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Materials and Structures
^ 3-D Dynamic Failure of Composite Materials
P. Geubelle,* C. Hwang
DOE/ASCI Center for the Simulation of Advanced Rockets
geubelle@uiuc.edu
http://ssm7.aae.uiuc.edu/PHG_GROUP/

In this project, researchers develop a numerical scheme, referred to as the spectral scheme, specially designed to tackle the complex phenomena associated with the delamination of fiber reinforced composite laminates. Special emphasis is placed on capturing with great detail the three-dimensional effects associated with the orthotropy of the surrounding medium. The issue of the existence of a limiting crack speed under both tensile and shear loading condition is addressed in this research.

^ Dimensional Stability of Composites: Process Simulation and Optimization
P. Geubelle,* S. White,* C. Tucker III* (Mech. & Indus. Engr.), Q. Zhu
National Science Foundation, CMI 96-10382
geubelle@uiuc.edu
http://ssm7.aae.uiuc.edu/PHG_GROUP

The objective of this project is to better understand the effect of the manufacturing process in the dimensional stability of composite parts. Of particular interest is the importance of capturing the evolution of the matrix material throughout the cure cycle on the final shape of the composite part. This particular project combines detailed 3-D coupled thermal, chemical, and mechanical finite-element simulations of the manufacturing process with optimization techniques to predict the final shape of the manufactured part.

^ Dynamic Fiber Pull-Out in Polymeric Composites
P. Geubelle,* J. Lambros, X. Bi
National Science Foundation, CMS-9712291
geubelle@uiuc.edu
http://ssm7.aae.uiuc.edu/PHG_GROUP

When a composite structure fails and a crack propagates perpendicularly to the fiber direction, a substantial amount of energy is dissipated in the progressive debonding and sliding on the fibers. Preliminary experimental investigations have shown that these two processes can be quite different under high strain rate conditions than in a quasi-static situation. In this combined experimental and analytical research program, the dynamic failure of a model composite is examined using a specially adapted version of the Split Hopkinson Bar apparatus and a special form of the finite-element scheme.

^ High-Performance Computing for 3-D Dynamic Fracture Problems
P. Geubelle,* M. Breitenfeld
DOE/ASCI Center for the Simulation of Advanced Rockets
geubelle@uiuc.edu
http://ssm7.aae.uiuc.edu/PHG_GROUP

The numerical simulation of 3-D dynamic fracture events is one of the most challenging computational issues in solid mechanics, due to the extreme refinement needed to capture continuously evolving geometries (as the fracture surface extends) and rapidly moving stress waves. The objective of this project is to develop and implement high-performance numerical tools used to simulate a variety of spontaneous dynamic fracture events (for which the crack path is not specified a priori but is part of the solution itself). Great emphasis is put on the implementation of the dynamic fracture codes on massively parallel computing platforms.

^ High-Speed Grinding of Ceramic Materials: Process Simulation and Damage Assessment
P. Geubelle,* S. Maiti
National Science Foundation CAREER, CMS-9734473
geubelle@uiuc.edu
http://ssm7.aae.uiuc.edu/PHG_GROUP

Many applications of structural ceramics require high dimensional accuracy and/or surface finish, and grinding is often required at the end of the manufacturing process. However, the brittle granular nature of ceramics renders the surface machining process more complicated than in metals, and surface cracks are often found to extend well into the ceramic part. High-speed machining has been proposed recently to reduce the grinding-induced subsurface damage. Researchers are using a special finite-element-based numerical scheme to simulate the grinding process, with special emphasis on the associated intergranular fracture process. This project involves collaboration with Professor Ghatu Subhash from Michigan Tech. That research group is conducting detailed experiments on the topic.

^ Intersonic Crack Propagation under Shear-dominated Loading Conditions
P. Geubelle,* Y. Huang* (Mech. & Indus. Engr.), D. Kubair
DOE/ASCI Center for the Simulation of Advanced Rockets
geubelle@uiuc.edu
http://ssm7.aae.uiuc.edu/PHG_GROUP

The issue of limiting crack speeds in dynamically failing structures is still an active topic of research. While it is accepted that the Rayleigh wave speed constitutes the theoretical limit under tensile (mode I) conditions, recent observations of dynamic failure in homogeneous specimens subjected to shear-dominated loading conditions indicated that intersonic crack motion (that is, higher than the shear wave speed but lower than the dilatational wave speed) is possible. The objective of this project is to investigate the subsonic-to-transonic transition process using a specially developed spectral scheme.

^ Mechanics of Live Propellant Failure
P. Geubelle,* C. Hwang, M. S. Breitenfeld, R. Fiedler, A. Haselbacher
DOE/ASCI Center for the Simulation of Advanced Rockets
geubelle@uiuc.edu
http://www.csar.uiuc.edu/

The initiation and propagation of one or more cracks in the solid propellant (SP) or along the grain/case interface can have dramatic repercussions on the rocket performance. By creating additional burning surfaces in the SP, the propagation of one or more cracks can greatly affect the pressure history in the rocket chamber. In some cases, this can lead to the complete failure of the rocket. Using 2-D and 3-D fully coupled explicit aeroelastic finite-element/finite-volume codes, researchers are investigating various accident scenarios associated with the presence of pre-existing cracks at various locations in the solid booster, with special emphasis on SP/liner interfacial failures.

^ Role of the Cohesive Failure Model in Quasi-Static and Dynamic Fracture
P. Geubelle,* Y. Huang* (Mech. & Indus. Engr.), D. Kubair
DOE/ASCI Center for Simulation of Advanced Rockets
geubelle@uiuc.edu
http://ssm7.aae.uiuc.edu/PHG_GROUP

Cohesive failure models are often used in the analytical and numerical study of spontaneous crack propagation. These simple models have been shown to capture some important dynamic fracture effects, such as maximum crack speed, crack branching, and unsteady crack energetics. The primary objective of this research project is to gain a better understanding of the importance of the often ignored rate-dependent effect on the spontaneous propagation behavior of a crack. The numerical tool used in this study is a spectral form of the boundary integral formulation of the elastodynamic relations, which allows for the incorporation of a wide range of cohesive models.

^ Analytical Determination of Optimum Viscoelastic Material Properties
H. H. Hilton,* C. E. Beldica*
University of Illinois; National Center for Supercomputing Applications, DAHC94-46-C-0005 (HPCMP-PET)

The influence of complex modulus shapes and parameters on creep, relaxation, and damping is being investigated. These moduli will be used to solve dynamic and static problems, such as bending, torsion, and flutter of lifting surfaces. The results will yield a categorization of viscoelastic material behavior in its relation to creep, relaxation, and damping. Both isotropic and anisotropic materials are being considered. Such an analytical catalog of material behavior then can be employed to fabricate real materials to conform to such modulus specification. Selection of these materials has direct application in the design of soundproofing, shock absorbers, composites, and helicopter blades.

^ Probabilistic Minimum Weight Analysis
H. H. Hilton*
University of Illinois

An analytical method has been developed for designing structures having a prescribed probability of failure so that the overall weight is minimal under combined loads. The solution is obtained for structures consisting of components having normal, Weibull beta-distributed applied and failure stresses, and is applicable to combined loading conditions. The loading conditions are such that general relations can be used to relate the mean stresses to the cross-sectional area. Weight comparisons with standard design procedures based on the margin of safety concept are made and indicate the possibility of substantial weight savings.

^ Random Viscoelastic Material Effects
H. H. Hilton*
University of Illinois; National Center for Supercomputing Applications, DAHC94-46-C-0005 (HPCMP-PET); DOE Center for Simulation of Advanced Rockets, 3341494 (ASCI)

Analytical studies are presented which extend the elastic-viscoelastic analogies to stochastic processes caused by random linear viscoelastic material properties. Separation of variable as well as integral transform correspondence principles is formulated and discussed in detail. The statistical differential equation of the moment characteristic functional is derived, but rather than solving the highly complex functional equation, the solutions are formulated in terms of the first- and second-order statistical properties. Gaussian, Weibull, and beta distributions are considered for the probability density distributions of creep and relaxation functions, and their effectiveness is evaluated.

^ Fundamental Problems in Dynamic Fracture Mechanics
J. Lambros,* J. Kimberley
National Science Foundation CAREER award, CMS-9874775
lambros@uiuc.edu
http://www.aae.uiuc.edu/Profs/Lambros/index.html

This project deals with the fundamental understanding of dynamic debonding in homogeneous and bimaterial systems that exhibit planes of preferential crack growth. Of particular interest is the effect of applied loading mixity in the development of contact regions aft of the propagating crack tip. In addition, limiting crack growth speeds in bimaterial and homogeneous material systems are investigated. The study is performed using optical interferometry in conjunction with high-speed photography to image in real time the deformation fields surrounding the propagating crack tip.

^ Fracture Mechanics of Crystalline Aluminosilicate Oxides
J. Lambros,* Z. Wang, R. F. Lobo (Univ. of Delaware)
Mobil Inc.; W.R. Grace and Co.
lambros@uiuc.edu
http://www.aae.uiuc.edu/Profs/Lambros/index.html

Zeolites are porous structures that are used in numerous industries, including the oil industry, as catalysts in reactive flows. It has been observed that upon continued use their catalytic ability diminishes. This is believed to be a consequence of mechanical loads on the zeolites. This particular project is focused on the determination of elastic and failure properties of such zeolites. Since the catalyst particles are usually of small size and primarily undergo compressive loading by contact with other particles, it was decided to test as small a zeolite crystal as possible in compression. A compressive load frame was custom built using a stepper motor actuator. Zeolite crystals were synthesized in the laboratory to facilitate testing. The average crystal size was around 500 microns and contained pores of the order of 5 nm. It was seen that the crystals are extremely brittle and are prone to excessive microcracking under continued compressive fatigue loading.

^ Fracture of Polymeric Matrix Composites
J. Lambros,* S. Prabhu
University of Delaware Research Foundation
lambros@uiuc.edu
http://www.aae.uiuc.edu/Profs/Lambros/index.html

The use of polymeric matrix fiber reinforced composites is constantly increasing in the aerospace industry. Such materials may provide several benefits, including significant weight savings and corrosion resistance, over traditional structural materials. However, because of their multiphase nature the failure processes composites undergo are considerably more complex than those in more traditional solids. This project deals with the numerical and experimental investigation of quasi-static fracture of polymeric matrix long fiber reinforced composites. Finite element simulations are used to determine the extent and amount of three dimensional deformation regions in cracked thick composite plates. Experimental confirmation of the numerical simulations is done by using full-field lateral shearing interferometric techniques to image the near-tip region in the cracked composite plates. The experiments are also used to determine regions of dominance of anisotropic asymptotic crack-tip fields and extract values of the composite fracture toughness. A study of the effects of fiber direction and applied load mixity is also performed.

^ High Strain Rate Properties of Advanced Materials for Use by the U.S. Navy
J. Lambros,* D. Heisig, Z. Li, J. R. Vinson (Univ. of Delaware)
Office of Naval Research, N00014-97-10638
lambros@uiuc.edu
http://www.aae.uiuc.edu/Profs/Lambros/index.html

Accurately determining the high strain rate mechanical response of polymers and polymeric matrix composites is of interest in this work. Dynamic experiments are conducted using either a compression or tension split Hopkinson bar. The bars provide a real-time signal of stress and strain in the sample. For a complete characterization of sample response, however, independent measurement of temperature is required. For this purpose, simultaneously with stress and strain, sample surface temperature is measured in real time using an HgCdTe high-speed infrared radiation detector array. The measured heat emitted during deformation is then related to the amount of mechanical work imparted in the sample during loading. For ductile polymers, such as polycarbonate, the amount of plastic work converted to heat is found to be around 50%. That is much less than occurs in most metallic materials (about 95%). For brittle systems, such as PlexiglasTM and composites, significant damage-induced heating is also seen to occur.

^ Numerical and Experimental Modeling of a Cellulose Cutting Process
J. Lambros,* M. Kompella
Hercules Inc.
lambros@uicu.edu
http://www.aae.uiuc.edu/Profs/Lambros/index.html

Cellulose is a plant-based polymer that has found widespread use in many applications ranging from toothpaste and drugs, to packaging and paints. In most cases cellulose is used in particle form (for example as a moisture retention agent). Thus, controlling the particle size while cutting raw cellulose is critical to its usefulness. The present project is concerned with determining micromechanical properties of cellulose fibers and using these properties in numerical simulations that accurately predict cellulose fiber failure. The fibers are about 2mm to 3mm in length and 50 microns in width. A special microtensile tester device, using a stepper motor for actuation and a high precision load cell for load measurement, was designed and constructed in-house. The microtensile tester was successfully used to measure elastic and failure properties of wood- and cotton-based cellulose fibers. The statistical distribution of fiber strength measured experimentally was then fit to a Weibull strength distribution model.

^ Quasi-static Fracture of Functionally Graded Materials
J. Lambros,* J. Abanto-Bueno
National Science Foundation, CMS-9712831
lambros@uiuc.edu
http://www.aae.uiuc.edu/Profs/Lambros/index.html

Functionally graded materials (FGMs) are materials that exhibit a spatially continuous variation of mechanical properties (graded metal-ceramic structures, for example). Such materials possess the benefits of metals (conductivity, ductility, and such) on one side and those of ceramics (including hardness, insulation, and stiffness) on the other side, without the adverse effects of an interface between the two solids being present. Understanding the fracture response of such graded microstructures is critical in developing design procedures for their beneficial use. In this work, researchers perform macroscopic fracture experiments on a model polymer-based functionally graded material. The full-field optical technique of digital image correlation is used to visualize the displacement field of an edge crack propagating in the FGM under remote loading. The experimental results are then used to extract values of the stress intensity factor and thus construct resistance curves for crack growth in FGMs. It is seen that FGMs possess a "built-in" fracture resistance curve by requiring constantly increasing amounts of energy to be supplied to the crack tip. The effect of the amount and orientation of material gradient is also investigated.

^ Thermomechanical Effects During Dynamic Fracture of Glassy Polymers
J. Lambros,* T. Bjerke

lambros@uiuc.edu
http://www.aae.uiuc.edu/Profs/Lambros/index.html

When a crack propagates rapidly through a solid, significant amounts of heat generation may occur. For the case of metals, the temperature increase surrounding a dynamically growing crack can be of the order of several hundreds of degrees. For polymeric materials this is expected to be less, but such materials are much more susceptible to even small temperature variations than metals. This project deals with the experimental and analytical investigation of heat generation during dynamic fracture of polymers. An HgCdTe high-speed infrared radiation detector array is used to measure in real time the temperature generated during dynamic crack growth in a nominally ductile polymer, polycarbonate, and a nominally brittle one, PlexiglasTM. Both symmetric (mode I) and predominately antisymmetric (mode II) crack growth is investigated. It is found that the temperature increase may be of the order of the glass transition temperature in both polymers and is substantially more for the shear dominated case. A cohesive failure-based formulation is used to theoretically predict the amount of heat generated at the dynamically propagating crack and to understand the partition of fracture energy into elastic deformation, plastic deformation, new surface generation and heat emission.

^ Damage Detection in Composite Materials Using Magnetostrictive Tagging
S. White,* J. Li
U.S. Army Construction Engineering Research Laboratory (CERL), DACA88-97-K-0001

Knowledge of the health of a material or structure is critical for timely maintenance and repair of components. This is especially difficult for composites because subsurface flaws are hidden from visual inspection. By incorporating smart material tags into the matrix of a polymer composite and then interrogating these tags, the state of health of a composite structure can be qualified throughout its processing and service life. TERFENOL-D, a magnetostrictive material, is used for tagging. The tagged material emits a magnetic signature that is proportional to the applied stress. The location of cracks is detected by local peaks in magnetic signature.

^ Design and Manufacture of Adaptive Structures
S. White,* J. Berman
University of Illinois

Adaptive structures and materials sense their environment and react to these sensory inputs in some logical fashion. The sensor/actuator materials for these applications can take many forms: shape-memory alloy wires, piezoelectric patches, fiber optics, and so forth. This research investigates the design and manufacture of shape-memory alloy composites, piezoelectric composites, and hybrids combining the two sensor/actuators. The approach builds upon investigations at the microlevel (interfacial bonding, residual stresses) to the macrolevel (structural mechanics of beams/tubes, process modeling). These types of materials have wide-ranging applications in the civil infrastructure, aerospace, and automotive industries.

^ Design, Manufacture, and Testing of Polymer Composite Bridges
S. White,* D. Parsons* (Civil & Environ. Engr.), J. Bignell
National Academy of Sciences, NCHRP-63

Polymer composite materials are particularly well suited for use in civil infrastructure applications due to their high specific strength and stiffness and resistance to environmental degradation. A unique design has been developed for large vehicular bridges based on integrated composite shells manufactured by filament winding. Scale models (1/10) of a 60-ft span bridge are manufactured and tested in static and dynamic loading conditions.

^ Magnetic Domain Imaging of Magnetostrictive Materials Subject to Coupled Field Loading
S. White,* J. Kamphaus
U.S. Army Construction Engineering Research Laboratory (CERL), DACA88-97-K-0001

Magnetostrictive materials such as TERFENOL-D are composed of small magnetic domains. The domains are distinguished by alignment of magnetic dipoles within regions of the material. The magnetostrictive behavior of TERFENOL-D arises through changes in the magnetic domains under the influence of stress and magnetic field. Magnetic force microscopy (MFM) is used to visualize the magnetic domains of TERFENOL-D while applying magnetic and mechanical loads.

^ Manufacture and Testing of Composite Couplings for Building Systems
S. White;* D. Parsons,* K. Hjelmstad* (Civil & Environ. Engr.); S. Singamsethi
National Science Foundation, CMS-9978588

The use of composite materials in civil infrastructure applications such as building superstructures is currently limited by the ability to connect composite components together. Standard fastening techniques, such as bolts, welding, and rivets, are not suitable for use with polymeric composites. A novel type of fastener utilizing composite materials formed into sleeves that nest with structural components has been designed. Manufacturing techniques suitable to high-speed, low-cost production are being developed using liquid molding processes. The novel fasteners are tested for mechanical performance under various types of loads.

^ Process Optimization for Dimensional Accuracy for Polymer Composites: Experimental Characterization of Warpage
S. White,* D. O'Brien
National Science Foundation, DMI 96-10382

The residual stresses induced during processing of polymer matrix composites manifest themselves as warpage in the finished structure. The overall objective is to develop models that can be used to predict these processing-induced deformations so that molds and curing schedules can be designed to account for warpage in the final structure. Experiments are being conducted to measure the processing-induced warpage in both flat plate and complex curvature geometries.

^ Process Optimization for Dimensional Accuracy for Polymer Composites: Material Characterization and Micromechanical Modeling
S. White,* P. Geubelle;* C. Tucker III* (Mech. & Indus. Engr.); D. O'Brien
National Science Foundation, DMI 96-10382

Residual stresses developed during the manufacture of composites have a strong influence on the final shape of the manufactured part. A precise understanding of the phenomena leading to the appearance of these residual stresses is the primary objective of this project, in which special emphasis is placed on characterizing the polymeric matrix composite during the manufacturing process. A full experimental characterization of the viscoelastic mechanical properties during cure is necessary to develop appropriate micromechanical models.

^ Self-Healing Composite Materials
S. White,* N. Sottos* (Theoret. & Appl. Mech.), J. Moore* (Chem.), P Geubelle, * M. Kessler, E. Brown, S. Sriram, D. Therriault
U.S. Air Force Office of Scientific Research, F49620-00-1-0094

Self-healing polymer composites are obtained by storing a repair agent in microcapsules that are dispersed throughout the matrix. Triggering of the repair process occurs when the cracks encounter an embedded microcapsule and break the shell material open. The repair agent stored inside the capsule is released into the crack plane and a rebonding of the fracture plane is initiated. Experiments are conducted to assess the capability of several candidate polymers for self-healing potential. The kinetics of the repair process are assessed and modeled, and the formulation of optimal repair agents is sought.

^ Shape-Memory Microflaps for Active Flow Control
S. White,* E. Loth,* P. Geubelle,* Q. Li, J. Kamphaus
Defense Advanced Research Projects Agency, F49620-98-1-0490; U.S. Air Force Office of Scientific Research, F49620-98-1-0381

Conventional bleeding of the boundary layer for supersonic inlet ducts utilizes fixed holes with active-passive transpiration. Improvement in efficiency can be obtained by using an active flow control device based on the development of shape-memory microflaps that open and close under certain operating conditions. Shape-memory composites and bilayers are proposed for fabricating these microflaps. This project entails significant experimental characterization of the constituent materials, analytical modeling of the constitutive and structural behavior, and mechanical-wind tunnel testing of performance.

^ Use of Corn Byproducts for Structural Composite Materials
S. White,* N. Sottos (Theoret. & Appl Mech.), T. Mackin (Mech. & Indus. Engr.)
University of Illinois

Two important issues have gained national priority in recent years: the development of alternative markets for corn and its byproducts and the revitalization of the U.S. civil infrastructure. These two issues are synthesized in the current project that focuses on the development of cheaper composite materials for civil engineering applications by using corn byproducts as reinforcements in polymer matrix composites. Husks, fiber stalks, kernels, and fiber silks are mechanically tested both individually and as embedded reinforcements in several different polymer matrices. Once a reasonable database has been established for the mechanical properties of corn byproducts, their potential for structural composites will be evaluated.


Summary of Engineering Research