Shape Memory Actuator: thermomechanical training and surface optimization of shape memory wires embedded in a polymeric matrix

In the first part of my work I studied the mechanical and thermal behaviour of NiTi wires and strips in order to find the best training cycle to achieve a good compromise between transformation temperatures, recovery forces and phase reversibility. Other parameters such as the stress-rate and maximum recovery deformations, useful to realize smart composites, were evaluated. DSC and DMTA analyses were performed on NiTi thin strips that were cold rolled with different thickness reductions. DSC is a well-known technique used to find the transformation temperatures of SMAs. Contrary to DSC, DMTA is not a traditional characterization method for shape memory alloys. Therefore, in this work I tried to find a correlation between the results of DSC and DMTA analysis, according to different hardenings induced by cold rolling. In the second part I dealt with the interface bonding optimization between polymer matrices and NiTi wires. Smart composites take advantage of the adhesion between the NiTi wires and the polymer matrix. Their mechanical properties depend strongly on the efficiency of stresses and deformations transfer at the interface between the wires and the surrounding matrix. This way, adhesion must be improved to avoid the degradation or premature failure of the actuation. I focused on the evaluation of the interface strength between NiTi wires and two kinds of thermosetting resins: polyester and vinylester. Different surface treatments were performed on the NiTi wires in order to increase the performance of the wire-resin interface adhesion. In particular, chemical passivation by using acid solution and functionalization by using silane coupling agents, were considered. Pull-out tests were carried out to quantify the improvement of the interface adhesion. Moreover, during my PhD I also dealt with the theoretical modeling of SMA materials and the mechanical behavior of smart composites during activation, but this topic is not discussed in this thesis.