Strain-ageing effects on the residual low-cycle fatigue life of low-carbon steel reinforcement

Giuseppe Loporcaro,Alberto Cuevas,Stefano Pampanin,Milo V. Kral

doi:10.1617/s11527-022-01885-0

Abstract

Low-cycle fatigue (LCF) failures can be expected when rebars are subjected to a small number of cycles at large stress/strain amplitudes. Major seismic events are often preceded or followed by other events of smaller/larger magnitude. During each event, the residual fatigue life of the steel reinforcement may reduce, eventually leading to failure of the bars due to the cumulative damage. Strain ageing may reduce even more the residual fatigue life of steel. This paper presents the results of an experimental testing campaign conducted on samples fabricated from New Zealand Grade 300E steel. A benchmark LCF life was obtained for 12-mm steel reinforcing bars subjected to axial-strain-controlled completely reversed cyclic tests at amplitudes ranging between 0.78 and 2.75%. The reduction in fatigue life attributed to strain ageing was obtained by comparing the benchmark fatigue life with that of steel reinforcing bars precycled up to 33% and 66% of the benchmark life and artificially strain aged for an equivalent period of 1 year at 15 °C. The Coffin-Manson and Koh-Stephens models were employed to evaluate the experimental results. The study demonstrates that, depending on the strain amplitude, strain ageing can cause a reduction in total and residual fatigue life ranging from 20 to 70%. Strain ageing should not be ignored during the assessment of earthquake-damaged steel reinforcing bars.

Highlights

IntroductionThe cyclic stress and strain induced by the seismic event may cause microscopic physical damage to the steel reinforcement
Steel reinforcing bars embedded in reinforced concrete members may undergo a small number of cycles at large inelastic amplitude in tension and compression as high as 6% strain, eventually fracturing the rebars due to a mechanism known as low-cycle fatigue (LCF) [1]
If the steel reinforcing bars yield in a seismic event, strain ageing is likely to occur in susceptible steels, which causes a change in the mechanical properties of the materials [5]

Summary

Introduction

The cyclic stress and strain induced by the seismic event may cause microscopic physical damage to the steel reinforcement. As the number of cycles increases, the microscopic damage accumulates and generates a crack that will eventually cause the material to fracture. This failure mode was recorded during experimental tests on circular bridge columns [2, 3]. If the steel reinforcing bars yield in a seismic event, strain ageing is likely to occur in susceptible steels, which causes a change in the mechanical properties of the materials [5]

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