Abstract
Shape memory alloys in the form of bars are increasingly used to control structures under seismic loadings. This study investigates the hysteretic behavior and the ultimate energy dissipation capacity of large-diameter NiTi bars subjected to low- and high-cycle fatigue. Several specimens are subjected to quasi-static and to dynamic cyclic loading at different frequencies. The influence of the rate of loading on the shape of the hysteresis loops is analysed in terms of the amount of dissipated energy, equivalent viscous damping, variations of the loading/unloading stresses, and residual deformations. It is found that the log-log scale shows a linear relationship between the number of cycles to failure and the normalized amount of energy dissipated in one cycle, both for low- and for high-cycle fatigue. Based on the experimental results, a numerical model is proposed that consists of two springs with different restoring force characteristics (flag-shape and elastic-perfectly plastic) connected in series. The model can be used to characterize the hysteretic behavior of NiTi bars used as energy dissipation devices in advanced earthquake resistant structures. The model is validated with shake table tests conducted on a reinforced concrete structure equipped with 12.7 mm diameter NiTi bars as energy dissipation devices.
Highlights
Earthquakes cause heavy casualties and property damage worth billions
Among the different types of energy dissipation devices developed in the past, those based on the use of shape memory alloys (SMAs) are appealing because, in addition to dissipating energy, they are able to regain their original shape after being deformed well beyond 6–8% strain [1,2,3,4,5,6,7,8]
This paper presents an experimental study aimed at (i) characterizing the hysteretic behavior and evaluating the ultimate energy dissipation capacity of large diameter bars made of NiTi alloys, and (ii) proposing a simple numerical model that can be implemented in a finite element code to perform non-linear time history analyses and obtain the seismic response of structures equipped with devices that use large diameter NiTi bars as a source of energy dissipation
Summary
Earthquakes cause heavy casualties and property damage worth billions. Designing structures to withstand the vibrations induced by seismic actions is of primary concern. Increasing the strain rate results in a vertical displacement (i.e. both loading and unloading transformation stresses increase) and significant influence on the shape of the hysteresis loops. This reduction of dissipated energy, together with the vertical displacement of the loops, reduces the equivalent viscous damping The reason for this behavior is the self-heating of the material associated with an increasing difficulty to transfer the heat generated between phase transformations at high strain rates [1,2,3]. Increasing temperature above Af causes a vertical displacement of the hysteresis loop (i.e., higher loading and unloading transformation stresses), while the energy dissipated per cycle remains almost the same. The results of the shake table tests were used to validate the numerical model proposed
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