Sponsored by: Jt. SMD/MSD Nuclear Materials and MSD Flow and Fracture Committees and FEMS (Federation of European Materials Societies)
Program Organizers: R.J. Arsenault, Department of Materials Science and Nuclear Engineering, University of Maryland, College Park, MD 20742-2115; David Cole, CRREL, 72 Lyme Rd., Hanover, NH 03755; Todd Gross, Department of Mechanical Engineering, University of New Hampshire, Durham, NH 03824; Gernot Kostorz, Institut für Angewandte Physik, ETH Hönggerberg, CH-8093 Zürich, Switzerland; Peter Liaw, Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996-2200; Sivan Parameswaran, NRC-Institute for Aerospace Research, Ottawa, Canada K1A 0R6; Howard Sizek, Inco Alloys International Inc., Huntington, WV 25705-1771
Wednesday, PM Room: Orange County 3
February 7, 1996 Location: Anaheim Marriott Hotel
Session Chairpersons: Gemot Kostorz, Institut für Angewandte Physik, ETH Hönggerberg, CH-8093 Zürich, Switzerland; Peter Liaw, Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996-2200
2:00 pm Invited
EFFECTS ON THE [[gamma]]/[[gamma]]' RAFT STRUCTURE ON THE CREEP AND FATIGUE BEHAVIOR OF MONOCRYSTALLINE SUPERALLOYS: H. Mughrabi, Universität Erlangen-Nürnberg, Martensstrasse 5, D-91058 Erlangen, Federal Republic of Germany
Nickel-base superalloys subjected to high temperatures under the action of an applied stress exhibit a marked directional coarsening of the [[gamma]] /[[gamma]]' microstructure. The latter is characterized by the transformation of the original [[gamma]] /[[gamma]] ' microstructure, consisting of cuboidal [[gamma]] ' particles in a [[gamma]] matrix, into the plate-like [[gamma]] /[[gamma]] ' raft structure. The present work summarizes some of the results obtained in recent years in the author's research group in high-temperature creep and fatigue studies on monocrystalline, [[gamma]] '-hardened nickel-base superalloys. In particular, attention shall be focussed on the following: 1) Development and consequences of [[gamma]] /[[gamma]] ' rafting during creep. 2) Formation of /' rafts during isothermal and thermomechanical fatigue. 3) Effect of different [[gamma]] /[[gamma]]' microstructures (' cubes, rafts parallel or perpendicular to the stress axis) on the isothermal fatigue behavior.
2:30 pm Invited
FATIGUE BEHAVIOR OF SINGLE CRYSTALS AND POLYCRYSTALS: P. Lukás, L. Kunz, Institute of Physics of Materials, Academy of Sciences of the Czech Republic, 616 62 Brno, Zizkova 22, Czech Republic
Fatigue characteristics (S-N curves, cyclic stress-strain curves and Coffin-Manson curves) of single crystals and polycrystals are compared from the point of view of their convertibility using a simple conversion factors both for the stress and the plastic strain values. The S-N curves for single crystals of f.c.c. and b.c.c. metals were found to be in very good agreement with the S-N curves for polycrystals converted by the Taylor factor M=3.06. The cyclic stress-strain curves for f.c.c. metals can be converted only to a first approximation, while the limited data for b.c.c. metals show a reasonable convertibility. The application of the conversion factor to the Coffin-Manson curves of f.c.c. metals does not lead to any satisfactory agreement. The explanation is based on the considerably different volume fraction of zones exhibiting higher cyclic slip activity in single crystals and in polycrystals.
3:00 pm
PHYSICAL PROCESSES OF FATIGUE CRACK CLOSURE OBSERVED NONDESTRUCTIVELY IN THE INTERIOR OF Al-Li 2090 SAMPLES: A. Guvenilir, S.R. Stock, School Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245
Understanding the physical basis of macroscopic, indirect measurements interpreted as fatigue crack closure requires nondestructive, microscopic quantification of the crack face separations as a function of applied load. Nondestructive sectioning by high resolution computed tomography allows in situ observation of the crack faces under applied load, and measurements of physical crack openings as a function of position on the crack face are summarized for three samples of Al-Li 2090. In situ loading during x-ray tomography allows direct observation of the position of crack faces at different loads. These measurements on the interior of the sample are discussed in terms of the grain morphologies and orientations, the crack face geometry, macroscopic measurements of crack growth rate and changes in load displacement curve. This research was supported by the US Office of Naval Research, and the tomography measurements were made in collaboration with J.H. Kinney's group at Lawrence Livermore National Laboratory and with M.D. Barker's group at Lockheed Missiles and Space Company. Some of the results were obtained at CHESS which is supported by the National Science Foundation.
3:20 pm
SOLID SOLUTION SOFTENING REVISITED: M.E. Fine, M. Meshii, Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208
Intrinsic solid solution softening has been well established experimentally in bcc and hcp metals and ionic crystals. It has been pointed out by Weertman that the stress field of a solute atom exerts a couple force on a screw dislocation and, thus, reduces the activation energy of the screw dislocation overcoming the Peierls stress barrier by the double kink mechanism. It is known that intermetallic compounds possess high Peierls stress increasing the flow stress. When the flow stress is near or greater than the fracture stress, brittle behavior results. Thus reducing the Peierls stress may be a way to make intermetallics less brittle. Alloy softening has been reported in some intermetallics. The possibilities for ductilizing intermetallics by solid solution softening will be discussed, applying what has been learned with bcc and hcp metals.
3:40 pm
CORROSION FATIGUE BEHAVIOR OF CARBON FIBER REINFORCED EPOXY MATRIX COMPOSITES UNDER COMBINED HYDROSTATIC AND AXIAL LOADING: J. Stolk, S.T. Mear, H.G. Wheat, Offshore Technology Research Center and the Center for Materials Science and Engineering, The University of Texas at Austin, Austin, TX 78712; H.L. Marcus, Institute of Materials Science, University of Connecticut, Storrs, CT 06269
Stress relaxation and load control fatigue tests of unidirectional, [90]12, carbon fiber-epoxy matrix composites were performed in air, seawater, and distilled water on the as-received composite material to obtain baseline da/dN vs. [[Delta]]K crack growth data. Samples which were soaked to saturation in seawater and distilled water were fatigued at ambient pressure in order to evaluate the effects of both moisture uptake and seawater chemistry on crack growth rates. In addition, both the as-received and saturated samples were fatigue tested inn elevated pressure (up to 4000 psi) seawater or distilled water to determine what effect, if any, the superimposed hydrostatic pressure had on the fatigue crack growth rate. Finally, post-failure scanning electron microscopy (SEM) was performed to determine the effects of the various test conditions on the failure modes of the specimens. The feasibility of using nuclear magnetic resonance spectroscopy (NMR) and magnetic resonance imaging (MRI) to characterize moisture uptake at the specimen crack tip was also examined.
4:00 pm
NEAR-THRESHOLD FATIGUE CRACK GROWTH BEHAVIOR OF NICKEL-TITANIUM SHAPE MEMORY ALLOYS: R.L. Holtz,Geo-Centers, Inc., 109033 Indian Head Highway, Fort Washington, MD 20744; K.Sadananda, M.A. Imam, C.A. Greene, CODE 6323, U.S. Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, DC 20375
Traditional interpretations of fatigue crack growth (FCG) mechanisms rely heavily on "crack closure" concepts. Among other things, plasticity and phase transformations have been invoked as contributing to crack closure. In order to delineate more clearly the role of plasticity and phase transformations on FCG, a comprehensive study of the FCG behavior of a Ni 49.9% - Ti 50.1% alloy with a martensitic transformation temperature (Ms) of 80deg.C was undertaken. Fatigue crack growth rate measurements were performed at frequencies of 10 Hz for stress ratios, R, from 0.1 to 0.9, in both vacuum and air, at temperatures ranging from room temperature up to 120deg.C, spanning the thermoelastic and pseudoelastic regimes. The threshold and near-threshold crack growth behaviors are discussed in the context of the two-parameter model of Vasudevan, Sadananda, and Louat [Materials Science and Engineering, A188, 1 (1994)] which avoid ad-hoc crack closure arguments.
4:20 pm
MICROMECHANISMS OF ROOM AND ELEVATED-TEMPERATURE FATIGUE CRACK GROWTH IN GAMMA TITANIUM ALUMINIDES: C. Mercer, M. Mehdirad, W.O. Soboyejo, Department of Materials Science and Engineering, The Ohio State University, 2041 College Road, Columbus, OH 43210-1179
The micromechanisms of room-and elevated-temperature fatigue crack growth in a wide range of gamma-based titanium aluminnides are elucidated in this paper. The differences between the fatigue crack growth behavior at room- and elevated-temperature are explained by considering the implications of different crack-tip deformation and shielding modes. The possible influence of alloying on oxide-induced wedging/closure is also examined within a micromechanics framework. The implications of the results are discussed for alloy/microstructure design and fatigue life prediction.
4:40 pm
SELF-ORGANIZATION AND SPATIO-TEMPORAL DISSIPATIVE STRUCTURES IN FATIGUED METALS: M. Glazov, C. Laird, Department of Materials Science and Engineering, School of Engineering and Applied Science, University of Pennsylvania, Philadelphia, PA 19104-6272
Strongly nonequilibrium systems of different physico-chemical nature may exhibit a tendency to self-organization either in the form of emerging temporal rhythms, or spatial scales, or both. We demonstrate that fatigued metallic alloys fall into this behavior as well. In the first part of the work the "classical" Portevin-Le-Chatelier(PLC) effect will be discussed as example of a temporal dissipative structure. The existing models of the PLC-effect (Kubin and Estrin, Ananthakrishna, McCormick) will be discussed along with their limits of applicability. A-non-linear dynamical model explaining the cyclic analog of the PLC-effect-the "Yan-Hong-Laird bursts" will be developed. We will also explain the "Neumann bursts" occurring in fatigued metals under the conditions of spatial coupling into the system of differential equations modeling the temporal instabilities in fatigued metals allows one to describe different types of dislocation patterning. Using the so-called "reaction+diffusion" approach (the finite-size analog of the Walgraef-Aifantis model) we will demonstrate that dislocation patterning in fatigue metals can be understood as a result of competition between the grain size and the characteristic length scales of the model. Finally, the prospects of future work will be briefly discussed.
5:00 pm
EVALUATION OF FRACTURE CHARACTERISTICS FOR SiC/SiC COMPOSITES UNDER HIGH TEMPERATURE CONDITIONS BY ACOUSTIC EMISSION METHOD: S.T. Kim, Y.T. Lee, Department of Mechanical Engineering, Yeungnam University, 214-1 Daedong, Gyongsan, Kyongpook, Korea
The temperature dependency of mechanical properties on woven-cloth SiC-fiber
reinforced SiC composite materials, which were fabricated by injection molding
method, was examined under slow speed (0.02mm/min) tensile test inn various
temperatures and atmospheric condition. Acoustic emission (AE) signal was
detected by piezoelectric AF sensor in real time. It was found that the AE
source location method, utilizing the longitudinal wave velocity, was effective
in identifying fracture points of the specimens tested at room temperature.
However, this method was not effective at elevated temperatures, because the
fracture occurred so fast by the oxidation of the materials. The strength
degradation of these composites at elevated temperature was turned out to be
due to the formation of SiO2 and TiO2 oxide particles.
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