A method for evaluating the service life of a power transmission tower bolt

Abstract

The present application belongs to the field of transmission line bolt loosening detection, and particularly relates to a transmission tower bolt service life evaluation method; including constructing a transmission tower finite element model and analyzing the dynamic results of the transmission tower under the action of wind load to determine the weakest part of the transmission tower; constructing a refined finite element model of the weakest part, obtaining a load-displacement curve through a cyclic load action experiment; connecting two steel plates with a bolt specimen of the weakest part and then installing the bolt specimen on a fatigue testing machine to conduct a bolt loosening experiment; statistically arranging the data of the bolt loosening experiment to obtain the relationship between the residual pre-tightening force of the bolt specimen and the amplitude cycle number, and output a D-N curve; using a rain flow counting method to count the number of different amplitude loads received by the bolt specimen in a unit time in the bolt loosening experiment; combining the statistical results of the rain flow counting method and applying a Miner linear fatigue cumulative damage theory to output the service life evaluation results of the bolt specimen of the transmission tower under the actual wind load.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of transmission line bolt loosening detection, and particularly relates to a transmission tower bolt service life evaluation method. BACKGROUND

[0002] The transmission tower line system bears the transmission, distribution, regulation and other functions of electric energy, and its safety and stability are very important. As an important supporting structure in the transmission tower line system, predicting its damage condition has great significance for maintaining safety. The bolt structure is an important structure of the transmission tower, and is widely used in various fields of industrial application due to its easy installation and convenient disassembly. The main problem of the bolt structure is that the bolt will loosen under the action of vibration or external force, causing the axial force of the bolt connection structure to decrease, resulting in failure of the bolt connection. This has a great influence on the reliability and safety of the working system of the transmission tower.

[0003] As one of the important connection structures in the transmission tower, the bolt will loosen in the structure vibration caused by wind force. If not discovered in time, the long-term influence of wind force will aggravate the bolt loosening, further leading to complete loosening of the bolt, which will cause damage or even falling of the member at the bolt loosening structure of the tower in subsequent strong wind and earthquake disasters, and further cause the tower to collapse. According to statistics, the fallen bolt nut can often be seen at the scene of the transmission tower collapse accident, and the analysis report of the tower collapse accident also shows that the bolt connection failure is the main cause of the tower collapse accident. SUMMARY

[0004] To solve the above problems, the application provides a transmission tower bolt service life evaluation method, which comprises the following steps:

[0005] S1. A finite element model of the transmission tower is constructed, the dynamic analysis result of the transmission tower under the action of wind load is calculated, and the weakest part of the transmission tower is determined according to the dynamic analysis result;

[0006] S2. A refined finite element model of the weakest part determined in step S1 is constructed, a load-displacement curve is obtained through a cyclic load action experiment, and four stages of bolt loosening are analyzed according to the load-displacement curve;

[0007] S3. The bolt specimen of the weakest part determined in step S1 is connected with two steel plates and then installed on a fatigue testing machine, a bolt loosening experiment is carried out, and the four stages of bolt loosening analyzed in step S2 are verified;

[0008] S4. The data of the bolt loosening experiment are statistically arranged, the relationship between the residual pre-tightening force of the bolt specimen and the amplitude cycle number is obtained, and a D-N curve is output as a standard for judging whether the bolt is loosened or not;

[0009] S5. Count the number of times the bolt specimen is subjected to different amplitude loads in unit time in the bolt loosening experiment by using the rainflow counting method;

[0010] S6. Apply the Miner linear fatigue cumulative damage theory to output the life assessment result of the bolt specimen of the power transmission tower under the actual wind load condition in combination with the statistical result of the rainflow counting method in S5.

[0011] Further, the wind load used in step S1 is:

[0012] S11. Obtain the natural wind environment data of the target area where the power transmission tower is located, and combine the design reference wind speed of the target area according to the national standard;

[0013] S12. Establish the fluctuating wind power spectrum according to the reference wind speed, and simulate the wind field of the target area by using MATLAB and the fluctuating wind power spectrum to obtain the wind load.

[0014] Further, the refined finite element model of the weakest part includes 4 identical bolts, 1 upper connecting plate, 1 left middle connecting plate, 1 right middle connecting plate, and 1 lower connecting plate; the right half of the left middle connecting plate is attached to the left part of the upper connecting plate and the left part of the lower connecting plate to form a first area; the left half of the right middle connecting plate is attached to the right part of the upper connecting plate and the right part of the lower connecting plate to form a second area; two threaded holes are provided in the first area and the second area, respectively, and are sequentially referred to as a first threaded hole, a second threaded hole, a third threaded hole, and a fourth threaded hole from left to right; one bolt is installed in each threaded hole.

[0015] Further, the bolt includes a bolt shank and a bolt head, the diameter and height of the bolt shank are 20 mm and 65 mm respectively, and the diameter and height of the bolt head are 30 mm and 12.71 mm respectively; the length, width, and height of the upper connecting plate are 235 mm, 115 mm, and 10 mm respectively; the length, width, and height of the middle connecting plate are 200 mm, 140 mm, and 12 mm respectively; the length, width, and height of the lower connecting plate are 235 mm, 125 mm, and 10 mm respectively.

[0016] Further, in the transverse distance, the distances from the first threaded hole, the second threaded hole, the third threaded hole, and the fourth threaded hole to the left edge of the upper connecting plate are 32.5 mm, 82.5 mm, 152.5 mm, and 202.5 mm respectively; in the longitudinal distance, the distances from the first threaded hole, the second threaded hole, the third threaded hole, and the fourth threaded hole to the upper edge of the upper connecting plate are 40 mm, 75 mm, 40 mm, and 75 mm respectively.

[0017] Further, step S2 of constructing the refined finite element model of the weakest part for the cyclic load experiment includes:

[0018] S21. Establish a space rectangular coordinate system according to the refined finite element model of the weakest part;

[0019] S22. Perform freedom degree constraint on the left half of the left middle connecting plate in X, Y and Z directions, so that it is fixed;

[0020] S23. Perform freedom degree constraint on the upper surface of the lower connecting plate in the Z direction, so that it can only move in the Z direction;

[0021] S24. Fix the left half of the right middle connecting plate, and apply a low-frequency displacement load to the upper surface of the right half of the right middle connecting plate, the low-frequency displacement load gradually increases, and the experimental results such as the load and displacement of the connecting plate right end node during the experiment are recorded.

[0022] The beneficial effects of the present application are:

[0023] The present application provides a method for evaluating the service life of bolts under wind load. The weak part of the transmission tower under wind load is determined through finite element simulation experiment; four stages of bolt loosening are determined through finite element simulation experiment and bolt loosening experiment; and the relationship curve between bolt residual pre-tightening force and cycle number (D-N curve) is arranged according to experimental data, which is used as a standard for judging bolt loosening; finally, the service life of the bolt loosening of the transmission tower under wind load is estimated according to the D-N curve and the Miner theory. The present application can effectively evaluate the service life of the bolts of the transmission tower under wind load according to the establishment of the finite element model of the corresponding tower combined with the actual wind field data, which is beneficial to the operation and maintenance of the transmission tower and prevents the tower from falling due to bolt loosening. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 It is a general flow chart of the bolt service life prediction method of the present application;

[0025] Figure 2 It is a finite element model diagram of the transmission tower of the embodiment of the present application;

[0026] Figure 3 It is a simplified diagram of bolt connection of the dangerous part of the embodiment of the present application;

[0027] Figure 4 It is a load amplitude value diagram of the bolt connection end of the embodiment of the present application;

[0028] Figure 5 It is a hysteresis curve diagram of the bolt connection of the embodiment of the present application;

[0029] Figure 6 It is a four-stage diagram of bolt loosening of the embodiment of the present application;

[0030] Figure 7A bolt vibration experiment device schematic view for an embodiment of the present application is shown in the figure.

[0031] Figure 8 A bolt residual pre-tightening force and cycle number relationship curve of the present application is shown in the figure.

[0032] Figure 9 A rain flow counting method schematic view of the present application is shown in the figure.

[0033] Figure 10 A two-node position on a tower body of an embodiment of the present application is shown in the figure.

[0034] Figure 11 A two-node relative displacement of an embodiment of the present application is shown in the figure.

[0035] Figure 12 A node load spectrum of an embodiment of the present application under a 30m / s wind speed condition is shown in the figure. DETAILED DESCRIPTION

[0036] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0037] The present application provides a power transmission tower bolt life evaluation method, as shown in the figure, comprising the following steps: Figure 1

[0038] S1. Construct a power transmission tower finite element model as shown in the figure, calculate the dynamic analysis results under the action of wind load, and determine the weakest part of the power transmission tower according to the dynamic analysis results. Figure 2

[0039] Specifically, the wind load acquisition process used in step S1 comprises:

[0040] S11. Obtain the natural wind environment data of the target area where the power transmission tower is located, and combine the national standard design target area reference wind speed.

[0041] S12. Establish a fluctuating wind power spectrum according to the reference wind speed, and use MATLAB and the fluctuating wind power spectrum to simulate the wind field of the target area to obtain the wind load.

[0042] S2. Construct a refined finite element model of the weakest part, obtain the load-displacement curve through a cyclic load action experiment, and analyze the four stages of bolt loosening according to the load-displacement curve, which is verified by subsequent experiments.

[0043] Specifically, as shown in the figure, Figure 3 ​​As shown, the refined finite element model of the weakest part includes 4 identical bolts, 1 upper connecting plate, 1 left middle connecting plate, 1 right middle connecting plate and 1 lower connecting plate; the right half of the left middle connecting plate is attached to the left part of the upper connecting plate and the left part of the lower connecting plate to form a first area; the left half of the right middle connecting plate is attached to the right part of the upper connecting plate and the right part of the lower connecting plate to form a second area; two threaded holes are provided in the first area and the second area respectively, and are sequentially referred to as a first threaded hole, a second threaded hole, a third threaded hole and a fourth threaded hole from left to right; one bolt is installed in each threaded hole.

[0044] Specifically, the bolt includes a bolt shank and a bolt head, the diameter and height of the bolt shank are 20 mm and 65 mm respectively, and the diameter and height of the bolt head are 30 mm and 12.71 mm respectively; the length, width and height of the upper connecting plate are 235 mm, 115 mm and 10 mm respectively; the two middle connecting plates are of the same size, and the length, width and height of each middle connecting plate are 200 mm, 140 mm and 12 mm respectively; the length, width and height of the lower connecting plate are 235 mm, 125 mm and 10 mm respectively.

[0045] Specifically, in the transverse distance, the distances of the first threaded hole, the second threaded hole, the third threaded hole and the fourth threaded hole from the left edge of the upper connecting plate are 32.5 mm, 82.5 mm, 152.5 mm and 202.5 mm respectively; in the longitudinal distance, the distances of the first threaded hole, the second threaded hole, the third threaded hole and the fourth threaded hole from the upper edge of the upper connecting plate are 40 mm, 75 mm, 40 mm and 75 mm respectively. The hole diameters of the four threaded holes are all 21.5 mm.

[0046] Specifically, in the transverse distance, the distances of the first threaded hole and the second threaded hole from the right edge of the left middle connecting plate are 30 mm and 80 mm respectively; in the longitudinal distance, the distances of the first threaded hole and the second threaded hole from the upper edge of the left middle connecting plate are 43 mm and 78 mm respectively.

[0047] Specifically, in the transverse distance, the distances of the third threaded hole and the fourth threaded hole from the left edge of the right middle connecting plate are 30 mm and 80 mm respectively; in the longitudinal distance, the distances of the third threaded hole and the fourth threaded hole from the upper edge of the right middle connecting plate are 43 mm and 78 mm respectively.

[0048] Specifically, in the transverse distance, the distances of the first threaded hole, the second threaded hole, the third threaded hole and the fourth threaded hole from the left edge of the lower connecting plate are 32.5 mm, 82.5 mm, 152.5 mm and 202.5 mm respectively; in the longitudinal distance, the distances of the first threaded hole, the second threaded hole, the third threaded hole and the fourth threaded hole from the upper edge of the lower connecting plate are 43 mm, 78 mm, 43 mm and 78 mm respectively.

[0049] In an embodiment, step S2 includes:

[0050] S21. Establish a space rectangular coordinate system according to the refined finite element model of the weakest part;

[0051] S22. Restrict the freedom of the left half of the left middle connecting plate in X, Y, Z directions, so that the left middle connecting plate is fixed;

[0052] S23. Restrict the freedom of the upper surface of the lower connecting plate in Z direction, so that the lower connecting plate can only move in Z direction;

[0053] S24. Fix the left half of the right middle connecting plate, and apply a low-frequency displacement load to the upper surface of the right half of the right middle connecting plate as shown in Figure 4 , and gradually increase the low-frequency displacement load. Record the load, displacement, and other experimental results of the right half of the right middle connecting plate during this process.

[0054] Specifically, step S24 applies different end face displacement loads to the refined finite element model of the weakest part to obtain the load-displacement curve of the node on the left half of the right middle connecting plate under cyclic loading as shown in Figure 5 , and analyze and compare it. It can be seen that when the displacement load is first applied, the critical condition for generating the first stage slip has not been reached, and the displacement load is small at this time, so the material is still in the elastic stage. Therefore, the deformation of the node at this stage is elastic deformation, so the hysteresis curve at the beginning is a straight line passing through the origin. As the displacement increases, the critical condition for the first stage slip is reached and the slip begins. During the application of the displacement load in the range of 0-1.5 mm, the bolted joint structure (refined finite element model of the weakest part) is always in the elastic deformation stage. As the displacement load amplitude gradually increases, the hysteresis curve of the node becomes more and more full, and the area of the hysteresis loop surrounded by the hysteresis curve also becomes larger and larger, indicating that the node can better resist vibration caused by external load and better absorb vibration energy to reduce the intensity of vibration, which can reduce noise and prevent damage to the component caused by vibration. At the same time, it also shows that the node can resist deformation in a larger range. As the displacement load increases to 3.15 mm, the hysteresis curve gradually shows Z shape and appears pinch shape. As the displacement load increases, the curve slope gradually decreases and the node bearing capacity decreases.

[0055] S3. After connecting two steel plates with the weakest part of the bolt specimen, install it on the fatigue testing machine and conduct the bolt loosening experiment.

[0056] Specifically, the weakest part of the bolt specimen is used to connect two steel plates, the pre-tightening force on the bolt specimen is adjusted to the same level, and then the tightened two steel plates are installed on a fatigue testing machine, as shown in Figure 7 The pre-tightening force of the bolt is adjusted to 25000N by using a torque wrench, and the fixed bolt is subjected to vibration test at different amplitudes by starting the transverse vibration testing machine. During the test, the transverse loading displacement, the residual pre-tightening force of the bolt, and the transverse load cycle number are recorded by using a data acquisition device.

[0057] The loosening test data is sorted out and the previous refined finite element model test results are verified. The loosening process of the bolt specimen connected steel plate structure under periodic shear load is divided into four stages as shown in Figure 6 .

[0058] Stage 1: The axial force is less than the slip load, and elastic deformation occurs;

[0059] Stage 2: The axial force exceeds the slip load, and relative sliding to eliminate the gap occurs, and the connection stiffness is greatly reduced.

[0060] Stage 3: The structural gap between the bolt and the bolt hole disappears, and extrusion deformation occurs between the bolt rod and the bolt hole wall, and the stiffness of the node connection is significantly increased compared with the previous stage;

[0061] Stage 4: The connection part is gradually damaged and eventually fails.

[0062] S4. Statistics and arrangement of bolt loosening test data, get the relationship between bolt specimen residual pre-tightening force and amplitude cycle number, output D-N curve.

[0063] Specifically, the experimental data such as residual pre-tightening force and amplitude cycle number in the bolt vibration test are counted as shown in Table 1, and the residual pre-tightening force and amplitude cycle number are plotted as a (D-N) curve as shown in Figure 8 , which is used as a standard for judging the bolt loosening life.

[0064] Table 1 Experimental data

[0065]

[0066]

[0067] S5. The rain flow counting method as shown in Figure 9 is used to count the number of different amplitude loads received by the bolt specimen in unit time in the bolt loosening test.

[0068] S6. The statistical results of the rainflow counting method in S5 are applied to the Miner linear fatigue cumulative damage theory to output the life assessment results of the power transmission tower bolt test piece under the actual wind load condition.

[0069] Specifically, the Miner linear cumulative damage theory is a theory for calculating cumulative damage in a linear method, which believes that: a) under the action of equal amplitude cyclic load, each cycle has the same damage to the material; b) under the action of variable amplitude cyclic load, the damage of cyclic load of different amplitudes to the material is relatively independent, and is irrelevant to the loading sequence; c) the critical fatigue damage of the material is 1.

[0070] If a component is under a certain constant amplitude stress level S i , the life (i.e. the number of cycles) to failure is N i , then the damage of the component when it is subjected to n i cycles can be defined as:

[0071] D i = n i / N i

[0072] If the component is subjected to n i cycles under k stress levels, then the total damage of the component can be defined as:

[0073]

[0074] When D≥1, it can be judged that the component reaches failure.

[0075] The load cycle number obtained by the rainflow counting method in S5 is brought into the Miner linear fatigue cumulative damage theory for calculation, and the life of the bolt test piece under the actual wind load condition is judged and predicted, as shown below:

[0076] Take the two nodes of the tower body as an example, the positions of the two nodes on the tower body are shown in Figure 10 . The relative displacements of the two nodes in each direction under the condition of 30 m / s wind speed are extracted respectively, and the data are imported into MATLAB, and the rainflow counting method is used to count the displacements to obtain the load spectrum as shown in Figures 4-12 .

[0077] The load spectrum is compared with the D-N curve of bolt loosening to find the cycle number under the displacement when the remaining 90% pre-tightening force is obtained, and then the damage can be calculated to obtain the fatigue life as shown in the following table.

[0078] Table 2 Node cycle cumulative damage table

[0079]

[0080] Its total damage D = ∑Di = 1.190, the fatigue life of the structure is T = 1 / D = 0.840.

[0081] That is, the bolt connection experiences 0.8401 cycles to reach the 90% pre-tightening stage under this load spectrum. The random wind is applied for 2 minutes, so it is considered that the residual pre-tightening force of the node will decrease to 90% under the condition of 1.68 minutes of random wind of 30 m / s.

[0082] And according to the judgment and prediction results combined with the overhead transmission line state maintenance guide DL / T1248-2013, the bolt connection of the transmission line tower is maintained and maintained to ensure the safety of the tower body of the transmission line.

[0083] In the present application, unless otherwise explicitly specified and limited, the terms "mounting", "setting", "connecting", "fixing", "rotating" and the like should be understood broadly, for example, can be fixedly connected, or can be detachably connected, or integrated; can be mechanically connected, or can be electrically connected; can be directly connected, or can be indirectly connected through an intermediate medium; can be the internal communication of two elements or the interaction relationship of two elements, unless otherwise explicitly limited, those skilled in the art can understand the specific meaning of the above-mentioned terms in the present application according to the specific circumstances.

[0084] Although the embodiments of the present application have been shown and described, those skilled in the art can understand that various changes, modifications, replacements and variations can be made to the embodiments without departing from the principles and spirits of the present application, and the scope of the present application is defined by the appended claims and their equivalents.

Claims

1. A method for evaluating the service life of a power transmission tower bolt, characterized by, The method comprises the following steps: S1. Constructing a finite element model of the power transmission tower, calculating the dynamic analysis results under the action of wind load, and determining the weakest part of the power transmission tower according to the dynamic analysis results; S2. Constructing a refined finite element model of the weakest part determined in step S1, obtaining a load-displacement curve through a cyclic load experiment, and analyzing the four stages of bolt loosening according to the load-displacement curve; The refined finite element model of the weakest part comprises four identical bolts, one upper connecting plate, one left middle connecting plate, one right middle connecting plate, and one lower connecting plate; the right half of the left middle connecting plate is attached to the left part of the upper connecting plate and the left part of the lower connecting plate to form a first area; the left half of the right middle connecting plate is attached to the right part of the upper connecting plate and the right part of the lower connecting plate to form a second area; two threaded holes are provided in the first area and the second area, respectively, and are sequentially referred to as the first threaded hole, the second threaded hole, the third threaded hole, and the fourth threaded hole from left to right; one bolt is installed in each threaded hole; S3. Connecting two steel plates with the bolt specimen of the weakest part determined in step S1, and installing the bolt specimen on a fatigue testing machine to perform a bolt loosening experiment and verify the four stages of bolt loosening analyzed in step S2, comprising: Stage 1: elastic deformation occurs when the axial force is less than the slip load; Stage 2: relative sliding occurs to eliminate the gap, and the connection stiffness is greatly reduced when the axial force exceeds the slip load; Stage 3: the structural gap between the bolt and the bolt hole disappears, and extrusion deformation occurs between the bolt rod and the bolt hole wall, and the stiffness of the node connection is significantly increased compared with the previous stage; Stage 4: the connection part is gradually damaged and eventually fails; S4. Statistically arranging the data of the bolt loosening experiment to obtain the relationship between the residual pre-tightening force of the bolt specimen and the amplitude cycle number, and outputting a D-N curve as a standard for determining whether the bolt is loose; S5. Using the rainflow counting method to count the number of different amplitude loads received by the bolt specimen per unit time in the bolt loosening experiment; S6. Combining the statistical results of the rainflow counting method in S5 and applying the Miner linear fatigue cumulative damage theory to output the life evaluation results of the bolt specimen of the power transmission tower under actual wind load.

2. The method for evaluating the service life of a power transmission tower bolt according to claim 1, characterized in that, The wind load used in step S1 is: S11. Obtain the natural wind environment data of the target area where the power transmission tower is located, and combine the national standard to design the benchmark wind speed of the target area; S12. Establish a fluctuating wind power spectrum according to the benchmark wind speed, simulate the wind field of the target area by using MATLAB and the fluctuating wind power spectrum, and obtain the wind load.

3. The method of claim 1, wherein, The bolt comprises a bolt shank and a bolt head, the diameter and height of the bolt shank are 20 mm and 65 mm respectively, and the diameter and height of the bolt head are 30 mm and 12.71 mm respectively; the length, width, and height of the upper connecting plate are 235 mm, 115 mm, and 10 mm respectively; the length, width, and height of the middle connecting plate are 200 mm, 140 mm, and 12 mm respectively; and the length, width, and height of the lower connecting plate are 235 mm, 125 mm, and 10 mm respectively.

4. The method of claim 1, wherein, In the lateral distance, the distances between the first threaded hole, the second threaded hole, the third threaded hole and the fourth threaded hole and the left edge of the upper connecting plate are 32.5mm, 82.5mm, 152.5mm and 202.5mm respectively; in the longitudinal distance, the distances between the first threaded hole, the second threaded hole, the third threaded hole and the fourth threaded hole and the upper edge of the upper connecting plate are 40mm, 75mm, 40mm and 75mm respectively.

5. The method of claim 1, wherein, In the lateral distance, the distances between the first threaded hole and the second threaded hole and the right edge of the left middle connecting plate are 30mm and 80mm respectively; in the longitudinal distance, the distances between the first threaded hole and the second threaded hole and the upper edge of the left middle connecting plate are 43mm and 78mm respectively.

6. The method of claim 1, wherein, In the lateral distance, the distances between the third threaded hole and the fourth threaded hole and the left edge of the right middle connecting plate are 30mm and 80mm respectively; in the longitudinal distance, the distances between the third threaded hole and the fourth threaded hole and the upper edge of the right middle connecting plate are 43mm and 78mm respectively.

7. The method of claim 1, wherein, Step S2 includes: S21. Establishing a space rectangular coordinate system according to the refined finite element model of the weakest part; S22. Restricting the degrees of freedom of the left half of the left middle connecting plate in X, Y and Z directions; S23. Restricting the degrees of freedom of the upper surface of the lower connecting plate in Z direction; S24. Applying low-frequency displacement load to the upper surface of the right half of the right middle connecting plate.

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