Portal de Periódicos da CAPES

Experimental investigation on safety and reliability of ball‐eye under bending load in electrical systems

2018; Institution of Engineering and Technology; Volume: 12; Issue: 15 Linguagem: Inglês

10.1049/iet-gtd.2017.1553

1751-8695

Zhanshan Xie, Yuan Zheng, Anni Wang, Qian Tian, Yuan Chen, Kan Kan, Yi Lu,

Vibration and Dynamic Analysis

IET Generation, Transmission & DistributionVolume 12, Issue 15 p. 3692-3698 Research ArticleFree Access Experimental investigation on safety and reliability of ball-eye under bending load in electrical systems Zhan-Shan Xie, Corresponding Author Zhan-Shan Xie xiezs2015@hhu.edu.cn College of Water Conservancy and Hydropower Engineering, Hohai University, Nanjing, 210098 People's Republic of China Department of Industrial Engineering, University of Padova, Padova, 35131 ItalySearch for more papers by this authorYuan Zheng, Yuan Zheng College of Water Conservancy and Hydropower Engineering, Hohai University, Nanjing, 210098 People's Republic of ChinaSearch for more papers by this authorAn-Ni Wang, An-Ni Wang State Grid Shenyang Electric Power Supply Company, Shenyang, 110811 People's Republic of ChinaSearch for more papers by this authorQian Tian, Qian Tian Anhui Science and Technology University, Bengbu, 233100 People's Republic of ChinaSearch for more papers by this authorYuan Chen, Yuan Chen North China Electric Power Research Institute Co. Ltd., Bingjing, 100045 People's Republic of ChinaSearch for more papers by this authorKan Kan, Kan Kan College of Water Conservancy and Hydropower Engineering, Hohai University, Nanjing, 210098 People's Republic of ChinaSearch for more papers by this authorYi Lu, Yi Lu North China Electric Power Research Institute Co. Ltd., Bingjing, 100045 People's Republic of ChinaSearch for more papers by this author Zhan-Shan Xie, Corresponding Author Zhan-Shan Xie xiezs2015@hhu.edu.cn College of Water Conservancy and Hydropower Engineering, Hohai University, Nanjing, 210098 People's Republic of China Department of Industrial Engineering, University of Padova, Padova, 35131 ItalySearch for more papers by this authorYuan Zheng, Yuan Zheng College of Water Conservancy and Hydropower Engineering, Hohai University, Nanjing, 210098 People's Republic of ChinaSearch for more papers by this authorAn-Ni Wang, An-Ni Wang State Grid Shenyang Electric Power Supply Company, Shenyang, 110811 People's Republic of ChinaSearch for more papers by this authorQian Tian, Qian Tian Anhui Science and Technology University, Bengbu, 233100 People's Republic of ChinaSearch for more papers by this authorYuan Chen, Yuan Chen North China Electric Power Research Institute Co. Ltd., Bingjing, 100045 People's Republic of ChinaSearch for more papers by this authorKan Kan, Kan Kan College of Water Conservancy and Hydropower Engineering, Hohai University, Nanjing, 210098 People's Republic of ChinaSearch for more papers by this authorYi Lu, Yi Lu North China Electric Power Research Institute Co. Ltd., Bingjing, 100045 People's Republic of ChinaSearch for more papers by this author First published: 19 June 2018 https://doi.org/10.1049/iet-gtd.2017.1553Citations: 2AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract In view of the frequent failure of the ball-eye (BE) in the transmission line, the force analysis of BE in extreme weather is carried out, and a set of bending fatigue test equipment was designed. It is the first time to carry out the fatigue test of BE treated by normalising, annealing and quenching and tempering process under bending loads of 2330, 3495 and 4560 N. Meanwhile, the bending fatigue life curve is plotted. From the macro and micro point of view, the characteristics and the content of the elements of the fracture, and the influence of the internal structure on the mechanical properties of three samples of different heat treatment processes are analysed. Conclusion indicates that the mechanical properties of the BE treated by normalising process are best, whose microstructure is fine and uniform 'ferrite + pearlite'. Comparing the life of BE withstood tension load, it is revealed that bending load is the main reason to accelerate the failure of BE. 1 Introduction The investigation on key components of transmission lines in the electrical systems and stranded steel wire by means of experimental and theoretical analysis have been reported [1–12], such as insulators, towers, overhead lines, but the research literature on the safety and reliability of the ball-eye (BE) have not been found. There is only one article written by the author of this paper about the mechanical properties of QP-16 type BE under the tensile load, in which the fatigue failure mechanism and life span of the OP-16 type BE are described in detail [12]. Recently, the BE (OP-16S) running on the lines caused a lot of fatal accidents from March 2002 to December 2012 (Fig. 1). Such as NingXia grid and Guangdong grid, Northeast power grid, Dong Liao 1 line 267 and 266# in China and so on. In order to find out the causes of the failure of the BE and improve the safety and reliability of the transmission line, force analysis, experimental research and performance analysis of BE will be carried out in the following contents. Fig. 1Open in figure viewerPowerPoint Fracture of BEs from the transmission line of China 2 Force analysis of the BE under extreme weather conditions 2.1 Evaluation of static load A 500 kV transmission line usually has an average span of 500 m, and its weight is carried by a double series of composite insulators. Therefore, the static load of a single BE is as follows: (1) In (1), Fstill is the static load of the BE, Mc is the mass of the conductor, MS is the mass of insulator strings. The safety factor of the ball head ring is as follows: (2) In (2), Fspecified load is the specified load of BE, Fspecified load = 160 kN. The power standard requires that the safety factor of the fittings is >2.5. Obviously, the result of the above calculation (2)shows that the BE can ensure the safety of the transmission line. 2.2 Wind load assessments of overhead lines under lateral wind When the wind speed is 25 m/s, the lateral load of quadripartion sub-conductor with the average span length of the 500 m can be evaluated by the following formula: (3) (4) In (3) and (4), the non-uniformity coefficient of wind pressure α = 0.75, wind pressure corresponding to the standard period W V, (kN/m2), v is the wind speed at a base height of 10 m, where V = 25 m/s. μ is the wind load figure coefficient of the wire, μ = 1.2. β is the coefficient of wind load adjustment for overhead lines, β = 1.1. d is the diameter of the wire, d = 0.036 m. B is the increase coefficient of the wind load when the conductor is covered by ice, B = 1. θ is the angle between the direction of the wind and the direction of the conductor, θ = 90°. 2.3 Limit resultant force of the BE (QP-16S) The resultant force of the BE under extreme weather conditions is as follows: (5) Safety factor of ball hanging ring under extreme weather conditions : (6) From the result of (6), obviously, under extreme weather conditions, the safety factor of the free rotating BE still meets the safety requirements of the power grid. 2.4 Evaluation of the bending load of the BE under the condition of non-free rotations This connection order and structure of BE on the power delivery systems can make the suspension insulator string swing along the direction of the transmission line (shown in Fig. 2). Although the load at the connection point is alternating and random, this structure can make the electric power fittings rotate freely and flexibly, and reduce the dynamic bending stress at the joint. However, there is no unlimit on the rotation angle of the foot of the BE in the insulator cap (Fig. 2c). The tilt angles of the BE in the nest are not allowed to exceed 9.5° (DS IEC 120-1987 and IEC120:1984 Third Edition, the connection size of Insulator cap, appendix A). Obviously, if the inclination angle is >9.5°, the ball neck of BE will be blocked at the exit of the cap, BE will produce bending stress. Moreover, this also makes the surface of the ball neck wear aggravated and fatigue failure of BE accelerated. Fig. 2Open in figure viewerPowerPoint Force analysis of the BE under extreme weather conditions The material of QP-16S type BE is No. 45 steel, whose diameter(d) is 21 mm, the tensile strength is 461.9 N/mm2, H the vertical distance of obstruction is 12.5 mm(Fig. 2e). Ball neck can bear the maximum bending load P : (7) In (7), Mmax is the maximum bending moment, N mm; W is a section of moment, mm3 ; Allowable stress of materials [σ] ≃ 0.4σb, N/mm2. The above formula is converted to (8) In fact, the lateral wind angle of suspension insulator is >35°, and the whole series of swing is not ideal, which is affected by each link constraints, the rotation of the BE in the insulator cap is often blocked (Figs. 2d and e). Moreover, if the tower is located in a windy area, when the wind speed reaches 30 m/s, for the transmission lines in the electrical systems whose average span is 500 m have quadripartion subconductors (LGJ-400/35 and LGJ-630/45), the wind load can be closed to 30–43 kN, whose magnitude is 2.2–3.14 times the upper limit of the bending load carried by the BE [10, 12]. 3 Introduction of test equipment and test parameters 3.1 Introduction to the bending fatigue test machine According to the characteristics of bending load of the BE on the power delivery systems, the bending fatigue testing machine (BFTM) is designed (as shown Fig. 3). The technical parameters of the system are shown in Table 1. Fig. 3Open in figure viewerPowerPoint Schematic diagram of bending fatigue testing machine and test load Table 1. Technical parameters of the BFTM Name Technical parameters Notes working voltage 380 V — impact frequency 5–40 Hz with adjustability range of impact force 500–5500 N with adjustability hydraulic clamping force >50,000 N — pressure sensor accuracy +1 kPa — control mode touch screen operation + PLC programme control — pressure of air supply 0.4–0.7 MPa — When the BFTM is working, the workpiece is clamped by hydraulic pressure, and the clamping force is >50 kN. Two high-temperature cylinders whose diameter is 100 mm were used as impact elements, its length is adjustable. Different impact forces can be achieved by adjusting intake pressure, precision pressure sensors (Fig. 3b), and energy storage tanks are employed to ensure accurate and stable output pressure. The pressure relief valve is located on the left side of the green light gas tank, its main function is to monitor and control the air pressure. During actual operation of the fatigue testing machine, the lower limit of the pressure of the locking device is 11.5 MPa, and the upper limit is 12 MPa. When the locking pressure reaches the lower limit, the system will automatically rise. Such as the loads were applied on foot of BE, which are 100, 300, 500 MPa, the floating error of impact load is 197 HB. 4 Analysis and discussion of experimental results 4.1 Macroscopic analysis of fatigue fracture The macroscopic morphology of annealed sample A is shown in Figs. 4a–d. The AP should strictly control the heating rate, holding and cooling time. Whose purpose is to eliminate the defects and residual stresses of the specimen, and refine the grain to improve the mechanical properties of the specimen. Fig. 4Open in figure viewerPowerPoint Macro morphology and micro morphology of failure of BE Figs. 4c and d show the fractograph of the specimen A, whose edge has a smaller area of the semi-circular zone, and the colour of which is dark and dull, the region is a fatigue zone. In the other zones there is no obvious plastic deformation, but there are a lot of shiny small facets, surfaces of the zone are roughness, which is the final fracture zone of brittle fracture. With the help of stereoscopic microscope, the fatigue zone of fracture is observed, it is found that the distance between fatigue cracks in that zone is much larger, and there is obvious rust mark between fatigue cracks, and the surface of the whole fatigue zone is relatively rough. As shown in Figs. 4a–d, under the action of alternating bending load (3945 N, r = −1), the surface quality of the galvanised layer of the whole specimen is uneven and poor, and the peeling phenomenon of the zinc coating from the ball foot to the ring is more serious, and in that zone there are different degrees of cracking, the cracking direction is parallel to the cross section of the rod. The tiny cracks and surface defects will be the source of the fatigue crack. Fig. 4e shows the macroscopic morphology of the specimen B treated by the condition of QTP, and this heat treatment process (HTP) is a combination of quenching process and tempering process. The quenching process refers to a process in which the specimen is heated, heat preservation and then rapidly cooled in a quenching medium. This process will increase the hardness of the specimen and the brittleness of the material. However, after quenching process, the internal stress and material brittleness and the hardness, strength and toughness of these specimens can be greatly reduced by tempering process. Compared with NP and AP, specimen B treated by QTP, whose hardness is still high and the brittleness is still very large. Under the alternating load of 3945 N, the specimen B first broke. The fracture of the specimen is flat, without obvious macroscopic plastic deformation, which shows that the brittleness of the specimen is larger. In addition, the fracture has obvious macroscopic fatigue stripe, such as cowrie pattern, and the appearance colour of the fracture is dark and dull, and the surface of the fatigue area is uneven. The area of fatigue zone is smaller than that of the total cross-sectional area, which accounts for <10%. The area of the instantaneous fracture zone is larger, and has obvious tearing and brittle fracture. The above failure characteristics reflect brittleness of the specimen B. The macroscopic morphology of specimen C fracture is shown in Figs. 4f and g, whose HTP is the normalising process. The normalising process can eliminate the coarse grain structure and Widmannstatten structure caused by overheating in castings, forgings and weldments, and make the pearlite refine, not only improve the mechanical properties but also conducive to the subsequent spheroidising annealing. The fracture of specimen C was characterised by multi source characteristics, having two obvious fatigue sources and obvious cowrie pattern, showing a typical fatigue fracture morphology, which are consistent with the characteristics of bi-directional bending fatigue failure. The extensive zone of the fracture is smaller, smooth and delicate, the colour and luster are black and dull, respectively. The surface of cross section is rough and granular, and it has obvious rust, final fracture region has obvious rust [12]. The research conclusions show that most of the samples are more or less defects, and these defects often become the source of fatigue, so the location and number of fatigue sources are different depending on the location and number of defects. Since the ring part of sample C is clamped by hanging plate and foot of BE is constrained by the ball wrist, the BE cannot swing freely along the vertical direction of the transmission line, so that there is a more obvious wear mark on the side of the ring. However, the crack source of fatigue failure appears to the opposite side of the wear surface, which reflects that the specimen C may bear the effect of long lateral force. If the ball foot of specimen C is subjected to cyclic bending stress for a long time, fatigue failure is inevitable. 4.2 Microscopic analysis of microstructure The mechanical properties of the metal depend on its chemical composition and microstructure, in which the microstructure is the intrinsic factor in determining the macroscopic mechanical properties of the metal and plays a decisive role in the metal properties. Therefore, it is necessary to analyse the microstructure of the BE. The microstructures of the annealed sample A are pearlite and a small amount of ferrite (Figs. 5a and b), but the distribution of these two microstructures is uneven, meanwhile, there are many abnormal growth phenomena of grain. From the metallographic structure analysis, the mechanical properties of the annealed specimens are not as good as those of normalising process, because its ability to resist cracking is slightly worse. The fracture of sample A is observed under microscopes, it is found that there are a lot of small cracks in the galvanised layer on the edge of the fracture. Some of these cracks extend into the matrix (Fig. 5a). In addition, the inclusions are found at the source of the crack. These impurities and cracks will lead to the stress concentration in the region. If the surface crack extends simultaneously to the substrate, which will lead to the generation of the fatigue source. Fig. 5Open in figure viewerPowerPoint Microstructures and test life of specimens (a) Microstructures of the edges of sample A, (b) Microstructures in the centre of the fracture of sample A, (c) Microstructures of cross section of sample B (×500), (d) Microstructures of cross section of sample B (×200), (e) Microstructures of cross section from sample C, (f) Microstructures of the edges of sample C, (g) Test life of the BE The microstructures of specimen B fracture are reticular ferrite and sorbite, but this microstructure is coarse and the grains are not uniform, and the sorbite structure is also appeared (Figs. 5c and d). The microstructure of the whole section is close to the microstructure of No. 45 steel treated by QTP in the other two samples (C1, C2), bainite structure was even found in the fracture, which shows that microstructure obtained by this HTP is not ideal, there is a difference between the ideal NP and the NP. Network-distributed ferrite is not only low in strength but also has a discrete effect on the continuity of the microstructure, which easily causes the crack to expand along the reticular ferrite. The appearance of the network-distributed ferrite in the matrix material will not only reduce the fatigue limit and impact toughness of the BE, and will result in a decrease in the ability to withstand alternating stresses and abrupt loads. In addition, the existence of the mixed structure of the coarse reticular ferrite and sorbite not only reduce the fatigue limit and impact toughness of the specimen but also easily lead to micro-crack initiation and expansion. Meanwhile, this kind of microstructure will reduce the overall mechanical properties of the material, so that the strength of the specimen itself is relatively high, but the toughness is reduced, the impact resistance is poor, and the ability to resist crack propagation is poor. However, in the above non-normalising BE specimens, reticular ferrite and sorbite were found, these organisations had a serious impact on the mechanical properties of the BE, which indicates that there are some problems of QTP of BE, which affects the internal structure of the BE, and increase its brittleness, reduces the mechanical properties of materials. Compared with the AP, the microstructure of normalising specimens is uniform, which are ferrite + pearlite (Figs. 5e and f), and the ratio of these two tissues is close to 1:1. In Figs. 5e and f, the white tissue is ferrite, and the black structure is pearlite. From the analysis of the distribution of ferrite, ferrite is distributed in the grain boundary grows into the crystal and forms a needle like ferrite with a certain orientation. If the ferrite is too much, the strength must be lower, but the toughness is good, when subjected to fatigue loads, the ability to absorb energy is stronger. As for pearlite, it is a composite microstructure, in essence, it is composed of ferrite and cementite structure. About the cementite structure, because of its high carbon content, on the one hand it helps to improve the strength of the specimen, on the other hand, it will reduce the plasticity and toughness of the specimen accordingly. No.45 steel is hypoeutectoid steel, and in the hypoeutectoid steel, pearlite and ferrite each accounted for 50% of the homogeneous tissue, so the tissue is more uniform. Therefore, from the applicable environment of the BE, the tissue produced by the NP is more favourable for the specimen to bear alternating stress and fatigue load. Thus, it is known from the above analysis that: normalising HTP can help to refine the microstructure of specimens and improve mechanical properties of specimens, but for carbon content <0.5% forging, commonly used NP instead of AP. The AP helps to reduce the hardness and improve the plasticity of the specimen. Quenching process helps to improve hardness and increase wear resistance of specimens. The fatigue lives of three different HTP specimens are shown in Fig. 5g. It can be observed in Fig. 5g that the fatigue resistance of NP is better. 4.3 Scanning electron microscopy analysis of fatigue fracture 4.3.1 Microscopic image analysis Back scattered electron imaging is an electronic imaging technology based on SEM. Back-scattered electrons are the primary electrons reflected by the sample, and the higher the atomic number of the sample, the brighter the contrast of the electron microscopic image. Otherwise, the image is dark. As can be seen from Figs. 6a–d, the contrasting area of the fatigue zone is dark, indicating that there is a certain amount of low atomic number (lower than Fe) in the region, such as common elements AL, O, P, Mg, Al, C, Si, Ca and so on. Fig. 6Open in figure viewerPowerPoint Microstructure under SEM (a) Microstructures of the fatigue source region, (b) Microstructures of the fatigue zone, (c) Electron backscattered image of the fracture source region, (d) Secondary electron image of the fatigue zone, (e) Fracture morphology of final zone, (f) Instantaneous fracture zone with fan-shaped river pattern, (g) Fracture morphology of final zone The microstructures of the fatigue source region of sample A are shown in Fig. 6a, a small amount of micro cracks can be seen at the bottom of some granular materials. Fig. 6b shows the microstructure of the fatigue zone of sample B, where the white highlighted zone is a non-metallic substance with poor conductivity and microcracks are present in the matrix of the fatigue extension zone. Fig. 6d shows the secondary electron image of the fatigue zone of the specimen C, and Fig. 6d also shows that there are some white dots in the fatigue inner loop, the fatigue extension zone is rough and has a radial step, the fracture has the characteristics of high cycle fatigue. Figs. 6e–g show the scanning electron microscope image of the final fault zone. As shown in Fig. 6e, the sample A has the characteristics of intergranular fracture. As shown in Fig. 6f, the instantaneous fracture zone of the sample B is a fan-shaped river pattern, which is a typical cleavage feature, and there are more secondary cracks on the cleavage fracture surface. Intergranular fracture and cleavage fracture are typical brittle fracture features. Fig. 6g shows that the fracture morphology of the sample C is brittle transgranular cleavage fracture, and there are a few intergranular cracks, which indicates that the fatigue zone is subjected to greater stress during the expansion process, and the crack propagation process is a kind of brittle crack. 4.3.2 Energy dispersive spectrometer analysis The fracture of all specimens were cleaned with acetone in an ultrasonic way and the microstructure of the fracture surface were observed near the fatigue source under scanning electron microscopy and analysed by energy dispersive spectrometer (EDS). For the sample A, white-dotted substance in the fracture source zone was analysed by EDS (Fig. 7a). Besides the Fe and Zn elements, it is found that these white-dotted substance contain a higher percentage of Si, Al and O elements, and the composition of the white-dotted substance is close to the composition of the gravel particles, and the percentage of element content in the fracture inclusions is shown in Fig. 7a. Fig. 7Open in figure viewerPowerPoint Percentage of element content of sample (a) EDS analysis of the inclusions from fatigue source of sample A, (b) EDS analysis of the inclusions from fatigue source of sample B, (c) EDS analysis of the inclusions from fatigue source of sample C For the sample B, it is found that the crack source contains not only Fe element but also the high proportion of O and Zn elements. However, the batch of BE material does not contain these two elements. The two elements should be brought into in the process of hot galvanising. This indicates that the defect of the specimen B is present before the hot dip galvanising, which is the original defect of the BE. During the course of the BE service, it is easy to form the crack source. In addition, the crack source also contains a small amount of Cl, Al, Mg, S, K, Si, and other elements, and the percentage of element content in the fracture inclusions is shown in Fig. 7b. As can be seen from the results of the EDS analysis (Fig. 7b), there are more impurities in the fatigue zone, which may be due to the fact that the BE is in a harsh environment, the additional lateral stress causes the BE to withstand the cyclic bending stress, the environmental factors lead to gravel pile up in the cracks. Meanwhile, the corrosive medium penetrates into the crack, thus accelerating the process of fatigue crack propagation. For the sample C, a radial pattern of the cross section was observed near the fatigue source under SEM, it is found that many impurities of the fracture not only contains Fe, Zn elements but also contain P, S, K, Ca, Si, Al and other elements, and the percentage of element content in the fracture inclusions is shown in Fig. 7c. These elements may be corrosion products formed by environmental corrosion factors. Supplementary specification : When the high-speed electrons collide with the material, the inner electrons of the material form vacancies, the outer electrons will radiate X-rays when they move to the vacancy. The wavelengths of X-rays of different materials are different, so they are called characteristic X-rays. The characteristic X-rays can be divided into K, L and M lines, which correspond to the radiations produced by the transition from outer electrons to the vacancy at the K, L and M layers, respectively. In Fig. 7, the K letter after the element represents the K-line of the characteristic X-ray. When the atomic number of the analysed element is <32, the K-line system is adopted. 4.4 Microscopic hardness analysis of BE According to the standard: 'GB/T 699-1999 improved carbon structural steel' and 'DL/T 439-2006 technical guidelines for high-temperature fasteners in thermal power plants', No. 45 steel treated by hot rolling or hot forging is directly supplied, whose the Brinell hardness value should be ≤229, the Brinell hardness value of No. 45 steel treated by the AP should be ≤197. The hardness index of No. 45 steel treated by high-temperature tempering and normalising shall be determined by the suppliers and customers. Indeed, as described in the above occupation standard, the hardness of the specimen is first met with the occupation standard, and it should be determined according to the requirements of both the supplier and the supplier, especially according to the working environment of the specimen. The hardness value of the specimen was tested by HBW-3000, which is shown in Fig. 8. Fig. 8Open in figure viewerPowerPoint Test results of Brinell hardness As can be seen from Fig. 8 that the hardness value of the No. 45 steel treated by the NP is slightly higher than that of the No. 45 steel treated by the AP, but it is lower than that of the No. 45 steel treated by QTP. If the hardness is too large, the specimen has the characteristics of strong brittleness and poor toughness, and the micro cracks are easy to generate and expand. Finally, the crack propagation results in the fatigue failure of the BE. If the hardness of the specimen is too large, it has the characteristics of strong brittleness and poor toughness. It is clear that the micro cracks of the specimen are easy to be formed and expanded, and leading to the breakage of the hanging ring. If the hardness is too large, the specimen has the characteristics of strong brittleness and poor toughness, and the micro cracks are easy to generate and expand. 4.5 Analysis of the results of the BE under two kinds of fatigue test methods Although the diameter of the ring of QP-16S is smaller than that of QP-16, that is the diameter of the ring is smaller than 1 mm, and the difference is only 1 mm, the size of other parts and technology are the same. In addition, the ring part of the ball head ring is fixed completely in the test, so the ring size does not affect the test results. Therefore, it is appropriate to compare the life of the QP-16S under the bending load with the life of the QP-16 under the tensile load. The fatigue test results of the BE under tensile and bending loads are shown in Fig. 9. In which, the experimental results of the BE treated by NP under tensile load are obtained from the article [12]. By comparison, under the action of the same stress cycle (2330, 3495 and 4560 N), the life of normal heat treatment specimens under bending load is 0.35, 0.76, 0.93 times of that of the same HTP under tensile load. Fig. 9Open in figure viewerPowerPoint Fatigue life of specimen As can be seen from Fig. 9, under the same stress level, the fatigue life of the BE is shorter than that of the BE under tensile load. In fact, the overhead line will vibrate under the action of wind, considering the safety coefficient of the BE, taking the load of 30 KN as an example, the stress which is the BE bearing the stress is 1010 N, which does not reach the stress amplitude in the test, and the stress ratio is also less than the stress ratio in the test. Therefore, tensile force is not the main factor. In addition, the wear on both sides of the BE is more serious, which also shows the effect of the bending load on the failure of the BE. Therefore, the bending stress is the main factor causing the hanging ring fracture in the actual operation. 5 Conclusions Compared with the mechanical properties of the BE of the other two processes, the BE with the NP has uniform internal structure, little hardness, and the best character of resistance to fatigue. Under the same stress amplitude, the life of the normalised specimen under bending load is 0.35–0.93 times of that under the tensile load, which indicates that the bending stress is the main factor in the fatigue fracture of BE. Before the BE is hung up the power grid, it is necessary to pay attention to whether the HTP of the BE meets the requirements of the power standard. Meanwhile, for the transmission line in the electrical systems which runs in the region where extreme weather occurs frequently should increase the frequency of the BE sampling, especially concerned whether the wear phenomenon occurs in the side of the BE. 6 Acknowledgments The authors of the paper express appreciation for the support from China Scholarship Council (grant no. 201606710045), the Fundamental Research Funds for the Central Universities (grant no. 2016B41914) and National Natural Science Foundation of China (grant no. 51579080), School grade research project of Anhui Science and Technology University (Grant No.SRC2016425), North China Electric Power Research Institute and the Institute of Metal Research Institute and high voltage Research Institute. 7 References 1Kumosa, L.S., Kumosa, M.S., Armentrout, D.L.: 'Resistance to brittle fracture of glass reinforced polymer composites used in composite (nonceramic) insulators', IEEE. T. Power. Deliv., 2005, 20, (4), pp. 2657– 2666 2Onur, Y.A., Imrak, C.E., Onur, T.Ӧ.: 'Investigation on bending over sheave fatigue life determination of rotation resistant steel wire rope', Exp. Techn., 2017, 41, (3), pp. 1– 8 3Kopsidas, K.: 'Assessing the power transfer performance of a lattice tower overhead line system', IET Gener. Transm. Dis., 2013, 7, (1), pp. 90– 100 4Othman, N.A., Piah, M.A.M., Adzis, Z. et al.: 'Space charge distribution and leakage current pulses for contaminated glass insulator strings in power transmission lines', IET Gener. Transm. Dis., 2017, 11, (4), pp. 876– 882 5Onur, Y.A.: 'Experimental and theoretical investigation of prestressing steel strand subjected to tensile load', Int. J. Mech. Sci.., 2016, 118, pp. 91– 100 6Karabay, S., Feyzullahoğlu, E.: 'Determination of early failure sources and mechanisms for Al 99.7% and Al-Mg-Si alloy bare conductors used in aerial transmission lines', Eng. Fail. Anal., 2014, 38, pp. 1– 15 7Onur, Y.A., İmrak, C.E.: 'Discard fatigue life of stranded steel wire rope subjected to bending over sheave fatigue', Mech. Ind., 2017, 18, (2), p. 223 8Mahdhi, N., Askri, B., Raouadi, K. et al.: 'Study of dielectric properties of composite insulators for use in medium voltage overhead lines', J. Electrostat, 2013, 71, pp. 892– 897 9Klinger, C., Mehdianpour, M., Klingbeil, D.: 'Failure analysis on collapsed towers of overhead electrical lines in the region Münsterland (Germany) 2005', Eng. Fail. Anal., 2011, 18, pp. 1873– 1883 10Xie, Z.S., Wang, Z.Q., Chen, Y. et al.: 'Analysis of damage mechanism for crimped composite insulators', Diangong Jishu Xuebao/Trans. China Electrotech. Soc., 2014, 29, (6), pp. 296– 302 11Prenleloup, A., Gmür, T., Botsis, J. et al.: 'Stress and failure analysis of crimped metal–composite joints used in electrical insulators subjected to bending', Compos. Part A Appl. S, 2009, 40, (5), pp. 644– 652 12Xie, Z., Zheng, Y., Wang, A.N. et al.: 'Experimental study on the fatigue failure mechanism of QP-16 type ball-eye under asymmetric load', Eng. Fail. Anal., 2016, 66, pp. 141– 153 Citing Literature Volume12, Issue15August 2018Pages 3692-3698 FiguresReferencesRelatedInformation