PSI - Issue 5

J. Morais et al. / Procedia Structural Integrity 5 (2017) 705–712 Morais J et l./ Structural Integrity Procedia 00 (2017) 00 – 0 0

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Fig. 7. Stacked stress-strain curves of several test series. The left graphic represents all test series performed with a cyclic frequency of 0.5 Hz and the right graphic represents all the test series realized with displacement amplitude of 10 mm.

Finally, Fig. 8 illustrates the device ’ s sensitivity to displacement amplitude (here shown as strain amplitude) and strain-rate relative to it’s damping ratio. Here we can see that both parameters have a decreasing effect on the equivalent viscous damping ratio of the device, in view of the specific conditions of the tests (room temperature, applied amplitude and cyclic frequency ranges, specific SMA tested, among others):

Fig. 8. Influence of strain amplitude (left) and strain- rate (right) on the device’s damping ratio.

 Strain amplitude sensitivity : as the strain amplitude increases, the E D also increases as expected. But since E S grows proportionally more than E D , the ratio E D /E S decreases and so does the damping ratio. This is true for the specific SMA wire in use. Other alloys with a lower martensitic transformation slope can attenuate this effect and even invert this tendency [as we believe was the case in Dolce et al. (2000)].  Strain-rate sensitivity : as expected, higher strain-rates result in lower damping ratios. This is due to the temperature changes during the test cycles. These temperature variations occur because the forward martensitic transformation is an exothermic process and the inverse transformation is an endothermic process [Dolce et al. (2000)]. This temperature differential influences the stress level associated with the forward and inverse martensitic transformation, changing the overall shape of the hysteretic cycle. ○ Thermal readings during the tests were carried out with a thermographic recorder. The maximum temperature reached was around 35ºC and the minimum was close to 20 ºC (see Fig. 9).