Hussein M. Zbib - US grants

This Research Initiation Award is for the study of the localization of deformation and softening that occurs in many materials. THis localization occurs in many applications of concrete and in penetration problems. The main aim of the proposed study is to develop the appropriate constitutive quations, especially those involving large rotations.

The research work will examine the mechanics of superplastic deformation with the intent of providing broadly applicable flow rules for use in the modeling of the superplastic forming process. Such a model can be used to devise processing methods which increase the productivity of the superplastic forming process and increase its range of application. The research study is a comprehensive one, including aspects of strain-rate path effects, anisotropy, grain coarsening, cavitation and interfacial friction modeling. The project involves verification of the research models in laboratory tests and in tests on production equipment in industry. Superplastic forming is used to shape high performance alloys in aerospace application and will find increased use in its applications to the forming of titanium aluminides and other, "next generation" aerospace materials. Better understanding of the behavior of materials during superplastic forming will allow the fabrication of high performance parts at lower cost and with higher reliability.

A theoretical, computational and experimental research program will be undertaken to develop a new framework for plasticity which will be capable of describing the deformation process when the length scale over which there are high strain gradients is too small for the conventional continuum theory of plasticity to explain, yet is large compared to the size of the relevant microstructure (e.g., dislocation structures or particles). The main focus of this investigation will be on the flow strength of materials containing elastic reinforcements (i.e., metal-matrix composites) and the manner in which these reinforcements interact with the matrix flow field over a wide range of size scales.

OIA-0116793 PI Eyad Masad Institution Washington State University Title "Acquisition of x-Ray Computed Tomgraphy System for the Modeling and Characterization of Materials with Nanostructure Abstract This MRI award is for the acquisition of an X-ray computed tomography (CT) system for non-invasive evaluation of the microstructure of engineering materials. The system is unique for the visualization of three-dimensional of microstructural features in the interior of opaque solid objects. The X-ray CT system will be used to perform detailed observations of the microstructural features associated with granular deformation. It will also be used in the development of new generation ceramics and metal matrix composites. The focus of the first research is on the development of a microstructure based continuum model to study the deformation and locialization in granular materials. The model is based on crystal plasticity but inclused two microstructure length scales; one associated with the plastic curvature (orientation re-distribution) and the other one is related to the porosity re-distribution. This study is unique in that the microstructure model parameters are determined directly from microscopic measurements. The study will lead to analytical methods for modeling strain localization not only in laboratory specimens, but also inpractical boundary value problems in geotechnical engineering. The outcome of this work will also have implications to the modeling of other types of materials that exhibit deformation instabilities and shear banding such as metals and composites. The engineering behavior of ceramics and metal matrix composite materials is controlled by the microstructure of several levels. Therefore, the second study combines microscopic and macroscopic principles and develops multiscale mosels for their description. The multiscale model parameters are determined directly from X-ray CT measurements. Use of such models will enable the design and development of new materials with tailored microstructures. In addition to the above major studies, a host of other research activities from different disciplines at Washington State University will benefit from the proposed system. The unique capabilities of the system will contribute substantially to the extramural funding, and encourage more collaboration with other institutions and research centers.

The deformation of soils in the field and laboratory are commonly observed to concentrate into shear bands formed during strain localization. While this mechanism is widely appreciated by engineers and researchers, it remains difficult to model, predict and analyze. The main difficulty can be attributed to the fact that while classical continuum mechanics models that do not include microstructure length scales can predict the onset of instability, they cannot predict the size and evolution of the shear bands. In particular, the classical theories of plasticity break down in the post-bifurcation regime.

The main objective of this study is to develop a microstructure based continuum model to study deformation and localization in granular materials. The model is based on crystal plasticity but includes two microstructure length scales; one associated with the plastic curvature (orientation re-distribution) and the other related to the porosity re-distribution, both of which can be directly quantified by experiments.

The study is unique in that the microstructure model parameters are determined directly from measurements. Granular specimens are hardened by impregnation with resin and their microstructure captured by means of non-invasive x-ray computer tomography. Evolution of the microstructure model parameters is monitored at various stages of shear deformation. The model will be implemented into a 3-D finite element code and used to identify deformation patterning, softening, and instabilities (shear banding and liquefaction) in boundary value problems.

The experimental and analytical program will lead to a better understanding of the phenomenon of bifurcation and localization and to analytical methods for analyzing strain localization not only in laboratory specimens, but also in practical boundary value problems in geotechnical engineering. The outcome of this work would also have implications to the modeling of other type of materials such as metals and composites that exhibit deformation instabilities and shear banding.

Of particular interest in this project are the mechanical properties of metals in systems that consist of structural components whose dimensions, or the dimensions of their substructure, lie in the range of tens of nanometers to tens of micrometers. The investigators will develop experiments, models and computational platforms to pursue reliable mechanical properties and prepare maps for use in the design and analysis of such systems. Innovations include advancements in bulge testing techniques for studying submicron structures, advancements in multiscale modeling based on molecular dynamics and dislocation dynamics analyses, and development of a crystal plasticity hardening law for submicron elements.

The potential performance levels of miniaturized systems made of submicron components, such as microelectormechanical systems and lightweight metal panels for automotive and aerospace application, can lead to new performance level and energy efficiency not achievable with current materials. The outcome of this project would have major impact on these emerging technologies by providing scientific bases for designing of such systems. Additionally, this project will involve graduate and undergraduate students in mentoring primary and secondary school students through a unique outreach program. The goal is to increase students' interest in science and engineering, broaden the background of doctoral students in outreach activities, and address issues of disparity that may be underlying concerns in attracting women and minorities to doctoral research.

Advancing and accelerating materials discovery depends on our ability to develop models that can allow rapid investigation of large materials spaces to optimize properties, and to design manufacturing processes that yield the desired mechanical behavior on the basis of verifiable simulations. Of particular interest are the material properties of systems which consist of multiscale structural components whose dimensions lie in the range of a few nanometers to a few hundred micrometers. This includes micro-electro-mechanical systems, micro implants and microelectronic devices. At small length scales the measured mechanical properties vary significantly with decreasing dimensions. While existing models can predict deterministic values, analytical models often implicitly assume that noise in the data is due to difficulties inherent in microscale testing techniques. However, experiments show a significant amount of stochastic behavior no matter how elegantly the experiments are performed. This award supports fundamental research to develop predictive models that account for stochastic phenomena to ensure that validation techniques and simulation tools are well paired. Further broader impact will be the creation of a diverse environment in our laboratories in terms of race, gender, and national origin. This project offers students and junior researchers opportunities to participate in a research and education experience in an interdisciplinary environment, tackling their thesis problems with co-advisement from faculty drawn from both mechanics and materials science.

The research activities will develop mechanics and materials science theories and testing schemes as metrics for material modeling and experimental validation. Our hypothesis is that plastic deformation in small volume is stochastic, serrated and heterogeneous. Such effects would arise from the stochastic nature of the underlying microstructure such as dislocations, grain size, interfaces and grain boundaries, from stress gradients arising from loading conditions and morphological defects, and the formation of localized and dislocation patterns. We will develop models that account for such phenomena so that validation techniques and simulation tools are well paired. Towards this end, this project will address the following five questions. 1) What are the underlying causes of the observed stochastic phenomena: onset of plasticity, localization, patterning, and serrated flow? 2) How do deformation mechanisms and defect-surface interactions contribute to strength, toughness, and damage in small volumes? 3) Under what morphological and microstructural conditions does the deformation behavior transition from deterministic to stochastic? 4) How can we quantify and model stochastic behavior across length scale? 5) How can we translate this understanding into a mesoscale stochastic size-dependent plasticity model?

The design and manufacture of lightweight materials having superior mechanical properties such as high strength is one of the key challenges for scientists and engineers. Current state-of-the-art materials show a drastic tradeoff between weight and strength, while manufacturing strategies for porous lightweight materials suffer from poor control over material architecture and limited material choices. This project investigates a novel additive manufacturing (AM) method that uses printing of nanoparticles to fabricate a new class of three-dimensional (3D) micro-architectured materials, which will possess the desired characteristics of low weight and high strength. The research will also incorporate multi-scale mechanical models that consider the effect of microstructures and length scales specific to AM. The research results will advance the field of AM by enabling rapid fabrication of 3D structures with custom architectures and materials that have a wide range of applications, including biomedical implants, porous membranes, tissue engineering, and energy storage. Minority and women undergraduate and graduate researchers will be recruited to work on the project and periodic activities will be carried out targeted to attract K-12 students into the manufacturing research profession.

The research focuses on the investigation of a novel additive manufacturing method that involves printing of metal nanoparticles dispersed into a solvent, followed by nanoparticle sintering to realize highly intricate and controlled 3D metal architectures that are lightweight and strong. The first objective of the project is to investigate the scientific principles governing the printing process. Models will be developed that identify the role of droplet condensation, solvent evaporation, and system dynamics in the formation of the 3D architectures. The models will guide experiments that will involve printing of 3D architectures from silver, nickel, or aluminum nanoparticles dispersed into a solvent such as ethylene glycol, and using an Aerosol Jet 3D printer. The second objective of this work is to identify the micro and nanoscale deformation mechanisms governing the mechanical behavior of the metallic 3D structures. Complex 3D lattices (with up to 94% porosity) and micro-pillars will be fabricated by printing, and tested under compression and bending. Multi-scale mechanical models will be developed that consider dislocation motion, stress and strain gradients, and variability in the microstructure. The models will predict optimal 3D designs that improve strength-to-weight ratio dramatically, which will be verified through mechanical tests. The result of this project will be a novel additive manufacturing platform that can create strong lightweight structures with architectural control of over five orders of magnitudes in length scale (tens of nanometers to several millimeters), and will potentially open up new research areas in the manufacturing of 3D architectures and modeling methods for mechanical behavior of additively manufactured parts.