Numerical analysis of high and low‐cycle flexural fatigue for reinforced concrete beams under full‐range of varying amplitudes

Structural reinforced concrete (RC) experiences cyclic loads during their service life, and in practice, the load amplitude can always change, which will lead to a mixed high and low‐cycle fatigue failure. This paper aims to investigate the fatigue behaviors of RC beams under different load levels and more importantly, their sequential effects. Based on the path‐dependent constitutive models, the structural degradation under different repeated loads are comprehensively investigated by numerical simulation of higher consistency with the reality. It is fairly indicated that a clear bi‐linear character may exist in the so‐called S‐N diagram, of which the high‐cycle fatigue failure is attributed to rupture of main reinforcement under tension, while the low‐cycle fatigue failure is controlled by the concrete failure in flexural compression zone. A simple design method to consider mixed low and high cycle fatigue is proposed to quantify this bi‐linear character. For the varying amplitude of cyclic loads, the high‐to‐low loading sequence may cause a shortened fatigue life. More interestingly, if a rather high heavy load (0.85 or 0.95 of the static capacity) is applied just once before ordinary fatigue loads (ex. 0.6), the deflection may significantly increase, while the stiffness and final fatigue life would be slightly reduced in view of nonlinear mechanics.