Reconstruction of Heat Sources Induced in Superelastically Loaded Ni-Ti Wire By Localized Deformation Processes

Abstract

Background: Shape memory alloys (SMAs) are phase transforming materials featuring strong thermomechanical couplings. Infrared thermography and heat source reconstruction (HSR) enable to track the calorific signature of deformation processes. Objective: The objective was to characterize the transformation processes in a superelastic nickel-titanium SMA wire subjected to a force-controlled superelastic tensile cycle. Methods: In-situ recorded thermographs were converted into spatiotemporal maps of heat sources using an inhouse developed post-processing method based on the heat diffusion equation resolved numerically for unknown heat sources. Results: Sequentially appearing patterns of localized transformation events of four types were identified and associated with martensite bands nucleations and their subsequent merging upon tensile loading. Analogically, the events associated with austenite bands nucleations and their subsequent merging were identified upon unloading. In addition, weak heat sources observed before and after the localized transformation events were associated with the homogeneous martensitic transformation. Conclusions:The intrinsic dissipation heat associated with the nucleation and merging events is estimated to be ~25% of the released/absorbed latent heat.

Concept of mechanocaloric granular material made from shape memory alloy

“…Rapid heating occurred in one of the small cylinders, featuring a temperature rise of more than 12°C. This value is close to the adiabatic temperature change of 13°C measured under axial loading on the same alloy [45]. Considering a typical value of 490 J/(kg•K) for the specific heat, this gives a latent heat of about 6 kJ/kg.…”

Applying Full-Field Measurement Techniques for the Thermomechanical Characterization of Shape Memory Alloys: A Review and Classification

“…Example 2: Performing Deformation Calorimetry in a Superelastic NiTi SMA Wire by IRT and Heat Source Reconstruction [37] As temperatures are impacted by heat diffusion within the specimen and heat exchange with the environment, another [36] option for the analysis of thermal maps is to retrieve the heat source distribution from the temperature distribution by using the heat diffusion equation. It is worth remembering that a heat source is defined here as the power density of heat produced or absorbed by the material itself (in W m -3 or in K s -1 when multiplied by the volumetric heat capacity as in Fig.…”

“…Ref. [37] is a typical example of this type of deformation calorimetry. In this study, the authors performed a superelastic cycle on a wire specimen made of stabilized NiTi SMA.…”

“…In addition to considering the heat source distribution instead of the temperature distribution, a difference with the preceding example is that the test is performed here at constant force rate instead of constant displacement rate. A consequence is that the plateau of the stress/strain curve is crossed at a Karlik, copyright 2020 [37] much higher rate, enforcing the specimen to transform quickly. Figure 2a shows the spatiotemporal distribution of the temperature change in this specimen during the test.…”

Prestress state evolution during thermal activation of memory effect in concrete beams strengthened with external SMA wires

“…The simulations are based on a SMA behavior in uniaxial tension considering three internal variables: wire temperature T , strain SMA and martensite volume fraction z . Phase changes in NiTi-based SMA wires are generally associated with the propagation of transformation fronts separating austenite and martensite [37][38][39][40], which leads to complex thermomechanical couplings in the case of heterogeneous temperature distribution. For the sake of simplicity, the three variables T , SMA and z were here considered as homogeneous within the wires.…”