Study On Ultrasonic Fatigue Of 40CrNiMoA Steel

At present,the existing studies on fatigue manly concentrated on low-cycle fatigue with cycles less than 10~7 and lack of thoroughly and systematically.But in many high-tech industrial fields,components have a fatigue life up to 10~8 cycles or more,so it is important to investigate the properties of very-high-cycle fatigue higher than 10~8 cycles.The restrictions of time and energy make it impossible to obtain material's fatigue property more than 10~7 cycles with conventional fatigue technique. Taking the specific ultrasonic welding probe with 40CrNiMoA steel as the research object,fatigue behavior of 40CrNiMoA steel with different microstructures under ultrasonic fatigue loading(20KHz,R=-1)was studied by ultrasonic fatigue testing and S-N curves in the range of 10~5～10~9 cycles were obtained in this paper.The fatigue fractures of the specimens were examined by scanning electron microscopy and transmission electron microscopy.The actual working frequency and vibration condition of the probes is accord with the ultrasonic fatigue testing,so the experimental results could truly reflect the fatigue property of the ultrasonic welding probes.Experimental results revealed that the 40CrNiMoA steel with different microstructures had two types of S-N curve,stepwise curve and continually decreasing curve.Specimens would be continued to fail over 10~7 cycles,and had no fatigue limit.As a result,fatigue property less than 10~7 cycles cannot provide the safety design data of the component have a fatigue life more than 10~7 cycles.The longer ultrasonic fatigue life of specimens with microstructures characterized by proper combination of both strength and toughness was observed,in less relationship with grain size,especially under low-stress conditions.And ultrasonic fatigue life of the specimen increased remarkably by surface treatment,particularly by nitridation. The observation of the fatigue fractures showed that the crack initiation took place at the defect of the specimen surface or sub-surface at the ultrasonic fatigue loading and the crack propagated towards the inside,the final fracture zone accounted for a small scale.The fracture had both the features of cleavage and that of quasi-cleavage.The fatigue mechanism considers that when there are no obvious inclusions,pores and similar defects,fatigue crack can initiate from the surface of the specimen in the very-high-cycle fatigue,due to cumulation of the irreversible plastic stain.By using 3-D finite element analysis,the stress intensity factors at the half-elliptic surface crack front in the ultrasonic fatigue specimen under ultrasonic vibration condition were calculated with displacement and J-integral approaches.The calculation results showed that the stress intensity factors at the crack front increased with the increase of the fatigue crack length.The stress intensity factor values obtained by the means of the J-integral approach had the same variation tendency as the results of displacement methods,and the error was less than 5%.