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
Monday, PM Room: Orange County 3
February 5, 1996 Location: Anaheim Marriott Hotel
Session Chairpersons: T. Gross, Department of Mechanical Engineering, University of New Hampshire, Durham, NH 03824; A.K. Vasudevan, Office of Naval Research, Arlington, VA
2:00 pm Invited
CRACKS ON INTERFACES: R. Thomson, Emeritus, NIST, Gaithersburg, MD 20899
Cracks on interfaces can cleave on the interface, cleave or branch to a cleavage plane off the interface, emit a (nonblunting) dislocations on the interface, or emit a blunting dislocation on a slip plane inclined to the interface. Criteria for all these events will be presented in terms of the balance between a driving force for the event and a lattice resistance to the event. The driving force can be written as an extension of the standard crack extension force for a crack. The lattice resistance for cleavage is related to the intrinsic interface surface energy, but is not identical to it for the branching crack. The lattice resistance for nonblunting emission is the unstable stacking fault, but involves both the surface and unstable stacking fault energies in the case of blunting emission. The role of chemical segregants on the interface will be discussed.
2:30 pm Invited
EXPERIMENTAL AND NUMERICAL STUDIES OF THE BDT TRANSITION IN NiAl: P. Neumann, P. Ochmann, H. Vehoff, Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Strasse 1, D-402337 Düsseldorf, Germany
In pre-cracked mono- and bi-crystalline samples of NiAl the fracture toughness was measured as a function of temperature and the distance between crack tip and grain boundary of the bicrystals. The results are compared to numerical simulations of dislocation emission from a crack tip in a vicinity of a grain boundary. The kinetics of dislocation motion and the image forces on the dislocations are fully taken into account. The grain boundary is either modelled as an impenetrable obstacle to the dislocations, a source of dislocations on other slip systems or a weak interface with a critical cohesive strength. The qualitative features of the experimental findings could be reproduced in the numerical simulations.
3:00 pm Invited
DISLOCATION STRUCTURES IN DEFORMED METALS: N. Hansen, Materials Dept., Risø National Laboratory, DK-4000 Roskilde, Denmark
The deformation of single crystals and polycrystals produces dislocation structures which evolve with continued deformation. These structures have been observed to be common to a broad range of materials, alloys, deformation modes and deformation temperatures. Recent work within this field is analyzed with emphasis on the characteristics of various dislocation boundaries and the causes of their formation taking into account the misorientation across the boundary, the boundary plane and the slip pattern in the adjoining crystallites. It is found that both the dislocations in the boundaries and the characteristic microstructural and crystallographic features of the boundaries relate to the actual slip pattern and to the tendency of dislocations to accumulate in low energy configurations. A basis has therefore been created for relating dislocation structures to the plastic deformation behavior over a large range of stress and strain.
3:30 pm
SLIT-LIKE INHOMOGENEITIES UNDER UNIFORM STRESS: CRACKS, ANTICRACKS AND QUASICRACKS: J.A. Hurtado, Department of Materials Science and Engineering. J. Dundurs, T. Mura, Department of Civil Engineering, Northwestern University, Evanston, IL 60208
Solutions for slit-like inhomogeneities in an infinitely extended isotropic solid subjected to a uniform stress at infinity are obtained by using Eshelby's equivalent inclusion method. First, two limiting cases are studied: Cracks (i.e. inhomogeneities with elastic moduli identically zero), and Anticracks (i.e. inhomogeneities with infinitely large elastic moduli). Solutions are obtained for different modes of uniform loading in plane strain (mode I and mode II), and antiplane strain (mode III). It is observed that the stress field of a mode I anticrack is the sum of the stress field of a mode I crack (which arises from a crack plane dislocation dipole distribution. Finally the most general case of inhomogeneities with finite non-zero elastic moduli, which here are called quasicracks, is considered and new solutions are provided. An interesting example of quasicracks in engineering practice is that of welded cracks where the elastic moduli of the welding material differ from those of the matrix.
3:50 pm
FATIGUE DAMAGE IN A 20 VOL% SiCw-6061 Al ALLOY MATRIX COMPOSITE: S. Homdee, J.G. Byrne, Department of Metallurgical Engineering, 412 W.C. Browning Bldg., University of Utah, Salt Lake City, UT 84112-1183
Both 20 vol % SiCw reinforced and unreinforced 6061-T6 Al alloy R. R. Moore specimens were fatigue tested under constant maximum alternating stress to establish S-N curves. The addition of SiC whiskers increased the fatigue strength and fatigue life of the composite relative to the unreinforced Al alloy. In the composite, the positron annihilation Doppler broadening technique was able to detect increasing fatigue damage up to failure, whereas in the unreinforced alloy fatigue damage could be detected only for the initial 8 to 17% of total fatigue life due to saturation of the Doppler line shape parameter used in the measurements. At low stress levels, the fatigue fracture process in the composite is localized near the fracture surface relative to the unreinforced alloy. As the stress level increases, the depth of the fatigue damage region beneath the fracture surface increases. Fatigue crack initiation in the composite occurs by the formation of microcracks near the interface between SiC whiskers and the aluminum alloy matrix beneath the free surface. Pull-out of SiC whiskers coated with metal matrix material was noted. The connection of these microcracks results in fatigue crack propagation. In the final fracture region, the fracture process consists of the formation and coalescence of voids in the matrix and at whisker ends in dense SiC cluster areas.
4:10 pm
PARTICLE SHAPE EFFECTS ON THE FRACTURE MORPHOLOGY AND DUCTILITY OF DISCONTINUOUSLY-REINFORCED COMPOSITES: N. Shi, S.G. Song, M.A.M. Bourke, J.A. Goldstone, Los Alamos National Laboratory, Los Alamos, NM 87545; G. Liu, Department of Metallurgical and Materials Engineering, Illinois Institute of Technology, Chicago, IL 60616
Particle shape effects on the fracture and ductility of a spherical and an angular particulate-reinforced 6061-Al matrix composite containing 20 vol. pct Al2O3 were studied using SEM fractography. The results were correlated to finite element method (FEM) simulations. SEM fractographic examinations show that during tensile deformation the spherical composite fails through void nucleation and linking in the matrix near the reinforcement/matrix interface. In contrast the angular composite fails through particle fracture and matrix ligament rupture. FEM results indicate that the failure modes for these two composites can be attributed to the differences in the development of internal stresses and strains. While the maximum strain in the matrix is similar under the same load, particle stress in the angular composite is considerably higher, inducing premature particle fracture which constitutes the composite failure mode.
4:30 pm
EFFECT OF YIELD STRENGTH AND ELASTIC MODULUS ON FRACTURE SURFACE INTERFERENCE OF REMOTELY LOADED MODE II CRACKS: T.S. Gross, R.U. Goulet, Department of Mechanical Engineering, University of New Hampshire, Durham, NH 03824-3591; D.A. Mendelsohn, Department of Aeronautical Engineering, Applied Mechanics and Aviation, The Ohio State University, Columbus, OH 43210
Fracture surface interference has been shown to shield the crack tip from a remote Mode II load as well as induce a Model I stress intensity factor. Specimens of 7075 Al in the T6 and O condition and specimens of M50 tool steel in the annealed and quenched and tempered condition were subjected to pure remote Mode II loads on a modified four point bend fixture. These four specimens were chosen to determine the extremes of the effect of elastic modulus and yield strength on the magnitude of Mode II shielding and induced Mode I stress intensity factor. The effect of differing boundary conditions on the loading pins will be discussed and a compliant modified four point bend fixture will be described.
4:50 pm
FATIGUE AND FRACTURE IN DUCTILE-PHASE REINFORCED, BRITTLE INTERMETALLIC LAMINATED COMPOSITES: D. Bloyer, K.T. Venkateswara Rao, R.O. Ritchie, Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720-1760
With their superior creep properties, several intermetallic alloys may become
light-weight alternatives to currently-used superalloys for many engine
applications, particularly associated with aerospace. Unfortunately these
materials invariably suffer from low intrinsic toughness and ductility at
ambient temperatures. Indeed, improvements in their damage tolerance has
necessitated the use of extrinsic toughening methods, i.e., by promoting
sources of crack-tip shielding, to improve their fracture behavior. Although
ductile-phase reinforcements are commonly used to this end, their effect on
fatigue behavior is less certain. Accordingly, in the present study, the effect
of metallic Nb reinforcements on the fatigue-crack propagation and fracture
toughness behavior of Nb3Al is examined in a medium-scale laminated composite
consisting of 125-um thick layers of Nb bonded to 500-um thick layers of Nb3Al.
It is shown that whereas the toughening response of such materials is far
superior to particulate-reinforced or micro-laminates of Nb/Nb3Al, the fatigue
properties are much less affected. The microstructural mechanisms underlying
such behavior, and the corresponding fatigue and fracture properties at
elevated temperatures (>1000deg.C), will be discussed. Work supported by the
Air Force Office of Scientific Research under the AASERT Program.
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