Study On The Damage Mechanism Of Bending Fretting Fatigue Of Integral Current Droppers In High Speed Railway Catenary System

Bending fretting fatigue is widely used in various fields of modern industry,such as aerospace,machinery,railway,automotive,electric power,high-altitude cable and other fields.It has become one of the main reasons for the failure in many key components.With the continuous growth of the high-speed railway operating mileage and speed,the high speed railway catenary failure caused by the fatigue failure of the key components always occurrs.The dropper is the "bracket" of the catenary.It is the transmitter of the vibration and force between the contact line and the message line,and is the key component for the safe operation of the catenary.Once the dropper fractures during service,the contact line will collapse and further affect the safe operation of the catenary system,and even bring immeasurable economic loss and safety threat to the humans.Hence,it is exceedingly crucial to avoid the premature failure of the droppers and improve the reliability of the catenary system,and it has a great engineering practical significance.In this paper,first of all,the failure analysis was carried out by collecting part of the broken droppers through investigation.Through finite element analysis and fracture analysis,the reasons for the droppers were found out.In this way,the existing problems of the droppers were put forward and relevant tests were designed to study the failure mechanism of them.Bending fretting fatigue was one of the main reasons for the fracture failure.And then,the raw materials of droppers(CuMg0.4 alloy)were adopted as the research object.Based on electrohydraulic servo fatigue machine and the rig of bending fretting fatigue test which was independent design.The tests adopted with the point contact configurations of cylindrical/cylindrical vertical cross.Under different bending fretting fatigue load,cycle numbers and current density,the CuMg0.4 alloy has been carried on this system.obtaining the bending fretting fatigue life curve of materials,namely the S ~ N curve.On the self-designed dropper bending fretting fatigue test rig,the integral dropper which was widely used in China’s high-speed railways has been tested.Under the conditions of different bending fretting fatigue amplitudes,current densities,the mode of clamp tube and the indentation depth of clamp tube,the bending fretting fatigue test was carried out.These micro-analysis methods of the steromicroscope(SM),the white light interferometer(WLI),scanning electron microscope(SEM),energy dispersive spectrometer(EDS),X-ray photoelectron spectroscopy(XPS)and electron probe microprobe(EPMA)were used to analyze the fatigue fracture surface,fretting damage area and mechanical properties in detail.The damage mechanism of bending and fretting fatigue of integral dropper used in high-speed railway has been investigated systematically.The main conclusions obtained as follows:(1)The fracture failure of the integral dropper at the clamp tube was mainly caused by bending fretting fatigue.The fundamental reason of failure dropper was that the crimping stress of clamp tube was too high,which induced the concentration of stress in the dropper sliced end,and resulted in service life of the copper wire reducing.It was suggested to optimize the design of the crimping method of the clamp tube based on the existing materials and the theory of fretting tribology.(2)under the condition of fretting,the fatigue life of bending fretting was lower than that of plain bending fatigue.The S~N curve of bending fretting fatigue presented the characteristic of "C" curve.The nose region of the curve was the mixed fretting regime(MFR),where cracks were most likely to initiate and expand and the fatigue life was the lowest.At the bottom of the "C" curve,there was a partial slip regime(PSR),with slight fretting damage and long fatigue life.At the top of the "C" curve was the slip regime(SR),where the relative displacement of the fretting operation was large,and the wear rate was higher than the nucleation rate of the mirc-crack,inhibiting the fatigue crack growth and extending the fatigue life.(3)the wear mechanisms of the bending fretting fatigue damage area were mainly abrasive wear,oxidation wear and delamination.In the initial stage of bending fretting fatigue,the material damage was relatively small,and a small number of ploughing grooves and few oxidative wear debris could be found in the contact damage area.With the continuous increased of cycle numbers,the damage in the contact area was gradually aggravated,and the denamination also appeared.The damage was more serious near the loading end,but no fatigue crack initiation was found.At this time,the wear mechanism was abrasive wear,oxidation wear and denamination.With the further increased of the cycle numbers,the damage in the contact area was further serious,and fatigue cracks appeared in the near loading section of the contact area,finally forming a macroscopic fatigue crack.At this time,the wear mechanism was still dominated by abrasive wear,oxidation wear and denamination.As the current intensity increased,the bending fretting fatigue life of the material gradually decreased.The temperature in the contact area of the sample gradually rised.Due to the thermal effect of the current,the size of the contact damage zone expanded with increasing of current intensity under the same normal load conditions.At the same time,the oxygen content near the loading end of the contact damage zone increased,and the oxidative wear went to worse.The denamination layer in the contact damage zone was gradually refined with the increased of current intensity.(4)At the same normal load and axial force of the dropper,the bending fretting fatigue life of droppers gradually reduce with the increase of fatigue amplitude.The fatigue characteristics and micro cracks were observed obviously in the fracture surface of droppers.The clamp tube near the loading end was seriously damaged.Especially,there were denamination in the clamp tubes which were contacted with principal line.Under the action of current intensity,when it increased,the thermal effect of the droppers was obvious.The temperature of them were obviously rised,the bending fretting fatigue life of the droppers string also decreased with the grew of the current intensity.The surface of the dropper was seriously oxidation wear.At this time,the wear mechanism was abrasive wear,oxidation wear and denamination.(5)Adopting the self-developed "a crimping device for accurately controlling the indentation depth of clamp tube ",the fretting fatigue optimization design of the integral dropper was made by changing the crimping method and the indentation depth.Under the same current intensity,when the "reverse crimping" with the indentation depth was 4.8mm,the bending fretting fatigue life of the dropper was the longest.And the " forward crimping" with the indentation depth was 5.0 mm,the bending fretting fatigue life of the dropper was the shortest.It was concluded that the new crimping method of clamp tube playrd a role in extending the service life of the integral droppers.