Experimental Assessment of the Stress-Ratio Sensitivity of the BS 4449 Fatigue Prequalification Criterion for 10- and 16-mm Steel Reinforcing Bars Under Constant-Amplitude Loading

The fatigue performance of reinforcing bars is essential to the durability and safety of reinforced concrete structures. BS 4449 assesses fatigue prequalification using a stress range of Δσ = 200 MPa, a stress ratio of R = 0.2, and a run-out criterion of 5 × 106 cycles for reinforcing bars with a nominal diameter not exceeding 16 mm. However, the influence of stress ratio variations under these conditions remains insufficiently documented. This experimental study examines the effect of stress ratio on the BS4449 fatigue prequalification results for 10- and 16-mm steel reinforcing bars under constant-amplitude loading. The bars were characterized through microstructural and inclusion analyses to provide a complete assessment of their metallurgical condition. Most specimens reached 5 × 106 cycles without failure. Two failures occurred: one at R = 0.3 for the 10 mm bars and one at R = 0.2 for the 16 mm bars. Most specimens maintained the required run-out across a broader range of loading conditions. SEM observations identified MnS inclusions as the predominant inclusion population. These inclusions may serve as crack initiation sites. These findings highlight the importance of steel cleanliness when interpreting fatigue qualification results under varying stress ratios.