Study On The Very-high-cycle Fatigue Behavior Of 40CrNiMoA Steel

Ultrasonic fatigue is characterized by high frequency,low amplitude,long fatigue cycles and short time,and is one of induced factors of the failure of aerospacecrafts and other mechanical components.However,the existing studies on fatigue manly concentrated on low-cycle fatigue with cycles less than 10~7,but were not made thoroughly and systematically.Little known is about studies of relationship between Very-high-cycle fatigue with microstructures of material.But in many high-tech industrial fields,fatigue failure of components occurs to up to 10~8 cycles or more.Therefore,it is of significance theory and practical meaning to study behavior of very-high-cycle fatigue more than 10~8 cycles.Taking the specific ultrasonic welding probe with 40CrNiMoA steel as the research object.The relationship between microstructures with ultrasonic fatigue behavior with 40CrNiMoA steel has been studied by the ultrasonic fatigue test(20kHz),and S-N curves over 10~8 cycles are obtained.Because of the actual working frequency and vibration condition of the probe as same as those the ultrasonic fatigue test,the testing results reflect the ultrasonic fatigue characteristics of the ultrasonic welding probe.The results show that the ultrasonic fatigue life of the specimen is markedly increased by surface treatment,particularly by nitridation.The longer ultrasonic fatigue life of specimens with microstructures characterized by proper combination of both strength and toughness is observed,in less relationship with grain size,especially under low-stress conditions.Moreover,it is also found that specimens of 40CrNiMoA steel continued to fail over 10~7 cycles,without the fatigue limit and 40CrNiMoA steel with different microstructure exhibit two types of S-N curve:a continuously decreasing curve and a stepwise curve.Analyses of fatigue fracture show that fatigue crack initiated at defects of specimen surface or sub-surface of specimen at the ultrasonic fatigue loading and the crack propagated towards the inside of the specimens.The fracture had both the features of cleavage and that of quasi-cleavage,with less proportion of the final fracture zone.It is considered through fatigue mechanism analyses that fatigue crack initiated from the surface of the specimen in the very-high-cycle fatigue condition when the specimens have no obvious defects inside them,such as inclusions,pores and so on.Due to inhomogeneity of deformation between the grains in polycrystal,plastic strain occurs to a few grains subjected to stress concentration,thus the resulting irreversible stain per cycle leads to fatigue failure through enough cycles.The modeling and modal analyses are conducted on the ultrasonic fatigue specimen of the crack from specimen surface using ANSYS system,determination and calculation of the stress intensity factor with 40CrNiMoA steel under ultrasonic vibration loading are presented.The calculation results show that the stress intensity factors at the half-elliptic surface crack front increase with an increase of the fatigue crack length.There is the same variation tendency of the stress intensity factor between two methods both energy and displacement,and the error was less than 5%.