Chandra S. Desai

Disturbed state constitutive modeling based on stress-strain and nondestructive behavior

The proposed disturbed state concept (DSC) is based on the idea that a deforming material element can be treated as a mixture of two constituent parts in the relative intact (RI) and fully adjusted (FA) states, referred to as reference states. During external loading, the material experiences internal changes in its microstructure due to a process of self-adjustment, and as a consequence, the initial RI state transforms continuously to the FA state. The self-adjustment process, which may involve relative motions of material particles that can lead to microcracking and damage or strengthening, can cause disturbances in the observed response with respect to the responses under the two reference states. Then, the observed response is expressed in terms of the responses for the RI and FA states that are determined from laboratory tests on material specimens. The DSC permits development of unified constitutive models that include, hierarchically, other previous continuum and damage models as special cases. Various aspects of the DSC are verified here with respect to laboratory behavior of two materials, a cemented sand and a ceramic composite. Some of the unique features of this study are that (1) the constitutive behavior and parameters can be obtained from the stress-strain-volume change behavior, and from the measurements of ultrasonic P-wave velocities, (2) correlations between mechanical and ultrasonic response can be established, (3) the concept can provide a description of the crack density, and (4) it can be simplified for predicting the remaining life of materials through definition of constitutive models and evaluation of design moduli affected by mechanical and environmental loading.

Elastoplastic model with damage for strain softening geomaterials

SummaryThe paper examines certain important aspects of a rate independent model that accounts for distributed damage due to microcrack growth. Material behavior is considered as a mixture of two elastic-plastic interacting components, one termed topical (undamaged), and the other termed damaged. Energy considerations show the equivalence of the two-component body to an elastic-plastic body containing cracks; the equivalence is considered in the Griffith sense. The mechanisms of failure are considered and discussed with respect to multiaxial stress paths. An explanation of failure, at the microlevel, is given. A series of laboratory tests on a concrete are used to illustrate the development of failure.

Thermomechanical Finite Element Analysis of Problems in Electronic Packaging Using the Disturbed State Concept: Part 1—Theory and Formulation

Accurate prediction of the thermomechanical cyclic behavior of joints and interfaces in semiconductor devices is essential for their reliable design. In order to understand and predict the behavior of such interfaces there is a need for improved and unified constitutive models that can include elastic, inelastic, viscous, and temperature dependent microstructural behavior. Furthermore, such unified material models should be implemented in finite element procedures so as to yield accurate and reliable predictions of stresses, strains, deformations, microcracking, damage, and number of cycles to failure due to thermomechanical loading. The main objective of this paper is to present implementation of such an unified constitutive model in a finite element procedure and its application to typical problems in electronic packaging; details of the constitutive model are given by Desai et al. (1995). Details of the theoretical formulation is presented in this Part 1, while its applications and validations are presented in Part 2, Basaran et al. (1998).

Numerical algorithms and mesh dependence in the disturbed state concept

The disturbed state concept (DSC) provides a unified approach for constitutive modelling of engineering materials including such factors as elastic, plastic and creep strains, microcracking, damage and softening, and sti⁄ening responses. The interacting mechanisms in the material mixture composed of the relative intact and fully adjusted states provide implicitly for various factors such as microcrack interaction and character- istic dimension. The DSC model can allow for well-posedness, reduction or elimination of spurious mesh dependence and localization. A number of problems are solved to illustrate convergence and uniqueness of the finite element procedures, localization, spurious mesh dependence, and validation with respect to observed behavior of simulated and practical problems. ( 1997 by John Wiley & Sons, Ltd.

Fundamental yet simplified model for liquefaction instability

A fundamental procedure is proposed for the identification of liquefaction in saturated soils based on the instability in the materials microstructure. The disturbed state concept (DSC) provides a unified constitutive model for the characterization of entire stress–strain behaviour under cyclic loading, and the values of disturbance at threshold states in the deforming microstructure provides the basis for the identification of liquefaction. The procedure is verified with respect to laboratory behaviour of two sands, saturated Ottawa and Reid Bedford. A mathematical analysis of the DSC constitutive matrix is also performed. Procedures for the application of the DSC for simplified analysis and design, and in finite element procedures are presented. It is believed that the proposed model can provide a fundamental yet simplified procedure for liquefaction analysis, and as a result, it is considered to be an improvement over the available empirical and energy-based procedures.

Thermomechanical Response of Materials and Interfaces in Electronic Packaging: Part I—Unified Constitutive Model and Calibration

The disturbed state concept (DSC) presented here provides a unified and versatile methodology for constitutive modeling of thermomechanical response of materials and interfaces/joints in electronic chip-substrate systems. It allows for inclusion of such important features as elastic, plastic and creep strains, microcracking and degradation, strengthening, and fatigue failure. It provides the flexibility to adopt different hierarchical versions in the range of simple (e.g., elastic) to sophisticated (thermoviscoplastic with microcracking and damage), depending on the user’s specific need. This paper presents the basic theory and procedures for finding parameters in the model based on laboratory test data and their values for typical solder materials. Validation of the models with respect to laboratory test behavior and different criteria for the identification of cyclic fatigue and failure, including a new criterion based on the DSC and design applications, are presented in the compendium paper (Part II, Desai et al., 1997). Based on these results, the DSC shows excellent potential for unified characterization of the stress-strain-strength and failure behavior of engineering materials in electronic packaging problems.

Implementation of DSC model and application for analysis of field pile tests under cyclic loading

The disturbed state concept (DSC) model, and a new and simplified procedure for unloading and reloading behavior are implemented in a nonlinear finite element procedure for dynamic analysis for coupled response of saturated porous materials. The DSC model is used to characterize the cyclic behavior of saturated clays and clay–steel interfaces. In the DSC, the relative intact (RI) behavior is characterized by using the hierarchical single surface (HISS) plasticity model; and the fully adjusted (FA) behavior is modeled by using the critical state concept. The DSC model is validated with respect to laboratory triaxial tests for clay and shear tests for clay-steel interfaces. The computer procedure is used to predict field behavior of an instrumented pile subjected to cyclic loading. The predictions provide very good correlation with the field data. They also yield improved results compared to those from a HISS model with anisotropic hardening, partly because the DSC model allows for degradation or softening and interface response. Copyright

Computational aspects of disturbed state constitutive models

Disturbed state concept (DSC) provides a unified basis for constitutive modelling including elastic, plastic and creep deformations, microcracking, damage and softening, stiffening, and cyclic fatigue under thermomechanical loading. It includes intrinsically regularization, localization, characteristic dimension and avoidance of spurious mesh dependence. It also leads to a new procedure for mesh adaptivity, particularly in zones experiencing significant microcracking and softening. The idea of the critical disturbance allows identification of the initiation of fracture and its growth as a consequence of microcrack coalescence. The DSC is implemented in nonlinear finite element procedures and a number of problems have been solved, and results are evaluated and compared with solutions by other methods so as to illustrate the foregoing capabilities of the model.

An interface model to describe viscoplastic behavior

Note: Sols Reference LMS-ARTICLE-1996-002doi:10.1002/(SICI)1096-9853(199604)20:4 3.0.CO;2-E Record created on 2006-11-09, modified on 2016-08-08

Analysis of the compression of structured soils using the disturbed state concept

The aim of this note is to quantify the influence of soil structure on the compression behaviour of natural soils using the disturbed state concept (DSC). The behaviour of the fully adjusted state is chosen to be that of the corresponding soil in a reconstituted condition so that the disturbance function is a direct measure of the effects of soil structure. A new DSC compression model is proposed. This model is able to describe the compression behaviour of structured soils under loading, swelling and reloading. Special versions of the proposed model are also described for situations (a) where the compression behaviour of the corresponding reconstituted soils is linear in the e–ln p′ space and (b) where the compression is one-dimensional. The ability of the proposed model and its various versions to describe the compression behaviour of structured soils has been verified. Copyright