Stein Sture

Assessment of Localized Deformations in Sand Using X-Ray Computed Tomography

The internal fabric and localized deformation patterns of triaxial sand specimens were investigated using computed tomography (CT). Three displacement-controlled, conventional, drained axisymmetric (triaxial) experiments were conducted on dry Ottawa sand specimens at very low effective confining stresses (0.05, 0.52, and 1.30 kPa) in a microgravity environment aboard the Space Shuttle during the NASA STS-79 mission. CT scanning was performed on these flight specimens, as well as on an uncompressed specimen and a specimen tested in a terrestrial laboratory at 1.30 kPa effective confining stress. CT demonstrated high accuracy in detecting specimen inhomogeneity and localization patterns. Formation of deformation patters is dependent on the effective confining stress and gravity. Multiple symmetrical radial shear bands were observed in the specimens tested in a microgravity environment. In the axial direction, two major conical surfaces were developed. Nonsymmetrical spatial deformation was observed in the 1-G specimen. Analysis tools were developed to quantify the spatial density change. Void ratio variation within and outside the shear bands is calculated and discussed.

Fundamental Issues of Smeared Crack Models

For numerical simulation of fracture in concrete and rock the “smeared crack approach” is receiving increasing attention. On one hand renewed attempts in terms of the fixed and rotating crack models resort to fracture mechanics in order to refine the traditional orthotropic crack formulation. Along this approach the original concept involving Mode I type cracking is being broadened to include mixed mode fracture interpretation of the shear retention factor, if the crack memory is fully retained. On the other hand, fracture energy-based plasticity models are advocated by the authors, as well as other investigators, which describe the degradation of strength due to tensile cracking and decohesion in shear in terms of isotropic and anisotropic strain-softening concepts.

Embedded localization band in undrained soil based on regularized strong discontinuity theory and FE-analysis

Abstract The paper presents a novel approach to the analysis of a developing localization zone in undrained soil considered as a mixture of a solid skeleton and fluid-filled pores, where the solid phase is considered as elastic-plastic. The basic feature is the concept of regularized displacement and pore pressure discontinuities, which are assumed to occur across an internal discontinuity surface. In this respect the present developments are extensions of those suggested by Simo et al. (1993), Comput. Mech. 12 , 277–296, and, more recently, by Larsson and Runesson (1995), J. Engng Mech. ASCE (in press). A finite element formulation is proposed on the basis of a mixed variational formulation in the spirit of the “enhanced strain” concept by Simo and Rifai (1990), Int. J. Numer. Meth. Engng 29 , 1595–1638. In this fashion, the localization zone is embedded into a “base” element for the ordinary analysis of the (non-localized) mixture problem.

CONCRETE OVER THE TOP, OR IS THERE LIFE AFTER PEAK?

Test results are presented that show the postpack response of concretes of 3 different strengths under triaxial stress states. These test results clearly show the transition from brittle to ductile behavior under increasing confinement, the residual strength, and the strain behavior at and after peak. Some evidence that indicates that Druckers postulate may be relaxed for concrete is presented also. Concrete of different strengths behave similarly, with increasing brittleness for higher strength concretes.

Tensile Strength of Unsaturated Sand

A theory that accurately describes tensile strength of wet sand is presented. A closed form expression for tensile strength unifies tensile strength characteristics in all three water retention regimes: pendular, funicular, and capillary. Tensile strength characteristically increases as soil water content increases in the pendular regime, reaches a peak in the funicular regime, and reduces with a continuing water content increase in the capillary regime. Three parameters are employed in the theory: internal friction angle (at low normal stress) ϕt , the inverse value of the air-entry pressure α , and the pore size spectrum parameter n . The magnitude of peak tensile strength is dominantly controlled by the α parameter. The saturation at which peak tensile strength occurs only depends on the pore size spectrum parameter n . The closed form expression accords well with experimental water retention and tensile strength data for different sands.

Integration in computational plasticity

Abstract Temporal integration of the constitutive relations is an important topic in computational plasticity. A brief review of recent developments is given in this paper. The Generalized Midpoint Rule and the Generalized Closest Point Projection Method are emphasized in particular. Algorithmic issues are discussed, especially with respect to the treatment of non-smooth yield surfaces. Numerical analyses support the conclusion that midpoint integration is preferred for small and moderate strain increments, while the Backward Euler rule (Closest Point Projection Method) is best suited when large strain increments can be anticipated, e.g. in zones of localized deformation and in large deformation plasticity.

FLUID CUSHION TRULY TRIAXIAL OR MULTIAXIAL TESTING DEVICE

The derivation of the constitutive relations of soils requires a testing facility that can apply a homogeneous and nearly ideal set of controlled boundary conditions to a specimen. A new multiaxial cubical test apparatus is described. If uses fluid or pneumatically pressurized flexible cushions to transmit a three-dimensional, independently controlled, and compressive stress state to a 102-mm (4-in.) specimen of geologic material. The deformations in three orthogonal directions are detected by a set of linear variable differential transforms. Flexible cushions allow nearly unrestrained deformations in the specimen. The deformations are uniform even at large strains. The specimen preparation and apparatus assembly procedures are straightforward. The simplicity of the multiaxial cubical cell and its easy operating procedures are emphasized. Typical stress-strain curves for straight-line stress paths are discussed. The apparatus appears to opeate especially well at low stress levels. (Author)

Implicit integrations in elastoplastic geotechnics

The paper presents the derivation of a fully implicit Newton algorithm for direct integration of constitutive equations, in extended stress-internal variable space, involving hardening or softening of a general dilatant isotropic elastoplastic geomaterial. All relevant derivatives are provided in tensor notation, thus facilitating implementation. The consistent, algorithmic tangent stiffness tensor is derived. The relative accuracy of a template algorithm is assessed on a number of examples by means of iso-error maps. We present a rather simple, one-increment example concerning convergence properties of the Newton iterative scheme at the global, finite element level, associated with the consistent tangent stiffness tensor for integrating the weak form of the equilibrium equations.

Effect of pore fluid compressibility on localization in elastic-plastic porous solids under undrained conditions

Abstract Conditions are derived for band-shaped localization, i.e. for the existence of discontinuous bifurcations of the incremental strain field, in elastic-plastic porous materials, under undrained conditions. It is assumed that the pores are filled with nearly incompressible liquid/gas mixture, and the significant parameter is the bulk modulus of the pore fluid. The extreme situation is complete incompressibility, which would correspond to full saturation of a liquid with an infinite bulk modulus. In this case the surprising result is obtained that, for plane strain and cylindrical stress states, the critical bifurcation direction is always 45° to a principal stress axis regardless of the adopted yield criterion. It is also shown in the paper that incompressibility usually (although not generally) has a stabilizing effect. For example, when the flow rule is associated, the critical hardening modulus is always non-positive. A discontinuity in strain rate is always accompanied by discontinuous pore pressurerate across the characteristic surface, and the magnitude of this jump is related to the strength of the displacement jump. One restrictive assumption used in the analysis is that the gradient of the yield criterion and the flow direction are coaxial. A numerical evaluation of the critical bifurcation direction and the corresponding hardening modulus is given for the Mohr-Coulomb criterion, which is representative for the behavior of cohesionless granular materials such as soil.

Analysis and calibration of a three-invariant plasticity model for granular materials

SummaryIn this paper we briefly review issues related to the characterization of properties of granular materials subjected to micro-gravity and one-gravity conditions at very low effective stress levels. We describe the development of a three-invariant plasticity model that resembles the model devised by Lade. An inverseidentification scheme where the analysis tools are used to extract constitutive model parameters from experiments is also discussed.ZusammenfassungÜbersicht: Über Resultate, die mit der Charakterisierung der Eigenschaften von granularen Stoffen in schwachen Schwerefeldern bzw. dem der Erde bei sehr kleinen Spannungen zusammenhängen, wird ein kurzer Überblick gegeben. Beschrieben wird ein mit drei Spannungsinvarianten formuliertes plastisches Stoffgesetz, welches dem von Lade entwickelten Modell ähnlich ist. Weiterhin wird ein inverses Identifikationsschema diskutiert, bei dem mit analytische Methoden benutzt werden, um die Parameter des Stoffmodells zu gewinnen.