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  • 1
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    Grain boundary oxidation and fatigue crack growth at elevated temperatures (1986)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Fatigue crack growth rate at elevated temperatures can be accelerated by grain boundary oxidation. Grain boundary oxidation kinetics and the statistical distribution of grain boundary oxide penetration depth were studied. At a constant delta K-level and at a constant test temperature, fatigue crack growth rate, da/dN, is a function of cyclic frequency, nu. A fatigue crack growth model of intermittent micro-ruptures of grain boundary oxide is constructed. The model is consistent with the experimental observations that, in the low frequency region, da/dN is inversely proportional to nu, and fatigue crack growth is intergranular.
    Keywords: METALLIC MATERIALS
    Type: NASA-CR-179529 , NAS 1.26:179529
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  • 2
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    Grain boundary oxidation and an analysis of the effects of pre-oxidation on subsequent fatigue life (1986)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: The effects of preoxidation on subsequent fatigue life were studied. Surface oxidation and grain boundary oxidation of a nickel-base superalloy (TAZ-8A) were studied at 600 to 1000 C for 10 to 1000 hours in air. Surface oxides were identified and the kinetics of surface oxidation was discussed. Grain boundary oxide penetration and morphology were studied. Pancake type grain boundary oxide penetrates deeper and its size is larger, therefore, it is more detrimental to fatigue life than cone-type grain boundary oxide. Oxide penetration depth, a (sub m), is related to oxidation temperature, T, and exposure time, t, by an empirical relation of the Arrhenius type. Effects of T and t on statistical variation of a (sub m) were analyzed according to the Weibull distribution function. Once the oxide is cracked, it serves as a fatigue crack nucleus. Statistical variation of the remaining fatigue life, after the formation of an oxide crack of a critical length, is related directly to the statistical variation of grain boundary oxide penetration depth.
    Keywords: METALLIC MATERIALS
    Type: NASA-CR-175051 , NAS 1.26:175051
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  • 3
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    Grain boundary oxidation and its effects on high temperature fatigue life (1986)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Fatigue lives at elevated temperatures are often shortened by creep and/or oxidation. Creep causes grain boundary void nucleation and grain boundary cavitation. Grain boundary voids and cavities will accelerate fatigue crack nucleation and propagation, and thereby shorten fatigue life. The functional relationships between the damage rate of fatigue crack nucleation and propagation and the kinetic process of oxygen diffusion depend on the detailed physical processes. The kinetics of grain boundary oxidation penetration was investigated. The statistical distribution of grain boundary penetration depth was analyzed. Its effect on high temperature fatigue life are discussed. A model of intermittent micro-ruptures of grain boundary oxide was proposed for high temperature fatigue crack growth. The details of these studies are reported.
    Keywords: METALLIC MATERIALS
    Type: NASA, Lewis Research Center, Turbine Engine Hot Section Technology 1986; p 407-414
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  • 4
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    The analysis of fatigue crack growth mechanism and oxidation and fatigue life at elevated temperatures (1988)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Two quantitative models based on experimentally observed fatigue damage processes have been made: (1) a model of low cycle fatigue life based on fatigue crack growth under general-yielding cyclic loading; and (2) a model of accelerated fatigue crack growth at elevated temperatures based on grain boundary oxidation. These two quantitative models agree very well with the experimental observations.
    Keywords: METALLIC MATERIALS
    Type: NASA-CR-182129 , NAS 1.26:182129
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  • 5
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    Literature survey on oxidations and fatigue lives at elevated temperatures (1984)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Nickel-base superalloys are the most complex and the most widely used for high temperature applications such as aircraft engine components. The desirable properties of nickel-base superalloys at high temperatures are tensile strength, thermomechanical fatigue resistance, low thermal expansion, as well as oxidation resistance. At elevated temperature, fatigue cracks are often initiated by grain boundary oxidation, and fatigue cracks often propagate along grain boundaries, where the oxidation rate is higher. Oxidation takes place at the interface between metal and gas. Properties of the metal substrate, the gaseous environment, as well as the oxides formed all interact to make the oxidation behavior of nickel-base superalloys extremely complicated. The important topics include general oxidation, selective oxidation, internal oxidation, grain boundary oxidation, multilayer oxide structure, accelerated oxidation under stress, stress-generation during oxidation, composition and substrate microstructural changes due to prolonged oxidation, fatigue crack initiation at oxidized grain boundaries and the oxidation accelerated fatigue crack propagation along grain boundaries.
    Keywords: METALLIC MATERIALS
    Type: NASA-CR-174639 , NAS 1.26:174639
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  • 6
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    Crack tip field and fatigue crack growth in general yielding and low cycle fatigue (1984)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Fatigue life consists of crack nucleation and crack propagation periods. Fatigue crack nucleation period is shorter relative to the propagation period at higher stresses. Crack nucleation period of low cycle fatigue might even be shortened by material and fabrication defects and by environmental attack. In these cases, fatigue life is largely crack propagation period. The characteristic crack tip field was studied by the finite element method, and the crack tip field is related to the far field parameters: the deformation work density, and the product of applied stress and applied strain. The cyclic carck growth rates in specimens in general yielding as measured by Solomon are analyzed in terms of J-integral. A generalized crack behavior in terms of delta is developed. The relations between J and the far field parameters and the relation for the general cyclic crack growth behavior are used to analyze fatigue lives of specimens under general-yielding cyclic-load. Fatigue life is related to the applied stress and strain ranges, the deformation work density, crack nucleus size, fracture toughness, fatigue crack growth threshold, Young's modulus, and the cyclic yield stress and strain. The fatigue lives of two aluminum alloys correlate well with the deformation work density as depicted by the derived theory. The general relation is reduced to Coffin-Manson low cycle fatigue law in the high strain region.
    Keywords: METALLIC MATERIALS
    Type: NASA-CR-174686 , NAS 1.26:174686
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  • 7
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    Morphological study of near threshold fatigue crack growth in a coarse grain aluminum alloy (1984)
    In:  Other Sources
    Details
    Publication Date: 2019-07-27
    Description: Fatigue crack propagation in the near-threshold region has been studied in coarse grain Al 7029 alloy. Over eighty percent of the crack surfaces are planar areas parallel to either 100-oriented or 111-oriented planes. The 100-plane crack surfaces show 'pine tree' morphological features formed by slip on two sets of intersecting planes. The 111-plane crack surfaces were planar and shiny. They were formed primarily by slip on a single dominant 111-oriented slip plane with sparse and very light secondary slip markings. Crack growth rates were measured and correlated with Delta-K.
    Keywords: METALLIC MATERIALS
    Type: Intl. Fatigue 84 Conference; Sept. 3-7, 1984; University of Birmingham
    Format: text
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  • 8
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    Grain boundary oxidation and an analysis of the effects of oxidation on fatigue crack nucleation life (1988)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: The effects of preoxidation on subsequent fatigue life were studied. Surface oxidation and grain boundary oxidation of a nickel-base superalloy (TAZ-8A) were studied at 600 to 1000 C for 10 to 1000 hours in air. Surface oxides were identified and the kinetics of surface oxidation was discussed. Grain boundary oxide penetration and morphology were studied. Pancake type grain boundary oxide penetrates deeper and its size is larger, therefore, it is more detrimental to fatigue life than cone-type grain boundary oxide. Oxide penetration depth, a (sub m), is related to oxidation temperature, T, and exposure time, t, by an empirical relation of the Arrhenius type. Effects of T and t on statistical variation of a (sub m) were analyzed according to the Weibull distribution function. Once the oxide is cracked, it serves as a fatigue crack nucleus. Statistical variation of the remaining fatigue life, after the formation of an oxide crack of a critical length, is related directly to the statistical variation of grain boundary oxide penetration depth.
    Keywords: METALLIC MATERIALS
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  • 9
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    The effect of pre-stress cycles on fatigue crack growth - An analysis of crack growth mechanism (1974)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: Cyclic prestress increases subsequent fatigue crack growth rate in 2024-T351 aluminum alloy. This increase in growth rate, caused by the prestress, and the increased rate, caused by temper embrittlement as observed by Ritchie and Knott (1973), cannot be explained by the crack tip blunting model alone. Each fatigue crack increment consists of two components, a brittle and a ductile component. They are controlled by the ductility of the material and its cyclic yield strength, respectively.
    Keywords: METALLIC MATERIALS
    Type: Engineering Fracture Mechanics; 6; Dec. 197
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  • 10
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    Fatigue crack growth at elevated temperature 316 stainless steel and H-13 steel (1976)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: Crack growths were measured at elevated temperatures under four types of loading: pp, pc, cp, and cc. In H-13 steel, all these four types of loading gave nearly the same crack growth rates, and the length of hold time had negligible effects. In AISI 316 stainless steel, the hold time effects on crack growth rate were negligible if the loading was tension-tension type; however, these effects were significant in reversed bending load, and the crack growth rates under these four types of loading varied considerably. Both tensile and compressive hold times caused increased crack growth rate, but the compressive hold period was more deleterious than the tensile one. Metallographic examination showed that all the crack paths under different types of loading were largely transgranular for both CTS tension-tension specimens and SEN reversed cantilever bending specimens. In addition, an electric potential technique was used to monitor crack growth at elevated temperature.
    Keywords: METALLIC MATERIALS
    Type: NASA-CR-135105 , MTS-HWL-4116-776
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