INVITEES

Angelillo: Traditional masonry structures behave very differently from modern structures and their detailed modeling is extremely challenging. There exists a rather crude model, the unilateral model of Heyman, which catches the very essence of masonry mechanics putting the theory of masonry within the realm of Perfect Plasticity. The most interesting aspect, from an engineering point of view, is the fact that both the strain and the stress can be singular. We report preliminary results on the automated numerical identification of singular stress states which are the solution of boundary-value problems in cases in which singular stresses are expected.

Davini: The bending stiffness and the Gaussian stiffness are the two crucial parameters governing the rippling of graphene. We have studied the continuum limit of a monolayer graphene sheet by starting from a molecular description of the lattice array and using Brenner’s second generation reactive empirical bond-order potential to give a nano-scale characterization of the atomic interactions. We outline a formal derivation of the continuum limit, with a quantitative evaluation of the bending and Gaussian stiffnesses that is in good agreement with that found by density Functional Theory.

Goddard: We present an outline of a unified treatment of nonlocal continuum mechanics, illustrating how weakly non-local models involving higher kinematic gradients, such as those presented by Kamrin and coworkers, arise from fully non-local models involving spatial functionals.

Herrmann: By considering spheres of different size, packings with bearing states, in which touching spheres roll on each other without slip, can be made to be space-filling. We will discuss the construction and mechanical properties of such space-filling bearings.

Luding: To bridge the gap between scales, micro-macro transition methods are necessary; these translate particle positions, velocities and forces into density, stress, and strain fields that are compatible with the conservation equations for mass and momentum of continuum theory. This results in a universal granular rheology that describes fluid and solid states and the jamming and unjamming transitions between them.

Makse: An analysis of contact network features of packings near the jamming and shear thickening transitions using graph theoretical tools indicates that the 3-core is a precursor of both transitions. It emerges suddenly and discontinuously around a coordination number of 3.35, in agreement with the random graph theory of k-core percolation of Pittel, Spencer and Wormald.

Roux: Cohesive particulate assemblies are not often investigated, although they exhibit a wider variety of microstructures and mechanical behaviors, due, in particular, to the cohesion-induced stability of very open, tenuous contact networks akin to colloidal aggregates and gels. DEM simulations and physical experimental permit the characterization of microstructures of static assemblies under stress. Features of particular interest include fractal microstructure, the role of rolling and pivoting resistance in contacts, and the identification of characteristic pore sizes.

Truskinowsky: Despite many important advances of continuum crystal plasticity, a sufficiently versatile computational approach that allows for natural coupling of different plastic “mechanisms”, while addressing realistic space and time scales, is still missing. We will discuss a new mesoscopic tensorial model dealing with large strains, while accounting correctly for both anisotropy and discreteness of the underlying lattice.

SENIOR PARTICIPANTS

Amorosi: We illustrate the links between the incrementally reversible soil response, as detected by accurate laboratory measurements carried out on clay specimens, the microstructural features that characterize the material at the states investigated experimentally, and the incorporation of these observations by a modelling approach developed within a thermodynamically consistent framework.

Artoni: We present measurements of self-diffusion coefficients in discrete numerical simulations of dense homogeneous and inhomogeneous shearing flows of nearly monodisperse, inelastic and frictional spheres. The inhomogeneous flow simulations show that the classical relationship of the kinetic theory of granular gases underestimates the magnitude of self-diffusion; but the scaling of the kinetic theory, in which the ratio between the self-diffusion coefficient and the square root of the granular temperature is a function of the volume fraction, is far better suited to describe the self-diffusion than that based on the relationship between the diffusion coefficient and the shear rate.

Delannay: In a recent experimental work, we introduced a characteristic friction height of the flow, Z, which can be interpreted as the height over which the flow experiences a significant friction from the side-walls. Numerical simulations reveal that the friction height rescaled by the flow depth is related to the average packing fraction of the flow through a universal relation. This relation, together with the two others that relate the stress ratio and volume fraction to the Froude number, combined with a simple force balance give a global scale model that allows the prediction of the sliding velocity for a granular flow in a smooth channel with a given inclination angle and particle holdup.

Giusteri: In the construction of continuum models it is of paramount importance to understand the dynamical way in which the microstructure preserves or acquires memory of the history of deformation. We present and discuss a class of mathematically local models that are able to capture the features of apparently different non-Newtonian effects on the basis of a few common principles related to memory loss or gain.

Herman: The part of the sea ice cover adjacent to the open ocean consists of relatively small floes with sizes often spanning several orders of magnitude. In response to oceanic and

atmospheric forcing, such ice behaves as an approximately two-dimensional, highly polydisperse granular material. Many known effects related to wave-ice interactions or sea ice response to wind/current forcing are not taken into account in existing models for such ice. The limitations of those models and their consequences will be discussed, together with recent developments in research on their improvement.

Issler: We investigate whether avalanches with very long run-out that are unexplained statistical outliers, can be understood as the result of several favorable factors with low combined probability. We propose that pore air from the snow cover seeping upward through the avalanche as the avalanche weight compresses the snow cover is responsible for strong fluidization and a concomitant reduction of friction and introduce a depth-averaged model that incorporates this mechanism.

Kroy: Conditions favoring bimodal sand transport, with fine-grain saltation driving coarse-grain reptation, give rise to the evolution of so-called mega-ripples with a characteristic bimodal sand composition. I will present a unified phase diagram for such bimodal transport and the ensuing mega-ripple morpho-dynamics that indicate that the quantitative signature of bimodal transport in the otherwise highly variable grain size distributions is the log-scale width of their coarse-grain peaks.

Magnanimo: Elastic wave propagation provides a non-invasive way to probe granular soils. Key mechanisms, related to the soil’s discrete nature, involve nonlinear, stress- and fabric-dependent elasticity, wave dispersion, and scattering. We apply micromechanical modelling to link the acoustic response to the multiple scales of natural soils in dry and saturated conditions. We carry out direct numerical simulations of wave propagation in saturated granular media to explore the pore-scale hydrodynamics and intergranular behaviour and compare the results with dry aggregates. Finally, we turn the attention to artificially improved soils and investigate the propagation of waves in dense granular mixtures made of soft and stiff particles, like rubber chips and sand.

Oger: Packings of beads confined in slowly tilted containers with a free surface are commonly used in laboratory experiments to model natural grain avalanches. However, slowly tilting the packing to the maximal stability angle results in a number of precursors that is too small to assess reproducible and permit robust statistical analyses. To avoid this limitation, we tilt the packing with successive oscillation cycles; this provides both reproducible precursor measurements based on large sample statistical inferences and a quasi-stationary state after few full cycles. We use a high-resolution optical camera and process the images of the packing free surface to identify the precursory events and employ two acoustic transducers on the lateral walls to record the acoustic emissions associated with the bead displacements.

Shen: We cover a representative set of different theories for waves propagating through sea ice and their dispersion relations, each showing different predictions for wave speed and attenuation through seas covered with ice ranging from meters thick continuous solid sheets through grease, pancake ice, fragmented floes, and damaged ice sheets, distributed with leads and ridges, to a frazil slurry.

Zurlo: Surface growth describes the continuous addition of mass on the external boundary of a solid body. In elastic systems, this process is usually accompanied by the onset of residual stresses, and in this talk we illustrate how their source, strain incompatibility, can be permanently accumulated during the deposition of prestressed layers. Zurlo and Truskinovsky (Phys. Rev. Lett., 119, 048001, 2017; Mech. Res. Commun. 93, 174, 2018) have developed a linearized theory of surface growth, which quantitatively relates the deposition protocols with post-growth states of stress. They have extended this analysis (Phy. Rev. E 99, 053001, 2019) to account for both physical and geometrical nonlinearities of an elastic solid. This new development reveals the shortcomings of the linearized theory, in particular, its inability to describe kinematically confined surface growth and to account for growth-induced elastic instabilities.

JUNIOR PARTICIPANTS

Bharath: We employ discrete element N-body local simulations to study the vertical structure of rings around central bodies that are either spherical or oblate. We analyze the static distribution of volume fraction and granular temperature through their depth for various choices of the gravitational potential, and investigate the role of self-gravity, as a function of the particle restitution coefficient. We focus on the importance of self-gravity in forming clumps, and test predictions of kinetic theory against the results of simulations over ranges of volume fractions and coefficients of restitution.

Guglielmi: We first report on clay microstructure and its evolution under different loading paths for clays that range from soft to stiff. We then compare microstructural analyses on several natural and reconstituted clays reported in the literature to our analyses of two Italian stiff clays. based on scanning electron microscopy, image processing, mercury intrusion porosimetry and swelling tests. Finally, we analyze the influence of composition, geological history, and loading paths upon the clay microstructure and the evolution of the clay microstructural features at different stages of one-dimensions and isotropic compression and shearing.

Pol: We address the kinematics and orientational order of confined, heterogeneous granular flows of anisotropic particles. A mix of experimental and DEM simulation data are presented, with a special focus on the rotational motion of particles. These data show that an inhibition of rotation is due to particle anisotropy, that the particle rotation depends on the shear rate, and that particle ordering is based on a correlation between particle local orientation and angular velocity fluctuations.

Prati: We analyze different morphologies induced on particle bed by both square and rectangular plates that oscillate above the bed and describe how the shape and the stiffness of the plate effect the shape of the deformed surface for several frequencies and amplitudes. We employ a high-speed camera to measure the displacements of the plates, confirming that the square plate is rigid and the rectangular plate is flexible. The camera used with a laser sheet characterizes the evolution of the topography of the bed. We also use a hydrophone to measure the distributions of pressure beneath the plates; these indicate that peaks in pressure are related to features of the bed deformation. Oscillations of a rigid, square plate induce only one heap, while oscillations of the flexible, rectangular plate induce more than one.

Tregaskis: In experiments and discrete numerical simulations, we show that if the incline an avalanche flows upon is changed from a smooth to a rough, there is a qualitative change in the interaction between the flow and an obstacle. On a rough incline, the friction between the grains and the incline depends on the flow thickness and speed, which allows both rapid (supercritical) and slow (subcritical) steady uniform avalanches to develop. For supercritical experimental flows, the material is diverted around a blunt obstacle by the formation of a bow shock and a static dead zone upstream of the obstacle. Downstream of the obstacle, a grain-free vacuum region forms; in contrast to flows on smooth beds, static levees form at the boundary between the vacuum region and the flow. In subcritical flows, the flow is diverted smoothly around the dead zone and the obstacle without forming a bow shock.

Vescovi: We investigate the phase transition in two unsteady, homogeneous flows of a collection of spheres in discrete element numerical simulations: shearing and cooling. In the former, we observe that, during the transient, fluid-like and solid-like behavior can be distinguished based on the size of the fluctuations in the coordination number and the pressure. In the latter, the material at the beginning of cooling can be shear-jammed, fragile, or unjammed; and the initial state determines the subsequent evolution of the dense assembly into, respectively, an anisotropic solid, an isotropic fluid, or an anisotropic fluid.