Research On Prediction Method Of Crack Growth Rate And Fatigue Life Of Aluminum Alloy Carbody In High Speed Train

Aluminum alloy welded structure is widely used for lightweight design of high-speed train carbody.Crack-like welding defects are easy to occur in welded structures,such as undercut,slag inclusion and pit.Crack-like defects will further expand under complex multi-axis loads,posing a potential threat to the safety and reliability of the carbody.Therefore,rationally formulating the inspection cycle of cracks and proposing welding quality control is an effective means to ensure the safety and reliability of train operation.The research on crack propagation characteristics and life prediction method of welded carbody aluminium alloy in this paper has important engineering value and practical significance for welding quality control,safe operation and maintenance cycle formulation of high-speed train.Firstly,the finite element model of the standard CT specimen of aluminium alloy in high-speed train carbody is established.The stress-strain fields of linear elastic and elastic-plastic crack tip are numerically simulated under tension loading.On the basis of fully considering the cyclic plastic zone at crack tip and the hysteresis effect of stress and strain,the crack growth rate of the CT specimen under constant amplitude cyclic loading is studied.The curve da/dN-(35)K is obtained,the data of stable crack growth zone is selected,and the material parameters C,m and threshold stress intensity factor range(35)K_t _h are obtained by fitting with Paris formula.Then,the crack growth rate of the CT specimen is calculated by using the parameters determined in this paper and the corresponding parameters given in IIW standard,and the comparative analysis of the two kinds of crack growth rates is carried out.Furthermore,the fatigue crack propagation life prediction method of aluminum alloy welded carbody of high-speed train is studied.The load spectrum of the high-speed train carbody is determined,combing with the distribution of structural stress and the selection principle of concerned points,the fatigue dangerous position of the aluminium alloy welded carbody is selected as concerned points.Then the surrogate model of the carbody is established by using Box-Behnken matrix design and polynomial fitting method,which realizes the transformation from the load spectrum of the carbody to the dynamic structural stress history of the concerned points.According to relevant standards,the cracking characterization of carbody welding defects is completed.The initial crack depth of each concerned point under two parameters is obtained by using the relevant parameters determined in this paper and the relevant parameters given in IIW standard.The crack propagation life from the initial crack depth to 2 mm is calculated,and the comparative analysis of the two kinds of crack propagation life is carried out.The results show that the crack tip singularity fields of linear elastic and elastic-plastic are consistent with the theory.The crack growth rate of CT specimen calculated by the parameters determined in this paper is larger than that calculated by the parameters given in IIW standard.The fatigue life calculated after the weld defect is characterized as crack under two parameters is compared and analyzed.The fatigue life calculated by using the parameters given in IIW standard is longer and the results of life prediction are dangerous.The crack propagation life of the carbody calculated by the parameters determined in this paper is compared with the standard overhaul schedule of the EMUs in China.In this paper,weld defects are characterized as fatigue life prediction method of cracks,which provides a reference for welding quality control and maintenance cycle formulation for aluminium alloy carbody.