The fatigue behaviors of metals are different under different in-service environment and loading conditions. This study was devoted to investigating the combined effects of high and low cycle fatigue loads on the performance of the low alloy steel Q345. Three kinds of experiments were carried out, including the pure high cycle fatigue (HCF) tests, the pure low cycle fatigue (LCF) tests, and the combined high and low cycle fatigue (HLCF) tests. The prediction formulae were proposed for the combined high and low cycle fatigue failure. Scanning electron microscopy (SEM) and stereo microscope were used to analyze the microstructure and fracture morphology due to different fatigue loads. Case study on the combined high and low cycle fatigue damage of a steel arch bridge was carried out based on the FE method and the proposed formula. The results show that the LCF life decreases evidently due to the prior HCF damages. The HLCF fracture surface is relatively flat near the crack initiation side, and rugged at the other half part. The fatigue damages at the bridge joints increase significantly with consideration of the pre-fatigue damages caused by traffic load. In the 100th anniversary of service, the fatigue damage index without considering the HCF pre-damage is only about 50% of the coupled damage value.
Mechanical properties, including the fatigue behavior of metals, are usually determined from damage-free specimens, but it is not well known how these properties change with respect to prior damages; hence, the present work aims to understand the remaining mechanical properties of low carbon alloy steel Q345q with pre-damages. Low-cycle fatigue tests on the damage free specimens, tensile tests on the low-cycle fatigue damaged specimens, and fatigue tests on the plastic deformed specimens were carried out, respectively. The low-cycle fatigue life prediction formula was proposed. The influences of different kinds of pre-damages on the residual mechanical properties were analyzed. Results show that the stable hysteretic loops in the low-cycle fatigue tests are well-stacked. The material illustrates Masing behavior, and it has a good energy dissipation capacity. The ductility of the low-cycle fatigue-damaged materials decreases significantly in comparison with the undamaged ones. The low-cycle fatigue lives of Q345q steel are almost unaffected, so long as the pre-applied tensile strain is lower than 10%.
For an RC beam, the strength of steel rebar, the bonding strength between the concrete and reinforcement, and the bite action between the aggregates will deteriorate significantly due to corrosion. In the present study, 10 RC beams were designed to study the impact of corrosion on the shear bearing capacity. The mechanism of corrosion for stirrups and longitudinal bars and their effects were analyzed. Based on the existing experimental data, the correlation between the stirrup corrosion factor and the cross section loss rate was obtained. An effective prediction formula on the shear bearing capacity of the corroded RC beams was proposed and validated by the experimental results. Moreover, a numerical analysis approach based on the FE technique was proposed for the prediction of the shear strength. The results show that corrosion of the reinforcements could reduce the shear strength of the RC beams. The corrosion of stirrups can be numerically simulated by the reduction of the cross section. The formulae in the literature are conservative and the predictions are very dispersed, while the predictions by the proposed formula agree very well with the experiment results.
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