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Bridging microstructural to macroscopic properties in failure of heterogeneous materials

Final Report Summary - TOUGHBRIDGE (Bridging microstructural to macroscopic properties in failure of heterogeneous materials)

What governs the resistance of materials? Can we control it and make structures and solids more reliable? And why do adhesives stick? Can we improve their performance? These questions are of primary importance for physicists who want to describe and understand the behavior of matter, even when driven close to its limit, i.e. its failure point. But also for engineers who design and build systems that must last and survive on the long run, specifically at a period where the economical and ecological constraints have reached an unprecedented level. Despite the importance of these challenges, there is nowadays no reliable model that can predict the failure properties of a material like its toughness, lifetime or strength from its microstructural features. Indeed, our current understanding of material failure relies on continuum mechanics of solids that do not take into account the central role played by material heterogeneities and defects. The project objective is to understand the role played by these microscale features on their macroscopic failure behavior, and integrate them into predictive models of material failure using concepts from both continuum mechanics and statistical physics. Our goal is thus to bridge small and large scale in material failure problems. The subsequent objective of the ToughBridge project is to show how a better understanding of the microscopic origin of the failure properties of materials can be used in engineering sciences for technological applications. We will see indeed that the scientific outputs of this research project can be used for the design of materials with improved failure properties and are also relevant to determine post-mortem the reason of the failure of a structure so it can be prevented in the future.
Three complementary tools have been used to explore the relationship between micro and macroscale in fracture problems. First, thanks to the support of the European Union, I opened and equipped an experimental laboratory devoted to the failure properties of heterogeneous materials in my host institution. In one of the experimental setups build in this lab, we designed adhesives with finely controlled microscale heterogeneities, and explored how patterning adhesives modify their macroscopic peeling behavior. Since thin film peeling shares many common features with the process of material failure, these experiments were very useful to understand how the behavior of a crack is dramatically changed by the presence of heterogeneities. In parallel to experimental studies, I have developed new numerical tools that describe how crack propagates through heterogeneous solids. These tools have been very useful to understand quantitatively the role of the microstructure of materials on their failure behavior. The observations made through the experiments realized in the laboratory and the numerical simulations have provided a better understanding of the process of fracture in heterogeneous materials that led us to the development of new theoretical models. In these models, we predict the complex spatio-temporal evolution of a crack or an ensemble of microcracks by describing how a fracture event taking place somewhere in the material affects the propensity of a material to break somewhere else. This approach that we used in the context of crack propagation and damage spreading in solids with heterogeneous failure properties turned out to be very efficient for predicting the effective resistance of heterogeneous materials. As so, we used it both for predicting the macroscopic failure properties of heterogeneous brittle materials that break through the propagation of a single crack and those of quasi-brittle materials that break through the spreading of damage or microcracks until catastrophic failure.
These new models have opened promising perspectives for engineering applications. First, we have implemented them in optimization procedures in order to find microstructures that optimize the failure properties of heterogeneous systems. We illustrated this approach by designing heterogeneous adhesives with new and improved peeling properties, and we hope on the long run that these ideas will be helpful for the design of better materials. Another application of this research lies in the post-mortem failure analysis of materials and structures. The investigation of the microscale failure processes led during this project has revealed that microcracking was generic to most of materials, and that this failure mechanisms let a clear geometrical signature on the statistics of fracture surfaces after full failure of a solid. Following this idea, I developed a method that allows measuring toughness of materials (that depends on the amount of microcracking involved during its failure) from the analysis of their fracture surface. This technique that led to a patent application is relevant for many industries (metallurgy, petroleum, aeronautics) and I am currently developing technological applications of this method in collaboration with industrial partners (Schlumberger, Constellium, CEA). Some representative images of this fractographic methods and of other discoveries made through the ToughBridge project have been provided in the document in attached file.
How did I organize and manage my research to achieve these results? Thanks to the support of the European Union, I was able to constitute a research team composed of undergraduates, PhD and post-doctoral students that worked under my guidance on these different questions. Over the three years of the project, I have welcomed about fifteen students, among which three post-doctoral scholars and three PhD students. This research group is unique both at the national and the international level, since it combines classical approaches and techniques in Fracture Mechanics with other ones issued from the Statistical Physics of Complex Systems. The students working in my group come from these different horizons, bringing different expertise to the research team. During my research, I also used and combined experimental, numerical and theoretical approaches in order to explore the mechanics and physics of crack propagation in heterogeneous materials that we subsequently used to design and develop new systems with improved properties. The strength of our research group lies in the diversity of competencies, all mobilized for the understanding of the physics and mechanics of fracture. Note that the future of my research group in the coming years is ensured. I was indeed recently granted a funding of 170 k€ from the city of Paris to support my research, and a PhD student just started his thesis under my guidance a few months ago. Combined with the fact that I have obtained a permanent CNRS position in 2010, I can pursue serenely the projects started during these last three years. I also would like to mention that I have been promoted in 2013 as director of the Solid & Structural Mechanics (MISES) group that composed of twenty faculties, so I hope I can play a positive role on the management of the research in my field at a larger scale. Note also that the ToughBridge project helped me to develop fruitful collaborations with several colleagues in my own laboratory at Institut Jean le Rond d’Alembert, such as with Pr. Leblond, Pr. Dascalu and Pr. Kondo through the co-mentoring of PhD students, as well as with researchers working in foreign universities, like Pr. Needleman (Univ. North Texas, US), Pr. Daraio (ETH Zürich, Switzerland), Pr. Ravichandran (Caltech, US) or Pr. Ravi-Chandar (Univ. Austin, US). All this collaborations were fruitful and led to common publications.
Overall, this ToughBridge project resulted in one patent application and the publication of twelve articles in international refereed journals, among which five in Journal of the Mechanics and Physics of Solids that is the leading journal in the field of Solids Mechanics and two in Physical Review Letters that is the leading journal in the field of Physics. Note that nine other articles are currently under review among which five in JMPS and two in PRL. The success of this project is also illustrated by the several presentations given during these last three years, with six invited talks given during international conferences and about twenty oral presentations during international conferences and seminars. Finally, I am actively implied in the science diffusion and dissemination through the organization of scientific events:
• Seminars: I am responsible of the Solid Mechanics Seminar in my institute since 2012
• Workshops: I have organized the “Mechanics and Physics workshop on effective toughness of heterogeneous materials” at Institut Jean le Rond d’Alembert in 2012
• Symposiums during international conferences: I have organized the “Statistical Physics & Fracture Symposium” at International Conference on Fracture in Beijing 2013.
• International conferences: I have organized the conference “Complexity in Mechanics” at Santa Barbara last year.
To disseminate widely my research findings, I have launched a website (www.laurentponson.com) that provides an easy-access overview of the main outputs of my works and my publications. 
I am also finishing the writing of a book entitled ‘Fracture Mechanics of heterogeneous materials’ that describes pedagogically the main research outputs of the ToughBridge project, and that will be published by Wiley & Sons in 2015. Finally, I have been implied in the dissemination of science through the training of the various students who worked in my research group, through traditional classes at ESPCI and UPMC or through the organization and participation to popular scientific events in museums or science centers such as Palais de la Découverte in Paris where I gave a popular science conference on fracture in 2012.
Overall, the support of the European Union through the ToughBridge integration grant was a triggering factor and an important element for building an experimental laboratory and gathering a research group that can lead ambitious research programs, at the forefront both at the national and International level in the field of physics and mechanics of material failure. I strongly believe that subsequent supports from the European Union is necessary and will lead to even more ambitious research programs with potential for strong impacts from scientific, technical and commercial perspectives.