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Experimental tests performed on superelastic shape-memory alloys (SMAs) show a significant dependence of the stress-strain relationship on the loading-unloading rate, coupled with a not negligible oscillation of the material temperature. This feature is of particular importance in view of the use of such materials in earthquake engineering, where the loading rate affects the structural response. Motivated by this observation and by the limited number of available works on the modelling of SMAs for seismic applications, the present article addresses a uniaxial constitutive model for representing the system rate-dependent thermo-mechanical behavior of superelastic SMAs. The model is based on a single internal scalar variable, the martensite fraction, for which different rate-independent evolutionary equations in rate form are proposed. Moreover, it takes into account the different elastic properties between austenite and martensite. The whole model is then thermo-mechanically coupled with a thermal balance equation. Hence, it considers mechanical dissipation as well as latent heat and includes temperature as a primary independent variable, which is responsible for the dynamic effects. The article also provides a description for the integration, in time, of the constitutive equation and presents the solution algorithm of the corresponding time-discrete problem. Finally, results from numerical analyses are reported and the ability of the model to simulate experimental data obtained from uniaxial tests performed on superelastic SMA wires and bars at frequency levels of excitation typical of earthquake engineering is assessed. [ABSTRACT FROM AUTHOR]

The use of special concentrically braced frames has increased since the 1994 Northridge and 1995 Hyogoken–Nanbu Earthquakes. However, past performance suggests limited ductility and energy dissipation in braced frame systems due to buckling of conventional braces. In order to address this limitation, three- and six-story concentrically braced frames with superelastic shape memory alloy (SMA) braces are studied to evaluate their seismic performance in comparison to traditional systems. SMAs are unique metallic alloys that have the ability to undergo large deformations while reverting back to their original undeformed shape providing recentering capabilities to the braced frame. Detailed analytical models of the frames with SMA braces are developed and two suites of ground motions are used to evaluate the structures with respect to interstory drift and residual drift. The results suggest that the SMA braces are effective in limiting interstory drifts and residual drifts during an earthquake, in part, due to the recentering nature of superelastic SMAs. [ABSTRACT FROM AUTHOR]

This paper presents and compares two different uniaxial constitutive models for super-elastic shape-memory alloys (SMAs), suitable to study the dependence of the stress-strain relationship on the loading-unloading rate. The first model is based on the inclusion of a direct viscous term in the evolutionary equation for the martensite fraction and it shows how the material response is bounded between two distinct rate-independent models. The second model is based on a rate-independent evolutionary equation for the martensite fraction coupled with a thermal balance equation. Hence, it considers mechanical dissipation as well as latent heat and includes the temperature as a primary independent variable, which is responsible of the dynamic effects. The ability of both models to reproduce the observed reduction of damping properties through the modification of the hysteresis size is discussed by means of several numerical simulations. Finally, the capacity of the constitutive equations to simulate experimental data from uniaxial tests performed on SMA wires and bars of different size and chemical composition is shown. [ABSTRACT FROM AUTHOR]

In this paper, the seismic performance of a three- and a six-storey steel frame equipped with different bracing configurations is assessed. The bracing systems consist of traditional buckling-restrained steel braces and superelastic Nitinol shape-memory alloy (SMA) braces. Background on the behaviour of SMAs is provided and a state-of-the-art review of the applications of such new materials in earthquake engineering is presented. A uniaxial constitutive model for superelastic SMAs is then implemented into the finite element platform OpenSees and nonlinear dynamic analyses are performed. Finally, the seismic performance of the structures under investigation is judged through the evaluation of several response quantities, to determine the efficacy of the new bracing system in reducing earthquake-induced vibrations. [ABSTRACT FROM AUTHOR]

Superelastic NiTi shape memory alloy (SMA) wires and bars are studied to determine their damping and recentering capability for applications in the structural control of buildings subjected to earthquake loadings. These studies improve the knowledge base in regard to the use of SMAs in seismic design and retrofit of structures. The results show that the damping properties of austenitic SMAs are generally low. However, the residual strain obtained after loading to 6% strain is typically <0.75%. In general, it is shown that large diameters bars perform as well as wire specimens used in non-civil-engineering applications. The results of a small-scale shake table test are then presented as a proof of concept study of a SMA cross-bracing system. These results are verified through analytical nonlinear time history analysis. Finally, a three-story steel frame implementing either a traditional steel buckling-allowed bracing system or a SMA bracing system is analyzed analytically to determine if there is an advantage to using a SMA bracing system. The results show that the SMA braces improve the response of the braced frames. [ABSTRACT FROM AUTHOR]