Defining fatigue-resistant conditions for elastocaloric applications

Limited fatigue life of elastocaloric materials is currently one of the bottlenecks that prevents practical applications of the elastocaloric technology. This is the case especially for tensile loading, which in contrast to compressive loading enables application of thin and slender elastocaloric elements that can provide fast and efficient heat transfer in elastocaloric device and thus improved performance. It is therefore very important to define operating conditions, such as mean strain and strain amplitude, that allow for fatigue-resistant operation of elastocaloric materials. For this purpose, a comprehensive study involving a broad spectre of loading conditions is necessary in order to provide a reference based on which any detailed life predictions can be made. The goal of our ongoing study is to analyse the fatigue life of superelastic binary NiTi alloy at different mean strains and strain amplitudes, ranging from compressive through mixed to tensile loading, while simultaneously monitoring the elastocaloric effect. Final goal is to create a Haig-like (in)finite life diagram in 3D space consisting of mean and amplitude strain together with the associated elastocaloric adiabatic temperature changes. Based on this, we will be able to define region(s) in the loading space which allow for a continuous fatigue-resistant operation while ensuring sufficiently large adiabatic temperature changes for use in elastocaloric cooling and heat pumping applications.