A fiber reinforced composite material creep performance testing device and testing method

Abstract

The present invention discloses a creep performance testing device and a testing method for fiber-reinforced composite materials. This method overcomes the shortcomings of traditional vertical creep testing devices, adopts a horizontal device to test creep performance, and can realize synchronous testing of a large number of test pieces. The test object is impregnated yarn, and FRP with different fibers and different resin types can be quickly prepared. The horizontal loading device is mainly composed of a reaction frame, a screw, a spring, a clamp, etc. The impregnated yarn is anchored by a friction-type clamp, and the impregnated yarn is tensioned by a jack. The spring is arranged at one end of the impregnated yarn, and the elasticity of the spring is used to ensure that the impregnated yarn produces free creep deformation under long-term stress. The method of the present invention can realize efficient creep performance testing of FRP with different fibers and different resin types, and has low processing cost and is easy to implement, which is of great significance for practical engineering applications.

Description

Technical Field

[0001] The present invention relates to the technical field of creep performance testing, and in particular to a creep performance testing device and a testing method for fiber reinforced composite (FRP) materials. Background Art

[0002] The phenomenon that the strain of FRP increases with time under long-term stress is called creep. Creep performance is one of the important properties of FRP, which is directly related to the long-term deformation of FRP reinforced structures and the relaxation of prestressed FRP.

[0003] Traditional FRP creep testing methods primarily involve conducting creep tests on FRP bars or sheets. However, this requires pre-production of the bars or sheets, making creep testing of FRP with different fiber types and resins time-consuming and costly. Existing creep testing methods are typically vertical, such as suspending a weight below the material (which can be combined with a lever) or using a creep testing machine. However, vertical testing methods require a large amount of space, complex equipment, and difficulty conducting a large number of creep tests simultaneously. Therefore, this method is less efficient for creep testing. Summary of the Invention

[0004] The main purpose of the present invention is to provide an efficient fiber reinforced composite (FRP) creep performance testing device and testing method.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0006] A fiber reinforced composite material creep performance testing device includes a creep test reaction frame, a fixture, a displacement measuring instrument, a first screw, a top plate, a U-shaped frame, a jack, a first vertical plate, a second vertical plate, a top pressure spring and a second screw; the first screw and the second screw are horizontally arranged on both sides of the creep test reaction frame, the inner ends of the first screw and the second screw are located inside the creep test reaction frame, and the outer ends of the first screw and the second screw are located outside the creep test reaction frame; a fixture is respectively provided at the inner ends of the first screw and the second screw; the top plate, the U-shaped frame and the first vertical plate are sequentially arranged on the first screw, and the top plate is fixed to the creep test reaction frame; the first vertical plate is adjustably fixed to the outer end of the first screw, and the jack is arranged between the U-shaped frame and the first vertical plate; the second vertical plate is adjustably fixed to the outer end of the second screw, the top pressure spring is sleeved on the second screw and is located between the creep test reaction frame and the second vertical plate; the displacement measuring instrument is arranged on the fixture connected to the second screw.

[0007] Ensuring that the dipped yarn and the clamp do not slip during creep testing is crucial for accurately measuring creep deformation. This invention uses a friction-type clamp to anchor the dipped yarn. The clamp's inner surface features a specially designed grid pattern, ensuring that slippage does not occur between the two during long-term creep testing. During the production of the dipped yarn, the yarn is flattened at both ends before curing, increasing the contact surface between the yarn and the clamp and ensuring a secure anchoring effect.

[0008] The rubberized yarn is tensioned using a jack. The tensioning method is similar to conventional prestressing. The force is transferred to the rubberized yarn through a U-shaped frame, top plate, vertical plate, screw, and other devices. During the tensioning process, the tensioning force is measured by a force sensor. After the tensioning is completed, the U-shaped frame, jack, force sensor, and tensioning end screw are removed.

[0009] A compression spring is placed outside the reaction frame at the opposite end of the yarn, away from the tensioning end. During tensioning, the yarn moves toward the tensioning end, transmitting force to the spring via a screw in the spring's center, causing it to compress. During long-term creep testing, the spring deforms as the yarn creeps, and the corresponding creep deformation can be measured using a dial indicator. During FRP creep, the product of the spring constant K and the change in spring length δ during creep should not exceed 10% of the tensioning force to avoid significant long-term stress changes. δ can be initially determined through trial testing or theoretical calculations.

[0010] After the creep test is completed, for a rough test of creep performance, the creep strain can be directly obtained by taking the measurement result of the dial indicator. For a precise test of creep performance, it is necessary to use the step-by-step method to correct the creep strain based on the measurement result of the dial indicator. The end time of the calculation step is 1min, 3min, 6min, 9min, 15min, 30min, 45min and 1h, 1.5h, 2h, 4h, 10h, 24h, 48h, 72h, 96h and 120h respectively, and then take it every 120h until the end of the test time. Considering the stress reduction caused by the contraction of the spring in each calculation step during the creep test, the creep strain correction for each calculation step is performed as follows:

[0011]

[0012] Among them, Δε i ' is the corrected creep strain of the i-th calculation step; Δε i is the creep strain measurement value of the i-th calculation step, which is equal to the ratio of the fixture displacement to the initial length of the free section of the dipped yarn; ΔL0 is the initial elongation of the spring; ΔL i is the elongation of the spring in the i-th calculation step, which is equal to the difference between ΔL0 and the fixture displacement.

[0013] It should be noted that the basic assumption of the above correction method is that the FRP is always in the linear creep stage, that is, the FRP creep strain in each calculation step is proportional to its stress. Therefore, the long-term stress of the FRP must not be greater than its creep rupture stress during the creep test time.

[0014] Compared with the prior art, the present invention has the following beneficial effects:

[0015] The present invention achieves the goal of efficiently testing the creep properties of FRP. On the one hand, the use of impregnated yarn for testing reduces the time and cost of FRP specimen preparation. On the other hand, the creep testing device involved in the present invention significantly reduces the space occupied by FRP creep testing, enables simultaneous testing of multiple groups of tests, and improves the efficiency of FRP creep performance testing. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 is a three-dimensional diagram of the creep testing device of the present invention;

[0017] Figure 2 Schematic diagram of the anchoring end and clamp of the dipped yarn in the present invention;

[0018] Figure 3 This is the FRP creep curve obtained by the method of the present invention.

[0019] In the figure: 1. Test frame; 2. Clamp; 3. Dial indicator; 4. First screw; 5. Top plate; 6. First nut; 7. U-shaped frame; 8. Jack; 9. First vertical plate; 10. Second nut; 11. Top pressure spring; 12. Second screw; 13. Third nut; 14. Free section of dipped yarn; 15. Anchor section of dipped yarn; 16. Inner surface of clamp; 17. Bolt hole. DETAILED DESCRIPTION

[0020] Reference Figure 1 As shown in FIG, the creep performance test device of fiber reinforced composite material (FRP) of the present invention mainly consists of a test frame 1 and a tensile component. Two clamps 2 are provided inside the test frame 1 to clamp the two ends of the FRP. Figure 2 During the preparation of the dipped yarn, when it is not solidified, a pressure plate is used to apply pressure to the anchoring section to form a Figure 2 The flat enlarged head shown in the figure increases the contact surface between the anchoring section and the clamp. Figure 2 The purpose of the grid pattern is to increase the friction coefficient between the clamp and the dipped yarn. It should be noted that the bolt hole 17 must not pass through the dipped yarn anchoring section 15.

[0021] The tensile assembly includes a first screw rod 4 and a second screw rod 12. The test frame 1 has symmetrical mounting holes on both sides thereof adapted to the first screw rod 4 and the second screw rod 12. A clamp 2 is mounted on each end of the first screw rod 4 and the second screw rod 12 adjacent to each other.

[0022] The outer wall of the first screw 4 is fitted with a first nut 6 through threaded engagement, and the end of the jack 8 away from the first nut 6 is fixedly mounted with a first vertical plate 9 for providing a reaction force to the jack during tensioning. A pressure sensor for measuring the pressure of the jack 8 is fixedly mounted on the end of the first vertical plate 9 close to the jack 8. The top plate 5 is used to withstand the pressure of the U-shaped frame 7. The inner surface of the U-shaped frame 7 is provided with a through hole that is compatible with the first screw 4. One end of the jack 8 is fixedly connected to the outer surface of the U-shaped frame 7. The outer wall of the second screw 12 is sleeved with a second vertical plate, and a top pressure spring 11 is fixedly mounted on the side of the second vertical plate close to the test frame 1. The other end of the top pressure spring 11 is fixedly connected to the outer surface of the test frame 1.

[0023] During tensioning using a jack, the second screw 12 moves toward the first screw 4. This compresses the pressure spring 11, which in turn provides a continuous thrust force that pushes the second screw 12 away from the first screw 4, thereby applying long-term stress to the impregnated yarn. A third nut 13 is threadedly mounted on the outer wall of the second screw 12 to limit its position. This nut is located on the side of the second vertical plate away from the pressure spring 11. After tensioning is complete, the first nut 6 is tightened to achieve long-term stress application to the FRP.

[0024] A dial indicator 3 is fixed on the inner surface of the test frame 1 to measure the moving distance of the fixture 2. The measuring end of the dial indicator 3 is in contact with one end of the fixture 2 close to the second screw 12. The moving distance of the fixture 2 can be measured by the dial indicator 3, and the creep deformation of the FRP can be obtained.

[0025] After the creep test is completed, the measurement data needs to be processed. The processing methods are divided into: rough test and precise test.

[0026] For a rough test of creep performance, the creep strain can be directly obtained by taking the measurement result of the dial indicator. For a precise test of creep performance, it is necessary to use the step-by-step method to correct the creep strain based on the measurement result of the dial indicator. The end time of the calculation step is 1min, 3min, 6min, 9min, 15min, 30min, 45min and 1h, 1.5h, 2h, 4h, 10h, 24h, 48h, 72h, 96h and 120h respectively. After that, it is taken once every 120h until the end of the test time. Considering the stress reduction caused by the deformation of the spring in each calculation step during the creep test, the creep strain correction for each calculation step is performed as follows:

[0027]

[0028] Among them, Δε i ' is the corrected creep strain of the i-th calculation step; Δε i is the creep strain measurement value of the i-th calculation step, which is equal to the ratio of the fixture displacement to the initial length of the free section of the dipped yarn; ΔL0 is the initial elongation of the spring; ΔL i is the elongation of the spring in the i-th calculation step, which is equal to the difference between ΔL0 and the fixture displacement.

[0029] It should be noted that the basic assumption of the above correction method is that the FRP is always in the linear creep stage, that is, the FRP creep strain in each calculation step is proportional to its stress. Therefore, the long-term stress of the FRP must not be greater than its creep rupture stress during the creep test time.

[0030] The inventors used basalt fiber reinforced composites (BFRP) and carbon fiber reinforced composites (CFRP) as the objects, and adopted the method of the present invention to carry out creep tests for 40 days with a stress of 0.5 times the ultimate strength as the initial tensile stress. The test results are as follows Figure 3 . In the figure, the dotted line represents the actual measured value of the dial indicator, and the solid line is the corrected value. It can be seen that this data processing method effectively takes into account the reduction of long-term stress as the spring deforms, and can achieve accurate testing of creep performance. On the other hand, since K·δ does not exceed 10% of the tension force, the creep strain error caused by spring deformation (that is, the error between the dotted and solid lines) is within 10%. Therefore, the actual measured value of the dial indicator can be directly used for a rough test of creep performance.

Claims

1. A creep performance testing device for fiber-reinforced composite materials, characterized by: The invention comprises a creep test reaction frame (1), a fixture (2), a displacement measuring instrument (3), a first screw rod (4), a top plate (5), a U-shaped frame (7), a jack (8), a first vertical plate (9), a second vertical plate, a pressure spring (11) and a second screw rod (12); the first screw rod (4) and the second screw rod (12) are horizontally arranged on both sides of the creep test reaction frame (1), the inner ends of the first screw rod (4) and the second screw rod (12) are located inside the creep test reaction frame (1), and the outer ends of the first screw rod (4) and the second screw rod (12) are located outside the creep test reaction frame (1); a fixture (2) is respectively arranged at the inner ends of the first screw rod (4) and the second screw rod (12); the top plate (5), the U-shaped frame (7) and the first vertical plate (9) are sequentially arranged on the first vertical plate (9). The screw rod (4) is fixed on the top plate (5) of the creep test reaction frame (1); the first vertical plate (9) is fixed on the outer end of the first screw rod (4) in an adjustable manner, and the jack (8) is arranged between the U-shaped frame (7) and the first vertical plate (9); the second vertical plate is fixed on the outer end of the second screw rod (12) in an adjustable manner, and the top pressure spring (11) is sleeved on the second screw rod (12) and is located between the creep test reaction frame (1) and the second vertical plate; the displacement measuring instrument (3) is arranged on a clamp connected to the second screw rod (12); the product of the spring coefficient K of the top pressure spring (11) and the spring length change δ during creep is not greater than 10% of the tension force loaded by the jack (8), and δ is preliminarily determined by trial test or theoretical calculation; A friction-type clamp is used to anchor the dipped yarn. The inner surface of the clamp has a grid-like pattern to ensure that there is no slippage between the dipped yarn and the clamp during long-term creep tests. When the dipped yarn is not solidified, its anchoring section is pressed into a flat shape and then solidified to ensure the anchoring effect between the dipped yarn and the clamp.

2. The creep performance testing device for fiber-reinforced composite materials according to claim 1, characterized in that: The displacement measuring instrument is a dial indicator.

3. The creep performance testing device for fiber-reinforced composite materials according to claim 1, characterized in that: A first nut (6) for fixing the top plate outside the creep test reaction frame (1) is provided on the outside of the top plate.

4. The creep performance testing device for fiber-reinforced composite materials according to claim 1, characterized in that: A second nut (10) is provided on the outer side of the first vertical plate (9).

5. The creep performance testing device for fiber-reinforced composite materials according to claim 1, characterized in that: A third nut (13) for adjusting the installation position of the second vertical plate is arranged outside the second vertical plate.

6. A testing method based on the fiber reinforced composite material creep performance testing device according to any one of claims 1 to 5, characterized by: Fixing the dipped yarn between the clamps of the first screw and the second screw; Applying a tensile force through a jack and measuring the displacement of the fixture on the second screw through a displacement measuring instrument; Correct the displacement obtained by the displacement measuring instrument: Among them, △ε' i is the corrected creep strain in the i-th calculation step; △ε i is the creep strain measurement value of the i-th calculation step, which is equal to the ratio of the fixture displacement to the initial length of the free section of the dipped yarn; ΔL0 is the initial extension of the top pressure spring; ΔL i is the elongation of the pressure spring in the i-th calculation step, which is equal to the difference between ΔL0 and the fixture displacement.

7. The testing method according to claim 6, characterized in that: The end time of the calculation step is 1min, 3min, 6min, 9min, 15min, 30min, 45min and 1h, 1.5h, 2h, 4h, 10h, 24h, 48h, 72h, 96h and 120h respectively, and then it is taken every 120h until the end of the test time.

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