Constitutive Modeling of Aluminum Sheets for Accurate Formability and Springback Predictions
Y. Huang,* P. Zhang, B. Chen
Ford Motor Co.; Alcan Aluminum

Development of constitutive models of aluminum sheets for automotive applications is the object of this research project.

Durability of Advanced Materials
H. Sehitoglu,* F. V. Lawrence, Jr.* (Civil & Environ. Engr.), D. F. Socie,* J. F. Stubbins* (Nucl., Plasma & Radiol. Engr.), K. J. Hsia* (Theoret. & Appl. Mech.), N. Chen, H. Hsieh, T. McGreevy, S. Andrews
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 with their counterparts represent excellent opportunities for engine, brake, and rotating components in the ground vehicle industry.

Exploration of New Design Avenues for High-Speed Ceramic Grinding
Y. Huang,* A. Chandra* (Iowa St. Univ.), G. Subhash* (Mich. Tech. Univ.)
National Science Foundation, NSF 42-87-93-000

The purpose of this research project is the exploration of new design avenues for high-speed ceramic grinding.

Fatigue Crack Growth and Crack Closure
H. Sehitoglu,* F. V. Lawrence, Jr.* (Civil & Environ. Engr.), D. F. Socie,* J. F. Stubbins* (Nucl., Plasma & Radiol. Engr.), K. J. Hsia* (Theoret. & Appl. Mech.), N. Chen, H. Hsieh, T. McGreevy, S. Andrews
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 and Fracture Toughness of Pressed and Sintered Powder Steels
P. Kurath*
Caterpillar, Inc.

Pressing and sintering of powder-based alloys is often an alternative to casting processing. Closer tolerances than can be achieved with casting result in reduced final machining costs. A more uniform microstructure and alloying heat treatment to achieve high hardness (for wear resistance applications) are additional advantages of powder metal alloys. Currently, full theoretical density cannot be achieved. The program addresses the penalty incurred with regard to fatigue life and fracture toughness because of a lack of theoretical density.

Fatigue of Welds and Adhesive Joints
H. Sehitoglu,* F. V. Lawrence, Jr.* (Civil & Environ. Engr.), D. F. Socie,* J. F. Stubbins* (Nucl., Plasma & Radiol. Engr.), K. J. Hsia* (Theoret. & Appl. Mech.), N. Chen, H. Hsieh, T. McGreevy, S. Andrews
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 currently being evaluated. Surface treatments, such as shot peening and laser dressing of the weld toe, are also being investigated as possible methods for improving the fatigue strength. Recently, 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.

Interface and Reliability Studies of MEMS and Microelectronics
T. Saif,* A. Haque
National Science Foundation, ECS 97-34368

The objective is to investigate in situ at an atomic to nanometer scale the fundamental mechanisms of failure in microelectronic components and micromechanical systems. Microinstruments, developed from micromechanical systems, as well as macroanalytical devices such as transmission electron microscopes are employed for the study. The study is initially directed to experimentally investigate the micromechanisms of failure of interfaces formed by a metal (aluminum) and a ceramic (silicon dioxide). The effects of environment, such as humidity, pressure, and temperature, on the mechanisms of failure are also studied.

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

The purpose of this research is to develop fatigue life prediction methods for notched components subjected to nonproportional multiaxial fatigue. To do this, 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. Results from these tests 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 Behavior of Ceramics
D. F. Socie,* K. Hsia,* G. Webster
U.S. Department of Energy, DE-AC02-76ER01198

The mechanical behavior of ceramics at temperatures exceeding 1200?C is being investigated in this study. Tests are conducted in both static and cyclic tension loading. Microstructural factors that contribute to the failure process are quantified in Al2O3 and Si3N4. A damage model for combined static and cyclic loading has been developed for durability assessment. State of stress effects is quantified with torsion tests of thin-walled tubes.

Mechanical Behavior of Thick Thermal Barrier Coatings
D. F. Socie,* E. Radja
Caterpillar, Inc.

Failure modes in thick thermal barrier coatings are studied to develop life prediction models suitable for design of low-heat rejection engines. These ceramic coatings are manufactured by a plasma spraying process. Special attention is given to the influence of the porosity in the material and how it governs the mechanical and thermal behavior of the coating system. Experiments are conducted on simple component geometries to validate the life estimation model.

Mechanics and Materials Issues in Mesoscale Compressors
H. Sehitoglu,* M. T. A. Saif,* E. Alaca
Defense Advanced Research Projects Agency

Mechanics calculations are conducted for thin membranes under the combined action of electrostatic forces and refrigerant pressures. Anisotropic plate theory will be used with variable support conditions at the ends of the membrane. The potential for instability in the form of bubbled mode will be explored. The aim of these calculations is to optimize the cross-sectional geometry of the membrane. Experiments are designed to assess the structural integrity of the membrane under conditions of fatigue loading. In this case the special concern will be on the bending stresses at the end where bonding to silicon occurs, and also crack growth behavior due to the modulus mismatch at the interface of dissimilar materials used.

Multiaxial High-Cycle Fatigue
P. Kurath*
Dayton Research Institute, 1-5-21170

With many components, it is desirable to ascertain if their actual service life is longer than that for which they were originally designed. Most fatigue test data is obtained from uniaxial specimens, and the extension of this data to more complex stress states has not been verified. Hence, long life multiaxial fatigue tests will be performed to evaluate existing design algorithms with an emphasis on high cycle fatigue. Existing multiaxial fatigue life model predictions often differ by orders of magnitude. The most appropriate algorithm for this life range will be identified or if necessary an alternate approach will be suggested.

Origins of Asymmetry in Phase Transformations
H. Sehitoglu,* K. Gall, R. Anderson
U.S. Department of Energy, DE-FG02-93ER14393

The basic information obtained from the work will generate improved understanding of transformation under stress, stress-strain behavior as a function of temperature, and fatigue conditions. Single crystals of different orientations (in solution treated and precipitated microstructures) of Nitinol are studied. Several unique experiments under combined shear stress-hydrostatic pressure are conducted. Based on these experiments, the work will set the background to evaluate the theories proposed, and lay the foundation for new ones with particular emphasis on complex changes in transformation strains.

Parallelization of a Polycrystal Plasticity Code for Determination of a Yield Surface
A. J. Beaudoin,* W. R. Reed
Naval Surface Warfare Center, N00174-99-M-0292

The calculation of a yield surface is a key step in characterizing the response of metals to applied load. In existing computer programs for prediction, the yield surface is 'probed' through examining the collective response of hypothetical metal crystals. This probing is inherently parallel; one probe does not rely on information from another probe. Procedures are being developed for yield surface calculation using distributed parallel computation. The resulting code is implemented on a cluster of low cost LINUX workstations.

Parameter Estimation of the Mechanical Threshold Stress Model via the Finite-Element Method and Inverse Analysis
D. A. Tortorelli,* A. J. Beaudoin,* S. Kok
U.S. Department of Energy through ASCI

The mechanical threshold stress (MTS) material model is being implemented in a small strain plasticity finite element code. This model has a very strong physical basis and includes temperature and rate effects. Parameter estimation will be performed via an inverse analysis. This will require sensitivity information obtained via the adjoint method.

Phase Transformations under Pressure Loadings
H. Sehitoglu,* I. Karaman, M. Balzer
National Science Foundation, CMS 94-14425

With unique experimental capabilities, the transformation behavior of advanced steels under different hydrostatic stress and principal stress level combinations are studied to elucidate the driving force for transformations. Pressure loadings permit isolation of hydrostatic stress and principal stress effects on transformations and produce homogenous stress fields allowing unambiguous interpretation of stress and strain results. Cyclic loading modifies the internal structure and will generate different transformation behavior compared to the monotonic (unidirectional) loading. Ultimately, the work will advance the analytical and numerical treatment of phase transformation phenomena applicable to materials processing and mechanical loadings in engineering service.

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

Statistical Fatigue Analysis
D. F. Socie,* K. Dahl
Ford Motor Co.

Long term monitoring of service usage to quantify customer variability involves collecting information in a rainflow histogram format on a single channel. Knowledge of this variability is an important component of statistically based design and reliability evaluations. In body and chassis structures, the phasing of the input loads determines both the magnitude and location of the stress concentrations that in turn govern the reliability and durability of the vehicle. For example, out-of-phase loading produces a racking or twisting motion that is not observed during in-phase loading. This research program is developing methods of pattern recognition for preserving the important phasing information.

Stress-Strain Response under Quenching of Al Alloys
H. Sehitoglu,* J. A. Dantzig,* X. Zhang
Ford Motor Co.

The purpose of this work is to develop a general model for the deformation of Al-Cu alloy under solutionized conditions. Experiments involve rapid cooling of laboratory specimens to suppress the nucleation of precipitates followed by deformation at temperature. The experiments show that the strength and the hardening of the solutionized material is significantly lower compared to the precipitation hardened cases.

Temperature-dependent Deformation of Plasma Sprayed Steels
P. Kurath*
Ford Motor Co.

Plasma deposition is a method currently under investigation to fabricate moderate to high hardness tooling in near net shape. This would eliminate subsequent heat treatment and associated warping due to nonsymmetrical residual stresses. Spot surface temperatures during deposition are as high as 700?C, with subsequent cooling to ambient temperature. In order to model the residual stresses, it is vital to model the time dependent mechanical response of the material. Baseline mechanical strength and durability of the product is also being investigated.

Thermomechanical Fatigue of Aluminum Alloys
H. Sehitoglu,* C. Engler, T. Smith, H. Maier, X. Qing
Ford Motor Co.

The thermomechanical fatigue resistance of a material often limits the lifetime of a component such as the cylinder head in engines. Isothermal tests performed at various temperatures, mechanical strain ranges, and strain rates may not capture many of the important damage micromechanisms under varying temperature and strain (i.e., TMF), and experiments and modeling of thermomechanical damage processes are needed. The study is developing a physically based life prediction method for the Al 319 and Al 356 alloys. The overall program considers the effect of the following process parameters on mechanical behavior: secondary dendrite arm spacing, effect of aging heat treatment, effect of porosity, and compositional effects.

Thermomechanical Fatigue of Cast Iron
P. Kurath*
Caterpillar, Inc.

During routine operation, many engine components can experience nonuniform or localized temperature changes. These temperature differences in conjunction with structural constraint can cause stresses to develop in addition to those caused by normal operating loads. The effects of these additional stresses and possible acceleration of fatigue damage from oxidation at extreme temperatures are being investigated. Extending the experimental observations from uniaxial testing into a multidimensional model applicable to actual components is also being investigated.

Twinning in Single-Crystal Steels
H. Sehitoglu,* I. Karaman
National Science Foundation, CMS 99-00090

Orientation and stress state dependence of twinning is studied with novel experiments in materials with low stacking fault energy. These materials include Hadfield and austenitic stainless steels with nitrogen additions. One of the unusual attributes of these steels is that during deformation, an upward curvature in stress-strain curves develops. Considerable tension-compression asymmetry develops in these classes of materials because of directionality of twinning. A micromechanics modeling effort, incorporating the twin volume fraction and twin evolution, will be undertaken for predicting the stress-strain response as a function of orientation, stress-state, and texture evolution.