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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EP

SP

SP

Transformation Plateaus

Fig. 3. Simulated stress-strain curves of both wire bundles performing the hysteretic cycle (left) and the combined external result (right). This loading path represents the basic dissipation mechanism of this device. SP is the starting strain point for both wire bundles and EP is the final strain point. These graphics are obtained by a very basic spreadsheet simulator created by the author.  Pre-strain : the damping effect on this type of Superelastic systems can be enhanced by introducing pre-strain in the SMA wires [Dolce and Cardone (2001)]. This initial strain applied to the Superelastic wires guarantees that they start and also work in the martensitic transformation plateau, even for low displacement values, thus improving the overall damping effect of the damper. Both wire bundles are pre-strained to the middle of the phase transformation plateau in order to maximize the usable range of the device. This configuration also exhibits the re-centering effect, albeit at a lower level. The re-centering effect refers to the device’s capability to return to the central position after an external actuation, when the load is released, similar to a regular spring [Dolce et al. (2000)]. But unlike a spring, due to the non-linear Superelastic behavior of the SMA, with this configuration there are multiple possible stable positions (internal force equilibrium), depending on the previous actuation path(see examples on Fig. 4). Another feature of this configuration, related to the Superelastic behavior, is that if the device is forced to the central position, there is a high probability that it will retain some residual stress, unless a specific loading path is used beforehand.

Fig. 4. Simulated stress-strain curves illustrating the re-centering effect of the device. Each illustrated curve represents the loading path of each wire bundle. SP is the starting strain point for both wire bundles. The device is in internal force equilibrium when both wire bundles stop around the horizontal line (same load on both wire bundles), situation visible on all three cases. In order to begin the validation process of this device and to better understand the SMA material behavior, we began by building a prototype version of the damper, on a smaller scale (see Fig. 5). The experimental tests and results obtained with this prototype are the main focus of this paper. This down-scaled device has the same features of the