Research Interests and Projects
The performance of materials as we perceive it with the naked eye is only the macroscopic manifestation of a myriad of complex mechanisms that occur on various length and time scales much smaller than those we experience in our everyday life. For many technological applications we need extraordinary materials that satisfy exceptional demands. Especially in aerospace technologies we need materials that can tolerate the extreme conditions of high or low temperature and pressure, radiation, or extreme mechanical loads and harmful vibrations, to name but a few. Before we can develop and create new materials with such superior properties, we must thoroughly understand the mechanics and physics of materials. In particular, we must understand the link between macroscopic properties and microstructure (from individual atoms all the way up to the macroscopic level). In our lab, we combine methods of theoretical, computational and experimental solid mechanics with the ultimate goal to accurately describe, to thoroughly understand, and to reliably predict the performance of materials, and - ultimately - to create novel engineered materials with exceptional properties.
Instabilities in solids are ubiquitous and often lead to mechanical failure or structural collapse, from the atomic level all the way to the macroscale. Examples include the buckling of structures, deformation localization, void nucleation and coalescence, but also phase transitions and domain switching. We aim at a fundamental understanding of how nano- and microscale instabilities in solids affect their macroscopic physical properties, and how a careful control of such fine-scale instabilities can result in solids with extreme effective mechanical properties. In other words, we take advantage of instabilities to achieve a beneficial material response. Our focus is on crystalline materials, including metals, ceramics, and composites (i.e., combinations thereof).
Computational research in our group comprises the development and application of new modeling techniques that bridge the scales in crystalline solids. A major focus is on atomistic techniques of molecular dynamics (MD) and, in particular, of coarse-grained atomistics. These enable us to model systems with (locally) atomistic accuracy but with significantly improved efficiency, so that we can target much larger length and time scales. The combination of theoretical advances and of high-performance computing facilitate simulations of length and time scales inaccessible to traditional MD and outside the realm of the continuum hypothesis. We employ this technique to bring atomistic accuracy to the meso- and macroscales, and to explore the physics of metallic and ceramic systems showing localization, failure, and instability.
At much larger scales, we employ methods of continuum mechanics to describe the response of solids with microstructures as well as of structural materials. This includes the (visco)elastic properties of stiffness, damping capacity, and the characteristics of wave propgation. Besides predicting the response of known materials, we also study new engineered materials systems with interesting or extreme properties based on an optimal microstructural control (see e.g. our work on ferroelectrics and composites). Beyond the elastic limit, we aim to understand inelastic mechanisms leading to plastic deformation and time-dependent behavior, e.g. in magnesium. Here, the observable material response is described by models of crystal plasticity that may lead to microstructural pattern formation, or by continnum dislocation theory and gradient plasticity approaches that capture intrinsic size effects. Deformation twinning is a further important mechanisms that we investigate by both phase field models and crystal plasticity extensions, e.g. for hcp materials.
Example results of an atomistic simulation of grain boundaries, a finite element simulation of polycrystalline magnesium, and a phase field description of deformation twinning.
In our experimental labs we fabricate ceramics and metal-ceramic composites, and we characterize their thermo-electro- mechanically-coupled performance. We are particularly interested in controlling the stiffness and damping of materials by inducing small-scale instabilities e.g. by temperature changes or by the application of electric fields. To this end, we use Broadband Electromechanically Spectroscopy (BES), a unique apparatus that was designed in our lab to measured the combined effects of mechanical vibrations, ambient temperature and pressure, and of time-varying electric fields applied to the specimen surface. This procedure enables us to quantify the effective macroscale dissipation in polycrystalline materials arising from the collective microscale mechanisms. Current research focuses on ferroelectric ceramics and metal/ceramic composites. In addition to our own experiments, we collaborate with a number of experimental groups whose experiments provide valuable data for comparison with our computational model predictions.
Our experimental lab in Caltech's Guggenheim building, home to the BES apparatus and other experimental equipment (images courtesy of Driver SPG).