Research On Hot Spot Stress Approach Of Fatigue Evaluation And Application For High Speed Car

The railway transportation in China is developing quickly with the aims that are high-speed,heavy load and light weight.This will cause much more requirements for the fatigue strength of the vehicle.The bogie frames and carbodies which belong to the raised-speed and high-speed vehicle are made from welded steel plate and aluminium alloy plate.This help reducing sole weight of the vehicle widely.However,raised speed, heavier load and duplicated forms of welded joints will make the load-bearing condition formidable for the vehicle.At the same time,various new materials are applied by bogie frames and carbodies continiously.Fatigue breakdown is the primary form of the welded structures under alternating load,and the mainly parts which breakdown are the welded joints.Therefore,the fatigue strength of welded joints confines the operating safety of the vehicle and researching fatigue evaluation approach becomes more important.The traditional nominal stress method is the main fatigue evaluation approach abroad.And the nominal stress is the most important approach while determine the fatigue strength of welded structure and carbody in our country.However, existing nominal stress approach of welded joint ignores the variety of welding details. The nominal stress will be difficult to be determined while the welding details are complicated,or some welding details do not belong to traditional welded joint. Consequently,the more reliable approach must to be brought forward on the basis that nominal stress S-N curve is got.Hot spot stress(HSS) approach is more applicable for high-speed vehicle fatigue evaluation,because its evaluation result is independent on the welding details and load form.Evaluation result by HSS is closer to real stress of welded joint than nominal stress approach;the result is much more exact and reliable therefore.Reports about fatigue evaluation by HSS for high-speed vehicle are quite few.In this paper,HSS calculation method by finite element and influencing factors are studied by the numbers. During this process,the welded joint belongs to high-speed car is the basis studying object,and the HSS theory is the basis flat.The fatigue capability data of new material welded joints applied for high-speed car are tested,which is the foundation for high-speed car fatigue evaluation by HSS approach.The fatigue evaluation by HSS approach for high-speed car is studied.The main research works in this paper are as follows: 1.Finite element model of typical weled joint is constituted;the element size and extrapolating method are resolved.The HSS calculation method and HSS concentration factor under different load form are analyzed.2.Geometry factors and welding details which maybe influence the HSS concentration factor are analyzed.The factors are:for butt joint,there are the height of welding seam and plate thickness,and for fillet welded joint,there are the size of welding seam,plate thickness,welding angle and erect board thickness.HSS eigenvalue is brought forward which is used to describe the influence to HSS concentration factor caused by geometry factors and welding details.Influence from load form is calculated.3.The nominal stress S-N and HSS S-N for fillet welded joint and butt joint of AQ400NH steel,SMA490BW steel and aluminum alloy 6005 are got.These three materials are applied to high-speed car in China.The fatigue capability data provide the basis for fatigue evaluation by HSS for high-speed vehicle.4.In practical application item,UIC615-4 Code is applied for analyzing the complete welded AQ400NH bogie frame by finite element method.And the bogie frame fatigue strength is evaluated by nominal stress.As the studid objects,two typical fillet welded joints which are connection area beween side beam top cover plate and lateral stopper of bogie frame and connection area beween lateral beam bottom cover plate and traction bar support, are evaluated by HSS approach.During the analysis,the sub-model technology is applied to separate the fillet parts from whole model,then the solid element models are established.On the basis of analysizing the box iron with rectangle hole,the sub-model method is validated.Then the fatigue evaluation by HSS approach for welded bogie frame is achieved.5.According to tested nominal stress spectrum of connection area beween cover plate and traction beam of carbody,and the nominal stress spectrum of connection area beween underframe of carbody and unti-hunting damper support,the equivalent HSS spectru are got by HSS eigenvalue of these two parts.Using HSS spectrum,the HSS fatigue strength can be evaluated.The fatigue evaluation by HSS approach for high-speed aluminium alloy car body is established then.