Fatigue Life In Very High Cycle Range Of Structural Steel

Conventional fatigue design point considers that structure components have boundless fatigue life-span above 10⁷ loading circulation cycle, corresponding design of fatigue strength based on the fatigue datum of 10⁷ cycles. But with the high-speed and large-scale development of modern mechanism in recent years,mang mechanism and engineering structure such as rolling stock structure and Component and part in high speed railway , nuclear power station radiator pipeline ,and engine component and part and so on can take place fatigue failure in ultra-long life regime exceeding 10⁷-10¹⁰ cycles. So conventional fatigue design standard and life prediction method already don't meet the activity acquire of mechanism equipment in ultra-long life regime. So it is necessary that testing some structural steel in common use by using ultrasonic fatigue test , and gain the data of ultrasonic fatigue test. It has very important project meaning for fatigue strength designing and safety appraise and life-span forecast.Fatigue behavior in ultra-long life regime (about 10⁴-10¹⁰ cycles) of 16 Mn structural steel , 304 stainless steel , 42 CrMo steel is studied by using ultrasonic fatigue test. Gaining three fatigue S-N curves of the three Materials in testing by ultrasonic fatigue test, and analyse the characteristics of three fatigue S-N curves. The fracture surfaces are analyzed using the scanning electron microscopy (SEM) and study the fatigue behavior and fracture mechanism of the three structure steels . analyzing crack initiation mechanism and crack propagation mechanism. The result of the experiment indicated The curves of the three structure steels are the stepwise shape, and no exit tradition significance fatigue limit. All of the fatigue crack initiated from the surface of specimen in 16 Mn structural steel , but fatigue crack initiated from the surface of specimen below 10⁷ circulation cycle and fatigue crack initiated from the flaws of second surface above 10⁷ circulation cycle in 304 stainless steel and 42 CrMo steel. The crack propagation mechanism is consistent from high-cycle fatigue regimes to ultra-long life regime in 16 Mn structural steel and 42 CrMo steel . The first propagation regime shape is obvious diversity to tradition definition of shape in 304 stainless steel,Then definiting the first propagation regime of it . Comparison of ultrasonic fatigue properties with conventional fatigue properties of 16 Mn structural steel showed that the ultrasonic loading frequency effect on fatigue property of 16 Mn structural steel,. discovering there is existing a very bad plasticity of cementite in framework of organization in the insides of 16 Mn structural steel is to lead to the main cause reducing ultrasonic fatigue property .Using the scanning electron microscopy (SEM) and X-ray diffractometer discover the austenite-martensite transformation occurs in crack pointed end of 304 stainless steel during the ultrasonic fatigue test and analyse cause of austenite-martensite transformation .The equation of the solid-liquid coupling vibration of pipe conveying fluid is deduced , and life time has carried out a forecast on the pipeline .Provides one kind of feasible method to the pipeline fatigue design.By analysing 40 Cr steel comparatively, 42 CrMo4 steel's ultrasonic fatigue test S-N curves, discover alloying element Mo having raised material surpassing the ultrasonic fatigue property . Effect in being deltamax-rization combining with Mo and, analyses the microcosmic mechanism raising material ultrasonic fatigue property.Microscopic analysis of this from the perspective of three structural steel not typical for internal initiation mechanism...