Analysis and experimental study on the cause of fracture of a Transmission line’s Drainage connecting plate

The transmission line is exposed to the outdoors for a long time, and the operation environment is complex, which makes the line equipment prone to corrosion, wore and heat, etc. The fracture of the drainage plate of the tensioning wire is one of the problems that often occur in the operation of the transmission line. Based on the systematic analysis of the problem of the fracture of the drainage plate of a transmission line, the causes of the fracture of the drainage plate are obtained, and the corresponding rectification measures are proposed, which has an important reference for the design, construction, operation and maintenance of the subsequent transmission line.


Introduction
With the rapid development of the national economy, the power load continues to grow and reach a new high, especially during the summer peak, some of the main lines of the west-east power transmission appear to be overheated or broken in different degrees, and there is a trend of increasing year by year.The drainage plate has become a major hidden danger affecting the safe and stable operation of overhead transmission lines [1][2][3][4].If the defect is not eliminated in time, it will directly affect the transmission quality of electric energy, resulting in power loss in light cases, and large-scale power outage accidents caused by line clamp disconnection [5][6].Taking a transmission cracking event in northwest China as an example, this paper analyzes the reasons for the breakage of the hoop and puts forward improvement measures by means of appearance inspection, material analysis and scanning electron microscopy, etc., so as to provide scientific basis for the investigation and treatment of similar problems in an all-round way.

2.1.Macroscopic inspection and structural dimension measurement
The samples of the drainage connection plate had three phases: A, B and C, and the fracture occurred in phase B.The drainage connection plate is divided into two sides.One side of the wire clamp with four bolt holes is marked as A-phase 1, B-phase 1, and C-phase 1, respectively.The side of the arrester with two bolt holes is marked as A-phase 2, B-phase 2, and C-phase 2, respectively.The thickness of three drainage connecting plates was measured respectively.The thickness of the two ends of the A-phase drainage connecting plate is 10.25mm and 9.99mm respectively, and the thickness near the bending part is significantly reduced.The thickness of the plate at the bending part is about 8.97mm, and the nearby material has plastic deformation.The right angles on both sides of the bending point are 88.29mm and 81.90mm, respectively.The specific measurement data are shown in Figure 1.

Figure 1.Side diagram of the A-phase drainage connection plate
The overall width of both sides of phase A is about 60mm, and there are four bolt holes on the front side of phase A-1.The distance between the bolt holes and the left and right ends of the drainage connection plate is about 12mm, but the distance between the upper and lower ends is uneven, and the distance between the upper and lower ends is between 2.53-3.95mm.The transverse distance between the threaded holes is about 26mm and the longitudinal distance is 30mm.There are two bolt holes on each side of phase A-2.The distance between the bolt holes and the left and right ends of the drainage connection plate is about 14mm, the distance between the upper and lower ends is about 24mm, and the transverse distance between the bolt holes is about 26mm.In the figure, it can be clearly seen that there is a dark indentation on both sides of the bending break point of the drainage connection plate, which is judged to be the location of the two support rollers during bending.It can be seen that the shape of the support roll is sharp, and the relative displacement with the surface of the plate body occurs during the bending process, resulting in mechanical damage at the support roll of the drainage connecting plate.As shown in Figure 2, there is a triangular groove in the inner bending bend, and the bottom is sharp, which is caused by the small bending radius of the indenter of the bending die.The B-phase drainage connection plate is broken into two pieces from the middle bend, and the side lengths are 76.25mm and 76.61mm, respectively.The thickness near the two ends of the connecting plate is about 6.6mm, and the thickness near the bending point gradually decreases, and the thinnest point is near the bending point.The thickness on both sides of the section is 6.43mm and 6.45mm, respectively, and the material has obvious plastic deformation.The specific measurement data is shown in Figure 2.

Figure 2.Side diagram of the B-phase drainage connection plate
A black area can be seen near the edge of the fracture at the inner bend, and the surface oxidation is serious, which is the original cracking position.In the figure, the shell grain generated by the crack fatigue expansion processcan be clearly seen on the crack surface, and the shell grain on the left side of the section is wider, and the right side is narrower, which proves that the alternating stress amplitude on both sides is different.There are obvious shear lips on the edge of the outer curved fracture, and there are small metal facets on the surface of the transient fracture area near the shear lip, showing cleavage fracture characteristics.The thickness near the ends of both sides of the C-phase drainage connecting plate is 6.62mm, and the thinnest part near the bending part is 6.37mm.There is obvious plastic deformation near the bending point, and a small crack is visible to the naked eye on the side of the bending point of the sample, with a length of about 2-3mm.The side lengths of both sides of the deflection point are 77.49mm and 74.11mm respectively, and the specific measurement data are shown in Figure 3.

Figure 3.Side diagram of the C-phase drainage connection plate
In the figure, it can be clearly seen that there is a dark indentation on both sides of the bending bend point, which is also the mechanical damage caused by the support roll in the bending process.As shown in Figure 3, there is a transverse crack at the bottom of the inner bend at the bend, because the indenter of the bending die is sharp and the bending radius is small, resulting in a sharp groove in the bending part of the drainage connection plate during the bending process.The stress during installation and operation forms a stress concentration at the bottom of the groove.A transverse crack is generated along the groove and expands in the depth direction during its operation.

Spectral Analysis
The components of A, B and C three phase drainage connecting plate samples were analyzed respectively, and the content of aluminum elements in the three samples exceeded 99.5%.The test results are shown in Table 1.The partial B-phase fracture was captured for SEM micro-morphology analysis, and its micro-morphology is shown in Figure 4, where Figure 4 (a) is a low-magnification image of the fracture.It can be seen that there are obvious fatigue shell patterns in the fatigue expansion area of the fracture, and the fatigue expansion area accounts for about half of the entire fracture.There are obvious river-like patterns and tearing steps in the instantaneous fracture area, which have the characteristics of cleavage fracture.Cut lip protrusion, the appearance is not clear due to depth of field.After enlarging the fatigue expansion area, it can be seen that the distance between the fatigue shell patterns changes from narrow to wide, indicating that the amplitude of alternating stress at the crack tip changes from small to large and the fatigue expansion rate changes from slow to fast with the increase of crack fatigue expansion depth.

2.3.SEM analysis
The energy spectrum analysis was carried out at the crack initiation location, fatigue extension area and instantaneous fracture area, and the results were shown in Table 2.It can be seen that the oxygen content of the three parts decreased gradually with the increase of the depth of the crack fracture surface from the inner curved surface.Due to the long exposure time in the air, the oxygen content in the crack initiation area is the highest, followed by the expansion area, and the oxygen content in the transient break area is the least.

2.4.Simulation Analysis
In order to better understand the cause of the fracture of the drainage plate, mechanical simulation analysis of the drainage plate was performed.The unbroken C-phase drainage connecting plate with original defects was selected for simulation analysis, as shown in Figure 5.The simulation model was established and the grid was divided.The mesh division near the bolt holes is finer, and the mesh is further subdivided for the cracks caused by the bending process, as shown in Figure 6.
The static analysis of the C-phase drainage connecting plate shows that when the C-phase 2 is fixed on the side and the lateral end of the C-phase 1 is strained in the horizontal bending direction, the stress concentration of the C-phase 1 exists in the bottom bolt hole and the internal bending crack, and the stress concentration is particularly prominent at the internal  This model simulates the stress and deformation trend of the drainage plate under the horizontal pull of the drainage wire when the drainage clip is fixed at the top of the arrester.If a vertical upward pull is applied at the same time, the trend is more pronounced.Therefore, the cause of the crack in the stress concentration area of the inner bend of the drainage plate can be accurately determined.

Comprehensive analysis
It can be seen from the macroscopic inspection that the pressure head of the bending die is sharp and the bending radius is small, resulting in sharp grooves in the bending part of the drainage connection plate during the bending process.The stress during installation and operation forms stress concentration at the bottom of the grooves, and transverse cracks are generated along the grooves during the long running process and expand to the depth direction.
It can be seen from the microscopic examination that the distance between the fatigue shell patterns changes from narrow to wide, which indicates that the amplitude of alternating stress at the crack tip changes from small to large and the fatigue growth rate changes from slow to fast with the increase of crack fatigue growth depth.The simulation of the drainage plate shows that the stress concentration in the inner bend of the drainage plate is the main reason for the crack.

Conclusion
Due to the sharp indenter and support roll of the crimping grinding tool and the small bending radius, the bending process leads to local plastic deformation and mechanical damage of the drainage connecting plate, and a sharp groove at the bottom of the triangle is formed at the inner bend.The thickness of B and C two-phase drainage plates is smaller than that of A phase, the bearing cross section is smaller, and the local stress and stress concentration are more obvious.
Because the drainage wire is not connected to the top of the arrester vertically, there is a certain sag of the drainage wire, which leads the drainage wire to form an oblique traction force on the drainage connection plate, which causes the Angle of the two right Angle sides of the drainage plate to increase, and the tensile stress and stress concentration are generated from the bottom of the sharp groove in the inner bend.The alternating stress caused by wind

Figure 4 .
Figure 4.SEM image of fracture of B-phase drainage connecting plate

Figure 5 .Figure 6 .
Figure 5.Grid distribution of C-phase drainage connecting plate

Figure 7 .
Figure 7.Static analysis of the original defect of the C-phase drainage connecting plate