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Theoretical and Applied Mechanics

Behavior of Engineering Materials
^ Mechanics of Cohesive Powder Compaction
G. Gioia*
University of Illinois Research Board
ggioia@uiuc.edu

This project is focused on the particle rearrangement process that dominates the first 30 to 45% of the compaction stroke during the densification of cohesive powders. The research is relevant to such applications as forming of ceramic components and pharmaceutical tablets. We have shown both experimentally and computationally that, contrary to currently prevalent notions, particle rearrangement is a highly inhomogeneous process. In fact, the evidence indicates that particle rearrangement occurs in the form of a phase transformation. We are studying the energetics associated with this phenomenon. Computational work will be based on a novel, mixed finite-element/discrete approach.

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

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 Thermal Barrier Coatings
K. J. Hsia;* T. C. Chiang* (Phys.); J. E. Greene* (Mater. Sci. & Engr.); Y. Huang,* D. F. Socie* (Mech. & Indus. Engr.); R. Panat, B. Chen
Critical Research Initiative Program
kj-hsia@uiuc.edu

Thermal barrier coatings (TBCs) have recently been developed to protect components in high-performance engines in order to increase operating temperature and, hence, energy efficiency. Preliminary results have shown that, with TBCs, the temperature in engine combustion chambers can be up to 100°C higher than the melting temperature of the base metal, leading to a tremendous increase in operating efficiency. This program aims at investigating the fundamental issues governing the durability of TBCs. The emphasis of the research will be on the linkage among coating and interface microstructures, micromechanisms of failure, and macroscopic behavior of TBCs.

^ 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)
kj-hsia@uiuc.edu

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 (such as aluminum) surfaces 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
kj-hsia@uiuc.edu

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.

^ 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)
kj-hsia@uiuc.edu

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. Using the principle of virtual work, we obtain the fracture toughness due to failure by cavity growth in the ductile layer. The results of this study can provide guidelines for generating optimized surface microstructures to achieve excellent strength and durability of adhesive bonding.

^ Modeling Material Failure under Impact Loading
K. J. Hsia,* H. H. Hilton* (Aero. & Astro. Engr.), W. Walter (Army Research Lab), G. Thiagarajan (NCSA)
Army Raytheon IC TAM

(In cooperation with the National Center for Supercomputing Applications)
kj-hsia@uiuc.edu

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.

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

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 also 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.

^ 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
kj-shia@uiuc.edu

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 with their counterparts represent excellent opportunities for engine, brake, and rotating components in the ground-vehicle industry.

^ 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
kj-hsia@uiuc.edu

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 anisotropy. 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
kj-hsia@uiuc.edu

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 University of Illinois at Urbana-Champaign fatigue data bank and with experimental work in the literature.

^ 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
kj-hsia@uiuc.edu

To develop fatigue life-prediction methods for notched components subjected to nonproportional multiaxial fatigue, we must relate the local stresses and strains 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 it will be verified from the results of the tests.

^ 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
kj-hsia@uiuc.edu

A comprehensive fatigue-damage model is being developed to address the following: 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.

^ 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)
sofronis@uiuc.edu

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, grain-boundary and surface diffusion, 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.

^ Micromechanics of Damage in Solid Propellants
P. Sofronis,* 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.

^ Numerical Modeling of Plastic Deformation and Fracture at a Blunt Notch in the Presence of Hydrogen
P. Sofronis,* Y. Liang
Bechtel Corp.
sofronis@uiuc.edu

The purpose of this work is to determine fracture criteria for hydrogen-enhanced cracking in nickel-base superalloys. Experimental observations suggest that embrittlement 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 in this project. 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.

^ Temperature and Hydrogen Effects on Ductile Crack Growth
P. Sofronis,* R. H. Dodds, Jr.* (Civil & Environ. Engr.)
NASA NAG 8-1751
sofronis@uiuc.edu

Crack-propagation processes are investigated in high-performance metallic components in engines that operate in highly demanding environments (such as hydrogen steam). Crack growth is modeled by using finite-element techniques that account for the temperature and hydrogen effects on void growth and interactions in the neighborhood of a notch. An essential part of this research involves the development of a thermodynamic model to describe the contribution of the plastic deformation to the fracture energy as a function of the impurity content. The main goal is to devise a criterion that allows a crack-growth numerical model having fully predictive capabilities to be developed.

^ The Mechanics of Hydrogen Embrittlement
P. Sofronis,* H. K. Birnbaum (Mater. Sci. & Engr.), Y. Liang, N. Aravas (Univ. of Thessaly)
U.S. Department of Energy, DE-FG02-96ER45439

(In cooperation with the Materials Research Laboratory)
sofronis@uiuc.edu

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.

^ Additive Patterning of Integrated Functional Materials on a Chip
N. R. Sottos,* D. A. Payne* (Mater. Sci. & Engr.)
National Science Foundation CMS 00-88206
n-sottos@uiuc.edu

A novel, additive method of patterning is investigated that enables integration of thin-film electroceramics on a chip, rather than as a discrete component added into the circuit and system. In this soft lithographic method, substrate surfaces are selectively functionalized by self-assembled monolayers through micro-contact printing, and manipulation of surface forces leads to subsequent lift-off and patterning. We seek to develop a fundamental understanding of the residual stresses generated in the films during patterning and the effects of these stresses on the novel oxide lift-off process, the patterned film structure, the electromechanical properties of the films, and the overall reliability of the device.

^ Characterization of Active Materials for Microactuators and Sensors
N. R. Sottos,* L. Lian
National Science Foundation, CMS 95-32038
n-sottos@uiuc.edu

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.

^ Determination of Thin-Film Interfacial Properties of Laser-Generated Stress Waves
N. R. Sottos,* R. L. Weaver,* J. Wang
University of Illinois
n-sottos@uiuc.edu

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 film (polyimide, copper, aluminum, and silicon). A shear wave is generated by an Nd:YAG laser with amplitude large enough to induce failure of 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 Labs, Motorola Advanced Technology Center
n-sottos@uiuc.edu

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 also investigated. Overall optimization of the microstructure for dimensionally stable structures is desired.

^ Interfacial Design of Composites for Improved Damage Tolerance
N. R. Sottos,* T. J. Mackin (Mech. & Indus. Engr.)
U.S. Air Force Office of Scientific Research, F4962-98-1-0084
n-sottos@uiuc.edu

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.

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

We are investigating the feasibility of developing a smart polymer-matrix composite that has the ability to self-repair internal cracks. The main concept being 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 experiment 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.

^ Mechanical Ignition of Condensed-Phase Energetic Materials
D. S. Stewart*
USAF Research Laboratory Armament Directorate, Eglin AFB, F08630-95-1-0004; U.S. Air Force Office of Scientific Research
dss@uiuc.edu

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. The new model is being applied to hot-spot initiation of HMX.

^ Laser-Generated Stress Waves for Determination of Thin-Film Interfacial Properties
R. L. Weaver,* N. R. Sottos,* J. Wang
National Science Foundation CMS 99-88127
r-weaver@uiuc.edu

We investigate, both theoretically and experimentally, the generation of high-amplitude compression waves due to the sudden deposition of heat from a YAG laser pulse in a thin metallic film between two solids. The resulting pulse, with a duration of 10 nsec, and a strain amplitude of the order of 1%, is measured using laser interferometry. Particular issues of concern include the effects of nonlinearity in the wave propagation and the corresponding development of shocks, and mode conversion at oblique interfaces with consequent generation of high-amplitude shear waves. It is anticipated that each of these effects will be important in ultimate application to the testing, by high-speed stress loading, of thin-film coatings.

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

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 autonomic fashion by release and/or transformation of other phases that are incorporated into the composite material in the form of microcapsules.


Summary of Engineering Research