Q345高周疲劳失效机理及固有耗散能研究

Q345 is a kind of ferrite and pearlite dual phase steel that is widely used on bearing force components in architectural structures and mechanical systems,the long term dynamic loading during their service life induces the fatigue fracture under the stress amplitude that far below the tensile strength of the material,which require the study of the fatigue failure of the material.The conventional fatigue test was carried out with the help of electromagnetic resonance fatigue test machine (140 Hz) to study the fatigue failure of Q345,the stress-life (S-N) curve of Q345 was collected in high cycle fatigue regime.The initiation and propagation of the cracks of Q345 under cyclic loading was studied by the scanning electron microscope (SEM),and the intrinsic dissipation energy during the fatigue failure of the material was investigated with the help of infrared camera additionally.The fatigue failure of low carbon steel Q345 under high frequency cyclic loading was induced by the micro cracks initiated from ferrites grains.The propagation of the micro cracks was influenced by the micro structure of the material,and apt to propagate along the ferrite grains and the grain boundaries,but could be easily hampered by the presents of pearlite grains in the crack tip.The presence of pearlite grains helped retard crack propagation,which made the fatigue crack tortuous.The variation of the temperature field was not distinct until the stress amplitude was higher than the fatigue limit in the high cycle fatigue regime,therefor,the fatigue limit can be quickly determined based on the temperature variation of the specimen surface under the cyclic loading.Furthermore,a model was established in thermodynamics framework to characterize the intrinsic dissipation energy of the material under high frequency cyclic loading, and the result showed that the relation between intrinsic dissipation energy of unit volume material and limited fatigue life loading presented to be nonlinearly.