Experimental Study And Theoretical Analysis On Low-Cycle Fatigue Performance Of Local Corroded Steel

Fatigue is considered to be one of the main reasons causing structural failure, fatigue failure is also a very common phenomenon in project. The actual steel components are often serviced under repeated loads, such as strong winds, earthquakes which may cause the occurrence of fatigue fracture. On the other hand, corrosion of steel structure happens easily in atmospheric environment. Studies show that corrosion of steel (especially local corrosion pit caused by non-uniform corrosion) has significant effect on its fatigue performance. Therefore, the effect of local corrosion on low-cycle fatigue performance was studied in this paper.In this paper, three groups including 24 specimens were designed, considering of the depth and longitudinal length of a corrosion pit. A simplified method that can be characterized the local corrosion pit was proposed based on the existed literature research and results of exploratory experimental. The corrosion test of 24 specimens was completed by using the method of electrochemical accelerated corrosion. Results of the corrosion test have verified that using the Faraday’s law to control the size of corrosion pit is feasible.Low-cycle fatigue tests of 24 corroded specimens and 2 smooth specimens have been finished with total axial strain control which the strain amplitude ranged from -0.1%to 0.3%. Cyclic stress response characteristics of part specimens and the effect of corrosion pit on fatigue life were investigated. It can be concluded that the existence of a corrosion pit leads a sharp decline on fatigue life of steel, the fatigue life decrease a lot with the increasing of pit depth when the depth is below 1.1 millimeter but maintain at a relatively low level with minor changes when it is above 1.1 millimeter, and that Q235 shows significant cyclic softening in low strain amplitude (<0.3%).The effect of various pit size (dmac、Lx、dmax/Lmax) of corrosion pits on stress concentration factor Kt under tension loading was discussed by establishing the FEM through ANSYS. Results show that Kt increases with the increasing of dmax/Lmax when dmax / Lmax is below 0.5, and the fatigue life decrease with the increasing of dmax/Lmax generally.With the aid of optical microscope and SEM, micrographs of specimen fracture were achieved and the influence of stress concentration on macro-patterns and microstructure for fatigue fracture sections of corroded steel was analyzed. The micrographs revealed that the macro-cracks, which increase with the increasing of Kt, were formed beginning from the bottom of corrosion pit, and the microstructure of fatigue extension area shows typical feature of stripes.