Evaluation of the Low-Cycle Fatigue Life in ASTM A706 and A615 Grade 60 Steel Reinforcing Bars

The low-cycle fatigue behavior of ASTM A706 and A615 Grade 60 (420 MPa) deformed reinforcing steel bars were evaluated experimentally. Although this study was initiated to evaluate the low-cycle fatigue of reinforcing bars in precast hybrid frame connections, the conclusions presented here may also apply to other relevant applications. Laboratory tests were performed under strain-controlled cyclic axial loading with nonzero mean strains. The deformed bars were subjected to constant-amplitude sinusoidal strains ranging from zero to peak strains that varied between 2 and 8% in different tests. All tests were performed on unmachined bar specimens. The experimental data were analyzed and compared with existing low-cycle fatigue models. Such models relate the total and plastic strain ranges to the number of cycles to failure. Relationships for calculating the tensile and compressive stresses corresponding to maximum strains are proposed based on the experimental results. Equations that relate dissipated energy to strain amplitudes and the number of cycles to failure are developed. This study demonstrates that the low-cycle fatigue responses of ASTM A706 and A615 mild steel bars are similar even though their monotonic ductility ratios are very different. The proposed low-cycle fatigue relationships for both ASTM A706 and A615 mild steel bars can be used to calculate maximum permissible strains in applications such as the precast/prestressed hybrid frames. The prediction of bar fracture due to low-cycle fatigue is an important consideration in the seismic design of reinforced and precast/prestressed concrete structures.