Effect Of Load Types On Short Fatigue Crack Behavior Of CuNi₂Si Alloy

CuNi₂Si is a precipitation-strengthened alloy,which has good mechanical properties and corrosion resistance,and is widely used in high-speed railway catenary components.With the continuous increase of the operating times of high-speed railway,the failures caused by fatigue of catenary components frequently occur,which seriously affects the safety of train operation.According to the existing fatigue failure cases,the catenary components are mainly subjected to bending,tension and compression,vibration and impact loads.And the initiation and propagation of short cracks usually occupy more than 80%of the fatigue life during the fatigue failure process.In this thesis,CuNi₂Si alloy is taken as the research object to study its short fatigue crack behavior under different load types.1.The short fatigue crack replica test of four load types including rotating bending and axial tension and compression specimens under constant load and overload were carried out.The experimental results show that short fatigue cracks in CuNi₂Si alloys usually nucleate at twins or large-angle grains.Crack initiation at the trigeminal grain boundary due to incompatible grain deformation is a common way.After the crack initiation,the path with low resistance will be chosen to propagate along the crystal.There will be an obvious slowdown in the crack propagation process,and the crack bifurcation caused by the stress concentration at the triple point of the grain boundary is one of the main reasons.In addition,fracture analysis shows that periodic overload can change the initiation mode of fatigue cracks from a single source of cracks to multiple sources of cracks,and the number of crack initiation cycles of the rotating bending specimen is relatively reduced.2.The average fatigue life of the specimen is reduced under the periodic overload.The average fatigue life of the rotating bending specimen and the axial tension and compression specimen under the periodic overload are 32.8%and 83.7%of the constant load,respectively.For rotating bending specimens,this difference is mainly due to the different number of cracks initiating on the surface of the specimen.When overloaded,several effective short cracks(ESFCs)will appear,and then propagate and merge,resulting in an instantaneous increase in the length of the cracks and a significant reduction in fatigue life.However,for axial tension and compression specimens,although overloading will cause the crack to grow slowly in the microstructure short crack(MSC)stage,it will accelerate the propagation of the physical short crack(PSC)stage to the rapid failure of the specimen.3.There are different critical fatigue crack growth dimensions under different load types.For rotating and bending specimens,the critical size is reduced from 800?m to 300?m after overload;For axial tension and compression specimens,the critical size is reduced from 1500?m to 700?m after overload.Through mathematical statistical analysis,the good statistical distribution of the dominant effective short crack scale(DESFC)under each load type is determined,and it is found that the periodic overload has the greatest impact on the axial tension and compression specimens,so that 90%of the fatigue life after overload is consumed within 300?m of the DESFC scale.4.Two commonly used short crack growth rate models are used to fit the fatigue short crack growth data.According to the fitting effect,the short fatigue crack model with multiple microstructure barriers can be used to better describe the growth rate of DESFC with the crack size,and it can also better reflect the periodic oscillation law in the propagation process.