Coupled functional and structural fatigue of shape memory alloy actuators

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The project aims for a better understanding of coupled functional and structural fatigue (CFSF) in NiTi-based shape memory alloys (SMAs) actuators. SMAs exhibit the fascinating ability to re-establish their initial geometry after a strong (apparently plastic) deformation when being thermally activated. Today, the thermal shape memory effect is exploited in different types of high-tech engineering applications, where SMAs are used for coupling, to move masses or to control walves (etc.). For application, it is often required that SMA actuators are able to provide a large number of functional cycles without suffering critical degradation. Therefore, CFSF is important, as it limits service life, especially when large numbers of functional cycles are required. CFSF is characterized by a functional degradation and simultaneously by the formation and growth of fatigue cracks. So far, CFSF has received insufficient scientific attentions although it governs service life in new actuator applications. In the present project, scientists from the fields of materials science and product design / production technology collaborate in order to establish a better understanding of CFSF and to improve fatigue life and component reliability. At the Chair for Materials Science and Engineering, materials science aspects of CFSF will be studied. The focus is placed on the identification of elementary CFSF mechanisms, and on the roles of microstructures and alloy compositions. At the Chair for Production Systems, CFSF fatigue life in SMAs and the effects of actuator operating conditions and actuator design aspects will be studied. It is important to highlight that different alloys and material states will be investigated which will be produced and tested in Bochum. This also includes actuator prototypes where these different SMAs will be integrated. A better understanding of CFSF allows to develop and to select optimized SMAs, and to adapt actuator design solutions and actuator operating condition for novel applications. In the proposed project, both partners join forces to provide answers which are important for the progress in shape memory technology.