Study Of Fatigue Property Of Q235 Steel Welding Joint In Ultra-High Cycle Fatigue Range

With technical development in modern industry, in many applications, the lifetime of mechanical components have to endure up to 109～1010 cycles of loading without failure. A number of recent studies, extending into the ultra-high-cycle fatigue (UHCF) or gigacycle fatigue range, show clearly for different steel and other metallic materials that even at stress amplitudes below the classical fatigue limit, fatigue life is finite. So the fatigue limit determined by the conventional fatigue tests can not provide the safety design data of the mechanical structures. Thus the fatigue property of metallic materials in the UHCF range tends to be an important subject in the mechanical design to ensure the long term safety of the mechanical structures.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 Q235 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.The experiment results showed that the S-N curve of Q235 steel, both smooth and welding joint specimen, displays the characteristic of"continually decreasing type"up to 109～1010 cycles and exhibits no traditional horizontal plateau beyond 107 cycles. Over 107 cycles and up to 109 cycles, fatigue failure still occurs. So it is very dangerous to design mechanical structures using data determined by the conventional fatigue tests.The inclusions in materials, called micro defect crack, is the nucleus of internal crack initiation. Compared with smooth specimen, the fatigue limit of welding joint specimen is smaller, endurance crack usually located in toe of weld. Weld bead defects reduce the fatigue strength of specimen.