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  • Metals and Metallic Materials  (5)
  • Astrophysics  (2)
  • Administration and Management  (1)
  • 1
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    A Cepheid Distance to NGC 4603 in the Centaurus Cluster (1999)
    In:  Other Sources
    Details
    Publication Date: 2018-06-08
    Description: In an attempt to use Cepheid variables to determine the distance to the Centaurus cluster, we have obtained images of NGC 4603 with the Hubble Space Telescope for 9 epochs (totalling 24 orbits) over 14 months in the F555W filter and 2 epochs (totalling 6 orbits) in the F814W filter.
    Keywords: Astrophysics
    Type: The Astrophysical Journal
    Format: text
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  • 2
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    Analyses of Fatigue and Fatigue-Crack Growth under Constant- and Variable-Amplitude Loading (1999)
    In:  CASI
    Details
    Publication Date: 2018-06-02
    Description: Studies on the growth of small cracks have led to the observation that fatigue life of many engineering materials is primarily crack growth from micro-structural features, such as inclusion particles, voids, slip-bands or from manufacturing defects. This paper reviews the capabilities of a plasticity-induced crack-closure model to predict fatigue lives of metallic materials using small-crack theory under various loading conditions. Constraint factors, to account for three-dimensional effects, were selected to correlate large-crack growth rate data as a function of the effective stress-intensity factor range (delta K(sub eff)) under constant-amplitude loading. Modifications to the delta K(sub eff)-rate relations in the near-threshold regime were needed to fit measured small-crack growth rate behavior. The model was then used to calculate small- and large-crack growth rates, and to predict total fatigue lives, for notched and un-notched specimens under constant-amplitude and spectrum loading. Fatigue lives were predicted using crack-growth relations and micro-structural features like those that initiated cracks in the fatigue specimens for most of the materials analyzed. Results from the tests and analyses agreed well.
    Keywords: Metals and Metallic Materials
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  • 3
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    Prediction of Crack Growth under Variable-Amplitude Loading in Thin-Sheet 2024-T3 Aluminum Alloys (1997)
    In:  CASI
    Details
    Publication Date: 2018-06-05
    Description: The present paper is concerned with the application of a "plasticity-induced" crack closure model to study fatigue crack growth under various load histories. The model was based on the Dugdale model but modified to leave plastically deformed material in the wake of the advancing crack. The model was used to correlate crack growth rates under constant-amplitude loading and then used to predict crack growth under variable-amplitude and spectrum loading on thin-sheet 2024- T3 aluminum alloys. Predicted crack-opening stresses agreed well with test data from the literature. The crack-growth lives agreed within a factor of two for single and repeated spike overloads/underloads and within 20 percent for spectrum loading. Differences were attributed to fretting-product-debris-induced closure and three-dimensional affects not included in the model.
    Keywords: Metals and Metallic Materials
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  • 4
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    Extreme Emission Line Galaxies in CANDELS: Broad-Band Selected, Star-Bursting Dwarf Galaxies at Z greater than 1 (2012)
    In:  CASI
    Details
    Publication Date: 2019-07-12
    Description: We identify an abundant population of extreme emission line galaxies (EELGs) at redshift z approx. 1.7 in the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS) imaging from Hubble Space Telescope/Wide Field Camera 3 (HST/WFC3). 69 EELG candidates are selected by the large contribution of exceptionally bright emission lines to their near-infrared broad-band magnitudes. Supported by spectroscopic confirmation of strong [OIII] emission lines . with rest-frame equivalent widths approx. 1000A in the four candidates that have HST/WFC3 grism observations, we conclude that these objects are galaxies with approx.10(exp 8) Solar Mass in stellar mass, undergoing an enormous starburst phase with M*/M* of only approx. 15 Myr. These bursts may cause outflows that are strong enough to produce cored dark matter profiles in low-mass galaxies. The individual star formation rates and the co-moving number density (3.7x10(exp -4) Mpc(sup -3) can produce in approx.4 Gyr much of the stellar mass density that is presently contained in 10(exp 8) - 10(exp 9) Solar Mass dwarf galaxies. Therefore, our observations provide a strong indication that many or even most of the stars in present-day dwarf galaxies formed in strong, short-lived bursts, mostly at z 〉 1.
    Keywords: Astrophysics
    Type: GSFC.JA.6335.2012
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  • 5
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    Managing Risk on the Final Frontier (2009)
    In:  CASI
    Details
    Publication Date: 2019-08-28
    Description: The National Aeronautics and Space Administration (NASA). Exploration Systems Mission Directorate (ESMD) has combined the Continuous Risk Management (CRM) discipline with innovative knowledge management (KM) practices to more effectively enable the accomplishment of work. CRM enables proactive problem identification and problem solving in the complex world of rocket science. while KM is used to improve this process.
    Keywords: Administration and Management
    Type: STI-09-183
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  • 6
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    Cryogenic Model Materials (2001)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: An overview and status of current activities seeking alternatives to 200 grade 18Ni Steel CVM alloy for cryogenic wind tunnel models is presented. Specific improvements in material selection have been researched including availability, strength, fracture toughness and potential for use in transonic wind tunnel testing. Potential benefits from utilizing damage tolerant life-prediction methods, recently developed fatigue crack growth codes and upgraded NDE methods are also investigated. Two candidate alloys are identified and accepted for cryogenic/transonic wind tunnel models and hardware.
    Keywords: Metals and Metallic Materials
    Type: AIAA Paper 2001-0757 , Aerospace Sciences; Jan 08, 2001 - Jan 11, 2001; Reno, NV; United States
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  • 7
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    Advances in Fatigue and Fracture Mechanics Analyses for Metallic Aircraft Structures (2000)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: This paper reviews some of the advances that have been made in stress analyses of cracked aircraft components, in the understanding of the fatigue and fatigue-crack growth process, and in the prediction of residual strength of complex aircraft structures with widespread fatigue damage. Finite-element analyses of cracked metallic structures are now used to determine accurate stress-intensity factors for cracks at structural details. Observations of small-crack behavior at open and rivet-loaded holes and the development of small-crack theory has lead to the prediction of stress-life behavior for components with stress concentrations under aircraft spectrum loading. Fatigue-crack growth under simulated aircraft spectra can now be predicted with the crack-closure concept. Residual strength of cracked panels with severe out-of-plane deformations (buckling) in the presence of stiffeners and multiple-site damage can be predicted with advanced elastic-plastic finite-element analyses and the critical crack-tip-opening angle (CTOA) fracture criterion. These advances are helping to assure continued safety of aircraft structures.
    Keywords: Metals and Metallic Materials
    Type: NASA/TM-2000-210084 , L-17955 , NAS 1.15:210084 , Aeronautical Fatigue; Jul 12, 1999 - Jul 16, 1999; Seattle, WA; United States
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  • 8
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    A Practical Engineering Approach to Predicting Fatigue Crack Growth in Riveted Lap Joints (2000)
    In:  CASI
    Details
    Publication Date: 2019-07-10
    Description: An extensive experimental database has been assembled from very detailed teardown examinations of fatigue cracks found in rivet holes of fuselage structural components. Based on this experimental database, a comprehensive analysis methodology was developed to predict the onset of widespread fatigue damage in lap joints of fuselage structure. Several computer codes were developed with specialized capabilities to conduct the various analyses that make up the comprehensive methodology. Over the past several years, the authors have interrogated various aspects of the analysis methods to determine the degree of computational rigor required to produce numerical predictions with acceptable engineering accuracy. This study led to the formulation of a practical engineering approach to predicting fatigue crack growth in riveted lap joints. This paper describes the practical engineering approach and compares predictions with the results from several experimental studies.
    Keywords: Metals and Metallic Materials
    Type: NASA/TM-2000-210106 , NAS 1.15:210106 , L-17956
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