Simulation Of Crack Propagation Rate In Tensile-Tensile Fatigue Test In Ultra-Fine Grain And Nanometre-Grain Low Carbon Steel Sheet With Lamellar Structure

Based on the simulation of tensile-tensile fatigue test in cold rolled low carbon steel with a layered microstructure of ultra-fine nanometer grain, the fatigue mechanism of this material was discussed.Low carbon steel sheet layered microstructure of ultra-fine nanometer grain was prepared by being cold rolled to 60%, 70% and 80% deformation rate (DR) after quenching. Mechanical properties and fatigue life of the steel sheet with three different DR were measured. Based on the experiment date, model of tensile-tensile fatigue fracture of steel sheet with the microstructure of layered grain was developed. With the aid of ANSYS 11.0, propagation of transgranular fatigue crack in the conditions of different loading was simulated. The simulated date of fatigue life, cruel of the propagation rate of macro fatigue crack and that of nanometer fatigue crack were obtained. The effects of DR and grain boundary on the propagation of micro fatigue crack were investigated, and the limit of the formula for crack propagation rate was also analyzed.The results show: the fatigue life of the three type of steel sheet under high load stress improves as the increasing of DR. the simulated S-N cruel greatly accords with that drawed from experiment date. The propagation rate of macro fatigue crack increases as the increasing of DR, and this tendency is basically identical with the date of experiment. That is to say, the developed model is reasonable and meets the need for further simulation.The simulated propagation rates of nanometer fatigue crack in steel sheet with three different DR under loading ranges form 6400 to 10000kN show that periodic peaks exist in the cruel drawed by propagation rate of nanometer fatigue crack VS its length, and the crack lengths are 80, 132 and 187nm that is absolutely corresponded to the grain thickness (80, 132 and 187nm) in steel sheet with the three DR. the propagation rate peak of fatigue crack in steel sheet with 80% DR decreases as the increasing of loading, this decrease indicates the increase of the resistant force of deformation. This character reveals the effect of interface of layered nanometer grains on the fatigue test and makes up the limit of experiment quantity.The formula of propagation fatigue crack used in the developed model is suitable for the simulation of the propagation rate of stress fatigue crack, but not for that of propagation rate of strain fatigue crack.