This study aims at developing a mechanistic model capable of predicting brittle-to-ductile transition behavior for some simple material systems. Of particular interest are those materials in which the brittle-to-ductile transition is dislocation-mobility controlled. The interactions between dislocations and the crack tip are investigated to evaluate the effect of shielding. Both the stationary crack and the propagating crack are studied to determine the effects of strain rate as well as crack velocity on brittle-to-ductile transition.

Brittle-to-ductile transition is the result of the competition between two atomistic processes at sharp crack tips, namely, separation of atoms and generation of dislocations. An experimental technique is developed to reveal the key parameters of brittle-to-ductile transition, including temperature, strain rate, and critical dislocation structure at the crack tip. An atomically sharp crack is propagated with various crack velocities against a temperature gradient, from the low-temperature brittle side toward the high-temperature ductile end. Crack-arrest temperature is determined as a function of crack velocity. Dislocation structure at the arrested crack front is studied with microanalysis techniques.

During rolling contact, the pressure build-up of the lubricating fluid under a roller can be as high as 1 GPa. Whether this pressure can be transmitted into surface defects, such as cracks, and how the high-pressure fluid affects material degradation are not known. This project aims at providing a quantitative evaluation of the effect of high-pressure fluid on surface crack propagation. Of particular interest are cyclic loading frequency effects and the behavior of the fluid under very high pressure.

The program is supported by the Partnership for a New Generation of Vehicle (PNGV) initiative to develop scientific understanding of surface preparation processes to enhance adhesive bond strength and durability for lightweight vehicles. Metal surfaces (e.g., aluminum) are usually chemically treated before being adhesively bonded together to form a structural component. The chemical treatment produces an oxide film with complicated microstructures. This project aims at providing a mechanistic understanding of the relationship between the microstructure of the oxide film and the interfacial failure strength of adhesive joints.

Factors that control the fatigue behavior of welded components are currently being studied. Analytical methods for estimating the total fatigue life of butt and fillet welds subjected to variable-amplitude loading histories are evaluated. Surface treatments, such as shot peening and laser dressing of the weld toe, are investigated as possible methods for improving the fatigue strength. A new model for estimating the fatigue life of weldments has been proposed for butt, T-joint, and cruciform weldments using the concepts of ``crack closure'' for cracks emanating from a notch. Results compare favorably with experimental data in the UIUC fatigue data bank and with experimental work in the literature.

The aim of this study is to develop a life prediction methodology for fatigue crack growth based on the changes in crack opening levels with maximum stress level, crack length, geometry, mean stress, and microstructure. The primary tool for the determination of opening stress is an elastic-plastic finite-element simulation of fatigue crack growth. Stress-strain behavior in the model accounts for slip at the microlevel as well as elastic anisotrophy. Fatigue crack growth data obtained under conditions of intermediate- and large-scale yielding, including low-cycle fatigue and biaxial loading, are successfully correlated only when closure-modified parameters are employed.

To develop fatigue life prediction methods for notched components subjected to nonproportional multiaxial fatigue, the local stresses and strains must be related to the global stresses and strains by some approximation procedure, such as Neuber's rule. Experimental tests on notched shafts subjected to proportional and nonproportional loading in tension and torsion are being performed. The results are being used to develop and verify the approximation procedure. Fatigue life estimates will then be made using an appropriate damage model that is based upon observations made during the tests. A life prediction scheme will be developed from the approximation procedure and the appropriate damage model and will be verified from the results of the tests.

Fiber-reinforced sheet molding compound is an attractive material used in ground-vehicle structural applications. It experiences cyclic loading in service, therefore, understanding the fatigue behavior as a function of processing conditions, chemistry of constituents, and loading conditions is important. The purpose of this work is to analyze some of the available fatigue data on these materials and to conduct experiments to identify the nature of damage mechanisms and to study cumulative fatigue damage. Tension-compression testing will be considered to gain insight into mean stress effects. In all these cases the fiber orientation of the molded part affects the progression of damage.

Recent developments in processing technology have resulted in advanced materials with lower fabrication costs and improvements in microstructural uniformity. To utilize the full potential of these materials, new design tools have to be developed in collaboration with industry. Examples of such materials include metal matrix composites and short reinforcement fibers in epoxy matrices. The metal matrix composites with higher elastic modulus, higher temperature capabilities, and lower weight compared to their counterparts represent excellent opportunities for engine, brake, and rotating components in the ground vehicle industry.

A comprehensive fatigue damage model is being developed to address the following issues: What governs the nucleation of a microcrack within a single grain or other suitable microstructural unit cell? What governs the growth of this microcrack into adjacent microstructural unit cells? When does the microcrack develop enough plasticity to sustain its growth? These elements will be combined into a model for the entire fatigue damage process.

It is no longer possible to specify a material without first considering its processing. In some applications, the so-called old materials processed in new ways are often more cost effective than some of the new advanced materials. Surface treatments such as carburizing and nitriding have been used for many years. Flexible manufacturing processes, such as those using lasers, now offer the potential to modify surfaces selectively to produce superior mechanical properties of traditional lower cost materials.

The microscopic mode of fatigue crack growth in ductile solids is strongly affected by the slip characteristics of the material, characteristic microstructural dimensions, applied stress level, and the extent of local tip plasticity. Cyclic crack growth is viewed as a process of intense localized deformation in slip bands near the crack tip. Computational modeling of the interactive roles of shear banding and fatigue crack growth is sought to understand better the mechanics of shear failure mechanisms in crystalline, semi crystalline, and noncrystalline solids.

A framework is developed for a unified treatment of the phenomenon of shear-flow localization during dynamic deformations of thermal viscoplastic materials. This framework recognizes the total kinetic energy of a deforming body as a single, scalar parameter capable of characterizing the complete localization history. A distinct evolutionary profile of the kinetic energy is associated with the evolution of severe localization and final material failure. Extension of the energy theory of localization to three-dimensional, finite deformations is expected to provide further insights into the evolution and propagation of localized zones as well as their role in promoting material damage and structural failure.

Damage evolution caused by shear-flow localization during high strain-rate deformations of multilayered media is examined. An energy-based localization framework is used to guide the analysis of failure mechanics in various configurations of multilayered media. A comprehensive computational program is underway to explore the mechanics of shear-band formation in composites and bi-materials, as well as to simulate the phase transformations associated with adiabatic shear-flow localization.

A new theory of localization that associates the evolution of shear bands with the system's total kinetic-energy evolution allows for consistent parametric studies of a multitude of effects on plastic flow localization. This effort seeks to compile a database of critical onset strains, critical failure strains, shear-band dimensions, and evolution profiles of various field variables for a wide spectrum of structural engineering materials. Such a database is largely lacking because of the absence of a consistent characterization of plastic flow localization at high rates of loading.

The torsional split-Hopkinson bar technique provides an ideal means for studying adiabatic shear-band formation in metals. In this technique a material specimen undergoes high-rate plastic deformation (200-10;s4 sec ^-1 ) under a well-defined state of stress and deformation. A novel experiment uses modern advances in photographic technology to resolve the evolution of deformation fields during the test. A set of experiments is designed to examine shear-band formation in a set of materials including a cold-rolled 1018 steel, 4340 steel, titanium, and aluminum alloys.

Ceramic materials often contain a thin layer of intergranular glassy phase that becomes a viscous fluid at elevated temperatures. Failures of ceramics under static and cyclic loadings at high temperatures are strongly dependent on the behavior of the viscous phase. The current project studies in detail the response of the intergranular glassy phase for different grain geometries, a range of volume fraction of the intergranular phase, and magnitude of viscosity. Failure processes under static and cyclic loading due to accumulation of intergranular damage in the form of grain-boundary cavities are also investigated.

Reinforcements are known to increase the creep resistance of metal and intermetallic matrix composite materials. However, at temperatures higher than approximately half the melting temperature of the matrix, the composite strength is limited and sometimes the strengthening imparted by the reinforcements is lost. The composite behavior is investigated by studying the degradation effects of stress-driven diffusion and slip along the reinforcement-matrix interface. The finite-element method is used to solve the relevant boundary value problems. Long reinforcements are better than short ones because they offer long diffusion circuits; however, they crack. The two competing effects are currently being investigated.

Hydrogen embrittlement is a severe environmental type of failure and the mechanisms involved are not well understood. Recent analytical results on the effect of hydrogen on dislocationdefect interactions are used to study the hydrogen-induced localization of plastic deformation into bands of intense shear. In hydride-forming systems, the finite-element method is used to predict the size of the hydrides accommodated by plastic deformation in the neighborhood of a crack tip. The results are used to establish criteria for the onset of crack propagation in engineering materials in the presence of hydrogen. A collaborative research effort is in place with the Atomic Energy Control Board of Canada and the Bettis Atomic Power Laboratory.

Powder densification is used to manufacture advanced structural materials and the promising class of nanocrystalline materials. The finite-element method is used to predict the deformation of a powder under general loading conditions. Scaling laws are detected for the macroscopic strain rate as a function of the macroscopic stress in the numerical results. The mechanisms considered include linear and nonlinear deformation effects, diffusion (grain boundary and surface), and grain-boundary slip. The coupling of these mechanisms is accounted for in the models, which are tested against experimental measurements from the densification of nanophase TiO;i2 powder compacts.

The purpose of this research is to understand the phe nomenology of hydride formation and fracture of the zirconium alloys used in the CANDU reactor pressure tubes. The primary focus is on modeling the stress-induced hydrogen transport and elastoplastically accommodated hydride formation and cracking resulting from tensile stresses. The thermodynamics of the accommodation is being assessed and the concept of terminal solid solubility of hydrogen as affected by the elastoplasticity stresses is thoroughly defined. The finite-element results are used to identify the temperature, strain rate, and initial concentration regimes in which hydrogen-induced failure is possible.

The purpose of this work is to determine the critical fracture criterion for hydrogen-enhanced cracking in nickel-base alloys, including temperature and strain-rate sensitivities using finite-element calculations for stress, strain, and hydrogen distributions ahead of notched and cracked geom etries. This investigation is also expected to lead to an increased understanding of the behavior of notched geom etries as compared with precracked geometries.

The formation of an interphase region with properties that differ from both the matrix and the reinforcement significantly affects the overall mechanical behavior of advanced composite materials. The current investigation focuses on how the properties of the interface/interphase region change with temperature in metal and intermetallic composites. Novel microdebonding experiments are developed to measure interfacial shear strength over a range of temperatures. Interferometric studies are performed to measure thermal deformations in the interphase. Ex- perimental results are combined with finite-element analysis to understand the behavior of the interface at elevated temperatures.

Micromechanical structure-property relationships are [fjdeveloped for woven glass/epoxy laminates for circuit- board applications. Residual stresses and warpage of the laminates caused by processing and thermal cycling are investigated as a function of the microstructure of the woven material. In particular, the effects of the number of threads, crimp, and the amount of twist are studied. The influence of the two-dimensional nature of the weave on the deformation behavior is investigated also. Overall, optimization of the microstructure for dimensionally stable structures is desired.

Shape-memory-alloy (SMA) wires have been successfully embedded in polymer matrix composite materials to provide active vibration and structural control. Experimental and analytical methods are developed to investigate the micromechanical behavior of such SMA composites. Interferometric methods are applied to measure in situ local deformations of embedded SMA wires, while photoelastic techniques are used to examine local stresses induced during actuation. Investigations focus on the influence of interfacial surface properties on the efficiency and repeatability of actuation. Theoretical models for predicting shape-memory behavior are developed and compared with experimental observations.

Trends toward smaller packages and higher circuit densities have made electronics packaging increasingly complex. Understanding the thermal and mechanical response of these various components is critical for optimizing manufacturing processes to yield maximum performance and reliability. The current work has resulted in the development of a unique capability to test small-scale electronic components using interferometric methods. This experimental setup enables in situ measurement of thermomechanical properties and constitutive response of various electronic components. Creep of electronics packaging components at elevated temperature, thermomechanical properties of thin films, and solder strains induced by thermomechanical loading of a package are investigated.

An extensive experimental test program is being carried out to measure the breaking strength of glass fiber bundles with different surface chemistries when systematically subjected to different relative-humidity levels. An acoustic-emission technique is used to correlate individual fiber breaks with loaddisplacement data during the test. The influence of the surface chemistry on bundle strength in each of the different environments, as well as the reversibility of the effect in a dry environment, is of primary importance.

As active materials are developed on smaller and smaller scales for the next generation of smart materials systems, and as the applications of these materials in micro-electro-mechanical systems (MEMS) require finer resolutions, the need to understand and predict material response becomes critical. An extensive experimental investigation is carried out to characterize the response of active materials in the form of thin films, fibers, and particulates embedded in a passive matrix or deposited on a substrate. Experimental data will be used to evaluate models reported in the literature for their ability to predict the response of active materials and provide guidelines for the development of improved models when necessary.

An effort continues to model the mechanical ignition of a condensed phase energetic material that is subjected to mild impact. The explosive crystal HMX has been the initial focus. A thermodynamic description of the stable, solid, liquid, and gas phases has been given. We have now developed a continuum mechanically based description to describe the mechanically induced shear deformation and the phase transitions from solid to liquid. We are engaged in localization studies and thermal ignition problems.

About 9% of the total corn crop grown in the U.S. is currently used for nonfood, industrial applications. In most of these applications, corn and its by-products are used as a low-grade filler material. The current investigation seeks to identify the feasibility of using corn for low-cost structural composite materials. Several corn parts, including the husk, the cob, the kernel, and the silk, as well as the starch and meal, are evaluated for effectiveness as a reinforcement in a polymer matrix. Extensive characterization of the corn reinforcement, fabrication, and testing of the resulting corn composites will be carried out.