The paper presents novel findings observed during the training process of superelastic, elastocalorically optimized Ni-Ti-based shape memory alloys (SMA). NiTiCuV alloys exhibit large latent heats and a small mechanical hysteresis, which may potentially lead to the development of efficient solid-state-based cooling processes. The paper starts with a brief introduction to the underlying principles of elastocaloric cooling, illustrating the effect by means of a typical thermodynamic cycle. It proceeds with the description of a custom-built testing platform that allows observation of temperature profiles and heat transfer between SMA and heat source/sink during high-loading-rate tensile tests. Similar to other SMA applications, a training process is necessary in order to guarantee stable performance. This well-known mechanical stabilization affects the stress-strain hysteresis and the cycle-dependent evolution of differential scanning calorimetry results. In addition, it can be shown here that the training is also accompanied by a cycle-dependent evolution of temperature profiles on the surface of an SMA ribbon. The applied training leads to local temperature peaks with intensity, number, and distribution of the temperature fronts showing a cycle dependency. The homogeneity of the elastocaloric effect has a significant influence on the efficiency of elastocaloric cooling process and is a key aspect of the specific material characterization.

Solid‐state cooling is an environmentally friendly, no global warming potential alternative to vapor compression‐based systems. Elastocaloric cooling based on NiTi shape memory alloys exhibits excellent cooling capabilities. Due to the high specific latent heats activated by mechanical loading/unloading, large temperature changes can be generated in the material. The small required work input enables a high coefficient of performance. An overview of elastocaloric cooling from basic principles, such as elastocaloric cooling cycles, material characterization, modeling, and optimization, to the design of elastocaloric cooling devices is presented. Current work performed within the DFG (Deutsche Forschungsgemeinschaft) Priority Program SPP 1599 “Ferroic Cooling”, which is focused on the development and realization of a continuously operating elastocaloric cooling device, is highlighted. The cooling device operates in a rotatory mode with wires under tensile loading. The design allows maximization of cooling power by suitable wire diameter scaling as well as efficiency optimization by implementing a novel drive concept. Finally, computer‐aided design (CAD) models of the discussed solid‐state air cooling device are presented.

Shape Memory Alloys (SMA) using elastocaloric cooling processes have the potential to be an environmentally friendly alternative to the conventional vapor compression based cooling process. Nickel-Titanium (Ni-Ti) based alloy systems, especially, show large elastocaloric effects. Furthermore, exhibit large latent heats which is a necessary material property for the development of an efficient solid-state based cooling process. A scientific test rig has been designed to investigate these processes and the elastocaloric effects in SMAs. The realized test rig enables independent control of an SMA's mechanical loading and unloading cycles, as well as conductive heat transfer between SMA cooling elements and a heat source/sink. The test rig is equipped with a comprehensive monitoring system capable of synchronized measurements of mechanical and thermal parameters. In addition to determining the process-dependent mechanical work, the system also enables measurement of thermal caloric aspects of the elastocaloric cooling effect through use of a high-performance infrared camera. This combination is of particular interest, because it allows illustrations of localization and rate effects -both important for efficient heat transfer from the medium to be cooled.The work presented describes an experimental method to identify elastocaloric material properties in different materials and sample geometries. Furthermore, the test rig is used to investigate different cooling process variations. The introduced analysis methods enable a differentiated consideration of material, process and related boundary condition influences on the process efficiency. The comparison of the experimental data with the simulation results (of a thermomechanically coupled finite element model) allows for better understanding of the underlying physics of the elastocaloric effect. In addition, the experimental results, as well as the findings based on the simulation results, are used to improve the material properties.

In diesem Beitrag wird über eine Parameteridentifikation für die Bestimmung der viskosen Eigenschaften von Kautschukmischungen berichtet. Bisher wurde die Kopplung der Zustandsgrößen durch die Anwendung von Korrekturverfahren berücksichtigt. Mit Hilfe eines Iterationsverfahrens werden die Materialparameter nun direkt ermittelt. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Superelastic Shape Memory Alloys (SMA) are typically used in applications where the martensitic phase transformation is exploited for its reversible, large deformation such as medical applications (e.g. stents). In this work, we focus on the mechanical and thermal behavior of a Nickel-Titanium SMA strip in bending mode. One possible application of this mode is to provide a restoring force when used in joints of SMA wire actuator systems making the need for an antagonistic SMA actuator redundant. In these applications mentioned above, typically only the mechanical properties are of interest while the temperature is considered constant, even though the martensitic phase transformation in SMA is a thermo-mechanically coupled process. As a part of the DFG (German Research Association) Priority Programme SPP1599 “Ferroic Cooling” which aims at advancing the development of solid state cooling devices, we have an equally large interest for the thermal evolution of Nickel-Titanium SMA during deformation and its induced phase transformation. In this paper we investigate the thermal and the mechanical response of a SMA beam during bending experiments in which the deformation is induced by holding one end of a SMA strip fixed while the other end is subject to a prescribed deflection. Sensors and high speed thermal cameras are used to capture reaction forces, deformations and temperature changes. We compare these experimental results with numerical simulation results obtained from Finite Element simulations where a thermo-mechanically coupled SMA model is implemented into a finite deformation framework.