The originality of our work on Shape Memory Materials lies in the study of fatigue and fracture of these materials. In fact, researchers in the MS group were the first to propose a three-dimensional fatigue criterion to predict the lifetime of a structure Shape Memory Materials. From the analogy with plastic accommodation, this new fatigue criterion is used to link the energy dissipated in the stabilized number of cycles to failure cycle. The calculation of the dissipated energy is based on the measurement of cyclic pseudo-elastic responses whose experiments were conducted by the MS group. In order to understand the fundamental mechanisms that govern the fatigue of shape memory materials, research on a smaller scale is necessary. The scientific objective is to define and identify the precursors of fatigue of Shape Memory Materials by measuring the acoustic emissions and analyzing different dissipative regimes during cyclic loading. The originality of this work is to interpret these fatigue phenomena by the theory of SOC (Self Organized Criticality). In particular we show that fatigue damage has the characteristics of SOC. The theory of SOC has been implemented to construct indicators of the dissipative regimes of the material. The transition from one regime to another can identify the precursors of fatigue failures. In addition, studies on the microscopic scale have led to the proposal of a scenario for the mechanisms of damage during cycling Shape Memory Materials. These studies have not only helped establish a relationship between the cyclical behaviors and the mechanisms involved (e.g., dislocations), but also to understand how these mechanisms can affect the phase transformation temperatures and mechanical properties.
Microscopic mechanisms involved in the cycling of a shape memory material
Modeling of crack propagation in MMF is also a topic in the MS group. The objective is to determine the fracture parameters of Shape Memory Materials: stress intensity factor, J-integral, rate of energy restitution. The difficulty lies in the consideration of the effects of phase transformation and the thermomechanical coupling on the tip of the cracks.
Isovalues of phase proportion at the tip of a cracked plate of Shape Memory Material loaded in mode I