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  • METALLIC MATERIALS  (9)
  • Humans
  • Models, Molecular
  • Nuclear Structure
  • 1985-1989  (8)
  • 1975-1979  (1)
  • 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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    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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  • 6
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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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  • 7
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    Resolved shear stress intensity coefficient and fatigue crack growth in large crystals (1988)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: Fatigue crack growth tests were carried out on large-grain Al 7029 aluminum alloy and the finite element method was used to calculate the stress field near the tip of a zigzag crack. The resolved shear stresses on all 12 slip systems were computed, and the resolved shear stress intensity coefficient (RSSIC) was defined. The RSSIC was used to analyze the irregular crack path and was correlated with the rate of single-slip-plane shear crack growth. Fatigue crack growth was found to be caused primarily by shear decohesion at a crack tip. When the RSSIC on a single-slip system was much larger than all the others, the crack followed a single-slip plane. When the RSSICs on two conjugate slip systems were comparable, a crack grew in a zigzag manner on these planes and the macrocrack-plane bisected the two active slip planes. The maximum RSSIC on the most active slip system is proposed as a parameter to correlate with the shear fatigue crack growth rate in large crystals.
    Keywords: METALLIC MATERIALS
    Type: Theoretical and Applied Fracture Mechanics (ISSN 0167-8442); 10; 111-122
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  • 8
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    Grain boundary oxidation and oxidation accelerated fatigue crack nucleation and propagation (1986)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Fatigue life at elevated temperatures is often shortened by oxidation. Grain boundary oxidation penetrates deeper than the surface oxidation. Therefore, grain boundary oxide penetration could be the primary cause of accelerated fatigue crack nucleation and propagation, and the shortened fatigue life at elevated temperatures. Grain boundary oxidation kinetics was studied and its statistical scatter was analyzed by the Weibull's distribution function. The effects of grain boundary oxidation on shortened fatigue life was analyzed and discussed. A model of intermittent microruptures of the grain boundary oxide was proposed for the fatigue crack growth in the low frequency region. The proposed model is consistent with the observations that fatigue crack growth rate in the low frequency region with hold time at K sub max is inversely proportional to cyclic frequency and that crack growth is intergranular.
    Keywords: METALLIC MATERIALS
    Type: NASA-CR-175050 , NAS 1.26:175050 , Minnowbrook Conference on Life Prediction for High Temperature Gas Turbine Materials; Aug 27, 1985 - Aug 30, 1985; Blue Mountain Lake, NY; United States
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  • 9
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    Grain boundary oxidation and fatigue crack growth at elevated temperatures (1986)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    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: Theoretical and Applied Fracture Mechanics (ISSN 0167-8442); 6; 85-94
    Format: text
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