Sia Nemat-Nasser - US grants

Coordinated theoretical and experimental research will be performed on the microstructure in granular materials and the interrelationships of microstructure to the overall constitutive behavior will be investigated. This research will identify and establish stress-fabric relationships; characterize fabric changes associated with stress rates; and develop appropriate constitutive relations.

A special triaxial loading system is purchased for studying granular materials (soils) under multi-axial stress conditions. The loading system will accommodate large samples (250mm high, 200mm inside diameter and 250mm outside diameter). The system is capable of independent application of confining pressure, axial compression, and torsion. A true triaxial state of stress with prescribed directions of the principal stresses can be achieved.

The research addresses a key issue in damage mechanics of brittle solids and plans to develop a new approach utilizing micromechanics concepts to understand the fracture and damage characteristics of brittle materials including granular solids and concrete. The research plan includes experimental and theoretical investigations of inelastic response and failure mechanisms of materials which contain specific microdefects. Controlled model experiments will be conducted where the geometry and other characteristics of the defects that produce micro cracking and inelastic flow are prescribed. This procedure will allow the observation of the evolution of the microstructure in the course of deformation. This will provide quantitative estimates of the macroscopic parameters which characterize stresses and strains. Model analyses will be performed utilizing rigoroud mechanics. Theoretical models will be developed and verified by experimental observations. The Principal Investigator is in intetnationally recognized expert in fracture mechanism of brittle and granular solids. The institution has an excellent laboratory facility to conduct this research. An award is recommended.

The research aims at developing a fundamental understanding of shear-band initiation and propagation. The materials that will be investigated, titanium and aluminum alloys, have great importance in the aerospace industry; they undergo shear localization during plastic deformation and this is an important practical problem. Systematic and integrated experimental and theoretical investigation of the phenomenon of shear-band formation will be carried out. This will lead to the prediction and control of shear instability and to the establishment of its effect on the strength and failure modes of a broad class of materials. Dynamic experiments will be performed, in order to generate shear bands under controlled conditions. The Hopkinson bar experimental technique will be used with both hat-shaped and double-notched specimens. Pulse amplitude and shape in the Hopkinson bar will be varied to control the extent of shear-band propagation. This will be followed by microstructural characterization involving optical, scanning, and transmission electron microscopy. The microstructure after controlled high-strain-rate deformation stages and prior to the onset of macroscopic shear localization, will also be systematically investigated in order to establish which internal defects can lead to the initiation of localization and to characterize the microstructural evolution. The microstructure at the tip of the shear band will be examined in order to establish the structural changes in this zone prior to shear band extension.

An Institute of Mechanics and Materials (IMM) is being established at the University of California at San Diego to integrate research and industrial applications in mechanics and materials. The Institute will foster interdisciplinary communication and liaison between academia, industry and governmental organizations. The principal activities will include short courses on frontier, interdisciplinary areas; workshops on specific industrial problems and innovative materials; and short and intermediate length visits of graduate students, post-doctoral fellows, faculty members, and scientists and engineers form government laboratories and industrial organizations. The Institute will not conduct extensive research projects, per se, but will aim to serve as an intellectual forum to catalyze the formation of research groups when new areas or methods of approach are identified. This will include novel techniques for materials development, synthesis and processing; characterization and identification of properties-microstructure relations; physically-based micromechanical and computational modeling; and constitutive relations for nonlinear response and failure analysis; as well as design and performance analysis of complete structural entities. Timely topics will be discussed in think-tank-style workshops which will also be used for periodic assessments of long-term goals in mechanics and materials science and engineering. An external Board of Governors of prominent senior scientists and engineers will direct the Institute activities through action subcommittees and regularly assess the Institute's progress. The educational efforts of the Institute will include all levels of engineers and scientists in academia, industry and government (both national and international); and a strong outreach program to high school students and their teachers on a nationwide basis wherever there is interest, with a deliberate effort to reach minorities and under-represented segments of the population, as well as the gifted.

9523959 Nemat-Nasser Major earthquakes can cause lateral and vertical motion of the surface soil layers, which may result in large permanent dis placements. A consequence is damaged underground pipelines and surface facilities such as roads, bridges, and buildings. In coastal regions, saturated soils may liquefy during an earthquake, resulting in damage to man-made facilities. The necessary scientific tools for assisting soil engineers and geophysicists concerned with liquefaction and the resulting soil failure are physically-based computational models with predictive capability. The aim of this research program is to develop such tools. The goal is to create science-based computational tools with predictive capability in order to address the liquefaction and soil failure phenomena. The following tasks are addressed: 1. Examine by direct observation the history of the deformation of lines and layers of lead-doped granules embedded in a soil sample, and relate this to the overall response. A special cell with X-ray attachment provides a tool to study the kinematics of deformation inside the sample. It provides the capability to obtain insight into the deformation of a saturated particulate medium during its liquefaction. It also permits the study of the inception and growth of shearbands. Preliminary observations suggest that even in a liquefied sample, shearbands can be induced, as the deformation becomes large, during each cycle. The development of such shearbands is examined as they develop, and how they change as the deformation cycle is reversed. Both drained and undrained samples are investigated. 2. Directly observe and quantify the microstructure in shearband zones. The special cell is used to create controlled shearbanding in both drained and undrained saturated samples. The sample is then impregnated with polyester resin, and solidified. In this manner, the microstructure of the shearbands within the sample can be captured. The sand mass is so lidified in situ, once a desired state is reached. Thin sections are cut from this solidified sample for microscopic analysis. During shear localization, the particles in the region which eventually forms the localized band may undergo large rotations, whereas particles adjacent to the eventual shearing zone may still retain their orientation and relative position that existed prior to the localization. By studying the microstructure of a zone adjacent to the localized region and comparing the deformation with that within the zone, a complete picture of the transition from a homogeneous to a highly localized heterogeneous deformation state can be documented, Pictures are taken of the shear zones and their neighborhoods for complete statistical and deterministic analysis. These results are used to guide the theoretical modeling of the deformation prior to shearband inception, during shearband formation, and within the shearband, once it has fully developed. ***

This research project addresses fundamental processes associated with the deformation and failure of granular materials; specifically strain localization, and the role that it plays in earthquake-induced ground failures. Experimental observations suggest that strain localization is an integral part of the ground failure mechanism, even in soil that has liquefied as result of strong ground shaking. The goal is to quantify this, based on existing experimental data, in order to develop a micromechanically-based unified computational constitutive model to analyze and quantify experimental observations. The model has broad application to many problems in granular materials, in particular to ground failure in earthquakes.

The program involves the creation of a science-based computational model through the following steps:

1. Starting with the micro-scale and grain-to-grain interaction, to
explicitly relate fabric and its evolution to macroscopic backstress and its evolution.

2. Based on grain-to-grain contact forces and the distribution density function of the contact normals (which characterizes the fabric), obtain explicit expressions for the resistance to the dilatant shearing observed and quantitatively documented in photoelasic experiments.

3. Based on the dilatant sliding model at the meso-scale, develop explicit constitutive relations involving just a few well-defined constitutive parameters.

4. Using an existing extensive experimental database, relate the constitutive parameters to test data.

5. Develop computational algorithms based on the plastic predictor-elastic corrector concept.

6. Using the constitutive algorithm in DYNA3D, quantify experimental results on shearbanding. This also will verify the effectiveness of the model for application to other problems.

The ultimate goal of the proposed research project is the development of a new class of self-sensing materials, which have the ability to mange data flow from the sizable number of embedded sensors by use of local processing techniques. To be addressed first will be the fabrication issues to include the electronic, sensing and connection elements within a fiber-reinforced composite. Initially the work will be directed to demonstrating a proof-of-concept self-sensing material, capable of monitoring temperature flow patterns with the composite. Miniaturized MEMS based sensors capable of measuring such quantities such as acceleration, rotation and acoustic emission will be developed Hierarchical algorithms will be developed to manage the data expected from the sensor network.

The 5-year research will be carried out by a multidisciplinary team made of Principal Investigator who is a civil/materials engineer and three Co-Principal Investigators who span mechanical, physics and applied mathematics. On-going research by the PIs in the area of fabrication of composites with tailored electromagnetic properties will provide a foundation for the proposed research. The proposed research includes collaboration with Lucent Technologies, which would provide substantial expertise and capability in MEMS fabrication and facilitate the technical transfer of the research outcome.

The project includes three specific outreach programs to engage high school and undergraduate students, and a plan to establish a Summer Research Program and summer internship for underrepresented students.

This is an NSF 03-512 project.
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