Experimental and Numerical Investigations of High-Cycle Fatigue Properties of HTRB630 High-Strength Steel Bars

To explore the application of heat-treated ribbed bar (HTRB) type HTRB630 high-strength steel bars in reinforced concrete bridges, the tensile test and high-cycle fatigue test are carried out. Then, the high-cycle fatigue properties of the specimens are analyzed and discussed from median stress-number of loading cycles ($S\text{-}N$) curve, $P\text{-}S\text{-}N$ curve, comparison with 500-MPa grade steel bars, and fatigue fracture mechanism. Finally, their high-cycle fatigue properties are investigated numerically, and the influence of stress ratio on their fatigue properties is discussed. The results indicated that the probability-stress-number of loading cycles ($P\text{-}S\text{-}N$) curve with a confidence level $\gamma =75\%$ and a reliability guarantee rate $P=97.7\%$ is more accurate than the median $S\text{-}N$ curves for the fatigue design of reinforced concrete bridges. In the $P\text{-}S\text{-}N$ curve, the stress ranges at 2 million and 10 million cycles are 237.03 and 203.38 MPa, respectively. In the median $S\text{-}N$ curve, the stress ranges at 2 million and 10 million cycles of fatigue life of steel bars are 262.77 and 236.08 MPa. The fatigue properties of the specimens are higher than those for the 500-MPa grade steel bars. The fatigue fracture of the specimens consists of a fatigue source zone, crack propagation zone, and transient fracture zone. Microscopically, it is a cleavage fracture in the crack propagation zone; it is micropore aggregation fractures in the instantaneous fracture zone. At a given fatigue life, the fatigue stress range decreases as the stress ratio increases. The fatigue properties of the specimens are better at a higher stress ratio with the same maximum stress.