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A major impediment to the development of physics-based multi-scale homogenization (composite) theories is the lack of appropriate methods to characterize the local anisotropic elastic-plastic properties of microscale constituents and interfaces at various hierarchical length scales in composite material systems. This work aims to address this gap by combining the information obtained from spherical nanoindentation with different indenter sizes, structure characterization methods (e.g., Orientation Image Mapping in Scanning Electron Microscope, X-ray microtomography),and sophisticated finite element models to arrive at reliable estimates of the elusive local properties. The methods developed have been critically validated in a series of material systems with increasingly complex internal structures, starting with single-phase cubic and hexagonal metals and progressing to multi-phase metals, freeze-cast polymer-ceramic composite systems with novel but highly complex hierarchical internal structures, and various biomaterials. Using these novel approaches, we were able to quantify reliably for the first time the local mechanical properties exhibited by various microscale constituents and interfaces.

Work on item response theory was extended to include two areas that had not been extensively researched previously. They include models for test items that require more than one ability for a correct response and models for the interaction between modules of instruction that have a hierarchical relationship. For both of these types of models, estimation procedures were developed for model parameters and extensive work was done to determine the appropriate interpretation of the parameter values. This report is a summary of work performed on these modules over a three year period. Keywords: Item response theory; Latent trait theory; Learning hierarchies; Multidimensional models.

The overall goal of our work is to develop new methods and materials for the fabrication of hierarchically structured, three-dimensional (3D) tissue scaffolds. Conventional scaffolds commonly lack substantial mechanical strength, and there is difficulty in controlling porosity, pore distribution, and pore interconnectivity. Additionally, the chemical nature of these scaffolds is typically homogenous. The ability to chemically modify selected areas on a scaffold is one method to direct cell growth in deliberate patterns; which could aid in the engineering of complex, functioning tissues. The general aim of this work is to address these issues through the application of stereolithography (SL) to the fabrication of hierarchically structured scaffolds. In order to achieve this goal, photopolymerizable materials must be developed that are both compatible with cell growth and with SL processing. SL methods are designed to produce arbitrary control over the physical structure of the part. In addition to physical structure control, control over the local surface chemistry of the scaffold is also desired. This would permit the use of both physical and chemical cues to control cell behavior in a tissue engineering construct. Chemical control could be achieved in SL methods by using photopolymerizable materials that can also be selectively chemically modified during the SL part building process. This paper provides an update on our work directed at using combined photoradical initiated polymerization and photoacid generator based chemical modification of a polymeric scaffold via multi-wavelength SL to produce hierarchically structured scaffolds.

Work on item response theory was extended to two areas not extensively researched previously, including models for: (1) test items that require more than one ability for a correct response (MIRT); and (2) interaction between modules of instruction that have a hierarchical relationship (HST). In order to develop the MIRT and HST models, the author evaluated characteristics of the models, developed estimation procedures for the parameter of the models, and evaluated the models on their ability to describe real test data. A summary of the research is presented here, and references are made to papers and technical reports containing more detailed descriptions of the research efforts. (Author/LMO)

The objective of this proposal was to develop novel materials and devices with dynamically variable refractive index properties, based on new hierarchically ordered composites and porous solids. The dominant innovation is the development of biologically inspired self-assembling inorganic/organic materials, whose versatile response properties can be designed and controlled, according to a variety of specific conditions, field stimuli, and performance criteria. Specifically, it was our goal to create mesostructured composite lens materials with large and reversible changes in their refractive indices (Delta-refractive index values of 0.5-1.0). This was to be demonstrated, by incorporating components with strong charge-transfer responses to applied optical (near-UV and near-IR) and electrical fields. Modes refractive index changes were obtained in thin film preparations of hybrid mesostructured materials (Delta-refractive index values of 0.1-0.27). However. we were unable to achieve An values close to or above 1.0 for isotropic light.

Research extended the SRG systems design approach to more complex materials by establishing methodology and fundamental principles for rational design of higher levels of structural hierarchy, while incorporating increased dynamic character in the form of adaptive system concepts via synergistic integration of materials science and applied mechanics. Research addressed both the methodology of rational design at new levels of materials complexity, and the fundamental principles of specific materials behaviors necessary to support quantitative design. This included principles of microstructural evolution during coherent carbide precipitation for efficient strengthening without embrittlement, control of metastable austenite precipitation for optimal-stability transformation toughening, behavior of pseudoelastic crack-bridge toughening, and shape-memory-based healing of internal composite damage. Exploratory research also addressed conceptual design of multifunctional metal/ceramic hybrid armor systems, and transformation-plasticity based matrix alloys for shearable tungsten heavy alloy composites.