Effects of strain aging in NiTi SMA wire for dampers

Abstract

The thermo-mechanical properties of Shape Memory Alloys (SMA) as smart materials suggest their application as actuators, but also as dampers, thanks to the high damping properties during the hysteresis of the thermo-elastic martensitic transformation. Damping in civil engineering is needed to minimize earthquake effects on buildings, and also to limit the damage produced by wind or traffic induced oscillations in stayed cables in bridges. In this work, we explore the consequences of strain aging the material at moderate temperatures (100 °C). A quasi-static stress and temperature aging of NiTi SMA wires of 2.46 mm diameter was done at 373 K. The experimental measurements establish a monotonic effect related to the introduced strain. Aging under larger strains modifies the hysteretic shape and induces an increase in the maximal stress on the transformation from 600 MPa to 800-1000 MPa. Aging at 373 K under 7-8% strain, the effect was up to 200 MPa in three months and near 400 MPa in eight months. The modified stress-strain curves suggest potential application as dampers under larger summer-winter temperature changes, i.e., from 233 to 323 K.