Seismic vibration control of frame structure using shape memory alloy

Abstract

The use of Shape Memory Alloy (SMA) in mitigating the seismic vibration response of civil infrastructure is gaining momentum. The name “Shape Memory” implies that it remembers its original formed shape. SMA has two basic properties, Super-Elasticity and Shape Memory Effect (SME). The “Super-Elastic” behaviour exhibited by SMA materials, permits a full recovery of strains up to 8% from large cyclic deformations, while developing a hysteretic loop. SME allows the material to recover the initial shape or position which in turn can be used as re-centering mechanism. The mechanism of shape recovery involves two crystallographic phases, Martensite and Austenite, and the transformations between them. The Austenite phase provides more stiffness than the Martensite phase. Phase transformation occurs between Martensite & Austenite depending upon temperature & stress. These unique properties result in high damping, combined with repeatable re-centering capabilities which can be used to advantage in several civil infrastructure applications, especially in seismic vibration control devices. Super-Elastic response of SMA has historically been the primary mode of interest of civil engineers as it occurs over a wide-range of temperatures; and also because SMA reaches activation temperature and becomes Austenite at the ambient temperature of civil engineering infrastructures. Thus the re-centering capability of SMA by generation of an activation force is not utilized. The use of high temperature SMA has enabled the re-centering mechanism to work. The SMA is heated by electrical current flow and the use of constant current in this purpose will result in greater power consumption which can be reduced significantly by passing pulsed current through the SMA using Pulse Width Modulation (PWM) technique. In this study, both the Super-Elastic and Shape Memory Effect has been taken into account by using SMA with high activation temperature. A Thermo-Mechanical SMA phenomenological constitutive model is used to simulate the SMA behaviour. The dynamic response data of a frame structure has been obtained from FE analysis by using the nonlinear FE software program MSC Marc. Then the frame is braced and reanalyzed; first using standard steel wire and then later using SMA wire, the seismic response of both the braced frames were measured. The SMA bracing is activated by joule-heating due to electrical current flow. The SMA is first activated by constant current, later using pulsed current. In this research work, From the FE solutions, the effectiveness of SMA braces as a seismic vibration control device and guidelines to optimum electrical input, considering appropriate stiffness and damping characteristics; is established. From the simulation result, it is evident that the use of pulsed current resulted in reduced energy consumption by the SMA, as well as mitigating the seismic vibrations on the frame structure.