Influence Of Notch Stress-concentration On The Ultra-high-cycle Fatigue Behaviors Of 5083 Aluminium Alloy

5083 aluminum alloy cannot be strengthened in heat treatment. The main alloy element of it is magnesium. It has high strength, good plasticity, good corrosion resistance and easy processability. It has been widely used as the high-speed train’s frame, body and surface plate, as well as the weldment of ships, cars and planes. When the trains are moving at high speed, their structures will bear high frequency load, which is caused by vibration. The number of the vibration cycles, which poses a serious threat to the structures’ safety and reliability, is as high as 108～1010. Due to the structure need to be, the high-speed trains’ components have some parts with discontinuous section shape, such as shaft shoulders, notches and holes. Under the action of external loading, these parts are likely to become a weak link and fracture source. This is so-called notch effect. As a consequence, the research about the notch effect’s influence to the fatigue property of 5083 aluminum alloy is very significant.The ultrasonic fatigue testing, whose fixed-frequency is 20kHz. Compared with the traditional fatigue test, ultrasonic fatigue testing can get the data of the material’s very high cycle fatigue properties more quickly. It can accomplish a fatigue test with 109 cycles in only 17 hours. In this paper, the experiment about 5083 aluminum alloy notch specimens, which had different stress concentration factors and shapes, was did by using ultrasonic fatigue test technology. Their fatigue life curves or S-N curves between 105 weeks and 1010 weeks were got. The shape effect’s and notch effect’s influence to the fatigue property of 5083 aluminum alloy was analyzed. Then the alloy specimens’ fracture surfaces were analyzed by using a field emission scanning electron microscope.The results showed that in more than 107 cycles, two kinds of 5083 aluminum alloy in different shapes samples still be destroyed, there was no traditional concept of the infinite fatigue life or fatigue limit. The test specimens of 109 cycles’ fatigue strength was significantly lower than those of 107 cycles’ fatigue strength. As a consequence, could not use the fatigue strength of 107 weeks time as the material’s fatigue strength criterion in the range of the very high cycle fatigue.Research on the shape’s influence indicated that, as the same as the smooth specimens, different shapes of 5083 aluminum alloy notch specimens’ S-N curve had different laws. The columnar notch specimens’ S-N curve showed a trend of continuous decline, while the tabular notch specimens’ S-N curve had two different trend line. In 106～1010 cycles, the tabular specimens’ fatigue life was shorter than the fatigue life of columnar samples. 106～1010 cycles are general components’ main range of service life, so using columnar sample’s test data to evaluate the fatigue life of tabular component is not safe.Research on the notch effect ’s influence showed that, under the condition of the same stress, the fatigue life of specimens, whose stress concentration factor were 2, were longer than the fatigue life of specimens, whose stress concentration factor were 3. It could come to the conclusion that the specimens who had more litter stress concentration factor would have longer fatigue life. But the value of 5083 aluminum alloy notch specimens’ fatigue notch factor and fatigue notch sensitive coefficient were not very large, it illustrated that the fatigue performance of this material was not very sensitive to the notch effect.Scanning electron microscopy analysis revealed that, Because the material’s internal structure was very close, 5083 aluminum alloy’s fatigue crack all produced in the samples’ surface, and no fisheye phenomenon was observed. Because of the influence of surface defects and stress concentration, there were more than one source of crack on the fracture. This martial had a traditional crack spreading area, and fatigue striations could be observed. In addition, since the stress concentration effect is more significant, the specimen’s theoretical stress concentration factor was bigger, its instantaneous fracture area was larger.The reasons of that, the tabular specimens’ fatigue life was shorter than the fatigue life of columnar samples in 106～1010 cycles, and the tabular notch specimens’ s-n curve had two different trend line, is worth to further research.