PSI - Issue 5

J. Morais et al. / Procedia Structural Integrity 5 (2017) 705–712

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Morais J et al./ Structural Integrity Procedia 00 (2017) 000 – 000

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by limiting the transmitted displacement (Seismic Isolation techniques) or by absorbing the energy of the seismic event (Energy Dissipation techniques). One energy dissipation technique consists on using dampers based on Shape Memory Alloy (SMA) wires. Shape memory alloys have many interesting properties that can be exploited in these applications, namely their Superelasticity, high fatigue resistance, near strain-rate independence (in certain conditions related to temperature control), among others [Dolce and Cardone (2001)]. Superelasticity is the main property explored in this type of devices and represents the material’s capability to change between metallurgical phases (austenitic and martensitic phases) due to stress application cycles. This property allows SMA based dampers to withstand very large strains (when compared to dampers based on other metallic materials) without any residual deformation upon unloading, while dissipating energy during the loading/unloading cycles. Fig. 1 represents a generic stress – strain tensile curve of a Superelastic SMA material, for the case where the SMA is above its austenitic phase transformation temperature (A f in the literature), i.e. the SMA material is in the austenitic phase at room temperature. This metallurgical state provides better properties for this type of dampening application [Dolce et al. (2000)]. Zone A and C correspond to the elastic deformation of the austenitic and martensitic phases, respectively. Zone B is the forward transformation plateau where the material is changing from the austenitic phase to the martensitic phase due to applied stress. Zone D represents the plastic deformation of the martensitic phase, after which if the SMA is unloaded there will be some residual strain (zone E). Zone F and G refer to the unloading path of the SMA while it still is in the elastic deformation region. Here the material recovers in the martensitic phase (F) and then begins the inverse phase transformation (G) when a certain stress level is reached, until it returns to its original austenitic phase with no residual strain.

Fig. 1. Generic stress-strain response of a SMA above temperature Af.

Based on the previous graphic, the following features of a SMA based damper can be discerned:

 Energy dissipation : since the forward and inverse transformation take place at different stress levels (zones B and G), a hysteretic cycle occurs. The energy dissipated by the SMA material is equal to the area under the hysteretic cycle [Dolce et al. (2000)]. This reduces the amount of energy transmitted to the structure under protection.  Control of the transmitted force : because the forward transformation plateau (B) has a relative low slope, the device can withstand a large range of deformation under near constant stress. Hence the transmitted load can be controlled based on the device ’ s characteristics.  Multiple stiffness stages: due to the erratic and unpredictable nature of seismic activity, it can be advantageous to have a device that: i) can stop undesirable movements for low load levels, due to external actuations on the structure unrelated to seismic activity like the wind (initial stiffness from zone A); ii) allow a good range of displacement for moderate earthquakes under near constant load (B); and iii) also resist further displacement of the structure if subjected to high intensity seismic activity (stiffness from zone C). These topics are some of the features that make SMA based dampers a good alternative to more conventional solutions. The final goal of this study is to devise several damper configurations, each tuned to a different set of