Evolution de la criticalité sous chargement cyclique comme indicateur de la fatigue – EVOCRIT

Fatigue is perhaps the most dangerous and simultaneously less well understood mechanism of mechanical failure encountered in a variety of modern structures ranging from nuclear reactors to micro-electronic connections in cell-phones. Failure in cyclic loading appears unexpectedly when the structure is operating in a safe and apparently steady state regime. Despite a long history of theoretical and experimental studies, modern technology still lacks reliable tools capable of predicting the catastrophic failure. The situation is particularly troublesome with early precursors of fatigue and there is an urgent need to find the way of monitoring microscopic evolution of damage. In this proposal we put forward a new set of ideas which open the way to experimentally distinguish various stages in the micro-development of fatigue. The approach is based on the study of the multi-scale statistical structure of the intermittent acoustic signal generated by the cyclically loaded solid. In our previous work we have succeeded in identifying the peculiar power-law nature of similar acoustic signals in several classes of materials in monotonic loading. In the current project, we plan to extend this approach to cyclic loading. In particular, we plan to develop an experimental method allowing one to discriminate between the power-law exponents and cutoffs generated by different deformation mechanisms. By tracing the proposed markers we shall be able to link the evolution in the structure of the acoustic signal to the changes in the collective behavior of defects. The originality of this project is in the interpretation of the statistics of the microscopic events using the concept of criticality. Self organized criticality and the ubiquity of power laws are the issues of great significance in contemporary science, giving a framework for understanding the emergence of complexity in a variety of natural systems, from earthquakes to turbulence. In this project we plan to extend this paradigm to fatigue and use the systematic changes in the structure of the critical (or near critical) acoustic signal as the measure of closeness to ultimate failure. Our main theoretical idea is to identify fatigue failure with the crossover between the two scale free regimes: dislocation-based and microcracking-based. We propose to conduct a series of decisive experiments on metals and shape memory alloys and develop a mathematical model capturing both the emergence of the power law signal and its eventual transformation.