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Session Chairperson: P.K. Liaw, Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996-2200
8:30 am INVITED
RELIABILITY OF FERROELECTRIC CERAMIC MULTILAYER DEVICES: Z. Suo, X. Gong, Mechanical and Environmental Engineering Department, Materials Department, University of California, Santa Barbara, CA 93106
In a multilayer actuator, each internal electrode terminates an edge inside the active ceramic. Around the edge, the nonuniform electric field generates an incompatible strain field, which, in its turn, generates stresses and may cause the ceramic to crack, leading to device failure. The industry has been exploring alternative electrode configurations to alleviate the stress concentration. The effort has been empirical and benefited little from numerical simulations. An inherent difficulty is that the actuator ceramics have nonlinear electro-mechanical interactions, of which no unified mathematical description is now available. This talk describes the basic phenomena, and a crack nucleation model that includes essential features of this nonlinearity. The model shows that, everything else being fixed, a critical layer thickness exists, below which a multilayer actuator will not crack around its internal electrode edges.
NON-LINEAR FRACTURE PROCESSES IN LAMINATED METAL COMPOSITES: Don Lesuer, Bob Riddle, Chol Syn, L-342, Lawrence Livermore National Laboratory, Livermore, CA 94551
Numerous fracture processes exist in layered materials. These processes, which include crack bridging, crack deflection, stress redistribution and local plane stress deformation, will, in general, increase the fracture toughness of layered materials over that observed for non-layered materials. However, their specific influence on toughness (namely their influence on initiation toughness, growth toughness and R-curve behavior) is a function of the specific fracture mechanism active during crack growth. Many of these mechanisms compete with one another. The active mechanism(s) are strongly dependent on the material properties of the component layers, laminate architecture (such as volume fraction of the component materials and layer thickness), and interface properties. This paper will discuss the various fracture mechanisms in laminated metal composites (LMCs) containing alternating layers of ductile and brittle materials. The layers between these component materials have sharp interfaces. Specific systems include LMCs of an aluminum matrix composite and a monolithic aluminum alloy as well as ultrahigh-carbon steel and brass. Simulations of crack growth in these materials have been done using finite element analysis to study the non-linear fracture processes involved. The influence of component material properties, interface properties and laminate architecture on these processes will be described.
A QUANTIFICATION OF THE FABRIC STACKING SEQUENCE EFFECT ON THE MECHANICAL PROPERTIES OF A 2-D WOVEN NICALON FIBER FABRIC REINFORCED SiC COMPOSITE: Wei Zhao, Peter K. Liaw, Dept. of Materials Sciences and Engineering, The University of Tennessee, Knoxville, TN 37996-2200; Nianni Yu, Dept. of Engineering Sciences of Mechanics, The University of Tennessee, Knoxville, TN 37996-2200; Elizabeth R. Kupp, David P. Stinton, The Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831
The mechanical behaviour of continuous fiber reinforced ceramic matrix composites is affected by several factors, such as: the type and thickness of fiber/matrix interface, the fiber strength statistical distribution and degradation after processing, the fabric stacking sequence of the laminate etc. For a 2-D plain woven Nicalon fiber cloth reinforced silicon carbide (SiC) with a SiC interfacial coating, little attention has been paid to the stacking sequence effect. In this paper, the fabric stacking sequence effects on the stress-strain distribution in flexural laminate bars, and the elastic properties of the laminate are calculated. A theoretical calculation model is established based on a combination of the classical laminated plate theory and an undulation model depicting fiber continuity and undulation shape, which is first developed by Chou et al. A closed-form FORTRAN program is compiled. Different combination of fiber lamina orientation and their stacking sequence are studied.
10:00 am BREAK
10:30 am INVITED
TOUGHENING AND FATIGUE PERFORMANCE OF BRITTLE AND SEMI-BRITTLE STRUCTURAL MATERIALS: John J. Lewandowski, Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, OH 44106
Significant efforts are being devoted to the cost effective processing of advanced structural materials with unique combinations of stiffness, strength, toughness, and fatigue resistance. Such efforts are necessarily multi-disciplinary and often combine the skills of academic investigators in Materials Science and Engineering, Mechanical Engineering, and Civil Engineering, as well as other related disciplines with those from both the materials producer and user community. Recent work at CWRU in conjunction with a variety of collaborators has begun to investigate the factors controlling the magnitude of performance enhancement possible in brittle and semi-brittle systems where a variety of approaches to toughening are being explored. The presentation will overview recent research by the P.I. and his collaborators on a number of different materials systems where toughening is being accomplished via the incorporation of ductile/tough reinforcements. The presentation will follow with documenting the fatigue performance of such systems.
INTERFACIAL INTERLOCKING FROM POROUS OXIDES: Z. Zhang, Z. Xing, J.K. Shang, Department of Materials Science and Engineering, University of Illinois, Urbana, IL 61801
Surface oxides on metals are often highly porous and the pore structure depends on oxidant and kinetics of oxidation. In this work, the oxide structure on aluminum-alloy surface was modified by chemical and electrochemical processes to produce different pore sizes and morphology. Interfacial bonds were then formed from these porous oxides and the crack growth resistance of the interfaces measured in air and corrosive environments. Significant differences were found among interfaces with different oxide microstructures. Micromechanical models were developed to determine the interlocking resistance from porous oxides. Based on the model, the role of interfacial interlocking in interfacial crack growth is discussed.
NOTCHED TENSILE PROPERTIES OF CERAMIC MATRIX COMPOSITES: F.W. Zok, High Performance Composites Center, Materials Department, University of California, Santa Barbara, CA 93106-5050
Fiber-reinforced ceramic matrix composites (CMCs) are being developed for a variety of high temperature components for gas turbine engines. Many of these components are complex in shape and will be subjected to local stress concentration under normal service conditions. The presentation will focus on the effects of stress concentrating features such as sharp slits and circular holes on the tensile properties of CMCs. Modeling and experimental efforts aimed at improving our understanding of the relationship between the mechanisms of inelastic deformation and their effects on notched strength will be reviewed. Results on MAS/Nicalon and woven SiC/SiC systems indicate that the conditions for fracture in the regions around holes are stochastic in nature and depend on the volume under stress. An approach for implementing weakest link statistics for predicting notched properties will be outlined. Recent work has revealed that inelastic deformation is also attainable in all-oxide CMCs with strong interfaces and porous matrices. The notched properties of these materials will be presented and their behavior compared with that of systems with weak interfaces. Research sponsored by ARPA-URI, Grant N00014-92-J-1808.
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