V. Torra, A. Isalgue, F. Martorell, Polytechnical University of Catalonia, Barcelona, Spain; F. C. Lovey, H. Soul, Centro Atomico Bariloche and Instituto Balseiro, Bariloche, Argentina
The work is focused in the application of SMA in damping of structures. Two levels are possible, the first relates low number of working cycles (i.e., 200) after several years or decades without any action as in the earthquakes in family houses. The second level centers in the higher number of cycles (i.e, 100000 or higher), for instance, related to the effects of wind or rain in stayed cables in bridges. The fatigue life for pseudoelastic NiTi SMA wire of 2.46 mm of diameter is studied by cycling in an hydraulic MST at different cycling amplitude, cycling frequency, room conditions surrounding the SMA i.e., in several liquids and in still or moving air. The analysis shows the action of relevant thermal effects associated to self-heating increasing the stress on the material and acting in fatigue life. The main results shows that the maximal stress (smax) is the more relevant macroscopic magnitude related to fatigue failure at N cycles. A relation between smax and N can be established (smax = f(N)). The stress value is clearly affected by several temperature effects (external room temperature and self-heating) under the action of Clausius-Clapeyron coefficient (for this NiTi alloy the value approaches 6.3 MPa/K). Using appropriate cycling method the fatigue life is improved and the evolution of hysteresis energy quantified. Appropriate model of the hysteresis behavior with representative internal loops can be built via a set of bilinear models. The routine is embedded in the ANSYS software permitting satisfactory simulation of dynamic effects in structures.
Summary: The use of SMA in damping of structures requires a clear description of the requirements of each expected application. In fact, the use as a damper for earthquakes in family houses need a reduced number of working cycles (i.e., 200) after several years or decades of complete inaction. At the contrary, the eventual damping of oscillations in stayed cables under the action of wind and rain requires high fatigue life. In working conditions, they are expected near 100000 oscillations/day. In this work, the fatigue life and the damping effects of a wire of NiTi (diameter 2.46 mm) in pseudoelastic state are studied and optimized via a “conditioning or training” treatment. The experimental analysis shows that the main effect affecting the fatigue life is the maximal applied stress as in classical materials and the self-heating effects on the material. The effect is clearly dependent on wire cross section and on working frequency and cycling amplitude of the cycles. Also, the external room temperature (i.e., summer-winter) need to be accounted. The stress effects of temperature actions (external and self-heating) are highly relevant in the fatigue life of NiTi by the action of the Clausius-Clapeyron coefficient close to 6.3 MPa/K. The hysteresis cycle and the smooth evolution of the hysteretic area in fatigue process are quantified for the eventual simulation requirements. The mechanical structure of a damper is outlined avoiding the parasitic effects associated to compressive part of the oscillation.
