperatures. Furthermore, it utilizes a conventional tensile-tensile testing fixture with a central through-hole and a central push rod, converting it into a fixture capable of applying alternating tensile and compressive loads without requiring the machining of other complex components, thus preserving the original functionality of the tensile-tensile testing fixture. The entire device is simple to assemble, significantly reducing the cost and time required for manufacturing testing fixtures. Attached Figure Description

[0017] Figure 1 This is a three-dimensional structural diagram of the clamp of the present invention; Figure 2 This is a schematic diagram of the longitudinal cross-sectional structure of the clamp of the present invention; Figure 3This is a schematic diagram of the fixture adapter. Figure 4 This is a front view of the test fixture; Figure 5 This is a schematic diagram of the three-dimensional structure of the test fixture; Figure 6 A schematic diagram of the structure of the central top rod; Figure 7 A schematic diagram of the actual assembly using this new type of fixture; Figure 8 The image shows the effect of using this new fixture to conduct a 650℃ fatigue crack propagation test on a CT test specimen with R=-1. Figure 9 The image shows the contact surface between the CT test specimen and the central push rod in a 650℃ fatigue crack propagation test with R=-1 using this new fixture. In the picture: 1- Hydraulic rod of testing machine, 2- Fixture adapter, 3- Pull rod of testing machine, 4- Testing fixture, 5- CT test piece, 6- Center push rod, 7- Pin. Implementation

[0018] The invention will now be further explained with reference to the accompanying drawings.

[0019] like Figures 1 to 5 As shown, the present invention provides a fixture for performing a high-temperature fatigue crack propagation test on a CT specimen with negative stress ratio, characterized in that it includes a testing machine hydraulic rod 1, a fixture adapter 2, a testing machine pull rod 3, a testing fixture 4, a central push rod 6, and a pin 7, wherein: The hydraulic rods 1 of the testing machine are a pair, arranged facing each other vertically.

[0020] The test fixtures 4 are a pair, arranged facing each other, and are tightly fitted to the ends of the hydraulic rods 1 of the two testing machines respectively; the CT test piece 5 is placed between the two test fixtures 4; a through hole is vertically opened in the center of the test fixture 4, and pin holes are opened on both sides of the test fixture 4.

[0021] The fixture adapter 2 is a pair, which are respectively connected to one end of the hydraulic rod 1 of the two test fixtures 4 near the test machine. Specifically, the inner wall of the fixture adapter 2 is provided with internal threads, and the outer wall of the test fixture 4 is provided with external threads. The fixture adapter 2 and the test fixture 4 are connected by threads.

[0022] The testing machine pull rod 3 is a pair, which are connected to two clamp adapters 2 respectively; specifically, the inner wall of the testing machine pull rod 3 is provided with internal threads, and the outer wall of the clamp adapter 2 is provided with external threads. The testing machine pull rod 3 and the clamp adapter 2 are connected by threads.

[0023] The central push rod 6 is inserted into the through hole of the test fixture 4, with one end tightly fitted to the end of the hydraulic rod 1 of the testing machine, and the other end tightly fitted to the CT test piece 5.

[0024] The pin 7 is inserted laterally into the pin hole of the test fixture 4.

[0025] The end of the hydraulic rod 1 of the testing machine is tightly fitted with the test fixture 4 and the central push rod 6, and is pressed tightly so that there is no gap between the test fixture 4, the pin 7, the CT test piece 5 and the central push rod 6, and it becomes a whole with the testing machine tie rod.

[0026] The central push rod 6 is in contact with the hydraulic rod 1 of the testing machine and the CT test piece 5, and is kept in a tight state through the hydraulic rod 1 of the testing machine to transmit the compressive load.

[0027] When the fatigue tester applies a tensile load, the tensile force is transmitted from the tester's tie rod to the fixture adapter via a thread; the fixture adapter is connected to the test fixture via a thread, transmitting the tensile force to the test fixture; the test fixture transmits the tensile force to the CT specimen via a pin.

[0028] When the fatigue tester applies a pressure load, the pressure is transmitted from the hydraulic rod of the testing machine to the central push rod; the central push rod then transmits the pressure to the CT specimen through the contact surface.

[0029] The method of using the clamp of the present invention includes the following steps: (1) First install the clamp adapter 2 on the experimental machine pull rod 3, ensuring that the clamp adapter 2 fits the experimental machine pull rod 3; (2) Install the test fixture 4 on the fixture adapter 2 and screw it to the bottom of the fixture adapter 2 to ensure that the upper and lower test fixtures 4 are in the same direction. Insert the center push rod 6 into the center hole of the test fixture 4. (3) Place the CT sample 5 between the two test fixtures 4, with the upper and lower ends of the two test fixtures 4 aligned with the pin holes of the test fixtures 4 respectively, and insert the pin 7 into the pin holes. (4) Apply pressure by manually pressing the hydraulic rod 1 of the testing machine; at this time, the gap of CT sample 5 disappears and tightens as the pressure rises.

[0030] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A fixture for performing high-temperature fatigue crack propagation tests on CT specimens with negative stress ratio, characterized in that: The components include a hydraulic rod (1), a fixture adapter (2), a testing machine tie rod (3), a testing fixture (4), a central push rod (6), and a pin (7), wherein: The hydraulic rods (1) of the testing machine are a pair, arranged facing each other vertically; The test fixtures (4) are a pair, arranged facing each other, and are respectively in close contact with the ends of the hydraulic rods (1) of the two testing machines; the two test fixtures (4) are used to place the CT test piece (5); the test fixtures (4) have a through hole in the center vertically, and the test fixtures (4) have pin holes on both sides; The clamp adapters (2) are a pair, and are respectively connected to one end of the hydraulic rod (1) of the two test clamps (4) near the test machine; The test machine pull rods (3) are a pair, which are respectively connected to two clamp adapters (2); The central top rod (6) is inserted into the through hole of the test fixture (4), with one end tightly fitted to the end of the hydraulic rod (1) of the testing machine and the other end tightly fitted to the CT test piece (5); The pin (7) is inserted laterally into the pin hole of the test fixture (4); The end of the hydraulic rod (1) of the testing machine is tightly fitted with the test fixture (4) and the central push rod (6) and pressurized to ensure that there is no gap between the test fixture (4), the pin (7), the CT test piece (5) and the central push rod (6), and becomes an integral part of the testing machine pull rod; The central top rod (6) is in contact with the hydraulic rod (1) of the testing machine and the CT test piece (5), and is kept in a tight state by the hydraulic rod (1) of the testing machine to transmit the compressive load.

2. The fixture for realizing the high-temperature fatigue crack propagation test of CT specimen with negative stress ratio according to claim 1, characterized in that: The inner wall of the fixture adapter (2) is provided with an internal thread, and the outer wall of the test fixture (4) is provided with an external thread. The fixture adapter (2) and the test fixture (4) are connected by threads.

3. The fixture for realizing the high-temperature fatigue crack propagation test of CT specimen with negative stress ratio according to claim 1, characterized in that: The inner wall of the test machine pull rod (3) is provided with an internal thread, and the outer wall of the fixture adapter (2) is provided with an external thread. The test machine pull rod (3) and the fixture adapter (2) are connected by threads.

4. A method of using the fixture for performing high-temperature fatigue crack propagation tests on CT specimens with negative stress ratio as described in claim 1, characterized in that: Includes the following steps: (1) First install the clamp adapter (2) on the experimental machine pull rod (3) to ensure that the clamp adapter (2) fits the experimental machine pull rod (3). (2) Install the test fixture (4) on the fixture adapter (2) and screw it to the bottom of the fixture adapter (2) to ensure that the upper and lower test fixtures (4) are in the same direction. Insert the center push rod (6) into the center hole of the test fixture (4). (3) Place the CT sample (5) between the two test fixtures (4), with the upper and lower ends of the two test fixtures (4) aligned with the pin holes of the test fixtures (4) respectively, and insert the pin (7) into the pin holes. (4) Apply pressure and perform pressure operation at the manual pressure rod of the hydraulic rod (1) of the testing machine; at this time, the gap of the CT sample (5) disappears and tightens as the pressure rises.

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