V. Torra, Polytechnical University of Catalonia, Barcelona, Spain; A. Isalgue, C. Auguet, F. Martorell, Polytechnical University of Catalonia, Esc. Tec. Sup. Eng. Camins, C. i Ports de Barcelona, Barcelona, Spain; F. C. Lovey, H. Soul, Centro Atomico Bariloche and Instituto Balseiro, Bariloche, Argentina; P. Terriault, Ecole de technologie superieure, Montreal, QC, Canada
This paper is centered in the required SMA behavior for their application to earthquakes damping in steel structures as in family houses and, also, in damping of stayed cables in bridges. In the earthquakes, the SMA requirements are associated to the relatively reduced number of oscillations. In fact, the earthquake (near one minute with frequencies close to 1 Hz) and the eventual replicas requires that the damper can realize around 1000 cycles of work without relevant changes and, also, that in the time interval between installation and earthquake (one or more decades) not appears changes related to diffusion processes related to meta-stability of the phases modifying the expected behavior of the material. The summer-winter temperature effects also require analysis. Usually, the dampers are situated inside the house and the amplitude of summer–winter wave temperature effects is smoothed. In the application to stayed cables in bridges the required behavior is substantially different. The working time scale is completely different, eventually the use of damper is immediate after the initial installation and the number of oscillations to be damped is, also extremely higher. In general, successful use of SMA requires a deep and clear knowledge of the effective conditions related to each application and, also, what are the required characteristics of SMA. For instance the use of pre-stressed material that furnishes better behavior in, for instance, damping processes is inappropriate for Cu-based alloys for their ability to stabilize the martensite with time and producing unexpected paths in the phase coexistence trajectories.
Summary: The properties of SMA (or smart materials), are associated to a first order phase named martensitic transformation between metastable phases from the upper temperature phase (austenite) to the lower temperature phase (martensite). The first order transformation (in coordinates stress, deformation and temperature) has hysteresis and permits different levels of applications, i.e., actuators and dampers. Two types of application can be considered in damping of structures in Civil Engineering. The first one the reduction of the damage effects induced by earthquakes. The second one is the increase of the lifetime for the stayed cables in bridges. The analysis of the experimental conditions required for each application and the associated needs of the SMA shows completely different conditions required for the SMA: several years or decades without any action (excepted the summer-winter room temperature parasitic effects) associated with one or two min of oscillations in the damping the earthquake affects or near 105 oscillations per day with pauses of several hours or days in the damping of stayed cables in bridges. This paper analyzes the fatigue behavior of a CuAlBe alloy (appropriate for earthquakes) and shows the results of some available experiments on a NiTi alloy for their eventual application to stayed cables.
