Study Of Ultra-Long Fatigue Testing At Non-Symmetric Loading

Many findings indicate that metals fatigue failure can be observed in ultra-long cycle regime beyond 10⁷ cycles. It is important that many structural components are working beyond 10⁷ cycles in service, even at 10¹⁰ cycles. So the design of fatigue strength based on the fatigue datum of 10⁷ cycles do not guarantee the security to industrial components. In recent years, it is interesting to us to develop the study of ultra-high cycle fatigue (UHCF) by means of ultrasonic fatigue testing technology. The technology develops rapidly in international and becomes a focus problem. But up to now, the ultra-long cycle fatigue is interested in the characteristic of S-N curve, the observation of fatigue failure surface, the behavior and fracture mechanism of ultra-long cycle fatigue. The experiments are carried out at symmetric cycle stress. But the industrial components and structures are often subjects to cyclically varying loads with mean stresses. It is well known that the fatigue process is sensitive to superposed tensile mean stress in both low-cycle and high-cycle fatigue regimes. So it is important to study the fatigue behavior at non-symmetric loading beyond 10⁷ cycles.The three-points bending and non-symmetric tension-compression ultrasonic fatigue testing system have been developed based on symmetric tension-compression ultrasonic fatigue testing system. The fatigue behavior of 50 steel with mean stress is tested by these experiments of ultrasonic fatigue at 20kHz. The effect of bending mean stress and tensile mean stress on fatigue behavior of 50 steel is discussed in this paper and achieve the model that describe the effect of mean stress under ultrasonic frequency loading. The fracture surfaces are analyzed using the scanning electron microscopy (SEM) and energy dispersive atomic X-ray (EDAX), and study the fatigue behavior and fracture mechanism of 50 steel. In addition, the specimens of medium tempering and high tempering of 50 steel are tested by the symmetric tension-compression fatigue system, and study the effect of heat treatment on ultra-long fatigue behavior.The results of non-symmetric tension-compression fatigue testing in 10⁴¹0¹⁰ cycles regime and three-point bending fatigue testing in 10⁶¹0⁹ cycles regime show that mean stress remarkable effect on fatigue strength of 50 steel under ultrasonic frequency loading. Along with increasing of mean stress, fatigue life decreases on...