Investigation of thermal, electrical and mechanical properties of shape memory polymer composites during electric activation

This thesis aims at the in-depth understanding of the electro-activation process of thermally-triggered SMCs. In particular, the material used is a shape memory poly(ε-caprolactone) filled with multi-wall carbon nanotubes (MWCNTs). This investigation is carried out in two main steps: first by focussing on the Joule resistive heating phenomenon and secondly by adding the effect of mechanical and shape memory solicitations. In the first step, we aim at developing tools that facilitate the prediction of the temperature increase due to resistive heating of a SMC parallelepiped upon the injection of an electric current. The resistive heating phenomenon is studied by means of surface temperature measurements, analytical formulas of increasing complexity and coupled 3D thermo-electric numerical models. After the careful calibration of an infrared camera, the temperature on the surface of the SMC as a result of an injected current at constant power could be recorded. The measurement of the electrical resistivity during resistive heating of the SMC reveals a non-linear and non-monotonic dependence on temperature. Although simple 1D analytical models assuming a temperature-independent electrical resistivity serve as a good first approximation of the resistive heating phenomenon, in this thesis we present other models of increasing complexity that offer more accurate solutions on the problem of resistive heating. Using these analytical models and 3D numerical models we showed the importance of describing the non-linearities of certain material parameters and the specific effect of the various parts involved in more complex experimental setups (such as grips in contact with the SMC). These techniques helped identifying material parameters that are crucial for predicting the temperature increase due to resistive heating: the temperature dependence of the resistivity, the thermal conductivity and the heat capacity. The second step deals with the experimental characterization of the thermo-electro-mechanical properties of the SMC during shape memory cycles that are electrically activated. For this investigation we have developed a bespoke tensile test bench that can be controlled at either constant displacement rate or constant load. The latter is achieved with the implementation of a fast and efficient fuzzy logic controller. This setup also includes the infrared camera and the required equipment for electrical activation. Both of them used in a proposed PI temperature controller of resistive heating that is able to maintain a constant temperature or thermal rate despite the changes in electrical resistivity of the SMC. Using this bespoke tensile test bench, the electrical resistivity of the SMC is shown to exhibit a non-linear and non-monotonic dependence on temperature throughout the conventional one-way shape memory cycles. This evolution with temperature is also found to be different upon heating than upon cooling, due to the melting and crystallization phase changes that happen respectively. Similarly, the electrical resistivity shows a non-monotonic evolution with applied tensile strain, showing a minimum value at a strain that can be related to strain hardening. The resistivity is also found to vary among successive shape memory cycles, suggesting that a first training cycle is necessary not only to stabilize the mechanical but also the electrical properties of the SMC. The two-way shape memory behaviour has also been demonstrated in this SMC upon electric activation. We showed that, under non-zero constant load, the 2D strain evolution with temperature is highly non-linear, due to the melting-induced contraction and crystallization-induced elongation happening around the transition temperatures of the SMC. The findings presented in this thesis provide insight into the electro-activation phenomenon and the behaviour of these thermally-triggered shape memory composites when subjected to resistive heating.