Fracture Failure Analysis And Finite Element Simulation Optimization Of Iron Accessories In Power Grid

With the continuous increase of China's total electricity production and consumption,higher requirements are put forward for the stability of the transmission grid.As the most important accessory of the transmission network,the stability of the power tie hoop is the key factor for the security of the whole grid.Aiming at the fracture failure of hoops in the cold region of Western China,the samples were selected as the research objects in the same batch.Fracture morphology analysis,chemical composition analysis,metallographic structure analysis,tensile test at room temperature,hardness test,impact test,bending(normal temperature and low temperature)test and in-situ tensile test were carried out.The reasons for the failure of the hoop under the special conditions in Northwest China are preliminarily explored.At the same time,the finite element analysis of the hoop is carried out,and two optimization measures of the hoop structure are put forward.Finally,the hoop structure after optimization is compared and analyzed.The research results can provide some theoretical references for the design and production of power cable hoops,and provide some theoretical guidance for the safe service of power cable hoops.The main research work in this paper is as follows:(1)Choosing a batch of hoops that have failed in a western power grid as the research object,the appearance size,fracture morphology,chemical composition and metallographic structure of hoops are analyzed,and the tensile test,hardness test,impact test,bending test and in-situ tensile test are also carried out.The shape and size of the hoop meet the requirements of relevant standards,but there are many micro-holes around the cracks in the cross-section.These micro-holes destroy the continuity of the hoop structure(mainly composed of pearlite and ferrite),and increase the possibility of brittle fracture of the hoop.In addition,secondary cracks occur during the fracture process,and the fracture surface is characterized by brittle fracture.The analysis of chemical composition shows that C,P and S elements in some hoop specimens exceed the standard.These elements can cause segregation and shrinkage in the material,which further indicates that the fracture is brittle fracture.ensile test results at room temperature show that the tensile strength and yield strength of hoop are better than those of Q235 D steel,but its section shrinkage and section elongation change sharply,and the extreme values are quite different.The elongation after fracture of individual specimens is lower than the standardrequirements.At the same time,there are leaks and surface corrosion in the galvanized layer of the hoop,and the chemical composition of the material and some defects in the process of processing,which further promote the fracture of the material.Hardness test shows that the hardness of the hoop meets the material requirements of Q235 D.Hoop impact test shows that part of the specimens are not qualified,most of the specimens are not qualified,and all of the specimens are not qualified,mainly because there are many holes and segregation in this batch of materials.In the bending test,the hoop performance under normal temperature meets the material requirements of Q235.In the low temperature environment,the galvanized coating cracked and the hoop did not meet the requirements.Finally,the in-situ tensile test of the hoop specimens shows that the microcracks first occur in the lamellar pearlite.With the increase of the load,the preferential microvoids in the pearlite are connected into one piece in turn,and the microcracks occur during the continuous loading process.When the hoop is bent at room temperature,the grain arrangement and structure inside the material are changed,which plays a certain role in the surface hardening of the hoop.At the same time,the work absorbed by the hoop during service is reduced,which indicates that the bending process of the hoop in the production process needs further improvement.(2)Finite element static analysis and high cycle fatigue simulation analysis of the hoop in service show that under static load,the maximum stress and strain value distribute in the screw hole and bending part of the hoop ear,and the arc surface is relatively small;the maximum stress at the screw hole is 76 MPa,the maximum strain value is 0.0033 mm,and the maximum stress at the bending part is relatively small.The maximum strain is 0.0001 mm with 32 MPa.The main deformation areas are located in the arc bending part and the bending part.The maximum deformation of the arc bending part is 0.015 mm and the maximum deformation of the bending part is0.012 mm.Fatigue analysis shows that the hoop can fully meet the design life requirements under 106 cycles of cyclic loading.However,the safety factor at the screw hole and bending is lower than that at other parts,which is a vulnerable position,and the strength of this area needs to be further increased.(3)Two optimization measures of hoop structure,reinforced hoop and surface strengthening hoop are put forward.The stress and strain distributions of the two optimized hoop structures are the same as those of the conventional hoops in service.However,through data comparison,it is found that the maximum stress and strain values of the surface of the reinforced hoop and the surface reinforced hoop are lessthan those of the conventional hoop in service.Therefore,the two hoop models after structural optimization have higher strength and stronger deformation resistance.Through deformation analysis,it is found that the deformation of the two optimized hoops at bending(fragile part)and arc middle section is less than that of the conventional hoops in service.High cycle fatigue analysis shows that the two optimized hoop structures can effectively improve the safety factor.For stiffened hoop,fatigue damage of the hoop transfers from the bending surface to the stiffener after loading,so that the working part of the hoop can be effectively protected by the stiffener.For the Surface-strengthened hoop,the effective surface area and hardened layer of the material are increased by surface rolling and hardening treatment,which can further improve the fatigue resistance of the bending part,and have higher strength and fatigue life than the conventional hoop in service.In addition,under cyclic loading,the cycle times of reinforcing rib hoop and surface reinforcing hoop can reach 106 times of design life.Therefore,the mechanical structure optimization and surface deformation hardening treatment of the hoop are helpful to improve the strength and fatigue resistance of the hoop.The improved hoop has a strong resistance to external loads and can effectively reduce the stress and deformation of the hoop.