Publications

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

The purpose of this presentation is to provide an overview of the science-based materials modeling activities at Sandia National Laboratories, California. The main mission driver for the work is the development of predictive modeling and simulation capabilities leveraging high performance computing software and hardware. Presentation will highlight research accomplishments in several specific topics of current interest. Sandia/California has been engaged in the development of high performance computing based predictive modeling and simulation capabilities in support of the Science-Based Stockpile Stewardship Program of the U. S. Department of Energy. Of particular interest is the development of constitutive models that can efficiently and accurately predict post-failure material response and load-redistribution in systems and components. Fracture and failure are inherently multi-scale and our philosophy is to include required physics in our models at all appropriate scales. We approach the problems from the continuum point of view and intend to provide continuum models that include dominant subscale mechanisms. Moreover, numerical algorithms are needed to allow implementation of physical models in high performance computing codes such that large-scale modeling and simulation can be conducted. Other drivers of our effort include the emerging application of micro- and nano-systems and the increasing interest in biotechnology. In this presentation, our research in fracture and failure modeling, atomic-continuum coupling code development, microstructure-material properties relationships exploration, and general continuum theories advancement will be presented. Where appropriate, examples will be given to demonstrate the utility of the models.

We report our conclusions in support of the FY 2003 Science and Technology Milestone ST03-3.5. The goal of the milestone was to develop a research plan for expanding Sandia's capabilities in materials modeling and simulation. From inquiries and discussion with technical staff during FY 2003 we conclude that it is premature to formulate the envisioned coordinated research plan. The more appropriate goal is to develop a set of computational tools for making scale transitions and accumulate experience with applying these tools to real test cases so as to enable us to attack each new problem with higher confidence of success.

This report summarizes materials issues associated with advanced micromachines development at Sandia. The intent of this report is to provide a perspective on the scope of the issues and suggest future technical directions, with a focus on computational materials science. Materials issues in surface micromachining (SMM), Lithographic-Galvanoformung-Abformung (LIGA: lithography, electrodeposition, and molding), and meso-machining technologies were identified. Each individual issue was assessed in four categories: degree of basic understanding; amount of existing experimental data capability of existing models; and, based on the perspective of component developers, the importance of the issue to be resolved. Three broad requirements for micromachines emerged from this process. They are: (1) tribological behavior, including stiction, friction, wear, and the use of surface treatments to control these, (2) mechanical behavior at microscale, including elasticity, plasticity, and the effect of microstructural features on mechanical strength, and (3) degradation of tribological and mechanical properties in normal (including aging), abnormal and hostile environments. Resolving all the identified critical issues requires a significant cooperative and complementary effort between computational and experimental programs. The breadth of this work is greater than any single program is likely to support. This report should serve as a guide to plan micromachines development at Sandia.

This article attempts to review the progress achieved in the understanding of scaling and size effect in the failure of structures. Particular emphasis is placed on quasibrittle materials for which the size effect is important and complicated. After reflections on the long history of size effect studies, attention is focused on three main types of size effects, namely the statistical size effect due to randomness of strength, the energy release size effect, and the possible size effect due to fractality of fracture or microcracks. Definitive conclusions on the applicability of these theories are drawn. Subsequently, the article discusses the application of the known size effect law for the measurement of material fracture properties, and the modeling of the size effect by the cohesive crack model, nonlocal finite element models and discrete element models. Extensions to compression failure and to the rate-dependent material behavior are also outlined. The damage constitutive law needed for describing a microcracked material in the fracture process zone is discussed. Various applications to quasibrittle materials, including concrete, sea ice, fiber composites, rocks and ceramics are presented. There are 377 references included in this article. © 1997 American Society of Mechanical Engineers.

This paper presents a nonlocal analysis of the dynamic damage accumulation processes in brittle solids. A nonlocal formulation of a microcrack based continuum damage model is developed and implemented into a transient dynamic finite element computer code. The code is then applied to the study of the damage accumulation process in a concrete plate with a central hole and subjected to the action of a step tensile pulse applied at opposite edges of the plate. Several finite element discretizations are used to examine the mesh size effect. Comparisons between calculated results based on local and nonlocal formulations are made and nonlocal effects are discussed.

Numerical simulations of perforation in steel plates involve the treatment of material failure during the perforation process. One way to model physical material separation is to delete failed elements from the analysis based on an appropriate failure criterion. Different algorithms were used in different transient finite element codes to delete failed elements. This investigation compares the results of PRONTO 2D and LS-DYNA2D codes for a specific steel plate perforation problem. Influences of the deletion algorithms on material parameters are discussed.

Application of conventional fracture mechanics concepts to treat crack growth and failure problems in geological media is discussed in this paper. Conventional fracture mechanics methods were developed mainly for metallic materials which exhibit nonlinearity associated mainly with plasticity type responses. Thus, these are not directly applicable to geological materials whose inelastic responses originate from inherent large-scale heterogenities, microcracking, strain softening, etc. Proposed fracture mechanics methods for geological materials and their associated problems are discussed. To demonstrate the utility of fracture mechanics concepts in geological applications, examples involving multiple-fracture generation in tight gas formations and oil shale blasting design are presented.

Pretest analysis of a heated block test, proposed for the Exploratory Studies Facility at Yucca Mountain, Nevada, was conducted in this investigation. Specifically, the study focuses on the evaluation of the various designs to drill holes and cut slots for the block. The thermal/mechanical analysis was based on the finite element method and a compliant-joint rock-mass constitutive model. Based on the calculated results, relative merits of the various test designs are discussed.

This investigation involves the development of a general two- dimensional continuum model to describe jointed rock mass. Chen recently developed a model for the analysis of rock mass containing two orthogonal joint sets. Development of the orthogonal joint set model followed the general formulation of Morland and the special single joint set implementation of Morland`s model by Thomas. Although the orthogonal joint set model has proven useful for analyzing field-scale problems, it remains restrictive in terms of the general field conditions. In this paper, the orthogonal joint set model has been extended to a more general model where the orthogonality restriction has been relaxed. Fundamental approaches remain the same for both models. However, as the general model becomes capable of treating physically more complicated problems, it becomes mathematically more complex. This complexity provides the potential to study more completely the interaction of various parameters representing the characteristics of jointed rock mass behavior. The equation governing the solution of the problem has been given, and example problems have been solved. The behavior of the rock mass predicted by the orthogonal joint set model has been compared to the general model. This model has been developed to aid in characterizing the site of the repository at Yucca Mountain, Nevada, for the potential geologic disposal of radioactive waste. Disposal of high-level nuclear waste is currently being considered by the Yucca Mountain Project, administered by the Nevada Operations Office of the US Department of Energy.

An isotropic continuum damage theory which accounts for the degradation of material strength under quasi-static loading conditions has been developed in the present investigation. The damage mechanism in this theory has been selected to be the interaction and growth of subscale cracks. The development of the theory follows closely the strain-rate dependent dynamic model advanced by the first author and his coworkers. Briefly, the cracks are activated by the maximum principal tensile strain and the density of activated cracks is described by a Weibull statistical distribution. The moduli of a cracked solid derived by Budiansky and O'Connell are then used to represent the global material degradation due to subscale cracking. Two additional material constants have been introduced in this model. These constants are determined from uniaxial tensile test data. The model has been implemented into a finite element code. Sample calculations involving the uniaxial and biaxial responses of plain concrete panels are presented to demonstrate the utility of the model. 7 refs., 2 figs.