| The high-speed railway(Abbreviated as HSR)is widely constructed and used around the world due to the advantage of high speed,convenience,comfort and safety.The catenary system is not only the medium to transmit electric energy for the vehicle,but also mechanically bears the slide-way of the pantograph,as well as suffers from the excitations of the pantograph.The droppers,as a key component of the electrified catenary system,are arranged between the loading cable and contact wire and played an important role in connecting and transferring loads.The loads of droppers mainly come from the gravity of contact wires as well as the catenary vibration caused by the pantograph passing or environmental factors.Due to the server operation environment of the HSR catenary,the fatigue failure of the dropper structure seriously affects the running safety of the HSR.This thesis focuses on the issues of the uncertainty of the fatigue failure mechanism and the difficulty in the fatigue life prediction of the droppers.Firstly,the failure problem of the droppers in service and the response between the pantograph-catenary systems are investigated.Secondly,it is preliminarily deduced that the impact loads exist in the pantograph-catenary system.Finally,the research on the impact fatigue failure mechanism and life prediction of the droppers is carried out as below:(1)Research on the impact load of dropper based on the pantograph-catenary dynamics.Based on theoretical analyses and field investigations,it was found that the dropper suffered relatively large transient impact loads after the catenary was swept by the pantograph.As the actual loads of the droppers are difficult to be measured,the compressed values of the droppers are calculated by the pantograph-catenary dynamic model.Taking the compressed values of the droppers as input,the impact tests of the droppers were carried out under the effect of the concentrated mass.The impact loads of droppers with different masses and different compression inputs were obtained,and can be further used as the load spectrum for the subsequent impact fatigue test.(2)The method of impact fatigue tests of droppers based on the electromagnetic vibration rig was proposed.It was found from test that the dropper response was relevant to the loading rate,and the droppers beard a large impact load during services.Therefore,it is necessary to study the fatigue performance of the dropper under impact loads.Through the analyses of the fatigue test standards and the existing test methods of droppers,it was found that the dynamic performance of the droppers was not considered in the standards and the existing fatigue test methods,which may lead to inaccurate test results.Thus,the impact fatigue test method of dropper based on the electromagnetic vibration rig is proposed in this work.The impact loads of droppers can be accurately simulated and makes up for the deficiency of the existing test methods.(3)Analysis of the fatigue failure mechanism for droppers under impacts.According to the proposed test method,the impact fatigue tests for droppers with tooth-shaped crimping and elliptical shaped crimping types were carried out respectively.The failure mechanism of different positions was obtained by analysing the damage morphology of the failure structure.For the droppers with tooth-shaped crimping,the stress concentration of the dropper lines at the crimping position was relatively larger,which leads to cracks in the contact position on the surface of the copper alloy wire,and even caused the structural crakes.Besides,the heart-shaped rings of tooth-shaped crimping droppers were large,and the contact between the hanging ring and the heart-shaped ring is relatively centered.The stress distribution of the lifting ring is well-distributed and the lifting ring was not easy to be damaged.Different from the elliptical crimped dropper,the heart-shaped ring is small and the pull of the current-carrying ring makes the contact position is located at one side of the hanging ring.The outer edge of the ring bears a larger tensile stress,causing an initial crack,and even crack on the hanging ring.However,the stress concentration of the dropper caused by the elliptical-shaped crimping is small and the dropper line was not easy to be damaged.(4)Research on the impact fatigue performance curve of droppers.Based on the statistics of test results,the relationship curves between the impact load of two kinds of droppers with different crimping forms and the fatigue life were obtained.It is found from the bi-logarithmic coordinates that the impact load is proportional to the fatigue life Through the finite element analysis of local parts of the dropper,the results showed that the tooth-shaped crimping causes a large stress concentration at the contact position of the two dropper lines,which was the fatigue risk position of the structure;When the heart-shaped retaining ring and the hanging ring of the elliptical crimping dropper contact eccentrically,the maximum stress point was located at the outer edge of the hanging ring,which was the fatigue risk position of the structure.Combining the analyses of damage parameters,a general fatigue performance curve of dropper with equivalent stress as damage parameter was gained.(5)Life prediction of droppers under impact fatigue load.Through the fatigue failure test of the dropper structure under multi-level impact loads,it was confirmed that the failure of the dropper structure is compliant to Miner’s linear cumulative damage law.Based on the study of the load spectrum in Section 1 and the impact fatigue performance curve in Section 4,a prediction method of the impact fatigue life of droppers was proposed combining the Miner’s Law.In summary,this thesis combines the experiment and finite element analyses to reveal the failure mechanism of dropper under impact loads,and to propose a method for predicting the impact fatigue life of droppers,which can provide theoretical supports to the structural optimization of droppers. |
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