Evaluation of the Low-Cycle Fatigue Life in Seven Steel Bar Types

The low-cycle fatigue (LCF) behavior of steel in energy-dissipating seismic connections is an important consideration, especially in light of the interest in performance-based seismic design. In this study, the LCF performance of seven steel bar types (AISI 8620, 1018, 1045, 1117, 1215, 4140 and ASTM A36 steel) was experimentally examined and compared. The bar specimens were subjected to sinusoidal strains of constant amplitude from zero to peak strains of 4%, 6%, or 8%. Equations that relate the applied strain amplitudes with the number of cycles to failure were developed and compared. In addition, relationships for calculating the total dissipated energy corresponding to the applied strain amplitude were proposed based on the experimental results. This study demonstrated that, in general, the LCF resistance of AISI 1045 steel type outperformed the other steel materials at a strain amplitude of $\pm 2\%$. However, at $\pm 3$ and $\pm 4\%$ strain amplitudes, the LCF lives of ASTM A36 and AISI 1117 bars outperformed all others, respectively. The results also showed that steel types of similar strength could have varying LCF resistance. The impact of chemistry on the LCF life at different strain amplitudes was assessed by correlating the contents of various elements with the LCF resistance data. The results indicated that the sum of Si, Cr, and Mn contents had the highest negative correlation with LCF life. The selection of steel types used in seismic detailing in buildings should consider LCF performance, particularly in designated yielding elements within seismic connections (such as in precast hybrid frames), which rely on repetitive yielding of bars for energy dissipation.