Low-cycle fatigue failure of reinforcing steel bars

A comprehensive experimental study was carried out to examine the low-cycle fatigue behavior of ordinary reinforcing bars used in reinforced concrete (RC) construction. The objective of the study was to gain a better understanding of low-cycle fatigue failure of the longitudinal steel reinforcement in potential plastic hinge zones of RC members subjected to seismic loads and to develop a fatigue life relationship to characterize the response. A special-purpose experimental setup was constructed to conduct fatigue tests of standard ASTM A 615 reinforcing steel ranging from No. 6 to No. 9 bars with a specified yield strength of 60 ksi (420 MPa). To transfer a force of sufficient magnitude to induce inelastic axial strains in the bar a "swaging" concept was employed to design and fabricate a special gripping mechanism capable of resisting both tension and compression without altering the virgin state of the deformed reinforcing bar specimens. A pair of aluminum strips was securely fastened between the specimen and the custom-built gripping blocks that were used to act as the force transfer media. Deformation patterns present on the specimen and deformation ridges incorporated onto the grips using a high-hardness weld diode allowed for the transfer of force via shear stress through the aluminum. The results of both monotonic tension tests and low-cycle fatigue tests using constant amplitude cyclic strain histories on four different bar sizes are reported in this paper. Preliminary findings indicate that fatigue life is influenced by the diameter of the bar and the geometry of the rolled on deformations. Fatigue life relationships based on total strain amplitude and energy are developed for use in damage and failure modeling. It is shown that fatigue life models based on strain are more reliable than energy-based models. Finally, the study points to the need for including low-cycle,fatigue behavior of the bars, in addition to monotonic properties, for applications that subject the bar to reversed cyclic loads.