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 due to lack of theoretical density.

Baseline fatigue testing is often conducted on smooth polished hourglass specimens. However, service applications often use the as-cast condition. Unlike wrought metals, there is a significant microstructural variation within the thickness of many cast components. A program has been initiated to ascertain the effect of the as-cast surface on fatigue life. Additionally, differences between uniaxial fatigue and bending fatigue will be addressed.

Aluminum alloys have been used in many engineering applications because they weigh less than other engineering metals. However, these alloys generally display a drastic reduction in mechanical properties at temperatures greater than 200°C. The ability of reinforcing materials in an aluminum matrix to improve the mechanical properties, specifically fatigue, to levels that allow engineering de- sign for a slightly broader temperature range is being investigated.

Most testing of induction-hardened materials is conducted on uniaxial samples that have a uniform strain field, and often failure originates from the ``core'' material. Most structural applications involve either bending or torsion, both of which have nonuniform strain fields. Typically with a nonuniform strain field, the core material is subject to smaller strains than the hardened material, and uniaxial tests may underestimate the service durability of actual components. Incorporation of residual stresses into an elastic plastic deformation/fatigue damage model is also anticipated.

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 due to 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.

The use of high-temperature CMCs requires holes, notches, attachments, and various joining procedures, all of which lead to stress concentration. Consequently, these stress concentrators are the most likely locations of failure in the material. Fortunately, fiber reinforcements impart a degree of ductility to CMCs that mitigates the stress concentration. Various CMCs are being investigated to determine their performance and notch sensitivities. Stress pattern analysis from thermal emission (SPATE) is being used to quantify damage progression; the results are used in the development of materials modeling.

The advent of relatively small, high-powered computers is creating new opportunities for real-time processing of experimental data. Video and photographic images of test specimens are captured during loading. Key features on the specimens are identified and tracked throughout the loading history, enabling full-field displacement measurements as a function of applied load. The displacement fields are converted into strains, followed by constitutive transformation into stresses. This procedure provides a new, noncontacting tool for experimental stress analysis.

Drop weight rebound testing is carried out to introduce subsurface damage in a laminated polymer composite through stress wave interaction at the laminate interface. Instrumented strikers and accelerometers record the loads and subsequent stress waves during impact, which is then correlated to NDE evaluation of the test specimen. Thermoelastic techniques are employed to evaluate subsequent stress redistribution brought about by the impact test. Knowledge of stress redistribution is then employed to invoke constitutive relations in a continuum damage mechanics model and its effect on composite properties such as compressive strength.

Glass-fiber-reinforced polyester matrix composites are widely used in commercial products and widely considered for use in the civil infrastructure. However, styrene, an EPA-regulated chemical, is freely emitted throughout composite fabrication. Various chemical additives are used to control the styrene emission, most of which decrease the interlaminar toughness of the composites. A novel fabrication process, whereby a chosen volume fraction of glass fibers are aligned perpendicular to the laminate interface, is being developed to greatly improve the delamination resistance. The goal is to develop a commercially viable processing methodology that improves interlaminar toughness while decreasing styrene emission.

The fiber push-out test is being used to evaluate interfacial properties in a sapphire-fiber-reinforced TiAl matrix composite. Sapphire fibers have a sinusoidal roughness that is known to affect the interfacial sliding properties. This interface roughness is being modified by the application of CVD carbon coatings. Various coating thicknesses are being applied to modify the roughness amplitude of the sapphire fibers, and the interface properties are evaluated using the fiber push-out test. The goal is to demonstrate that interface properties can be ``designed'' to improve composite performance.

Fiber-optic cables are finding increasing use in line communications systems. During installation, the fiber-optic cables are subjected to a variety of stresses, arising from bending, tension, and torsion. These stresses may affect signal transmission in the fiber-optic lines. This project investigates transmission losses in two technologically important cable systems, and evaluates those losses using a composite paradigm, wherein the load sharing among the constituents in the cable will affect the transmission losses.

Extended lifetimes in biomedical implants depends upon careful control of the material microstructure. Microstructural effects are especially important in polymeric materials whose melting temperatures are at or near body temperature. A controlled study of UHMWPE implant materials is being conducted to explore relationships between processing, sterilization, and microstructure.

Improvements in fuel economy depend, to some extent, upon improvements in tire technology. Rolling resistance is directly related to cyclic hysteresis in elastomeric tire materials. Cyclic hysteresis can be adjusted by the addition of fillers into the elastomeric matrix. Experiments, modeling, and computer simulation are being used to develop the next generation of high-traction, high-durability, low rolling resistance materials for the automotive tire industry.

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.

In the first phase of the work, we monitor the transformation of austenite to martensite in carburized steels under axial torsional stress states. The evolution of transformation strains, the anisotropy of transformation strains, and the stress-strain response were monitored. Initial experiments in axial-shear loadings indicate the requirement of a resolved tensile stress for transformation to proceed. Transformations under confining pressures approaching 1000 MPa are experimentally studied. In the second phase of the work, fatigue under high pressures is studied for 1070 steel, which is a nearly full pearlitic structure.

T he 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 aluminum 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.

Based on a stress invariant hypothesis and a stress/strain relaxation procedure, an analytical approach is forwarded for approximate determination of residual stresses and strain accumulation in rolling contact. For line rolling contact problems, the proposed method produces residual stress distributions in favorable agreement with the existing finite element findings. We study ratchetting behavior of 1070 steel under uniaxial tension-compression and axial-shear loadings experimentally. Strain ratchetting direction exhibits a complex dependence on the previous loading history, including nonconsistence with the mean stress direction. Different models to predict this phenomenon are proposed and compared to experiments.

Analysis of fiber clustering and fiber misalignment on strength of carbon fiber epoxy matrix composites has been conducted. Experiments are being performed to understand matrix versus fiber failure mechanisms. The composite material is subjected to uniaxial tension and uniaxial compression in the temperature range 25°C to 175°C at different strain rates. The influence of matrix strength and matrix modulus on tensile and compressive strength is investigated.

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. Different materials that exhibit a shape change and a volume change are selected for the work. 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.

The purpose of this research is to identify the significance of crack closure in fatigue crack growth. For a given material, crack growth rates may differ for the same stress intensity range, depending on the crack length, stress range, specimen thickness, and geometry considered. The constraint effect at the crack tip due to specimen geometry and thickness, stress field parallel to crack, and material hardening behavior is identified. Early studies of crack closure considered small-scale yielding cases. However, it is well known that crack closure is significant at high applied stresses (approaching the yield stress) where large-scale yielding conditions prevail. This aspect of the problem is examined in this work.

Fatigue of Welds and 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 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.

Fatigue Crack Growth and Crack Closure 10]
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.

Life Prediction Methods for Notched Members under Nonproportional Multiaxial Fatigue
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.

Fatigue Life Prediction of Composites
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.

Durability of Advanced Materials
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.

Probabilistic Methods
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.

Processing Existing Materials to Enhance Performance and Reduce Cost
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 kinetics and the significance of transformation of retained austenite to martensite under strain and load reversals will be established. The effect of slightly elevated temperature exposures and periodic overloads on results will be examined. X-ray diffraction is used to monitor changes in the microstructure. In the second phase of the project stresses that develop in bearings due to shrink fit, tightening force, and volumetric strains will be established. The volumetric strains and their evolution with time and the change in material properties would be available from the first phase of the study. The changes in the shape of the bearing cone will be determined. The results will aid in understanding factors that contribute to cone bore growth in bearings.

The purpose of this work is to evaluate the silicon carbide particle size effect on the high-temperature strength of aluminum alloys. Volume fractions of SiC in the range 5% to 30% are considered under isothermal and thermomechanical fatigue loading conditions. The particle sizes vary in the range 5 µm to 30 µm. Thermomechanical fatigue experiments in the range 50°C to 300°C are also underway. Noncontinuum models are being developed to account for particle size effects.

Laser-induced stresses are studied to better understand high-power laser ablation of solids. Significant stresses (>> GPa) in laser targets can result from thermal expansion, phase changes, momentum recoil of ablated species, shock wave generation, and even radiation pressure. The combination of high stresses and velocity gradients during short-pulse laser ablation can give rise to significant mechanical stresspower. Stresspower is directly related to a local energy balance and is of theoretical and practical interest when studying the highly energetic events occurring.This project measures stresspower acoustically in and above the target to better understand laser-energy coupling to solids.

Experimental studies of surface crack growth under extreme pressures (1 GPa) are being conducted. Apparatus has been constructed to allow Mode I and Mode II crack growth studies in small specimens loaded in combined tension, torsion, and pressure. Both Si₃N₄ and 52100 steel are being investigated for high-performance bearing applications.

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 are quantified with torsion tests of thin-walled tubes.

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.

This project is part of a large international program to design and build the next generation ITER fusion reactor. Numerical simulations of the thermal and mechanical behavior of the first wall are being performed to assist with mechanical property experiments to evaluate different wall materials and designs, including the wall layer construction and cooling water channel geometry. The effects on temperature, stress, and life of manufacturing problems, such as incomplete contact at tube/plate junctions, are being investigated. The project also aims to examine the effect of transient thermal and mechanical loading cycles, to ensure adequate fatigue life of the first wall.

Corn is an economically important agricultural product in the state of Illinois and the U.S. Furthermore, corn is a natural composite material, constructed with aligned fibers in an organic matrix, and exhibits the key attractive feature of a high-quality composite: it is strong, yet lightweight. The present research project is investigating the use of corn and corn by-products as filler and second phase reinforcement in several polymer matrix materials. The express goal of the project is to develop a viable structural composite material that utilizes corn as either a primary or secondary reinforcement phase.