Prof. Stavros Kourkoulis
National Technical University of Athens (NTUA), Athens, Greece
Title of the paper: Predicting Crack Initiation through Natural Time Analysis and Non-Extensive Statistical Mechanics
Abstract: Prediction of impending catastrophic events governs research efforts across a broad spectrum of disciplines. Despite intensive study, reliable forecasting at the field scale is not yet achieved. At the laboratory scale, where environmental “noise” can be controlled, promising progress has been made, yielding theoretical models capable of identifying qualitative pre-failure indicators. This lecture presents two novel analytical frameworks applied to Structural Health Monitoring (SHM) data to detect and evaluate pre-failure indices signaling the entrance of a loaded specimen or system into its critical stage prior to fracture: (i) The first approach is based on the concept of Natural Time (NT), which offers significant advantages in studying the dynamic evolution of complex systems by extracting the maximum possible information and identifying the instant a system enters its critical stage. Consequently, NT has gradually become a valuable tool for forecasting catastrophic natural events and a compelling alternative for analyzing Structural Health Monitoring (SHM) data. Today, it is applied across a broad variety of disciplines, including the Mechanics of Materials, Medicine, Physics, and Engineering. In NT analysis, a time series is reduced strictly to the sequential order of events and their associated energy, discarding conventional timestamp data. This design focuses attention on system changes induced by external loading rather than the loading itself, which operates independently of the system and may start, stop, or emit energy bursts unpredictably. Hence, NT is in sharp contrast with the hitherto used conventional time, which is modeled as the one-dimensional continuum of the real numbers. By decoupling events from continuous time, NT effectively tracks how energy variations drive a system toward critical, potentially extreme states. (ii) The second approach is based on concepts of “Non-Extensive Statistical Mechanics” (NESM), a discipline founded on the idea of entropies violating the additivity principle (the entropy of a system is not equal to the sum of the entropies of the individual parts constituting the system). They are usually expressed in terms of the entropic index, $q$, a measure of the system’s non-extensivity, accounting for non-independent, long-range interacting subsystems with memory effects. In order to highlight the potential of the above approaches, traditional tools used for the analysis of Acoustic Emission data are considered comparatively, including the Ib-value, the interevent time intervals, the interevent Euclidean distances, the energy content of the acoustic signals, the frequency, and the average frequency of the acoustic events, etc. The analysis reveals that both NT analysis and NESM provide reliable signals warning about upcoming catastrophic crack initiation and propagation.
Bio: Stavros Kourkoulis is a Full Professor of Experimental Mechanics at the Department of Mechanics of the National Technical University of Athens (NTUA), where he also serves as a member of the Board of Directors, following his tenure as Dean of the School of Applied Mathematical and Physical Sciences (SAMPS) from 2018 to 2023.
He holds an M.Sc. in Mechanical Engineering (1984) from the School of Mechanical Engineering and a Ph.D. in Mechanics (1989) from the School of Applied Mathematical and Physical Sciences (SAMPS) of NTUA, the latter completed under the supervision of Academician Professor P.S. Theocaris.
In addition to his role at NTUA, where he directs the Laboratory for Biomechanics and Biomedical Physics and heads the Laboratory Unit for Natural Building Stones, he has been a Professor of Mechanics at the School for Technical Education of the Officers of the Engineering Corps of the Greek Army since 2014. Previously, he served as Director of both the Department of Mechanics and the Laboratory for Testing and Materials (2016–2018) of SAMPS/NTUA.
His research interests span the experimental strength of materials, theory of elasticity, rock mechanics, the conservation and restoration of stone monuments, and musculoskeletal biomechanics. He has authored approximately 250 peer-reviewed journal papers, three books in Greek, and numerous conference papers, while additionally serving as Editor for five conference proceedings and Guest Editor for six special issues.
His editorial contributions include serving as Co-Editor-in-Chief for Fracture and Structural Integrity and as an Editorial Board member for Theoretical and Applied Fracture Mechanics and Forces in Mechanics, alongside reviewing for over 120 international journals. Dedicated to academic mentorship, he has supervised 10 Ph.D. theses and more than 60 M.Sc. and diploma dissertations. He has successfully led more than 30 national and international research projects as Scientific Responsible, with a total budget exceeding three million Euros.
Currently, he is the President of the Greek Society of Experimental Mechanics of Materials (GSEMM), and member of the Executive Committee of the European Structural Integrity Society (ESIS).