Abstract
A mechanical model for pseudoelastic behavior of NiTi wires is proposed with the aim to predict the behavior of Shape Memory Alloys(SMA) damping wire elements in model structures. We have considered at first a simple linearwise stress-strain relationship to describe the basic isothermal behavior of the SMA members. Then, this basic model is modified in order to include the effect of the strain rate. The model is based on detailed experimental characterization performed on a Ni rich NiTi superelastic wire which included the study of the localized character of the deformation and the local heat generation associated with the stress induced martensitic transformation occurring in these alloys. Heat conduction along the wire and heat interaction with the surroundings was also considered. In that way, the resulting local temperature field around the transformation front is assessed and its effect on the progression of the transformation is evaluated. It is shown how the simple mechanical model reproduces the global mechanical behavior, including the existence of a maximum in the damping capacity with the transformation rate.
Highlights
In certain temperature ranges, shape memory alloys (SMA) as Niquel-Titanium (NiTi) or Copper based (CuZnAl, CuAlNi, CuAlBe, etc) exhibit the pseudoelastic effect (SE)
It is considered that σ−ε trajectories for different strain rates could be obtained evaluating the change of temperature at the interfaces region due to latent heat of martensitic transformation exchange, and considering the change in the stress with respect to an empirical cycle measured at isothermal conditions
When considering a pseudoelastic tension cycle on a NiTi wire with very low strain rate and the ambient temperature fixed at a value T, the induced transformation is slow, and the latent heat delivered by the transformation does not change significantly the local temperature of the wire, as it is maintained in thermal equilibrium with the surroundings
Summary
Shape memory alloys (SMA) as Niquel-Titanium (NiTi) or Copper based (CuZnAl, CuAlNi, CuAlBe, etc) exhibit the pseudoelastic effect (SE). The hysteresis appeared in the pseudoelastic cycles is due to friction associated to interface movement and plastic work associated to the grain accommodation This implies an energy dissipation that motivates the study the SMA as an interesting alternative for dampers design for seismic or mechanical vibrations control[2]. Taking a reference isothermal σ−ε cycle, when transformations or retransformations conditions are reached, the interfaces will move giving rise to latent heat exchange with surroundings This will change the temperature profile along the wire and because of the Clasius-Clapeyron relation the stresses needed to proceed with the transformation or retransformation will change[12]. It is considered that σ−ε trajectories for different strain rates could be obtained evaluating the change of temperature at the interfaces region due to latent heat of martensitic transformation exchange, and considering the change in the stress with respect to an empirical cycle measured at isothermal conditions. Where ∆W is the hysteresis area and W is the maximum strain work made during the pseudoelastic loading
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