Thermodynamics of Shape Memory Alloy Wire: Modeling, Experiments, and Application

Thermo-Mechanical Behaviour of NiTi at Impact

“…At impact no more nucleations were observed so that only the phase transformation fronts arising from the mentioned nucleations appear. This feature shows that the martensitic transformation at impact is inhomogeneous, similarly to that observed at very low strain rates, on the order of 10 -4 s -1 , when the deformation may be considered as an isothermal process (Shaw & Kyriakides, 1995), going against the trend shown in some works (Shaw & Kyriakides, 1997;Chang et al, 2006), where the phase transformation fronts are multiplied as the strain rate is increased in the range 10 -4 -10 -2 s -1 . This change of trend will be discussed in the next section.…”

“…The characterization in the intermediate range (1 -10 3 s -1 ), has been recently possible (Zurbitu et al, 2009a) thanks to the improvement of the conventional instrumented tensile-impact test which is able to measure with high accuracy mechanical properties at impact strain rates (Zurbitu et al, 2009c). Regarding the thermal evolution as a function of the strain rate it is limited to the low range (10 -5 -10 -1 s -1 ) (Chrysochoos et al, 1995;Li et al, 1996;Chang et al, 2006;Pieczyska et al, 2007), which offers the chance to www.intechopen.com study the thermo-mechanical behaviour of NiTi at impact strain rates using the new set up of the conventional instrumented tensile-impact test in combination with thermographic techniques, Fig. 3.…”

Phase transformation fronts propagation during the stress induced martensitic transformation at impact strain rates in NiTi shape memory alloy wires

“…The confluence of the forward transformation fronts originates a discontinuity in the crystal lattice which is favourable for the nucleation of reverse SIM transformation. This feature shows that the SIM transformation at impact strain rates is inhomogeneous, similarly to that observed at very low strain rates, 10 -4 s -1 or lower [8,9], and no multiple transformation fronts appear as is observed when strain rate is on the order of 10 -4 -10 -2 s -1 [5]. It is well known that when the deformation is performed at very low strain rates, there is enough time to allow all the transformation heat to be exchanged with the surroundings being the temperature in the sample almost constant.…”

“…This leaves cooler regions in the sample where stress necessary to generate another nucleation is lower than necessary to continue the propagation of the active fronts, so that new nucleations appear. As strain rate is increased, the deformation process is closer to the adiabatic condition, and self-localized heating may be more intense, generating more and more new fronts [5]. Nevertheless, in this work it is shown that this explanation is not valid at impact strain rates since no multiple transformation were observed.…”

“…For instance, these alloys are highly attractive for energy absorption/storage, impact damping or seismic protection [1][2][3]. Nevertheless, the thermomechanical behaviour of these alloys at impact strain rates, on the order 1-10 2 s -1 , is not yet well known and only a few works deal with the dynamics of propagating phase boundaries at impact strain rates [4], while the maximum strain rate at which the transformation fronts have been observed via infrared radiation has been 10 -2 s -1 [5,6]. In the present work, the phase transformation fronts at impact strain rates, on the order 10 s -1 , were observed on the surface of NiTi wires via thermographic observations.…”