Showing papers on "Viscoplasticity published in 1984"

Evaluation of Viscoplastic Cap Model

Abstract: A time‐dependent plasticity model for rocks and soils based on Perzyna's elastic/viscoplastic theory is presented, wherein the plasticity yield function is patterned after the inviscid cap model developed over the last decade by Sandler et al. Three aspects are addressed: (1) Theoretical review and development; (2) numerical solution strategies suitable for a displacement‐based finite element method; and (3) illustrative examples of model behavior and comparisons with experimental data. The numerical solution algorithm employs a one‐parameter time integration scheme which provides options for explicit or implicit methods. Nonlinear equations resulting from the implicit method are solved by a Newton‐Raphson iterative technique, and efficiency versus accuracy studies are reviewed. Parameter identification and comparison of the model with experimental data is demonstrated for hard limestone in a variable loading history, soft sedimentary rock in triaxial creep tests, and well‐graded sand in slow and rapid lo...

The Anisotropic Deformation of Yield Surfaces

Abstract: Extensive experimental evidence indicates that yield surfaces exhibit significant anisotropic hardening. Subsequent yield surfaces are both translated and deformed in stress space. A viscoplasticity theory which describes anisotropic hardening by using a Lagrangian deformation mapping of the elastic domain is applied to the quantitative modelling of observed yield surface deformation of 1100-0 aluminum subject to non-proportional loading in tension-torsion space.

An analytical solution for axisymmetric tunnel problems in elasto‐viscoplastic media

Abstract: An analytical solution is presented for the time-dependent stresses and displacements in plane strain around a circular hole when it is loaded by an axisymmetric internal and far-field pressure. The material is assumed to be elasto-viscoplastic with dilatant plastic deformations according to a non-associated flow rule. Strain softening is considered by a modified St. Venant slider which is characterized by Mohr-Coulomb yield conditions for both the peak and the residual strengths.

Dissipative materials as models of thixotropy and plasticity

Abstract: We consider a class of simple materials characterized by a history-dependent viscosity tensor which we call purely dissipative. These materials encompass several existing models of thixotropy and viscoplasticity, including a generalized Oldroyd-Bingham material. We lay down a theoretical framework for classifying these and other simple materials which extends and modifies the well-known scheme of Noll. Finally, we show how a dissipative model of the fluid-like state associated with plastic yielding in solids can be employed to formulate the usual plastic yield condition and flow rule in a single equation.

A finite element analysis of metal‐forming problems with an elastic‐viscoplastic material model

Abstract: A finite element formulation for elastoviscoplasticity problems in the presence of large strains and deformations is presented in this paper. The formulation is capable of using any of a class of combined creep-plasticity constitutive models with state variables for the description of material behaviour. The specific problem considered is plane strain extrusion using the constitutive model originally proposed by Hart. Numerical results are presented for sample problems and the important effects of extrusion speed and friction (at the die–workpiece interface) on the residual stresses in an extruded sample are investigated. The computer program developed here is quite general and can be modified to solve problems for other manufacturing processes like rolling, sheet forming, etc.

Some critical uniaxial experiments for viscoplasticity at room temperature

Abstract: An austenitic AISI Type 304 stainless steel, a ferritic A533B pressure vessel steel and a Ti-7Al-2Cb-1Ta alloy were tested using a servocontrolled MTS axial-torsion testing machine. Tests involved changes in strain rate between 10−8 and 10−3 s−1 and intermittent creep periods of less than 1200 s duration. The tests show that inelastic work is not a suitable repository for modeling strain (work)-hardening and the Bauschinger effect is found to be rate dependent. Upon an increase in stress level, creep rate can decrease. This anomaly can be reproduced by a theory of viscoplasticity based on overstress previously proposed by the first author and his co-workers.

A Class of Elastic‐Plastic Constitutive Laws for Brittle Rock

Abstract: The inelastic behavior of fissured rock masses is due primarily to microcracking from the tips of preexisting fissures and frictional sliding on fissure surfaces. Consequently, the macroscopic inelastic response is inhibited by an increase of hydrostatic compression and exhibits volume change and strain softening. By generalizing the type of laws often used in metal plasticity, Rudnicki and Rice introduced a class of simple constitutive laws that incorporate these features and is useful for studying the inception of rupture. An important aspect of this generalization is that normality of the inelastic strain increment vector to a yield surface in stress space, as assumed in classical metal plasticity, is not appropriate. More detailed consideration of the preferential activation of sliding on differently oriented fissure surfaces during a program of loading suggests that, although this class of laws will be suitable for describing loading in which stress components increase nearly in proportion to a singl...

Constitutive Restrictions for Idealized Elastic-Viscoplastic Materials

Abstract: Etude des restrictions sur les equations constitutives de materiaux elastoviscoplastiques en presence de deformations finies On utilise un modele qui inclut le dechargement et permet une definition convenable de la deformation plastique

Failure analysis of structures by continuum damage mechanics

Abstract: In this paper, principles of Continuum Damage Mechanics are applied to the failure analysis of structures. First, constitutive equations of elastic, plastic or viscoplastic materials coupled to damage laws are presented. Then, their implementation in a in-house finite element program is described. Within this framework, a numerically predicted macrocrack is the set of points in the structure where damage has reached its critical value. This procedure is applied to the prediction of macrocrack initiation and propagation.

Determination of near-surface plastic deformation in sliding contacts

Abstract: It is pointed out that substantial plastic deformation occurs on and near the contact surfaces, when two solid bodies slide against each other without lubrication. It has been found that this deformation plays an important role in the tribological behavior of the sliding contact. The present investigation has the objective to develop an analytical model to predict the near-surface plastic deformation resulting from a single pass of one metallic surface over another. A finite element viscoplasticity program was written relating velocities to forces in a two-dimensional domain. The program was employed in the study of plastic deformation during a single pass of a hardened tool steel slider over a copper rub specimen. It was found that essentially the only material set in motion by the slider was directly under the contact zone. The agreement between values obtained in the analysis and experimental data is reasonably good.

Inelastic deformation of thin cylindrical shells under axisymmetric loading

Abstract: The response of a thin cylindrical shell of elastic-viscoplastic material to internal pressure is considered. Small displacements and the validity of the kinematical Love-Kirchhoff-hypothesis are presupposed. Further it is assumed that the inelastic behavior of the shell material is governed by a unified constitutive model with internal state variables, where the total strain tensor can be decomposed additively into an elastic and an inelastic part. Under these assumptions the governing differential equation for the radial displacement is derived. A general solution is obtained by the method of variation of parameters and adjusted to different boundary conditions.

Constitutive Modeling of Engine Materials.

Abstract: : This investigation examines the capability to predict the growth of cracks in critical structural components operating at elevated temperatures where time-dependent behavior occurs. A viscoplastic constitutive model for INCO 718 was developed to determine this behavior in the area of the crack tip. The model constants were obtained from monotonically increasing, cyclic, and sustained loading. Therefore, the model has the capability to account for load history effects as well as material damage accumulation. Theoretical predictions are compared to the experimental results. (Author)

Explicit forms for the tangent modulus tensor in viscoplastic stress analysis

Abstract: Viscoplastic constitutive models typically lead to compliance (strain–stress) relationships, which must be inverted for use in the finite element displacement method. Computational requirements can be increased significantly for models of practical size due to the large number of matrix inversions which are necessary. This paper describes a method of obtaining the required stress–strain relations analytically, thus eliminating the need for numerous matrix inversions in the solution. The technique is applicable to a number of commonly-used viscoplastic models, as demonstrated in the examples.

High-temperature constitutive modeling

Abstract: Thermomechanical service conditions for high-temperature levels, thermal transients, and mechanical loads severe enough to cause measurable inelastic deformation are studied. Structural analysis in support of the design of high-temperature components depends strongly on accurate mathematical representations of the nonlinear, hereditary, inelastic behavior of structural alloys at high temperature, particularly in the relatively small strain range. Progress is discussed in the following areas: multiaxial experimentation to provide a basis for high-temperature multiaxial constitutive relationships; nonisothermal testing and theoretical development toward a complete thermomechanically path dependent formulation of viscoplasticity; and development of viscoplastic constitutive model accounting for initial anisotropy.