Research On Low-Cycle Fatigue Performance Of Reinforcing Bars Accompanied By The Impact Of Buckling

A lot of earthquake damage and experimental phenomenon indicates that buckling of longitudinal reinforcing bar in reinforced concrete column appears obviously before the fatigue fracture. It is important for longitudinal reinforcing bar to have good buckling performance and fatigue performance at the same time. The studies on buckling performance and fatigue performance have developed well respectively at China and abroad, but there is a lack of the method about how to consider the influence of buckling on fatigue. Therefore, the fatigue model of buckled longitudinal reinforcing bar is inaccuracy. It is necessary to obtain a fatigue model which takes buckling into account. The establishing of fatigue model considering buckling is of great significance to predict the time when longitudinal reinforcing bar fracture accurately.The original fatigue model which doesn't consider buckling is based on the fatigue test of reinforcing bar in this research. The simulation about the cyclic loading test of reinforcing bar considering buckling has been done based on the OpenSees finite element analysis software. The force-displacement curve of the longitudinal reinforcing bar, the local strain on the middle cross section and many other correlated data are extracted from the simulation. After modifying the original fatigue model in three stages on the basis of the local plastic strain amplitude on the middle cross section of the longitudinal reinforcing bar, the modified fatigue model III which is in terms of average plastic strain amplitude is set up in the end. Meanwhile, the strength degradation formulation is obtained on the base of the data of cyclic loading test of reinforcing bar considering buckling and the modified fatigue model III. The modified fatigue model IV is set up with the average plastic strain amplitude of reinforcing bar in the cyclic loading test considering buckling.The accumulative fatigue damage factors of reinforcing bar in two cyclic loading test considering buckling completed by different researchers are calculated with modified fatigue model III and modified fatigue model IV. The modified fatigue model III and modified fatigue model IV are modified further by considering yield strength of longitudinal reinforcing bar on the basis of one of the cyclic loading test, and the modified fatigue model III-y and modified fatigue model IV-y are set up. The accumulative fatigue damage factors of longitudinal reinforcing bars in column low-cyclic loading test are close to 1.000 with the modified fatigue model III-y and modified fatigue model IV-y which consider yield strength of longitudinal reinforcing bar. In addition, the strength degradation factors calculated by the strength degradation formulation are close to the test results.It is proved that the modified fatigue model and the strength degradation formulation considering buckling have a better result in simulating fatigue performance considering buckling than the original fatigue model. It is obvious that the modified fatigue model in the text can simulate fatigue performance considering buckling in concrete member more accurately.There are some innovation points in this article.(1)The influence buckling has on the fatigue is considered reasonably.(2)The relationship between local strain and average strain in the cyclic loading test of reinforcing bar is obtained.(3)The modified fatigue model and strength degradation model calculated directly by average plastic strain amplitude are obtained.