Study On Very-high-cycle Fatigue Behavior Of E36 Steel

In the modern mechanical structure, service life of welded structure general will reach 109~1010 cyclic times,but the existing standard of fatigue design in international welding society rules that after 107 cyclic times will enter an infinite life stage. Designing 109~1010 cycles fatigue life based on this standard is very dangerous. So study the performance of very-high cycle fatigue on welding joint is of great significance.Now much of the research for very-long life is for base metal, and the flaw also is mainly non-metal inclusions. Research for welding joint in very-long life , especially the impact of welding defects,few research is carrying out in our county. This subject is developed by using TJU-HJ-type I ultrasound fatigue test devices on E36 steel welding joint to study very-long life fatigue property. Comparing the test results of different welding process; analyse fracture, and explore the fracture mechanism. Classify defects which as a source of fatigue crack, and studied the influence of different defects on life.The welded joint S-N curve is got by ultrasound fatigue test results under different stress levels, which show the data points in very-long life stages is more decentralized, and still decline after 1.0 x 109 cyclic times. Defects in the source of weld fatigue crack include body type defect (porosity and slag) and face type defect (general surface crack), Face defect is much more serious than body defect. The size of the body defect affects the whole fatigue life, and the quantitative relationsh is got between inclusion size and fatigue life. Hydrogen plays an important role in crack propagation stage, and the "white dot" is found in the weld fracture cracking from the surface. It can cause local embrittlement and accelerate crack expansion and form GBF area. The size of GBF directly affect the fatigue life, and quantitative relations is also got between size and fatigue life. Welding defect’s influence on fatigue life is large in very-long life phase, But considering the economy, in the actual industrial production, it is not necessary to reduce defect size not to count the cost. Because the study of welding process to reduce defect size is more difficult, On the other hand the influence is certain when defect size reduced to a certain degree...