3-D Dynamic Failure of Composite Materials
P. Geubelle,* C. Hwang
DOE/ASCI Center for the Simulation of Advanced Rockets

In this project, we 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 in 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.

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. Selections of these materials has direct application in the design of sound proofing, shock absorbers, composites, and helicopter blades.

Damage Detection in Composite Materials Using Magnetostrictive Tagging
S. White,* J. Li, J. Kamphaus
U.S. Army Construction Engineering Research Laboratories, 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, etc. 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.), D. Thierrault
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 (1È) of a 60-ft span bridge are manufactured and tested in static and dynamic loading conditions.

Dimensional Stability of Composite Parts-Finite-Element Simulation and Optimization of the Manufacturing Process
P. Geubelle,* S. White,* C. Tucker III* (Mech. & Indus. Engr.), Q. Zhu
National Science Foundation, CMI 96-10382

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,* X. Bi, J. Lambros (Univ. of Delaware)
National Science Foundation, CMS-9712291

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.

Failure of Layered Brittle Systems
P. Geubelle*
University of Illinois

Layered systems are being considered more frequently in a wide range of applications involving time-dependent loading conditions. They are used, for example, in the protection of structures subjected to impact loading conditions for their ability to absorb an important part of the kinetic energy of the impactor, thereby reducing the damage of the substrate. Layered systems have also been proposed as passive damping systems of various structures subjected to harmonic loading. This research project is aimed at the investigation of the dynamic failure of this type of structure, using a special cohesive-based finite-element scheme.

Health Maintenance of Composite Materials
S. White,* N. Sottos* (Theoret. & Appl. Mech.), P. Geubelle,* J. Moore* (Chemistry), M. Kessler
Campus Critical Research Initiative Program; U.S. Air Force Office of Scientific Research, F49620--00-1-0094

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 (to heal themselves from damage). Just as radioactive tracers are used in medical diagnostics, magnetostrictive tags are used to locate damage in a composite material. Should damage occur, then 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.

Health Maintenance of Composite Materials Self-Healing Thermosetting Polymers
S. White,* N. Sottos* (Theoret. & Appl. Mech.), P. Geubelle,* J. Moore* (Chemistry), S. Suresh
Campus Critical Research Initiative Program; 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.

High-Performance Computing for 3-D Dynamic Fracture Problems
P. Geubelle,* M. Breitenfeld
DOE/ASCI Center for the Simulation of Advanced Rockets

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, i.e., 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

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 recently proposed to reduce the grinding-induced subsurface damage. We 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 the Professor Ghatu Subhash from Michigan Tech whose 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

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 (i.e., 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.

Magnetic Domain Imaging of Magnetostrictive Materials Subject to Coupled Field Loading
S. White,* J. Kamphaus
U.S. Army Construction Engineering Research Laboratories, 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. A novel experimental technique called Magneto-Optical Kerr Effect (MOKE) microscopy is used to image 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*
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. Fasteners are tested for mechanical performance under various types of loads.

Mechanics of Live Propellant Failure
P. Geubelle,* C. Hwang, A. Acharaya, D. Balsara,
B-H. Liou

DOE/ASCI Center for the Simulation of Advanced Rockets

Process Optimization for Dimensional Accuracy for Polymer Composites-Experimental Characterization of Warpage
S. White,* P. Geubelle,* C. Tucker III* (Mech. & Indus. Engr.), 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 in order to develop appropriate micromechanical models.

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.

Role of the Cohesive Failure Model in Quasi-Static and Dynamic Fracture
P. Geubelle,* Y. Huang* (Mech. & Indus. Engr.), D. Kubair
DOE Center for Simulation of Advanced Rockets

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.

Shape-Memory Microflaps for Active Flow Control
S. White,* S. Krishnan
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 bi-layers are proposed to be used to fabricate 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.

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, leading, in some cases, to the complete failure of the rocket. Using a fully coupled explicit aeroelastic finite-element/finite-volume code, we 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. 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 minimum 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.

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 stalk, 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.