Influence Of Surface Quality And Loading Mode On The Fatigue Behavior Of Clean 42CrMo High Strength Steel In The Vhcf Regime

Engineering components and structures are generally subjected to cyclic and alternating loading.For example,the high-speed rail,airplane and bridge always bear more than 107 cycles loading.Traditionally,the highest loading amplitude that material can endure at the 107 cycles is the fatigue limit.The infinite life will appear under the fatigue limit.However,with the improvement of the relevant technology,the structure or material also can be failure when its loading reaches up to 107 cycles under the traditional fatigue limit.The structure or material bear more than 107 cycles is called very-high cycle fatigue(VHCF)phase.In the practical engineering,when the structure fatigue performance is analyzed,the influences of surface quality and loading conditions for fatigue analysis are required to be considered.42CrMo high strength steel is widely applied.It always loaded(ore than 107 cycles in the practice.It is important to investigate the VHCF behavior of this material.To reduce the influence of inclusion,the clean 42CrMo high strength steel is chosen.This dissertation investigates the behavior of clean 42CrMo high strength steel in the HCF-VHCF regime by USF-2000.The influences of surface quality and loading mode are studied.This study can guide the processing and increase the application of the material.Based on these thoughts,the following studies are carried out:(1)The influence of surface quality on the fatigue behavior of the clean 42CrMo high strength steel in the HCF-VHCF regime is studied.The material is processed to the specimens of VHCF.All the specimens are divided into two groups,then processed by two different surface treatments to obtain two different surface qualities.The experiment of super-sonic fatigue is achieved.The data are processed and the fatigue behavior of this material is analyzed.The appearance of fracture is investigated by scanning electron microscope(SEM).The main components of the fracture are investigated by energy dispersive spectroscopy(EDS).The initial location of crack is found and the behavior of crack initiation is analyzed.(2)The influence of loading mode on the fatigue behavior of the clean 42CrMo high strength steel in the HCF-VHCF regime is analyzed.All the specimens are divided into two groups and experimented in the constant amplitude(CA)and variable amplitude(VA)loading mode,respectively.The data are processed and the fatigue behavior of this material are conducted.The appearance of fracture is investigated by SEM.The main components of the fracture are investigated by EDS.The initial location of crack is found and the behavior of crack initiation is analyzed.The effect of the low stress amplitude in the loading sequence on fatigue behavior is analyzed.(3)A new method is proposed to study the fatigue damage accumulation.From the beginning of the normal fatigue damage accumulation,the damage accumulation model and S-N curve are combined.The method of considering damage condition is done.Based on the damage condition,the relevant models by different damage accumulation are deduced.The relationship between damage condition and fatigue limit is worked out.The preliminary test is designed to verify the model.Resolutions are designed to resolve the problem that it may appear in the test.