Abstract
While it has been known for decades that even for quasi-static loading, increments in strain rate lead to increased stresses in the inelastic range, this effect is often ignored. However, accurate knowledge of the stress-strain characteristics of reinforcing bars is an indispensable prerequisite for the safe design of new structures and the realistic assessment of the structural safety of existing ones, and the strain rate dependency of the reinforcing steel characteristics should thus be accounted for in many situations. An exemplary case is quality control: Reinforcing bars produced today are periodically tested to check their conformity with specifications, determining their stress-strain characteristics in standard tensile tests. However, the applied quasi-static strain rates may vary considerably, but are not commonly reported. Hence, the results are subject to considerable uncertainty. Another relevant case is the structural safety assessment of existing structures affected by local corrosion of the reinforcement: Their cross section (and hence, stiffness) varies considerably along the bar axis and consequently, the strain rate in corroded sections is significantly higher than in non-corroded sections, leading to higher yield stress and tensile strength.This study investigates the effect of quasi-static strain rates on the stress–strain characteristics of modern reinforcing bars based on a comprehensive experimental campaign. In four series of experiments, 41 tensile tests on three different types of reinforcing bars were conducted, applying strain rates between 0.004 ‰/s and 1.0 ‰/s. Compared to the static stress, an increase of up to 8% in the dynamic stress was observed, depending on the type of reinforcing bar. Based on these observations, a simplified model for the strain rate dependency was developed and validated against experimental data, showing excellent agreement.
Highlights
Numerous studies investigated the influence of strain rate on the mechanical behaviour of steel and recognised an increase of stresses in the inelastic range with higher strain rates
For the slow strain rate of 0.004 ‰/s, the decay kept being logarithmic even after 45 min, without showing indications of a decrease of the slope. This leads to the conclusion that strain rate before stopping influences both, the magnitude of the stress decay and its rate. It indicates that a duration of the machine stop of 2 min, as commonly used in materials testing, is not sufficient for tests conducted at low strain rates to reach steady state and determine the static stress
Note that the static stress r0ðeÞ for experiments with strain rates lower than 1 ‰/s was calculated using the procedure described in Section 3.2., which may affect the accuracy, and that the given regression coefficients are strictly speaking only valid for the tested samples or production batches of reinforcing steel
Summary
Numerous studies investigated the influence of strain rate on the mechanical behaviour of steel and recognised an increase of stresses in the inelastic range with higher strain rates. The effect of quasi-static strain rate may be relevant in experimental campaigns on structural concrete, in the field of corrosion investigations: Reinforcing bars with non-constant cross section over their length, as commonly investigated in corrosion studies, exhibit different strain rates and different strengths in the intact and corroded sections, respectively This is due to the varying cross section reduction along the bar axis and the correspondingly varying tensile stiffness EA. Contrary to the numerous studies on the influence of medium and high strain rates on the stress-strain characteristics of steel in the past, the data set is scarce for modern reinforcing steel at quasi-static strain rates This missing data impedes a proper separation of the effects of different phenomena on the stress-strain characteristics of corroded reinforcing bars as described above, and complicates a reliable comparison of test results from different laboratories in production quality control. A simplified model is developed to describe the strain rate dependency of the mechanical characteristics of modern reinforcing bars in the inelastic range
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