Smart Materials and Structures 

 

 

 

This is one of the key themes of the Department of Mechanical Engineering. On the one hand, nearly half of the researchers in the department are working on this theme; on the other hand, this theme helped initiate several intergroup collaboration (MS and DFA), such as the dynamics of shape memory materials and energy harvesting.

The active materials have unusual behaviors compared to "traditional" materials. Their properties can be significantly changed by external controlled stimuli such as stress, temperature, electrical or electromagnetic field. Several industries, particularly the industries of space, aerospace, vehicles and biomedical devices, are interested in these materials.


Two types of materials are studied in the MS group:

1. Traditional Shape Memory Materials (NiTi): these materials can have very high reversible deformations under elastic deformations (pseudo-elasticity), or their permanent deformation disappears simply by raising the temperature (shape memory effect). These behaviors are due to a martensitic solid-solid phase transformation. We developed, within the MS group, a complete and unified three-dimensional model for structure design of Shape Memory Materials. The model is complete because it takes into account all the phenomena (the one-way and two-way shape memory effects, pseudoelasticity, etc.) via two variables - the martensite volume fraction and the strain tensor associated with the martensite orientation - enough to describe all these aspects into a single framework. In addition, our model can describe the cyclic behavior of Shape Memory Materials and calculate the asymptotic state of structures required for fatigue designs. It is also possible to take account of any plastic deformation during cycling. Modeling also takes into account the effects of thermomechanical coupling on the loading-rate dependent behaviors. This allows studies on the dynamic behaviors of Shape Memory Materials (with the DFA group); very interesting dynamic responses, such as period doubling, bifurcations and chaos, have therefore been revealed. Moreover, studies of regulation and stability of standard models for Shape Memory Materials are made. In particular, variational models of Shape Memory Materials and their improvement with gradient theories (introduction of characteristic lengths) are made.

 

Pseudo-elastic cyclic response of a shape memory material

 

Design of a stent made from Shape Memory Materials (Nitinol: NiTi): isovalues of phase proportions

 

2. Magnetic Shape Memory Materials: these are multifunctional materials which exhibit better dynamic responses than traditional Shape Memory Materials. They can be used as actuators, dampers and energy harvesters. Their behaviors can be explained by the effects of magneto-mechanical coupling on the reorientation of martensite variants. We modeled the materials’s behaviours with a multi-scale and multi-physics coupling approach. The originality of our work lies not only in the theoretical framework (similar to that for traditional Shape Memory Materials), but also in the analysis of their dynamic responses and fatigue behaviors.

Biaxial experiments on magnetic shape memory alloys (NiMnGa)