Effect Of Ultrasonic Frequency On Giga-Cycle Fatigue Properties For GCr15 Steel

Ultrasonic fatigue testing system delivers accelerated testing of materials fatigue in giga-cycle region. However, for Iron and Steel Materials, on the one hand, ultrasonic vibration may lead to "frequency effect", that is, the stress that materials practical endure is less than theoretical loading stress, in that dislocation velocity of materials is far less than ultrasonic loading speed, resulting to increase fatigue life of materials.On the other hand, the Aggravation of inner friction induced by the ultrasonic vibration, probably induces "temperature effect", namely, temperature rising in the experimental process that leads to thermal damage of materials, resulting in decrease fatigue life of materials.Many mechanism and engineering structure, such as bogie frame, automobile, engine, generator, marine structure and bridge etc., bears alternating load over 109 cycles in service period. It is essential to research giga-cycle fatigue properties of these structures and equipments to guarantee safe running in the long term. Therefore, it is necessary to determine the appropriateness of ultrasonic fatigue test for researching the effect of frequency on giga-cycle fatigue properties for structural materials.In order to investigate the effect of frequency on giga-cycle fatigue properties for GCr15 steel, ultrasonic fatigue (test frequency:20kHz, continuous loading and interval loading) and cyclic axial loading (test frequency:80Hz) tests with stress ratio R=-1 were carried out for GCr15 steel using hourglass-shaped specimens.During fatigue test, the specimens used for ultrasonic fatigue were cooled with dry air and water, respectively, to examine the effect of temperature raising inside specimens.The fractography observation results show that the fatigue fracture occurred at surface flaws, a surface inclusion or an internal inclusion in the whole stress amplitude level under 80Hz cyclic axial loading, whereas it mainly occurred at an internal inclusion in the whole stress amplitude level under ultrasonic continuous loading with dry air cooling. Under the condition of dry air cooling, when an inclusion induced fatigue facture locates at the deeply interior of specimen, a burned area surrounding the inclusion was observed by optical microscope. Under ultrasonic continuous loading with water cooling, it mainly occurred at an internal inclusion in the high stress amplitude level. Under ultrasonic interval loading with air cooling, it all occurred at an internal inclusion in the whole stress amplitude level.Experimental results show that the ultrasonic loading frequency increased the fatigue life of GCr15 steel, and the high temperature originated in ultrasonic vibration decreased the fatigue life of GCr15 steel.