Characterization of Active Materials for Microactuators and Sensors
N. R. Sottos,* L. Lian
National Science Foundation, CMS 95-32038

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-electromechanical 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.

Constitutive Response and Failure Behavior of Energetic Materials
P. Sofronis,* S. P. Meyer, N. Aravas (Univ. of Thessaly)
DOE Center for Simulation of Advanced Rockets

A numerical simulation approach is used to understand the major issues of deformation and fracture in solid propellants. The methodology of Hill for polycrystalline aggregates is used to assess the constitutive interactions of the propellant constituents (binder, fuel, oxidizer) by the finite-element method. The adverse effect of tiny voids around the crystalline oxidizer, which form during fabrication or by long-term chemical reactions, is investigated. In this context, the effect of the propellant hardening rates on the localization of the material deformation into bands of intense shear is analyzed. A central goal of this research is to devise a criterion for the onset of unstable crack advance on the basis of local material damage.

Crack Initiation and Fracture Toughness in Ferroelectric Materials
K. J. Hsia,* M. Busche
University of Illinois

Crack initiation in such ferroelectric ceramics as PZT and barium titanate can be induced by either mechanical loading or electric field. The fracture toughness of the material may also depend on the applied electric field level as fracture occurs. The current project uses an indentation method to measure the fracture toughness of barium titanate single crystals as a function of applied electric field. The results will be used to develop fracture theories for ferroelectric materials.

Creep Resistance of Composite Materials
P. Sofronis,* P. B. R. Nimmagadda
University of Illinois; National Science Foundation, MSS 92-10686

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, and asymptotic solutions at the sharp fiber corners are sought to assess the corner singularity effects on the numerical calculations.

Determination of Thin-Film Interfacial Properties of Laser-generated Stress Waves
N. R. Sottos,* R. L. Weaver,* J. Wang
University of Illinois

Thin films are crucial components in a wide range of multilayer microelectronic and optical devices. The size scales and dissimilar nature of the constituents present challenges in regard to thermomechanical integrity and reliability. We are investigating the feasibility of using laser-generated shear waves to probe the interfacial strength of several types of thin films (polyimide, copper, aluminum, and silicon). A shear wave is generated by an Nd:YAG laser with amplitude large enough to 'decohere' the film interface from a silicon substrate. The magnitude of the resulting shear stress is calculated from high-speed interferometric displacement measurements.

Dimensional Stability of Woven Glass/Epoxy Laminates for Circuit-Board Applications
N. R. Sottos,* P. Shrotriya
Motorola Inc.

Micromechanical structure-property relationships are developed 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.

Durability of Adhesive Bonds-Accelerated Testing and Modeling
K. J. Hsia;* J.-K. Shang* and J. Economy* (Mater. Sci. & Engr.), M. Liu
National Science Foundation, CMS-9872306

This project is supported by the NSF initiative 'Long Term Durability of Materials and Structures: Modeling and Accelerated Techniques.' The project aims at investigating long-term durability of adhesive bonds by performing accelerated testing combined with mechanistic modeling. The accelerated testing will be conducted using a novel ultrasonic loading device. As the loading frequency increases, local heating and viscous response of the adhesive may change the failure mechanisms and stress distributions. The study will focus on providing mechanistic understanding of failure mechanisms to develop models for predicting long-term durability.

Durability of Advanced Materials
H. Sehitoglu* (Mech. & Indus. Engr.), F. V. Lawrence, Jr.* (Civil & Environ. Engr.), D. F. Socie* (Mech. & Indus. Engr.), J. E. Stubbins* (Nucl., Plasma & Radiol. Engr.), K. J. Hsia,* N. Chen, H. Hsieh, S. Andrews, T. McGreevy
Fracture Control Program

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.

Effects of Surface Microstructure on Strength and Durability of Adhesive Bonding
K. J. Hsia,* A. Pearlstein* (Mech. & Indus. Engr.), A. Scheeline* (Chemistry), J.-K. Shang* (Mater. Sci. & Engr.)
U.S. Department of Energy, DE-FG02-96ER45607 (In cooperation with the Materials Research Laboratory)

The program is supported by a U.S. Department of Energy 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.

Experimental Study of Brittle-to-Ductile Transition in Cleavage Fracture
K. J. Hsia,* B. D. Ferney
National Science Foundation, CMS 95-22661

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.

Fatigue Crack Growth and Crack Closure
H. Sehitoglu* (Mech. & Indus. Engr.), F. V. Lawrence, Jr.* (Civil & Environ. Engr.), D. F. Socie* (Mech. & Indus. Engr.), J. E. Stubbins* (Nucl., Plasma & Radiol. Engr.), K. J. Hsia,* N. Chen, H. Hsieh, S. Andrews, T. McGreevy
Fracture Control Program

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.

Fatigue of Welds and Adhesive Joints
H. Sehitoglu* (Mech. & Indus. Engr.), F. V. Lawrence, Jr.* (Civil & Environ. Engr.), D. F. Socie* (Mech. & Indus. Engr.), J. E. Stubbins* (Nucl., Plasma & Radiol. Engr.), K. J. Hsia,* N. Chen, H. Hsieh, S. Andrews, T. McGreevy
Fracture Control Program

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.

Health Maintenance of Composite Materials
S. R. White,* M. R. Kessler, N. R. Sottos, P. Geubelle (Aero. & Astro. Engr.), J. S. Moore (Chemistry)
Campus Critical Research Initiative Program; U.S. Air Force Office of Scientific Research

Health maintenance refers to the development of active materials that possess self-assessment capability (determining their state of health) and are able to self-repair. Just as radioactive tracers are used in medical diagnostics, magnetostrictive tags are used to locate damage in a composite material. Should damage occur, self-repair is accomplished in an automonic fashion by release and/or transformation of other phases that are incorporated into the composite material in the form of microcapsules.

High-Temperature Static and Cyclic Fatigue Failure in Ceramic Materials
D. F. Socie* (Mech. & Indus. Engr.), K. J. Hsia*
U.S. Department of Energy, DE-FG02-96ER45439 (In cooperation with the Materials Research Laboratory)

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.

Influence of Fiber Surface Chemistry on E-Glass Bundle Strength
N. R. Sottos,* A. K. Davis
Advanced Glassfiber Yarns

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 load--displacement 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.

Interfacial Design of Composites for Improved Damage Tolerance
N. R. Sottos,* T. J. Mackin
U.S. Air Force Office of Scientific Research, F4962-98-1-0084

The relationship between interfacial properties and damage tolerance in high-temperature composites is investigated. A range of micro- and macro-level experimental techniques are utilized to characterize local damage and stress concentrations in a composite and to correlate the damage with interfacial properties. Model composite systems with controlled changes in the interfacial properties are used to elucidate the relationships between micromechanical properties and damage tolerance. The research should result in a detailed picture of damage evolution based upon micromechanical models that rationalize the evolution of damage in terms of interfacial properties and relate the evolution of damage to the macroscopic stress redistribution.

Interfacial Fracture due to Cavitation in a Ductile Adhesive Layer
K. J. Hsia,* S. Zhang
U. S. Department of Energy, DE-FG02-96ER45607 (In cooperation with the Materials Research Laboratory)

As part of the program to understand the relationship between microstructure of pretreated surfaces and interfacial strength of an adhesively bonded component, this project studies one particular failure mechanism cavitation within a ductile adhesive layer. A new method utilizing fluid mechanics solutions is developed. The plastic deformation field surrounding a growing cavity is approximated by a fluid flow field. By using the principle of virtual work, the fracture toughness due to failure by cavity growth in the ductile layer can be obtained. The results of this study can provide guidelines for generating optimized surface microstructures to achieve excellent strength and durability of adhesive bonding.

Life Prediction Methods for Notched Members under Nonproportional Multiaxial Fatigue
H. Sehitoglu* (Mech. & Indus. Engr.), F. V. Lawrence, Jr.* (Civil & Environ. Engr.), D. F. Socie* (Mech. & Indus. Engr.), J. E. Stubbins* (Nucl., Plasma & Radiol. Engr.), K. J. Hsia,* N. Chen, H. Hsieh, S. Andrews, T. McGreevy
Fracture Control Program

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.

Mechanical Ignition of Condensed Phase Energetic Materials
D. S. Stewart,* E. Fried
USAF Wright Laboratory Armament Directorate; Eglin AFB, F08630-95-1-0004; U.S. Air Force Office of Scientific Research

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 complete continuum, thermomechanical model of HMX has been formulated that models the phase changes from solid to liquid to vapor to reacted products. Calculations have been carried out for shear bands and longitudinal impact. The equilibrium phase diagram is being computed.

Mechanics of Powder Densification
P. Sofronis,* R. S. Averback (Mater. Sci. & Engr.), S. Subramanian
U.S. Department of Energy, DE-FG02-96ER45439 (In cooperation with the Materials Research 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 TiO2, porous alumina, and Ti-48Al powder compacts.

Modeling Material Failure under Impact Loading
K. J. Hsia,* H. H. Hilton,* W. Walter (Army Research Lab)
Army Raytheon IC TAM (In cooperation with the National Center for Supercomputing Applications)

The project aims at introducing high performance computing technologies into research related to the U.S. Army's mission. Failure behavior of metal plates of armored vehicles under explosive loading is studied as an example. Numerical simulations based on mechanics constitutive laws are performed to predict the failure modes.

Numerical Modeling of Plastic Deformation and Fracture at a Blunt Notch in the Presence of Hydrogen
P. Sofronis,* A. Taha
Bechtel Corp.

The purpose of this work is to determine fracture criteria for hydrogen-enhanced cracking in nickel-base superalloys. Experimental observations suggest that embrittle-ment in these systems results from the combined action of hydrogen-enhanced decohesion at particle/matrix interfaces and shear flow localization in the uncracked ligaments in between. The finite-element method is used to calculate the stress, strain, and hydrogen distributions ahead of notched and cracked geometries and around grain-boundary precipitates. The effects of temperature and strain-rate sensitivities are accounted for. An important aspect of this research is the study of the synergy between decohesion and plastic flow localization in promoting material degradation in the presence of hydrogen.

Oscillatory Crack Propagation in Brittle Materials
K. J. Hsia,* B. D. Ferney, M. Liu, A. Needleman (Brown Univ.)
National Science Foundation, CMS 95-22661

It has been observed that, by slowly lowering a glass plate through a heater and into cold water, a crack may grow along a straight path, or take an oscillatory path, or branch into multiple cracks. The crack growth patterns depend on the temperature difference between the heater and the cold bath, and on the dipping speed. The mechanisms controlling the change of crack growth patterns are not well understood. This project combines an experimental study and numerical simulations to identify the critical processes at the crack tip that give rise to different crack growth patterns.

Probabilistic Methods
H. Sehitoglu* (Mech. & Indus. Engr.), F. V. Lawrence, Jr.* (Civil & Environ. Engr.), D. F. Socie* (Mech. & Indus. Engr.), J. E. Stubbins* (Nucl., Plasma & Radiol. Engr.), K. J. Hsia,* N. Chen, H. Hsieh, S. Andrews, T. McGreevy
Fracture Control Program

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.

Self-Repair in Polymer Matrix Composite Materials
N. R. Sottos,* S. R. White,* P. H. Geubelle (Aero. & Astro. Engr.), J. S. Moore, E. Brown
University of Illinois; U.S. Air Force Office of Scientific Research

We are investigating the feasibility of developing a smart polymer matrix composite that has the ability to self-repair internal cracks. The main concept we have pursued is a crack-filling technique in which a crosslinking agent is stored in tiny microcapsules embedded in a polyester matrix. A crack approaches the microcapsule and ruptures it, releasing the encapsulated repair agent. After a sufficient period of time, the crack faces are bonded together and the repair process is complete. Two levels of experiments are carried out to assess the self-repair process: micromechanical controlled crack propagation experiments to observe rupture of the microspheres and macroscale impact and fatigue tests.

The Mechanics of Hydrogen Embrittlement
P. Sofronis,* H. K. Birnbaum (Mater. Sci. & Engr.), A. Taha, N. Aravas (Univ. of Thessaly)
U.S. Department of Energy, DE-FG02-96ER45439 (In cooperation with the Materials Research Laboratory)

Hydrogen embrittlement is a severe environmental type of failure and the mechanisms involved are not well understood. Recent numerical results on the effect of hydrogen on dislocation--defect interactions are used to study the hydrogen-induced localization of plastic deformation into bands of intense shear and the hydrogen effect on crack tip blunting. 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.

The Potential Use of Corn and Its By-Products for Structural Composite Materials
S. R. White,* N. R. Sottos, T. J. Mackin (Mech. & Indus. Engr.)
University of Illinois

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.