Investigation On The Fatigue Crack Propagation Behavior Of Al Alloy Used In High Speed Train Structure

The service safety of structures used in high speed trains has attracted widely attentions recently with the fast development of high speed trains in China.One of the key structures in high speed trains is the chassis,which will inherently get damage during its service due to the cyclic stresses and corrosive environment.With the formation and propagation of little cracks,the residual strength and service life would consequently decrease.Thus,how to compatibility monitor the fatigue damage process by the criteria of damage tolerance design and the concept of evaluation,and further investigate the fracture mechanical properties of key functional materials,is an important topic for the structur safety of high speed trains.In the direction of engineering application,this dissertation made an investigation on the fatigue crack propagation(FCP)behavior of structural material that used in high speed trains.After testing the fracture mechanics performance and analyzing the reliability of the structure material(Al alloy),this dissertation foucsed on the effects of service experience on the fracture mechanical properties,the 'coaxing' effect of pre-cyclic stresses(PCS)on material,and the methods of evaluating the FCP behaivior,with expecting that this dissertation would privode some fundational information for the design reliability and operation safety in high speed trains.A three dimentional(3D)finite element method(FEM)model was devloped based on the phsical geometry of chassis.In order to ensure the computing accuracy,some simplifications were made so as to focus only on the detialed analysis of stress and displacement.In the model setting,the loading capacity and the restrainted bondary conditons were determined by actual situation.The stress and displacement distribution were simulated by the FEM method.The predictions of FEM model showed good agreement with the experimental data,indicating the FEM model is reliable.The resluts provided the energining background and design criteria for the following FCP investigation of materials subjected to service experience.Aimed on the key structure materials that experienced a long term service in high speed trains chassis,this dissertation experimentally investigated the fatigue mechanical properties,and analyzed the change tendency of material properties.The da/dN-?K curves in the FCP zone illustrated a turning phenomenon,by which a piecewise Paris' curve fitting method was produced to describe the FCP behavior.The reason of the turning phenomenon was later explained by analyzing the FCP behavior in the region near the FCP threshold.The morphologies of the fracture surface were observed by a scanning electron microscopy,with the purpose to explore the microcosmic mechanism on the changed performance of service materials,then comprehensively assessed the influence of service experience on the fracture mechanics performance of high speed train structures.Since the da/dN-?K curves in the FCP zone experienced a turning phenomenon,this dissertation further applied pre-cyclic stress(PCS)to simulate the effect of service experence.The tensile and fracture mechanics tests for materials that had experienced a certain number of PCS cycles were conducted in this dissertation.The effects of PCS on material properties and the reason of the 'coaxing' effect were investiaged here.Based on the FCP behaivor of mateirals that subjected to PCS,a more persuasive life prediction method was produced to compare with the experimental data.Finally,this study quantitatively analyzed the change of micro-defect and microstructures of the material that has experienced PCS,and explored the possible methods to improve the strength and service life of high speed train structure.This dissertation reviewed the widely accpeted methods for p-da/dN-?K curves fitting with fixed confidence and reliability,and analyzed its unreliable aspects.Acorrding to the researches on the the distribution of dispersion for the experimental data,two statistical analysis methods were proposed.The two methods were able to describe the probability distribution of FCP behaivor in different regions of stress intensity factor.This dissertation designed a software for the probability statistical analysis of FCP rate on the Java Platform.This software contained a traditional method and three new proposed methods,which was proved to be an efficient and convenient calculation tool for the assessment of the FCP behavior and FCP life of engineering materials.A 3D stress intensity factor model was developed by the FEM at the end of this dissertation.This model was used to calculate the distribution of stress intensity factor along the thickness direction of the crack tip.Through comparing the model resluts with experimental data,it is proved that the model plays a modifed role on the caculating of the stress intensity factor for the crack tips that has a thickness effect.The proposed 3D stress intensity factor model so as to provide an accessment model for the FCP behavior of materials.