Study On Fatigue Property Of Structural Steel Under Variable Amplitude Loading

In recent years, with the improvement of running speed, engineering structures had a longer service life. At the same time, fatigue caught attention of people. With increasing frequency of cyclic loading, structural steel’s fatigue performance which was under the influence of low stress amplitude had caught people’s attention. Because of limit of experimental technique and theoretical basis, the research of metal structure’s fatigue performance is far from enough. Mostly of material’s research was based on constant amplitude. In fact when structure working, the stress amplitude was derricking. So it’s necessary to investigate the performance of fatigue performance under derricking stress amplitude. Thanks to the improvement of stored program control fatigue test machine and new detect technology, the fatigue test had a large development. Doing study on derricking stress amplitude had an important significance in fatigue prediction theory. In case to study on the regularity of strengthening effect which is developed by small stress amplitude, this paper develop experiment of 50CrVA structure steel. The research achievement as follows:Finite element analysis was used to design specimen of fatigue experiment. By using finite element analysis, specimen had succeed to avoid buckling when under compressive stress of axial direction. Solution by finite element method was compared to theory solution of reduced model to verify that specimen would not buckling.Scheme of constant amplitude experiment was designed basing on the metal fatigue test standards. The experimental result showed that the S-N curve had tendency of successive declining. Basquin equation was used to fit the S-N curve. The fatigue limit was estimated by lifting and lowering method. This experiment had provide data to follow-up experiment.When doing experiment, on-load duration and magnitude of stress amplitude were regulated to study on derricking stress amplitude fatigue performance. The result of different on-load durations showed that strengthening effect need enough times to affect on material. Small stress amplitude had two characteristics when it worked on 50CrVA:destructive and amplifier. The result of different small stress amplitudes showed that different small stress amplitudes had something to do with strengthening effect of material. It was not significant when small stress amplitudes was very low. With increasing of small stress amplitude, strengthening effect was more than more significant. When small stress amplitude passed the peak value, strengthening effect was getting weak. Comparing different points of high stress amplitude showed that the tendency of strengthening effect curves was the same.Scanning electron microcopy was conducted to examine the fatigue fracture surface. It showed that the fatigue fracture was brittle fracture. all fatigue cracks emanated from the surface of specimen. The fatigue fracture surfaces which had no strengthening effect were the same. The fatigue fracture surfaces which had strengthening effect were apparently different.