Study Of Fatigue Property Of Welded Joint Under Combined High-and Low Cycle Fatigue Range

The traditional definition of fatigue limit for 107 cycles, and the use of conventional technology to meet the requirements of fatigue test。However, with modern industrial and technological development, many of the industrial sector (the current structure of welded joints and the fatigue design and evaluation are based on low-cycle fatigue load test on the basis of the data) In fact a number of important structural welding works, such as aircraft, nuclear reactors, turbine, automotive, gas turbine engines and turbines, and so on, to bear in the high rate of low-frequency alternating load process, from time to time in varying degrees to the low rate of high-frequency vibration. Existing norms in the project estimate the actual structure of fatigue life, often do not consider the harmful effects of high-frequency vibration, the only high-frequency components in the main stacking alternating load, according to single-cycle treatment, or simply be completely ignored. If the vibration is only dependent on the load on the average, because of its smaller amplitude of the structure of the relatively minor damage. If, however, is superimposed on low-frequency load on the cycle, and its material injury caused by the complex will greatly speed up the project structure in particular, welded structure damage and failure, a serious threat to the entire structure of the equipment or the use of reliability and safety Life.Since performing experiments of the ultra-high-cycle regime in the range of 109~1010 cycles using a conventional fatigue testing method is very time consuming and expensive, so we studied and developed the ultrasonic fatigue testing system to accelerate fatigue test. In this paper, based on the ultrasonic fatigue testing system, the fatigue behavior of 16Mn including smooth and welding joint specimen was studied, and the fracture surface of fatigue specimen was examined by scanning electron microscopy (SEM).Specimen design is critical for optimum ultrasonic fatigue testing, dog-bone-shaped specimen is usually designed in ultrasonic fatigue testing. An analysis was carried out to calculate the resonance length, strain and displacement along the length of fatigue specimen with different geometries and for various resonant frequencies.Fatigue test of 16Mn steel welded joints has been carried out under low cycle fatigue(LCF), high cycle fatigue(HCF) and combined cycle fatigue(CCF) respectively, in order to investigate the fatigue properties of welded joints being subjected to CCF loading. The stress ratio was 0.5 and the ratio of HCF cycles to LCF cycles was about 260:1. The loading frequency of HCF was about 20kHz and performed by an ultrasonic fatigue testing system. The results indicate that fatigue strength of welded joints has been decreased further under CCF. HCF is detrimental to fatigue life of welded joints under CCF loading. With the increasing of the ratio of HCF loading to LCF loading, fatigue life of welded joints becomes shorter.