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69. Micromechanics of fracture in graphite using the scanning electron microscope
ABSTRACTS
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tical method and the compliance analysis method. The test results show, that while a strong influence of the grain-size on the bending strength of unnotched specimens exists, such an influence disappears by introducing an artificial notch. In that case, a relationship between grain-size and the fracture energy does not exist. 68. Deformation and fracture of thin-walled graphite tubes under biaxial states of stress* G. T. Yahr, R. S. Valachovic and W. L. Greenstreet (Oak Ridge National Laboratq, Oak Ridge, Tennessee). Thin-walled graphite tubes were tested under five different stress states to examine stress-strain response and failure characteristics. The strains are in good agreement with theoretical predictions based on a small-deformation elastic-plastic continuum theory. The stresses and strains at fracture are compared with theoretical predictions of several theories of failure. *Research sponsored by the National Aeronautics and Space Administration under interagency agreement 40-18269, NASA-SNSO-C Order SNC-79, and performed at the Oak Ridge National Laboratory operated by Union Carbide Corporation for the U.S. Atomic Energy Commission.
hiicromechanics of fracture in graphite using the scanning electron microscope R. A. Meyer and J. D. Buch (The Aerospace Corporation, Los Angelse, California). An investigation has been undertaken to understand the mode of failure of graphites as a function of their microstructure and to correlate the experimental observations with a theory of failure based on the coincidence alignment of microstructural defects. SEM observations were made of the initiation, propagation, and joining up of cracks. The coincidence alignment of defects, including pores and fabrication heterogeneities as well as the easy cleavage of filler particles along the grain, determines the size of the crack, and ultimately the failure stress and strain.
69.
70. A new theory of the role of porosity in graphite fracture J. D. Buch (The Aerospace Corporation, Los Angeles, California). A model of fracture of graphite which incorporates various microstructural effects, e.g. grain cleavage, porosity, etc, is described. Entire calculated stress-strain curves, up to and including fracture are included. 71. The room temperature fracture of polycrystalline graphites C. A. Anderson (Materials Science, Westinghouse Research Laboratories, Pittsburgh, Pennsylvania) and E. I. Salkovitz (Metallurgical and Materials Engiueering, University of Pittsburgh, Pennsylvania). The fracture mechanism for polycrystalline graphite was determined. Large cracks were shown to develop during deformation. By quantitatively assessing the effects of particle sizes, densities, preferred orientation and stress axis directions on each of the parameters of the brittle fracture equation containing such a crack, a fracture criterion was derived. 72. The room temperature deformation of polycrystalline graphite C. A. Andersson (Materials Science, Westinghouse Research Laboratories, Pittsburgh, Pennsylvania) and E. K. Salkovitz (Metallurgical and Materials Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania). The tensile deformations of four graphites were correlated to physical properties. The deformation mechanism was ascertained and the total strain at a given stress was found to be comprised of three components. A combined theoretical+!mpirical equation was developed which allows the stress-strain curves to be determined from the density, the preferred orientation and the stress axis direction. 7% The effect of distribution of crystallite size on the thermal conductivity of graphite B. T. Kelly (U.K.A.E.A. Reactor Fuel Laboratories, SpringFelds Works, Salwick, Preston, England). The effect of a Gaussian distribution of crystallite sizes parallel to the basal planes (La) on the basal conductivity of graphite is calculated assuming that all phonon scattering is due to crystallite boundaries and phonon-phonon interaction. A standard deviation of 5000 A on a mean crystallite size of 21,000 A is found to change the apparent crystallite size by less than 10 per cent. 74. The effect of the anharmonicity of the bond-bending coefficient on the thermal expansion coefficient of graphite parallel to the hexagonal axis B. T. Kelly (U.K.A.E.A. Reactor Fuel Laboratories, Sp-ingjields Works, Salwick, Preston, England). Calculations of the hexagonal axis expansion coefficient of a graphite crystal QL,up to 1000°C are presented
679
tical method and the compliance analysis method. The test results show, that while a strong influence of the grain-size on the bending strength of unnotched specimens exists, such an influence disappears by introducing an artificial notch. In that case, a relationship between grain-size and the fracture energy does not exist. 68. Deformation and fracture of thin-walled graphite tubes under biaxial states of stress* G. T. Yahr, R. S. Valachovic and W. L. Greenstreet (Oak Ridge National Laboratq, Oak Ridge, Tennessee). Thin-walled graphite tubes were tested under five different stress states to examine stress-strain response and failure characteristics. The strains are in good agreement with theoretical predictions based on a small-deformation elastic-plastic continuum theory. The stresses and strains at fracture are compared with theoretical predictions of several theories of failure. *Research sponsored by the National Aeronautics and Space Administration under interagency agreement 40-18269, NASA-SNSO-C Order SNC-79, and performed at the Oak Ridge National Laboratory operated by Union Carbide Corporation for the U.S. Atomic Energy Commission.
hiicromechanics of fracture in graphite using the scanning electron microscope R. A. Meyer and J. D. Buch (The Aerospace Corporation, Los Angelse, California). An investigation has been undertaken to understand the mode of failure of graphites as a function of their microstructure and to correlate the experimental observations with a theory of failure based on the coincidence alignment of microstructural defects. SEM observations were made of the initiation, propagation, and joining up of cracks. The coincidence alignment of defects, including pores and fabrication heterogeneities as well as the easy cleavage of filler particles along the grain, determines the size of the crack, and ultimately the failure stress and strain.
69.
70. A new theory of the role of porosity in graphite fracture J. D. Buch (The Aerospace Corporation, Los Angeles, California). A model of fracture of graphite which incorporates various microstructural effects, e.g. grain cleavage, porosity, etc, is described. Entire calculated stress-strain curves, up to and including fracture are included. 71. The room temperature fracture of polycrystalline graphites C. A. Anderson (Materials Science, Westinghouse Research Laboratories, Pittsburgh, Pennsylvania) and E. I. Salkovitz (Metallurgical and Materials Engiueering, University of Pittsburgh, Pennsylvania). The fracture mechanism for polycrystalline graphite was determined. Large cracks were shown to develop during deformation. By quantitatively assessing the effects of particle sizes, densities, preferred orientation and stress axis directions on each of the parameters of the brittle fracture equation containing such a crack, a fracture criterion was derived. 72. The room temperature deformation of polycrystalline graphite C. A. Andersson (Materials Science, Westinghouse Research Laboratories, Pittsburgh, Pennsylvania) and E. K. Salkovitz (Metallurgical and Materials Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania). The tensile deformations of four graphites were correlated to physical properties. The deformation mechanism was ascertained and the total strain at a given stress was found to be comprised of three components. A combined theoretical+!mpirical equation was developed which allows the stress-strain curves to be determined from the density, the preferred orientation and the stress axis direction. 7% The effect of distribution of crystallite size on the thermal conductivity of graphite B. T. Kelly (U.K.A.E.A. Reactor Fuel Laboratories, SpringFelds Works, Salwick, Preston, England). The effect of a Gaussian distribution of crystallite sizes parallel to the basal planes (La) on the basal conductivity of graphite is calculated assuming that all phonon scattering is due to crystallite boundaries and phonon-phonon interaction. A standard deviation of 5000 A on a mean crystallite size of 21,000 A is found to change the apparent crystallite size by less than 10 per cent. 74. The effect of the anharmonicity of the bond-bending coefficient on the thermal expansion coefficient of graphite parallel to the hexagonal axis B. T. Kelly (U.K.A.E.A. Reactor Fuel Laboratories, Sp-ingjields Works, Salwick, Preston, England). Calculations of the hexagonal axis expansion coefficient of a graphite crystal QL,up to 1000°C are presented