Multiaxial Fatigue Life Prediction Based on High-Cycle Uniaxial Fatigue Test of Steel Pipe Weldments with Welding Defects

Welding defects are unavoidable for welded structures, which can lead to fatigue damage even under the random wind load with small amplitude. It is therefore necessary to explore the effect of welding defect on the fatigue properties of welded steel pipes. Three groups of welded steel pipe specimens were designed according to welding defect conditions, i.e. specimens without welding defect (Group I), specimens with incomplete fusion (Group II), and specimens with welding porosity (Group III). Uniaxial tension-compression and torsion high-cycle fatigue tests were carried out. S-N curves of uniaxial tension-compression and torsion tests were obtained by cyclic loading with equal stress amplitude. The test results show that the high-cycle fatigue strength of weldments is obviously lower than that of base metal with the same strength under uniaxial tension-compression and torsion loading. In addition, the welding defects result in a decrease in fatigue strength, while the decrease extent by welding porosity is greater than that by incomplete fusion. Finally, because of the inherent multiaxial loading characteristics of welded structures, the high-cycle multiaxial fatigue life of steel pipe weldments was also predicted by using the modified Wohler curve method based on the uniaxial fatigue test results. It can be found that when the stress amplitude is constant, the fatigue life of welded steel pipe decreases and the modified Wohler curves move downward more quickly with the increase of damage parameter defined as the ratio of normal stress amplitude to shear stress amplitude on the critical plane, which means that normal stress amplitude will accelerate the cracks growth and result in faster failure of the weld materials.