Phenomenological modelling of damage in polymer blends

To describe the constitutive behaviour of a certain class of polymer blends an elastoperfectly-viscoplastic and creep damageable material characterization is proposed. For a composite of 80 % Polystyrene and 20 % Ethylene Propylene Diene Monomer rubber (PSIEPDM) the specific parameters are determined from tensile tests in a particular range of strain velocities. To investigate the applicability of the model, the results of a finite element analysis for a laterally loaded thin plate (plane stress) with a circular hole are compared to measurements. Numerically calculated values are in reasonable agreement with reality; discrepancies can be ascribed to noise in experimental data. The finite element approach is evaluated with respect to the occurrence of mesh-dependence. Mesh-refinement shows convergence of solutions, attributable to the stabilizing influence of the viscous contribution in the constitutive equations

An Experimental study of the initial volumetric strain rate effect on the creep behaviour of reconstituted clays

Clayey soils tend to undergo continuous compression with time, even after excess pore pressures have substantially dissipated. The effect of time on deformation and mechanical response of these soft soils has been the subject of numerous studies. Based on these studies, the observed time-dependent behaviour of clays is mainly related to the evolution of soil volume and strength characteristics with time, which are classified as creep and/or relaxation properties of the soil. Apart from many empirical relationships that have been proposed in the literature to capture the rheological behaviour of clays, a number of viscid constitutive relationships have also been developed which have more attractive theoretical attributes. A particular feature of these viscid models is that their creep parameters often have clear physical meaning (e.g. coefficient of secondary compression, Cα). Sometimes with these models, a parameter referred to as initial/reference volumetric strain rate, has also been alluded as a model parameter. However, unlike Cα, the determination of and its variations with stress level is not properly documented in the literature. In an attempt to better understand , this paper presents an experimental investigation of the reference volumetric strain rate in reconstituted clay specimens. A long-term triaxial creep test, at different shear stress levels and different strain rates, was performed on clay specimen whereby the volumetric strain rate was measured. The obtained results indicated the stress-level dependency and non-linear variation of with time

Modelling of Short-Term Interactions Between Concrete Support and the Excavated Damage Zone Around Galleries Drilled in Callovo–Oxfordian Claystone

peer reviewedProduction of energy from nuclear power plants generates high-level radioactive nuclear waste, harmful during dozens of thousand years. Deep geological disposal of nuclear waste represents the most reliable solutions for its safe isolation. Confinement of radioactive wastes relies on the multi-barrier concept in which isolation is provided by a series of engineered (canister, backfill) and natural (host rock) barriers. Few underground research laboratories have been built all over the world to test and validate storage solutions. The underground drilling process of disposal drifts may generate cracks, fractures/strain localisation in shear bands within the rock surrounding the gallery especially in argillaceous rocks. These degradations affect the hydro-mechanical properties of the material, such as permeability, e.g. creating a preferential flow path for radionuclide migration. Hydraulic conductivity increase within this zone must remain limited to preserve the natural barrier. In addition galleries are currently reinforced by different types of concrete supports such as shotcrete and/or prefab elements. Their purpose is twofold: avoiding partial collapse of the tunnel during drilling operations and limiting convergence of the surrounding rock. Properties of both concrete and rock mass are time dependent, due to shotcrete hydration and hydromechanical couplings within the host rock. By the use of a hydro-mechanical coupled Finite Element Code with a Second Gradient regularization, this paper aims at investigating and predicting support and rock interactions (convergence, stress field). The effect of shotcrete hydration evolution, spraying time and use of compressible wedges is studied in order to determine their relative influence

Temperature and rate dependent theory of plasticity of crystalline solids

The paper shows a new approach for formulation of temperature and rate dependent models of plastic behaviour of crystalline solids. It has several particular objectives. The first is to give a very brief overview of recent developments of large strain constitutive laws for crystalline rate dependent plasticity.The second is to investigate how the main mechanisms on crystalline slip system can be incorporated in the general framework of continuum rate dependent large plastic deformation constitutive structure. The internal state variables have been precisely interpreted. Based on main mechanisms for crystalline slip system the evolution equations for the internal state variables introduced are proposed. The third objective is to illustrate how to include additional features such as work hardening, thermal softening etc. in constitutive structure proposed. Next objective is to focus attention on temperature dependence of plastic behaviour. The final goal is the discussion how these propositions can be extended to develop theories for large deformation,temperature and rate dependent, polycrystalline behaviour

Adiabatic shear band localization fracture of solids in dynamic loading processes

The main objective of the present paper is the application of a recently developed viscoplastic-damage type constitutive theory for high strain rate flow process and ductile fracture to the problem of shear band localization and fracture of dynamically loaded inelastic bodies experiencing strain rates ranging between 103 - 104 s-1. In the first part of the paper an adiabatic inelastic flow process is formulated and investigated. The Cauchy problem is examined and the conditions for well-posedness are discussed. The relaxation time is used as a regularization parameter. The viscoplastic regularization procedure assures the unconditionally stable integration algorithm by using the finite element method. The second part of the paper is devoted to the numerical investigation of the three-dimensional dynamic adiabatic deformations of a steel thin tube twisted in a split Hopkinson bar at nominal strain rates ranging 103 - 104 s-1.On présente l'application d'une récente théorie de rupture viscoplasique avec des microendommagements au problème de localization des bandes de cisaillement dans les corps non-élastiques soumis à la charge dynamique aux grands taux de déformation d'ordre 103-104 sec-1. Dans la première partie de l'article un processus adiabatique d'écoulement non-élastique est formulé et étudié. On fornlule ensuite le problème de Cauchy et on précise les conditions pour ce qu'il soit bien posé. Le temps de relaxation est utilisé comme un paramètre de régularisation. La régularisation viscoplastique assure la stabilité de l'algorithme d'intégration à l'aide de la méthode des éléments finis. La deuxième partie de l'article est consacrée à 1'étude numérique d'une 3D déformation dynamique adiabatique d'un cylindre mince en acier tordu à l'aide du bar d'Hopkinson avec les taux de deformation de l'intervalle 103 - 104 sec-1