Modeling of plain concrete structures based on an anisotropic damage formulation

In this contribution, a strain-based constitutive law for concrete within the framework of continuum damage mechanics is proposed. The model allows for the multi-axial simulation of predominantly tensile loaded plain concrete structures. A second-order integrity tensor is chosen as the internal damage variable to consider the phenomenon of load-induced anisotropy. The model is implemented in the finite element method. Hence, a fracture-energy based regularization approach is included to overcome the mesh-dependence of the computational results. The proposed model is applied to the simulation of well-known experiments with plain concrete specimens, which are often used to demonstrate the applicability of damage constitutive laws. In this context, attention is focused especially on problems regarding the accurate implementation of the complex loading and boundary conditions. Oversimplifications are discussed and proposals for a more realistic involvement of the test setups in finite element models are submitted.     

A new scalar potential formulation for three-dimensional magnetostatic problems

A new scalar potential formulation for three-dimensional problems is described. This formulation avoids cancellation errors within ferromagnetic objects and discontinuities and nonuniqueness of a scalar potential outside these objects. These deficiencies are peculiar to reduced and total scalar potential formulations, respectively. The finite-element discretization is applied to the new scalar potential formulation, and a novel approach to smoothing and extension of finite-element solutions is presented. Some numerical results obtained using the new scalar potential formulation are reported.     

Numerical implementation of anisotropic damage mechanics

This paper describes implementation of anisotropic damage mechanics in the material point method. The approach was based on previously proposed, fourth‐rank anisotropic damage tenors. For implementation, it was convenient to recast the stress update using a new damage strain partitioning tensor. This new tensor simplifies numerical implementation (a detailed algorithm is provided) and clarifies the connection between cracking strain and an implied physical crack with crack opening displacements. By using 2 softening laws and 3 damage parameters corresponding to 1 normal and 2 shear cracking strains, damage evolution can be directly connected to mixed tensile and shear fracture mechanics. Several examples illustrate interesting properties of robust anisotropic damage mechanics such as modeling of necking, multiple cracking in coatings, and compression failure. Direct comparisons between explicit crack modeling and damage mechanics in the same material point method code show that damage mechanics can quantitatively reproduce many features of explicit crack modeling. A caveat is that strengths and energies assigned to damage mechanics materials must be changed from measured material properties to apparent properties before damage mechanics can agree with fracture mechanics.     

Thermodynamic formulation of finite linear viscoelasticity for anisotropic media

Summary The theory of irreversible thermodynamics of continuous media with fading memory is used to formulate general constitutive equations of finite linear viscoelasticity. Specific forms for transversely isotropic and orthotropic media based on both the mechanical and thermodynamic theories are found.

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Zusammenfassung Die Aufstellung allgemeiner Materialgleichungen der endlichen linearen Viskoelastizität wird mit Hilfe der irreversiblen Thermodynamik kontinuierlicher Medien mit Erinnerungsschwund durchgeführt. Spezielle Fassungen für transversal isotrope und orthotrope Medien, basierend auf der mechanischen und der thermodynamischen Theorie, werden angegeben.

     

A soil damage model expressed by a double scalar and its applications

Poroelasticity refers to the study of the mechanics of porous elastic materials that are saturated with compressible or incompressible fluids. When considering saturated poroelastic geomaterials, their consolidation response can be influenced by the evolution of damage in the porous skeleton. The objective of this paper is to examine the problem of consolidation response of damage-susceptible poroelastic geomaterials. Firstly, a new constitutive model of soft soils expressed by isotropic double scalar damage variables is developed and incorporated into Biot’s consolidation finite element equations via EDAPD program. Then, the EDAPD program is applied to analyze a soft subgrade reinforced by surcharge preloading technology. The comparison between the numerical predictions and the experimental data shows that the isotropic double scalar damage model presented in this paper is effective and feasible in analyzing the consolidation problem of damaged porous media.     

Micromechanical identification of anisotropic damage evolution laws

This paper deals with the establishment of anisotropic conjugate force based damage evolution laws in the framework of Rice's (1971) ‘normality structure’. The damage variable is the second-order crack tensor (Kachanov, 1980), which represents preexisting Griffith microcracks in a solid. The principal results include the deduced damage surfaces, potentials and kinetic equations for the basic internal variables and damage tensor during isothermal processes. The generalized pth order crack tensors and qth order energy release rates are introduced. The deduction in this paper is fully independent of the specific form of the free energy or Gibbs energy functions, so the deduced damage evolution laws have a wide applicable range including plasticity. Using the deviatoric stress as the conjugate force, the two well-established anisotropic yield surfaces, Karafillis and Boyce (1993) and Hill (1950), are recovered from the deduced damage surface. This revised version was published online in July 2006 with corrections to the Cover Date.     

Axisymmetric formulation for boundary integral equation methods in scalar potential problems

Axisymmetric geometries often appear in electromagnetic device studies. The authors present an original formulation for Boundary Integral Equation methods in scalar potential problems. This technique requires only 2D boundary in the r-z plane and evaluation of the equations only on those boundaries.     

Modeling of delamination using a discretized cohesive zone and damage formulation

Delamination initiation and growth are analyzed by using a discrete cohesive crack model. The delamination is constrained to grow along a tied interface. The model is derived by postulating the existence of a maximum load surface which limits the adhesive forces in the process zone of the crack. The size of this maximum load surface is made dependent on the amount of dissipated crack opening work, such that the maximum load surface shrinks to zero as a predefined amount of work is consumed. A damage formulation is used to reduce the adhesive forces. Mode I, II and III loading or any combined loading is possible. An analytical solution is obtained for a single mode opening and the implications of this result on the governing equations is discussed. The delamination model is implemented in the finite element code LS-DYNA and simulation results are shown to be in agreement with experimental results.     

A finite element approach to anisotropic damage of ductile materials in large deformations Part II: finite element formulation and applications

A finite element analysis model for material and geometrical non-linearities due to large plastic deformations of ductile materials is presented using the continuum damage mechanics approach. To overcome limitations of the conventional plastic analysis, a fourth-order tensor damage, defined in Part I of this paper to represent the stiffness degradation in the finite strain regime, is incorporated. General forms of an updated Lagrangian (U.L.) finite element procedure are formulated to solve the governing equations of the coupled elastic–plastic-damage analysis, and a computer program is developed for two-dimensional plane stress/strain problems. A numerical algorithm to treat the anisotropic damage is proposed in addition to the non-linear incremental solution algorithm of the U.L. formulation. Selected examples, compared with published results, show the validity of the presented finite element approach. Finally, the necking phenomenon of a plate with a hole is studied to explore plastic damage in large strain deformations. This revised version was published online in August 2006 with corrections to the Cover Date.     

An anisotropic damage model for tensile fatigue

Fatigue damage in materials is considered to be the effect of material degradation, and the dispersion in fatigue life is attributed to variability in microstructure. This paper presents a numerical model to simulate fatigue damage evolution using continuum damage mechanics to characterize material degradation. An explicit microstructure topology representation is achieved using Voronoi tessellations. Unlike conventional models which use a scalar approximation for damage, this model treats the damage variable as an anisotropic tensor. The model is used to simulate tensile fatigue failure in thin steel specimen. The fatigue life estimations from the model compares well with published experimental results. The results predict a high variability in fatigue life that is characteristic of metals and alloys, as compared with the existing isotropic damage models available in the literature. The model was also used to study the influence of material inhomogeneity on fatigue life dispersion.     

Finite element analysis of anisotropic damage mechanics problems

Elastic constitutive relationships for anisotropic damage mechanics have been developed in this paper. Implementation of these constitutive equations in the finite element analysis is explained. Validation of these relations is provided in the form of comparison of numerical results with the available experimental results. The application of these relationships to an anisotropic damaged foundation problem is discussed.     

Boundary integral equation formulation for Interface cracks in anisotropic materials

We present a boundary integral formulation for anisotropic interface crack problems based on an exact Green's function. The fundamental displacement and traction solutions needed for the boundary integral equations are obtained from the Green's function. The traction-free boundary conditions on the crack faces are satisfied exactly with the Green's function so no discretization of the crack surfaces is necessary. The analytic forms of the interface crack displacement and stress fields are contained in the exact Green's function thereby offering advantage over modeling strategies for the crack. The Green's function contains both the inverse square root and oscillatory singularities associated with the elastic, anisotropic interface crack problem. The integral equations for a boundary element analysis are presented and an example problem given for interface cracking in a copper-nickel bimaterial.     

An anisotropic elastic formulation for configurational forces in stress space

A new variational principle for an anisotropic elastic formulation in stress space is constructed, the Euler–Lagrange equations of which are the equations of compatibility (in terms of stress), the equilibrium equations and the traction boundary condition. Such a principle can be used to extend recently obtained configurational balance laws in stress space to the case of anisotropy.     

A finite element formulation for highly anisotropic incompressible elastic solids

A numerical approach is developed for the solution of problems of materials with extremely strong directions. Small deformations of a transversely isotropic linear elastic solid, reinforced by a single family of inextensible fibres, are considered. The kinematic constraint equations of incompressibility and inextensibility in the fibre direction lead to the appearance of an arbitrary hydrostatic pressure and an arbitrary tension stress in the constitutive equations. A Galerkin approach is used to discretize the virtual work and weak form of the constraint equations. Independent interpolation of the displacement, pressure and tension fields leads to a mixed system of equations, with characteristic zero-diagonal terms. The assumption of plane stress conditions in the plane of the fibres results in a simplified displacement-tension formulation, analogous to the primitive-variable formulation of Stokes flow. A mixed penalty approximation is then employed to solve for displacement and tension stress fields. Computations are carried out using a biquadratic displacement element with discontinuous bilinear tension stress interpolation. The formulation is used to solve a number of simple beam problems and the results compared to closed-form solutions.     

Non-singular antiplane fracture theory within nonlocal anisotropic elasticity

In the present paper, the distributed dislocation technique is applied for the analysis of anisotropic materials weakened by cracks. Eringen's theory of nonlocal elasticity of Helmholtz type is employed. The non-singular screw dislocation within anisotropic elasticity is distributed to model cracks of mode III. The corresponding dislocation density functions are evaluated using the proper crack-face boundary conditions. The nonlocal stress field within a plane weakened by cracks is determined. The crack opening displacement is also discussed within the framework of nonlocal elasticity. The stress singularity of the classical linear elasticity is removed by the introduction of the nonlocal theory of elasticity. The general anisotropic case and the special case of orthotropic material are studied. The effect of material orthotropy is presented for a crack which is not necessarily aligned with the principal orthotropy direction.     

A model for anisotropic viscoelastic damage in composites

A model for continuous damage combined with viscoelasticity is proposed. The starting point is the formulation connecting the elastic properties to the tensor of damage variables. A hardening law associated with the damage process is identified from available experimental information and the rate-type constitutive equations are derived. This elastic damage formulation is used to formulate an internal variable approximation to viscoelastic damage in the form of a non-linear Kelvin chain. Elastic and viscoelastic equations are implemented into a finite element procedure. The code is verified by comparison with closed-form solutions in simplified configurations, and validated by fitting results of experimental creep tests.     

An anisotropic damage model with dilatation for concrete

An anisotropic damage model for concrete is developed within the general framework of the internal variable theory of thermodynamics. The rate of change of the compliance tensor is described in terms of kinetic relations involving a damage parameter whose increment is governed by the consistency equation associated with a pressure-dependent damage surface in stress space. The use of the compliance tensor implies that damage is reflected through a fourth-order tensor. Dilatation is obtained as a consequence of damage, and permanent deformation due to damage is addressed via a simple evolution equation. The theory is capable of accommodating the anisotropy induced by microcracking and is very suitable for computer implementation.