Duplex self-centering device for fatigue test and two-parameter progressive test method

Abstract

The invention discloses a duplex self-centering device for a fatigue test and a two-parameter progressive test method, the duplex self-centering device is provided with two bearing plates at the central position of a fatigue testing machine, and the bearing plates are fixed through bolts and nuts; a spherical base plate, a polytetrafluoroethylene ring and a steel wire anchoring part are sequentially arranged on the concave spherical surface of the bearing plate; the steel wire anchoring part is composed of a casting body and a steel wire, and the steel wire penetrates through the polytetrafluoroethylene ring, the spherical base plate and a hole in the center of the concave spherical surface of the bearing plate and is connected to a clamp of a fatigue testing machine to be clamped. The fatigue test adopts the device, and two parameters of stress amplitude and anchoring length are utilized for progressive accumulation test. Synchronous testing of the two steel wire anchoring test pieces is achieved through the duplex structure design, the testing efficiency is greatly improved and the testing cost is reduced while the testing precision is guaranteed, so that batch testing is met to obtain the actually-needed steel wire anchoring S-N curve, and a basis is provided for anti-fatigue design of bridge cable steel wire anchoring.

Description

Technical Field

[0001] This invention relates to the testing technology for the anchorage performance of steel wires in bridge cables, and particularly to a double-type self-centering device and a dual-parameter progressive testing method for fatigue testing. Background Technology

[0002] The in-depth implementation of the national strategies of "building a strong transportation nation" and "building a strong manufacturing nation" has continuously promoted the construction of major cross-river, cross-sea, and mountainous transportation corridors, driving the development of cable-stayed bridges towards larger spans. Currently, the Changtai Yangtze River Bridge, the world's largest span cable-stayed bridge already completed, has a main span of 1208 meters; the Zhangjinggao Yangtze River Bridge, the world's largest span suspension bridge under construction, has a main span of 2300 meters. As the core load-bearing component of such bridges, the cables, composed of a large number of steel wires, including the main cables, suspenders, and stay cables, need to increase in strength in tandem with the increase in span. In the past decade, my country's bridge cable strength has achieved a leading and rapid leap forward. The steel wire strength of the stay cables of the completed Changtai Yangtze River Bridge has reached 2100 MPa, while the steel wire strength of the main cables of the under-construction Zhangjinggao Yangtze River Bridge will reach 2200 MPa.

[0003] Meanwhile, the cable transmits force through its anchoring systems at both ends. Therefore, increasing the strength of the cable wires places equally high demands on the safety of the cable anchorage, mainly encompassing two aspects: anchorage load-bearing capacity and fatigue resistance. In terms of load-bearing capacity, this is typically achieved by developing new anchoring materials or increasing the anchorage length, with safety verification conducted through anchor pull-out tests. Research in this area is relatively mature. On the other hand, during bridge operation, the cable anchorage is highly susceptible to cumulative damage due to alternating effects from vehicles, wind, and temperature, potentially leading to fatigue failure and impacting the overall bridge structural safety. Therefore, accurately testing and evaluating the fatigue performance of cable wire anchorages is of significant practical importance.

[0004] However, current testing and analysis methods for the fatigue performance of cable anchorages lack effective means, resulting in a lack of direct data support for related theoretical research and engineering applications. The main reasons are: first, cable wire anchorage specimens have many parameters, and different parameter combinations generate more working conditions, meaning the sample size for testing is often large; second, such fatigue tests are time-consuming and expensive, and traditional individual testing methods cannot meet the testing needs of a large number of specimens; and third, fatigue testing is extremely sensitive, requiring the specimen to be kept centered to avoid the influence of additional bending moments, thus requiring very high assembly precision of the testing equipment. Therefore, it is essential to develop testing equipment and methods that can improve testing efficiency, save testing costs, and ensure specimen centering for a large number of cable wire anchorage fatigue specimens. Summary of the Invention

[0005] To address the issues of insufficient device accuracy and low testing efficiency in wire anchor fatigue testing, this invention provides a dual-type self-centering device and a dual-parameter progressive testing method for fatigue testing.

[0006] This invention discloses a double-unit self-centering device for fatigue testing of steel wire anchorage. Two pressure plates are vertically positioned at the center of the fatigue testing machine. Four bolts pass through the four corners of the two pressure plates, which are then secured with nuts. One pressure plate has a concave spherical surface with a central hole. A spherical pad, a polytetrafluoroethylene (PTFE) ring, and a steel wire anchor are sequentially arranged on the concave spherical surface of the pressure plate. The steel wire anchor consists of a casting and a steel wire. The steel wire passes through the PTFE ring, the spherical pad, and the hole in the center of the concave spherical surface of the pressure plate, connecting to a clamp on the fatigue testing machine. The other pressure plate has the same configuration, but the steel wire is connected to a clamp on the fatigue testing machine.

[0007] Furthermore, the diameter of the holes in the center of the pressure plate and the spherical pad is slightly larger than the diameter of the steel wire.

[0008] Furthermore, the diameter of the hole in the center of the PTFE ring is the same as the diameter of the steel wire.

[0009] Furthermore, the bolts are threaded at both ends and smooth in the middle.

[0010] This invention provides a dual-parameter progressive test method for fatigue testing of steel wire anchorage, using the aforementioned double-unit self-centering device. The specific experimental steps are as follows:

[0011] Step 1: Prepare a batch of steel wire anchors and group and number them according to stress amplitude and anchorage length from small to large. Each group of tests starts with the specimen with the smallest number, and each group of specimens has several corresponding replicas.

[0012] Step 2: Assemble the tooling as required and set the stress amplitude.

[0013] Step 3: Hold the free end of the steel wire of the longer steel wire anchor with the clamp on the fatigue testing machine, and hold the free end of the steel wire of the shorter steel wire anchor with the clamp on the lower clamp.

[0014] Step 4: Start the fatigue testing machine and apply fatigue load cyclically until the specimen fails, and record the number of load cycles C.

[0015] Step 5: Remove and disassemble the fixture, take out the two test pieces, replace the damaged wire anchor with the next wire anchor in the same group, reassemble and invert the fixture, and continue loading until all wire anchors in the same group except the last one are damaged by loading.

[0016] Step 6: Complete the test of the replicated specimen, record the number of load cycles, and process the discrete experimental data.

[0017] Step 7: Set the next stress amplitude and repeat steps 3-5 until all specimens in each group fail except for the last one.

[0018] Step 8: Summarize the number of fatigue load cycles C, and calculate the fatigue life N of the specimen under different stress amplitudes and different anchorage lengths.

[0019] Step 9: Plot the fatigue curves (SN curves) for different anchorage lengths with stress amplitude σ as the ordinate and the logarithm of the fatigue life of the specimen lgN as the abscissa.

[0020] Furthermore, the installation steps in step 2 are as follows:

[0021] Step 2.1: First, pass one of the pressure plates through the four bolts, with the concave spherical side of the pressure plate facing inward, and then install the inner nut.

[0022] Step 2.2: Pass one steel wire anchor to be tested through the polytetrafluoroethylene ring, the spherical pad, and the aforementioned pressure plate in sequence.

[0023] Step 2.3: Tighten another set of inner nuts at the position of the thread on the other end of the bolt. Then, pass one of the steel wire anchors to be tested through the PTFE ring, the spherical pad and another pressure plate in sequence as in Step 2.2. Then, pass this pressure plate through the bolt and adjust the pressure plate appropriately by adjusting the inner nuts. Finally, tighten the nuts on the outside of the top plate.

[0024] Step 2.4: Set the stress amplitude of the fatigue testing machine.

[0025] The beneficial technical effects of this invention compared to the prior art are as follows:

[0026] 1. This invention reduces friction between the wire anchor and the spherical pad by incorporating a polytetrafluoroethylene (PTFE) ring. The diameter of the PTFE ring's hole is the same as the diameter of the wire in the anchor, both smaller than the hole diameter in the spherical pad. This allows the wire anchor to translate on the pad under the pressure of the fatigue testing machine's clamps. The spherical pad also allows the wire anchor to rotate under eccentricity. Thus, through a combination of translation and rotation, the wire anchor is automatically centered relative to the loading direction, avoiding the adverse effects of additional bending moments caused by eccentricity and improving the accuracy of the fatigue test.

[0027] 2. This invention utilizes a modular, double-loading fixture to simultaneously conduct fatigue tests on two wire anchors. Considering both stress amplitude and anchorage length, a progressive testing method is employed to accumulate the fatigue life of the wire anchors under different parameter combinations. Therefore, it significantly improves fatigue testing efficiency, providing the possibility of obtaining sufficient test data to plot SN curves that can guide actual structural design. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the loading process of the present invention.

[0029] Figure 2 This is a schematic diagram of the overall structure of the dual-type self-centering device of the present invention.

[0030] Figure 3 This is a structural diagram of the pressure plate of the double-link self-centering device of the present invention.

[0031] Figure 4 This is a schematic diagram of the placement of the test specimen for the double-unit self-centering device of the present invention.

[0032] Figure 5 This is a schematic diagram of the bolts of the double-jointed self-centering device of the present invention.

[0033] Figure 6 This is a schematic diagram of the spherical pad of the double-linked self-centering device of the present invention.

[0034] Figure 7 This is a schematic diagram of the polytetrafluoroethylene ring of the double-unit self-centering device of the present invention.

[0035] In the diagram: 1-fatigue testing machine, 2-pressure plate, 3-nut, 4-steel wire anchor, 401-cast body, 402-steel wire, 5-bolt, 6-PTFE ring, 7-spherical pad.

[0036] Figure 8 This is a schematic diagram of the SN curves under different anchorage lengths obtained from the dual-parameter progressive test of this invention.

[0037] Figure 9 This is a flowchart of the dual-parameter progressive test of the present invention. Detailed Implementation

[0038] The present invention will be further described in detail below with reference to the accompanying drawings and specific implementation methods.

[0039] A double-unit self-centering device for fatigue testing of steel wire anchorage according to the present invention, as shown in the invention. Figure 1 , Figure 2 As shown, two pressure plates 2 are vertically installed at the center of the fatigue testing machine 1. Four bolts 5 pass through the four corners of the two pressure plates 2, and they are fixed with nuts 3. One pressure plate 2 has a concave spherical surface with a central hole. A spherical pad 7, a polytetrafluoroethylene ring 6, and a steel wire anchor 4 are sequentially installed on the concave spherical surface of the pressure plate 2. Figure 4 As shown, the wire anchor 4 consists of a casting 401 and a wire 402. The wire 402 passes through the hole in the center of the concave spherical surface of the polytetrafluoroethylene ring 6, the spherical pad 7, and the bearing plate 2, and is connected to the clamp on the fatigue testing machine 1. The other bearing plate 2 is set in the same way, but the wire 402 is connected to the clamp on the fatigue testing machine 1.

[0040] Furthermore, such as Figure 3 , Figure 6 As shown, the diameter of the hole in the center of the pressure plate 2 and the spherical pad 7 is slightly larger than the diameter of the steel wire 402.

[0041] Furthermore, such as Figure 7 As shown, the diameter of the hole in the center of the polytetrafluoroethylene ring 6 is the same as the diameter of the steel wire 402.

[0042] Furthermore, such as Figure 5 As shown, bolt 5 has threads at both ends and a smooth middle.

[0043] This invention discloses a dual-parameter progressive test method for fatigue testing of steel wire anchorages, using the aforementioned double-unit self-centering device. The specific experimental steps are as follows: Figure 9 As shown, specifically:

[0044] Step 1: Preparation There are several wire anchors, where m is the number of stress amplitudes to be developed, and the stress amplitudes are numbered sequentially from smallest to largest (σ1~σ2). m ); n is the number of repeated tests for each group; k is the number of anchorage lengths to be measured, and the wire anchors are numbered in ascending order of anchorage length (A). 1,1 ~A k+1,m ).

[0045] Step 2: Pass the four bolts through the four corner holes of a pressure plate, with the concave spherical side facing inward, and tighten the nuts on the inside.

[0046] Step 3: Install wire anchor A i+1,j (The initial values ​​of i and j are both 1) They pass through the central hole of the polytetrafluoroethylene ring, the spherical pad and the pressure plate in sequence.

[0047] Step 4: Tighten the inner nut on the other side of the bolt according to the thread position.

[0048] Step 5: Install wire anchor A i,j The bolt passes through the central hole of the PTFE ring, the spherical pad, and another pressure plate in sequence, and is then placed on the inner nut. After adjusting the inner nut for proper leveling, the outer nut is screwed on.

[0049] Step 6: Set the fatigue test stress amplitude σ j .

[0050] Step 7: Install wire anchor A i+1,j The free end of the steel wire is placed in the middle of the upper clamp of the fatigue testing machine, and then the upper clamp is clamped, so that the anchor moves to the test loading direction under the push of the clamp.

[0051] Step 8: Adjust the spacing between the upper and lower clamps of the fatigue testing machine, and place the wire anchor A. i,j The free end of the steel wire is placed in the middle of the lower clamp of the fatigue testing machine, and then the lower clamp is clamped, so that the steel wire anchor moves to the test loading direction under the push of the clamp.

[0052] Step 9: After the specimen has automatically leveled and stabilized, clear the system, start the fatigue testing machine, and cyclically load until a certain wire anchor A is reached. i,j Failure occurs (by default, the specimen with the shorter anchorage length fails first), and the number of cyclic loading cycles C is recorded at this point. i,j .

[0053] Step 10: Loosen the upper and lower clamps and remove the tooling; loosen the upper and lower nuts and remove the wire anchor A. i,j and A i+1,j .

[0054] Step 11: Let i = i + 1, repeat steps 3 to 10 until i = k, ending the test of all wire anchor lengths under this stress amplitude, where specimen A k+1,j It does not need to be destroyed during loading.

[0055] Step 12: Take the same specimen and repeat steps 3-11 n times, recording the value C of the number of cyclic loading cycles each time. i,j Take the average value .

[0056] Step 13: Let j = j + 1. If j ≤ m, repeat steps 3 to 11 to conduct fatigue tests on wire anchors of various anchorage lengths under other stress amplitudes.

[0057] Step 14: After completing all tests, summarize the fatigue cycle loading count dataset. The fatigue life dataset of wire anchors under various stress amplitudes and anchorage lengths was obtained through calculation. (1≤i≤k, 1≤j≤m).

[0058] Step 15: Plot a scatter plot of all data with fatigue stress amplitude (σ) as the ordinate and the logarithm of fatigue life (lgN) as the abscissa, and fit the curve corresponding to different anchorage lengths, i.e., the SN curve, as shown below. Figure 8 As shown.

[0059] This invention discloses a double-unit self-centering device for fatigue testing of wire anchorages. The "double-unit" principle and process are as follows: the "double-unit" setup allows for simultaneous fatigue testing of two wire anchorages. If one specimen fails during testing, it is removed and replaced with another similar specimen, and the loading cycle continues. This enables fatigue testing of two wire anchorages with different parameters on a single fatigue testing machine, significantly reducing testing time compared to traditional individual testing methods. This is extremely advantageous for fatigue testing, as similar tests are very expensive per run, and wire anchorage testing involves two parameters, resulting in a large number of test specimens. Therefore, the "double-unit" testing provides an important prerequisite for conducting fatigue tests on a sufficient number of specimens.

[0060] The principle and process of its "self-centering" is as follows: Due to errors in materials and manufacturing, the force axis of the steel wire anchor under test is difficult to be coaxial with the tensile force of the fatigue testing machine. In this case, the matching of the concave spherical surface of the bearing plate 2 and the spherical pad 7 allows the steel wire anchor to rotate under the bending moment caused by eccentricity. Adding a polytetrafluoroethylene ring 6 between the spherical pad 7 and the steel wire anchor allows the steel wire anchor to translate under the bending moment caused by eccentricity. Therefore, the automatic centering of the steel wire anchor with respect to the loading direction is achieved through a combination of "translation + rotation". Eccentricity can cause additional bending moments in the steel wire anchor during fatigue testing, leading to stress concentration and reducing the fatigue life of the steel wire anchor (in severe cases, by an order of magnitude), deviating from the actual situation. Therefore, "centering" is a very important and difficult-to-control sensitive factor in similar fatigue tests. The "self-centering" setting of this invention avoids the adverse effects of additional bending moments caused by eccentricity, ensuring the accuracy of the fatigue test.

[0061] It should be noted that, for those skilled in the art, it is obvious that the present invention is not limited to the details described above, and that the present invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered exemplary and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations within the meaning and scope of equivalents of the claims are intended to be included within the present invention, and no reference numerals in the claims should be construed as limiting the scope of the claims.

Claims

1. A double-unit self-centering device for fatigue testing of steel wire anchorage, characterized in that, Two pressure plates (2) are vertically set at the center of the fatigue testing machine (1). Four bolts (5) pass through the four corners of the two pressure plates (2) and are fixed with nuts (3). One pressure plate (2) has a concave spherical surface with a central hole. A spherical pad (7), a polytetrafluoroethylene ring (6), and a steel wire anchor (4) are set sequentially on the concave spherical surface of the pressure plate (2). The steel wire anchor (4) consists of a casting (401) and a steel wire (402). The steel wire (402) passes through the polytetrafluoroethylene ring (6), the spherical pad (7), and the hole in the center of the concave spherical surface of the pressure plate (2) and is connected to the lower clamp of the fatigue testing machine (1) for clamping. The other pressure plate (2) is set in the same way, but the steel wire (402) is connected to the upper clamp of the fatigue testing machine (1) for clamping.

2. The double-unit self-centering device for fatigue testing of steel wire anchorage according to claim 1, characterized in that, The diameter of the hole in the center of the pressure plate (2) and the spherical pad (7) is slightly larger than the diameter of the steel wire (402).

3. The double-unit self-centering device for fatigue testing of steel wire anchorage according to claim 1, characterized in that, The diameter of the hole in the center of the polytetrafluoroethylene ring (6) is the same as the diameter of the steel wire (402).

4. The double-unit self-centering device for fatigue testing of steel wire anchorage according to claim 1, characterized in that, The bolt (5) has threads at both ends and a smooth middle.

5. A dual-parameter progressive test method for fatigue testing of steel wire anchorage, characterized in that, The specific experimental steps for using the dual-unit self-centering device as described in any one of claims 1-5 are as follows: Step 1: Prepare a batch of steel wire anchors (4), and group and number them according to stress amplitude and anchorage length from small to large. Each group of tests starts with the specimen with the smallest number, and each group of specimens has several replicas. Step 2: Assemble the tooling as required and set the stress amplitude; Step 3: Hold the free end wire (402) of the longer wire anchor by the upper clamp of the fatigue testing machine (1), and hold the free end wire (402) of the shorter wire anchor by the lower clamp. Step 4: Start the fatigue testing machine (1), apply fatigue load cyclically until the specimen fails, and record the number of load cycles C; Step 5: Remove and disassemble the fixture, take out the two test pieces, replace the damaged wire anchor (4) with the next wire anchor (4) in the same group, reassemble and invert the fixture, and continue loading until all wire anchors (4) in the same group except the last wire anchor (4) are loaded and destroyed. Step 6: Complete the test for the replication group, record the number of load cycles, and complete the discretization of the test data; Step 7: Set the next stress amplitude and repeat steps 3-5 until all specimens in each group except the last one fail. Step 8: Summarize the number of fatigue load cycles C, and calculate the fatigue life N of the specimen under different stress amplitudes and different anchorage lengths. Step 9: Plot the fatigue curves (SN curves) for different anchorage lengths with stress amplitude σ as the ordinate and the logarithm of the fatigue life of the specimen lgN as the abscissa.

6. The dual-parameter progressive test method for fatigue testing of steel wire anchorage according to claim 5, characterized in that, The installation steps in step 2 are as follows: Step 2.1: First, pass one of the pressure plates (2) through 4 bolts (5), with the concave spherical side of the pressure plate (2) facing inward, and then install the inner nut (3). Step 2.2: Pass one steel wire anchor (4) to be tested through the polytetrafluoroethylene ring (6), the spherical pad (7) and the above-mentioned pressure plate (2) in sequence. Step 2.3: Tighten another set of inner nuts (3) at the position of the thread on the other end of the bolt (5), and then pass a steel wire anchor (4) to be tested through the polytetrafluoroethylene ring (6), the spherical pad (7) and another bearing plate in sequence according to the method in step 2.

2. Then pass this bearing plate through the bolt, and adjust the bearing plate appropriately by adjusting the inner nuts; finally, tighten the nuts (3) on the outside of the top plate (2). Step 2.4: Set the stress amplitude of the fatigue testing machine (1).

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