Low‐Cycle Fatigue Behavior of Reinforcing Steel

TLDR

The study proposes a methodology to detect incipient failure in reinforcing steel and introduces energy‑based fatigue models linking stress/strain to dissipated energy. ASTM A615 grade 40 and ASTM A722 high‑strength thread bars were tested under axial‑strain‑controlled reversed cyclic loading from yield to 6 % strain amplitude on virgin specimens to simulate seismic behavior, and the data were analyzed with existing fatigue models relating stress–strain to life. The results show similar modulus of toughness and low‑cycle fatigue life for low‑ and high‑strength bars, leading to the conclusion that current design codes unnecessarily restrict the use of high‑strength thread bars in seismic‑resistant elements.

Abstract

ASTM A615 grade 40 ordinary deformed‐steel reinforcing bars with a specified minimum yield strength of 276 MPa (40 ksi) and ASTM A722 high‐strength prestressing thread bars with a specified ultimate strength of 1,083 MPa (157 ksi) were experimentally evaluated for their low‐cycle fatigue behavior under axial‐strain‐controlled reversed cyclic tests with strain amplitudes ranging from yield to 6%. All tests were performed on virgin (unmachined) specimens to closely simulate seismic behavior in structural concrete members. A methodology is suggested to identify incipient (first‐cracking) failure of test specimens. The experimental data were evaluated with existing fatigue models, which related stress‐strain quantities to the failure life. Additional energy‐based fatigue models are proposed that relate various stress and/or strain quantities to the dissipated energy. The study demonstrates that the modulus of toughness and low‐cycle fatigue life for both the low‐ and high‐strength materials are similar. Based on fatigue considerations, it is concluded that existing design codes are overly restrictive in not permitting the use of high‐strength thread bars in seismic‐resisting elements.