Abstract of doctoral thesis - Andronikos Loukidis

"Weak electric signals and acoustic emissions as a tool to investigate criticality of mechanical status of materials"


Fragmentation phenomena in heterogeneous and cementitious materials are a fairly complex and complex process, involving broad space-time behaviours. The fragmentation process is related to the creation, development and aggregation of microcracks in macros, which eventually lead the material to failure. The mechanical behaviour and progression of damage to heterogeneous materials subjected to compressive axial stress are of great interest in a number of application domains.

In the framework of this doctoral thesis, a series of laboratory experiments will be carried out on specimens of brittle materials which will undergo mechanical stress until final failure. During those laboratory experiments, Acoustic Emissions (AE) and the generated weak electric signal will be recorded through the Pressure Stimulated Current (PSC) technique. Considering the quantitative and qualitative characteristics of the PSC signals and in conjunction with the analysis of the Acoustic Emissions parameters, an estimate can be made as to which levels of the applied mechanical strain the first microcracks begin to appear in the material and how they are then extended to main part of the material. This assessment can provide important information about the generation and spread of microcracks, in materials subjected to mechanical load, and ultimately driven into permanent macrocracks conditions, especially during the phase just before the final failure of the material.

At the same time, it will be explored in connection with the above, the emergence of the critical stage that precedes the imminent rupture with the help of natural time. Natural time analysis has the advantage of allowing the study of the dynamic evolution of a complex system and thus determining when the system approaches a critical point.