MODELLING, MATERIALS AND PROCESSES INTERDISCIPLINARY SYMPOSIUM FOR SUSTAINABLE DEVELOPMENT

In this talk, Material Intelligence (MI) will be introduced as a novel concept and key enabling technology for insect-scale robotics for new engineering applications. MI is defined here to be the science, methodology and application of materials with the abilities to sense and respond to stimuli, and adapt to/learn from their environments for robotic applications to accomplish desired tasks. With a delocalized suite of functions MI enables intelligent robotic systems to be constructed at the insect scale where conventional sensors and actuators (such as electromagnetic, pneumatic or hydraulic motors) are too bulky to be employed. Through the discovery of new materials exhibiting stimuli-induced chemo/physio-mechanical reactions or phase transformations, and development of methods for their integration to achieve compact material systems with intelligent capabilities, MI enables robotic devices to be built at the insect scale. MI will be illustrated in this talk using visible-light-driven, dual-responsive materials such as manganese-based oxides, which exhibit high actuation performance and electrical resistivity changes under light illumination. Utilizing these properties, compact micro-robotic devices capable of self-sensing and responding to visible light to perform complex motions along multi-selectable configurational pathways are fabricated. Intelligent robotic functions including self-adapting load lifting, object sorting, and on-demand structural stiffening are demonstrated in these devices. This talk will also present novel enabling techniques including direct printing of robots using open-electrodeposition and key chemo-mechanics principles for analyzing robotic performances. The concepts demonstrated here lay down a solid foundation for creating robotic intelligence using multi-stimuli-responsive materials.

In 1921 Griffith published his seminal paper, basically describing the theory of Linear Elastic Fracture Mechanics. Today, 100 years later, this theory shows new generalizations and implications that we will discuss in this Keynote. Understanding fracture mechanics in several disciplines, from nano- to earthquake- engineering including medicine (e.g. bone fracture), is indeed vital and is currently limiting our technologies and lives.

Many important properties of crystalline materials are controlled by the dislocation core. There have been many attempts to remove the elastic field singularities at the dislocation core. Three of the most common methods for regularizing the elastic fields are: (1) considering a cutoff parameter, (2) spreading the Burgers vector in all directions as proposed by Cai et al., (2006. A non-singular continuum theory of dislocations. J. Mech. Phys. Solids, 54, 561–587), and (3) using gradient elasticity. Each of these methods requires an extra parameter with the dimension of length. We show that these characteristic length parameters can significantly affect the results of the discrete dislocation simulations. By comparing with the results of atomistic simulations, we show how the core energy should be included if an arbitrary constant is chosen for the characteristic parameters for each of these three nonsingular theories of dislocations.

Materials failure has for decades been considered one of the paradigmatic multiscale phenomena, involving processes from the atomic to the systems scale. On the continuum level, a well established approach is provided by the laws of fracture mechanics established by Griffith's seminal work exactly a century ago. However, the relationship between key concepts of fracture mechanics such as fracture toughness on the one hand, and parameters characterizing the microstructure of materials from the atomic to the grain scale on the other hand, remains poorly understood. The same is true for the transition from diffuse accumulation of damage to the formation and propagation of a macroscopic crack. Data analytic approaches may offer new pathways towards closing the gap between discrete and continuous descriptions of material microstructures undergoing failure under load. We illustrate this on a range of examples from the atomic to the geo-scale. On the atomic level, we show how machine learning methods can be used to identify local atomic configurations prone to irreversible change under load, and how continuum mechanics concepts can provide essential 'domain knowledge' in approaching this task. On the mesoscale, we demonstrate how network theoretical concepts can be used to identify potential failure locations in load-carrying structures that can be mapped onto networks transmitting linear momentum. Finally, on the macroscale, we discuss how macroscopic monitoring data can be used to predict imminent failure under load.

The development of materials with enhanced mechanical properties and ionic conductivity constitutes a major challenge in the area of solid polymer electrolytes (SPEs) for lithium batteries. We utilize high functionality star polymers as nanostructured additives to liquid electrolytes for the development of SPEs that simultaneously exhibit high modulus and ionic conductivity. We discuss two different cases of multiarm stars used. When high functionality PMMA stars are dispersed in low molecular weight PEO, the SPEs exhibit two orders of magnitude higher conductivity and one order of magnitude higher mechanical modulus compared to the linear PMMA analogues due to the formation of a highly interconnected network of pure liquid electrolyte that leads to high conductivity. When mikto-arm star copolymers are introduced (with PS and PEO arms), SPEs are obtained with high modulus and high ionic conductivity (close to those for practical use) due to their self-assembled morphology of highly interconnected structures formed within the PEO host. The intramolecular nanostructuring of the mikto-arm star particles and their self-assembly within a homopolymer matrix are studied by molecular dynamics simulations as well. The functionality and the arm lengths lead to an intramolecular nanostructure of the stars, which influences the overall morphology. These miktoarm stars form percolated interconnected assemblies within the PEO host as opposed to simple cylindrical micelles formed when linear diblock copolymers of equivalent characteristics are introduced into the same host.
* In collaboration with E. Glynos, P. Petropoulou, G. Nikolakakou, D. Chatzogiannakis, L. Papoutsakis, E. Mygiakis, A. D. Nega, G. Sakellariou, W. Pan, E. P. Giannelis, P. Bačová and V. Harmandaris
# Acknowledgements: This research has been co-financed by EU and Greek national funds (Action RESEARCH – CREATE - INNOVATE).

Crystal-based dislocation plasticity, as put forward originally by G.I. Taylor, for the stress-strain behavior of aluminum, and by E. Orowan and M. Polanyi, for temperature and strain rate influences described in terms of thermal-activated dislocation mobility, has led through much work to the present understanding of the mechanical properties of crystals. E.O. Hall and N.J. Petch added a polycrystal grain size contribution to strength properties via dislocation pile-ups being obstructed at grain boundaries. Modern research efforts are extending the work to micro- and nano-scale, individual crystal and polycrystal grain size, dimensions. Extremely high rate shock and isentropic deformation properties are being modeled via thermally activated constitutive relations.

We studied the uniaxial compression behavior of micro- and nanoparticles of several elemental metals (Au [1], Ni [2], Ag [3], Mo [4]) and alloys (Ni-Fe, Ni-Co [5], Au-Ag). The particles were obtained by solid state dewetting of thin metal films and multilayers deposited on sapphire substrates. The high homological temperatures employed in the dewetting process ensure the low concentration of dislocations and their sources in the particles. The particles compressed with a flat diamond punch exhibit purely elastic behavior up to very high values of strain approaching 10%, followed by a catastrophic plastic collapse. The uniaxial yield strength of the particles defined as an engineering stress at the point of catastrophic collapse reached the astonishing values of 34 GPa and 46 GPa for the smallest faceted particles of Ni and Mo, respectively. The atomistic molecular dynamic simulations of the particle compression demonstrated that the catastrophic plastic yielding of the particles is associated with the multiple nucleation of dislocations at the facet corners or inside the particles. The latter, homogeneous nucleation mode resulted in higher particle strength. The size effect in compression was observed both in the experiments and in atomistic simulations, with smaller particles exhibiting higher compressive strength. In contrast with the solute hardening observed in bulk alloys, alloying the pure metal nanoparticles with a second component resulted in significant decrease of their strength. Finally, we produced Au-Ag core-shell nanoparticles by coating the single crystalline Ag nanoparticles with a polycrystalline Au shell. The core-shell nanoparticles exhibited much lower strength than their single crystalline pure Ag counterparts. We related this decrease in strength with the activity of grain boundaries in the polycrystalline Au shell.

The talk ventures to describe a high-risk proposal to extend classical laws of mechanics and physics by enhancing them with a Laplacian term accounting for nonlocality and underlying heterogeneity effects. The approach is motivated by a robust gradient model of the classical theory of elasticity which in the last two decades has been shown very useful in eliminating undesirable singularities and interpreting size effects. Implications to a variety of unsettled questions across scales and disciplines are outlined.

An assessment of current efforts to design next-generation renewable batteries is provided. Mechanics and electrochemistry aspects are presented on materials, geometry/topology, and size of both electrodes and binder. The feasibility of current laboratory research to be transferred to pilot and industrial scale is discussed.An assessment of current efforts to design next-generation renewable batteries is provided. Mechanics and electrochemistry aspects are presented on materials, geometry/topology, and size of both electrodes and binder. The feasibility of current laboratory research to be transferred to pilot and industrial scale is discussed.

Even though it was recognized almost 100 years ago that plasticity is discrete in both space and time, deformation models were and still are primarily based on homogenizing flow. Driven by novel experimental techniques, intermittent plasticity of a variety of crystalline and amorphous solids has received renewed interest because it seems to have a lot in common with entirely different physical processes, such as magnetic domain switching or earthquakes. More specifically, statistical investigations of intermittent deformation revealed scale-invariance, which is a paradigm shift away from traditional concepts that homogenize plastic flow and rely on well-defined average quantities. This development also demonstrates scale-free distributions of plastic fluctuations that have the same power-law exponent, therefore suggesting a universal phenomenon is at play. In this talk we will review key aspects of these developments, followed by recent observations that favor a return to the more materials-specific behavior, non-trivial scaling, and the emergence of scale-dependent plasticity.
We will base our discussion on mechanical characterization that trace plastic instabilities in small-scale crystals in real time, allowing us to assess the underlying collective dislocation dynamics, that is dislocation avalanches. We will discuss results from fcc and bcc single crystals, and in particular focus on slip-size magnitude distributions, their involved time scales, slip-velocity distributions, avalanche shapes, and their temperature-dependent scaling. Furthermore, we discuss the appearance and disappearance of discrete plastic behavior at the small scale, and what implications this may have for macroscopic bulk plasticity. In summary, our results demonstrate both scale-free and scale-dependent plasticity in one and the same material, allowing us to rationalize both Gaussian and non-Gaussian plastic fluctuations.

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