Vibrational dynamics of confined granular material

By means of two-dimensional contact dynamics simulations, we analyze the vibrational dynamics of a confined granular layer in response to harmonic forcing. We use irregular polygonal grains allowing for strong variability of solid fraction. The system involves a jammed state separating passive (loading) and active (unloading) states. We show that an approximate expression of the packing resistance force as a function of the displacement of the free retaining wall from the jamming position provides a good description of the dynamics. We study in detail the scaling of displacements and velocities with loading parameters. In particular, we find that, for a wide range of frequencies, the data collapse by scaling the displacements with the inverse square of frequency, the inverse of the force amplitude and the square of gravity. Interestingly, compaction occurs during the extension of the packing, followed by decompaction in the contraction phase. We show that the mean compaction rate increases linearly with frequency up to a characteristic frequency and then it declines in inverse proportion to frequency. The characteristic frequency is interpreted in terms of the time required for the relaxation of the packing through collective grain rearrangements between two equilibrium states

Short-time dynamics of a packing of polyhedral grains under horizontal vibrations

We analyze the dynamics of a 3D granular packing composed of particles of irregular polyhedral shape confined inside a rectangular box with a retaining wall sub jected to horizontal harmonic forcing. The simulations are performed by means of the contact dynamics method for a broad set of loading parameters. We explore the vibrational dynamics of the packing, the evolution of solid fraction and the scaling of dy- namics with the loading parameters. We show that the motion of the retaining wall is strongly anharmonic as a result of jamming and grain rearrangements. It is found that the mean particle displacement scales with inverse square of frequency, the inverse of the force amplitude and the square of gravity. The short- time compaction rate grows in proportion to frequency up to a characteristic frequency, corresponding to collective particle rearrangements between equilibrium states, and then it declines in inverse proportion to frequency

Force transmission in a packing of pentagonal particles

We perform a detailed analysis of the contact force network in a dense confined packing of pentagonal particles simulated by means of the contact dynamics method. The effect of particle shape is evidenced by comparing the data from pentagon packing and from a packing with identical characteristics except for the circular shape of the particles. A counterintuitive finding of this work is that, under steady shearing, the pentagon packing develops a lower structural anisotropy than the disk packing. We show that this weakness is compensated by a higher force anisotropy, leading to enhanced shear strength of the pentagon packing. We revisit "strong" and "weak" force networks in the pentagon packing, but our simulation data provide also evidence for a large class of "very weak" forces carried mainly by vertex-to-edge contacts. The strong force chains are mostly composed of edge-to-edge contacts with a marked zig-zag aspect and a decreasing exponential probability distribution as in a disk packing

Thermomechanical couplings in shape memory alloy materials

In this work we address several theoretical and computational issues which are related to the thermomechanical modeling of shape memory alloy materials. More specifically, in this paper we revisit a non-isothermal version of the theory of large deformation generalized plasticity which is suitable for describing the multiple and complex mechanisms occurring in these materials during phase transformations. We also discuss the computational implementation of a generalized plasticity based constitutive model and we demonstrate the ability of the theory in simulating the basic patterns of the experimentally observed behavior by a set of representative numerical examples

Mobility and the city improvement district: Frictions in the human-capital mobile assemblage

In this paper, we interrogate the role of the city improvement district (CID) in the intervention and management of mobility within the context of the South African city and the case study of the Groote Schuur Community Improvement District (GSCID), a public–private urban governance scheme situated in Cape Town’s middle income southern suburbs. Using the theoretical lens of bodily-scale mobility, we investigate the CID’s activation and management. This is useful, as we will demonstrate, because it is through the mobility and immobility at the scale of the body, where the CID’s mandate is operationalised and it is through the control of mobility that the CID’s mission, discourses and activities are linked. This work demonstrates that CIDs, as elite-driven urban renewal initiatives closely aligned with capital interests, employ exclusionary spatial practices that have the potential to shape the twenty-first century urban experience in significant ways. We conclude by theorising the co-constitutive nature of human mobilities and capital as the ‘human-capital mobile assemblage’ and by arguing that the CID occupies an ambivalent place in the contemporary city.IS

Modélisation du comportement élastique de matériaux composites à fibres courtes

International audienc

Analyse expérimentale et modélisation numérique des couplages thermomécaniques dans les matériaux solides

International audienceIn the first part, the theoretical and experimental framework used to present the thermomechanical behaviour of solid materials is briefly recalled. The main feature of the experimental approach relies on the use of thermographical techniques allowing us to deduce, from the thermal data, the distribution of heat sources arising during the mechanical transformation. In the particular case of homogeneous thermomechanical tests, an energy balance can be performed and used to derive the behavioural constitutive equations. When heterogeneities occur, the infrared images facilitate the analysis of localization mechanisms. In the second part, basic aspects of homogenization techniques are reiterated. Related to thermomechanical couplings, homogenization improves the description of the behaviour of materials and structures in which microstructural phenomena have a significant influence at the macroscopic scale. Several finite element simulations are shown concerning the thermoviscoelasticity of polymers, the thermoelasticity coupled with damage in composites, and the pseudoelastic behaviour related to the solid-solid phase change of shape memory alloys.Dans une première partie, le cadre théorique et expérimental adopté pour étudier le comportement thermomécanique des matériaux solides est brièvement rappelé. L'originalité de l'approche expérimentale développée repose sur l'utilisation de techniques thermographiques permettant de déduire des données infrarouges la distribution des puissances calorifiques accompagnant le processus de déformation. Dans le cas d'essais thermomécaniques homogènes, il est possible de dresser des bilans complets d'énergie utiles au développement des lois de comportement. Dans le cas d'essais non homogènes, les images infrarouges peuvent être utilisées pour étudier les phénomènes de localisation. Dans une deuxième partie, les notions de base en homogénéisation sont présentées. Associée aux couplages thermomécaniques, l'homogénéisation permet de mieux décrire le comportement de matériaux et de structures où des phénomènes physiques microscopiques ont une forte incidence sur le comportement macroscopique. Plusieurs simulations numériques par éléments finis sont ensuite présentées; elles concernent la thermoviscoélasticité des polymères ou la thermoélasticité avec endommagement des composites, et le comportement pseudoélastique avec changement de phase des alliages à mémoire de forme