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SWISS MECHANICS SEMINARS

# A Glimpse into Mechanics in Switzerland

### Mechanics of spontaneously arrested laboratory earthquakes

##### Prof. David Kammer

Computational Mechanics of Building Materials, ETH Zurich

Date/Time: October 14, Thursday, 16:00-17:00

Abstract: Earthquakes, which we experience as ground shaking, consist of sudden relative motion between tectonic plates. The underlying mechanical process involves three phases – the initiation of local slip, its growth along the tectonic fault, and its arrest. This rupture-like process is governed by physics at multiple length scales, making it a highly complex phenomenon that remains only partially understood. This is particularly true for earthquake arrest, which directly affects its magnitude. The current understanding of earthquake arrest is almost exclusively based on remote measurements because most laboratory experiments are too small to allow rupture arrest to occur naturally. However, recently developed large-scale laboratory experiments on granite blocks provide the necessary fault length to generate laboratory earthquake ruptures that not only nucleate and propagate, but also arrest, spontaneously. These experiments provide an opportunity to reexamine and better understand the physics governing earthquake arrest conditions. In this talk, we will discuss various analytical and numerical models that enable in-depth analysis, interpretation and extrapolation of results from such large-scale laboratory earthquake experiments. The results suggest that rupture arrest (at least in the laboratory) is controlled by the driving force rather than by the resistance, as often assumed. Further, we will discuss fault fracture energy, which is a key parameter in the arrest of earthquake ruptures. We will present a minimal numerical model with scale-invariant fault fracture energy in accordance with laboratory observations. However, when applying seismological approaches to estimate the fault fracture energy, it appears to be scale-dependent, similar to field observations, despite being scale-invariant. Therefore, the model reconciles conflicting observations from the field and the laboratory, and provides a pathway for more realistic models of earthquake arrest mechanisms.

### A virtual lab tour of the flexible structures laboratory at EPFL

##### Prof. Pedro Reis

Institute of Mechanical Engineering, EPFL Lausanne

Date/Time: November 11, Thursday, 16:00-17:00

Abstract: It used to be that academic seminars involved a researcher visiting a host institution to deliver a talk and meet with colleagues (do you remember those days?!). Times have changed, at least temporarily, but this situation is also opening opportunities. In this ‘talk’, we will be inviting you for a virtual tour of our Flexible Structures Laboratory (fleXLab) at EPFL in Switzerland. We will show you some of our experimental facilities and share some of our recent research activities, focusing on the mechanics of magneto-active structures. Multiple members of our team will be involved in this tour. Research at our fleXLab focuses is centered in the general area of the mechanics of slender structures, which leverage their post-buckling regime for novel modes of functionality. Methodologically, we recognize scaled high-precision model experiments as a powerful tool for discovery in mechanics, supported by theory and computation, in a vision of science-enable engineering and engineering-motivated science. Recently, we have become fascinated with active structures made out of magneto-rheological elastomers that can be actuated in the presence of an external magnetic field. After introducing some recent advances in experimentation, modeling, and computational for this class of systems, we will present a series of concrete examples. Specifically, we will discuss (i) (re)programmable mechanical metamaterials with programmable memory; (ii) magneto-active beams and Kirchhoff-like rods; and (iii) magnetic shells with tunable buckling properties.

This virtual lab will involve the participation of a few members of the fleXLab at EPFL.

### Machine learning based plasticity modeling

##### Prof. Dirk Mohr

Date/Time: December 9, Thursday, 16:00-17:00

Abstract: Machine learning offers a data-driven approach to the development of constitutive models as an alternative to classical physics-based modeling.  Recent applications of machine learning in the context of metal plasticity are presented ranging from temperature and rate dependent hardening laws to 3D constitutive models for anisotropic solids. In addition to developing mechanics-specific neural network architectures, new robot-assisted experimental procedures are presented that generate “big data” for the identification of machine-learning based plasticity and failure models from experiments.

### Friction and wear in light of elastic interactions between micro contacts

##### Prof. Jean-François Molinari

Date/Time: February 17, Thursday, 16:00-17:00

Abstract: It is well known that man-made and natural surfaces are rough, with roughness observed over many length scales. An important consequence is that the real contact area is much smaller than the nominal contact area, and is made of micro contacts that vary in size and shape. It is well known that elastic interactions between nearby micro contacts matter. Elastic interactions are felt over long distances, affect the location and average size of micro contacts, and influence the tribological properties. In particular, in the case of adhesive and abrasive wear, we show how crack shielding mechanisms between nearby asperities promote the formation of larger debris, thereby providing a mechanistic understanding of the transition for mild to severe wear at a critical load.

While these results were initially observed through molecular dynamics simulations, we will discuss our recent efforts at generalizing those early observations with computationally efficient continuum solvers, through the boundary-element method or the finite-element method incorporating phase-field modeling of fracture.

Ultimately, elastic interactions help revise the definition of a contact asperity, by incorporating nearby contact junctions into an effective contact area. The presentation will also explore optimization strategies in order to maximize elastic interactions for more efficient grip or scraping tools technology.

### Harnessing the dynamics of acoustically driven two-phase fluid media

##### Prof. Outi Supponen

Institute of Fluid Dynamics, ETH Zurich

Date/Time: March 10, Thursday, 16:00-17:00

Abstract: Bubbles oscillate volumetrically under the effect of pressure fluctuations, such as those produced by sound waves. When driven into a violent collapse, they can yield strong sound emissions, high-speed jets, and extreme heating – a behaviour known as cavitation. Here, we will present ongoing research efforts to reach a fundamental understanding of the intriguing dynamics of bubbles across a wide range of scales from a single bubble level to that of bubble clouds. For this, we generally combine ultra-high-speed experiments with theory. More specifically, we will report on our progress in temporally resolving the oscillations and translations of microbubble ultrasound contrast agents, elucidating the role of cavitation in ultrasound-mediated tissue adhesion, reaching extreme flow focusing using tandem bubbles and discussing effective capillarity in ultrasound-driven bubble clouds. The broad aim of this research lies in the quest for harnessing the power of acoustically driven two-phase fluid media for a variety of engineering applications, including medical ultrasound diagnostics, drug delivery, lithotripsy, sonochemistry, surface cleaning and micro-fluidics.

### The birth and death of buoyant hydraulic fractures

##### Prof. Brice Lecampion

Geo-Energy Lab – Gaznat Chair on GeoEnergy, EPFL Lausanne

Date/Time: April 14, Thursday, 16:00-17:00

Abstract: Propagation of tensile fractures ubiquitously occurs at depth in the upper earth crust due to fluid pressurization associated either with natural causes (magma overpressure, dehydration reaction in subduction zones, hydrothermal systems etc.) or direct anthropogenic fluid injection (well stimulation for hydrocarbons or geothermal fluids production). The growth of such fluid-driven fractures in a solid under pre-existing compressive stresses is governed by the coupling between lubrication flow inside the growing fracture and linear elastic fracture mechanics of the solid. In nature, the initial stress field becomes more compressive with depth, leading to a buoyancy force generated by the difference between the vertical gradient of the minimum stress (acting perpendicular to the fracture plane) and the fluid weight. Once the fracture grows beyond a critical length scale, this buoyancy force strongly affects its propagation. In particular, the ratio between the energy dissipated by viscous flow and fracture surfaces creation sets the dynamics and elongation of such three-dimensional buoyant fractures. The expectancy and variety of shapes of these fractures will be illustrated in the light of typical material and injection parameters encountered in nature, engineering applications and laboratory experiments. Finally, we will explore how typical variation of in-situ stress and material properties at depth can stop the other-wise self-sustained growth of these fractures.

### Architected materials

##### Prof. Dennis M. Kochmann

Mechanics and Materials Laboratory, ETH Zürich

Date/Time: May 12, Thursday, 16:00-17:00

Abstract: Architected materials (often referred to as mechanical metamaterials) have gained tremendous attention over the past decade. Across engineering disciplines, the modeling, design, fabrication, and characterization of such cellular solids, which derive their properties from small-scale structural architecture, has resulted in a myriad of materials systems with as-designed, optimized mechanical properties – from high stiffness- and strength-to-weight ratios to energy absorption and wave guidance all the way to active and smart reconfigurability. The dream has been to revolutionize our approach to selecting materials for engineering applications: away from property look-up tables for available materials, towards the on-demand creation of novel architected materials with controllable or extreme properties and functionality. Focusing on mechanical properties, we will discuss to what extent this has become the new reality and what challenges we are facing. We will highlight recent examples of modeling and reverse-engineering architected materials with tunable stiffness, toughness, wave motion, shape morphing, and more, and we present experimental examples of their realization and application. This sets the stage for discussing what has been achieved and what lies ahead.

### Register

Please register to enroll for the SwissMech Seminars.