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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Asteroid collisions: Target size effects and resultant velocity distributions

To study the dynamic fragmentation of rock to simulate asteroid collisions, we use a 2-D, continuum damage numerical hydrocode which models two-body impacts. This hydrocode monitors stress wave propagation and interaction within the target body, and includes a physical model for the formation and growth of cracks in rock. With this algorithm we have successfully reproduced fragment size distributions and mean ejecta speeds from laboratory impact experiments using basalt, and weak and strong mortar as target materials. Using the hydrocode, we have determined that the energy needed to fracture a body has a much stronger dependence on target size than predicted from most scaling theories. In addition, velocity distributions obtained indicate that mean ejecta speeds resulting from large-body collisions do not exceed escape velocities.

Ryan, Eileen V.↗
NASA NTRS1985

Depth and size effects on cosmogenic nuclide production in meteorites

The galactic cosmic particle radiation (GCR) can cause changes in condensed extraterrestrial matter in different ways. It can lose energy via ionization processes of induced nuclear reactions which lead to a wide variety of stable and radioactive cosmogenic nuclides. Heavy particles incur radiation damage in minerals such as olivine and pyroxene. Light particles predominantly tend to induce nuclear reactions, causing the development of a secondary particle cascade of neutrons, protons, pions and gamma-rays and the production of cosmogenic nuclides. Such processes are described by various models, which predict the depth and size dependent production of cosmogenic nuclides.

Englert, P.↗
NASA NTRS1998

A Specimen Size Effect on the Fatigue Crack Growth Rate Threshold of IN 718

Fatigue crack growth rate (FCGR) tests were conducted on IN 718 in the solution annealed and aged condition at room temperature in accordance with E647-87. As part of each test, the FCGR threshold was measured using the decreasing Delta K method. A new heat of material was being tested and some of this material was sent to a different laboratory which wanted to use a specimen with a 127 mm width. Threshold data previously had been established on specimens with a width of 50.8 mm. As a check of the laboratory, tests were conducted at room temperature and R equal to 0.1 for comparison with the earlier data. The results were a threshold significantly higher than previously observed. Interchanging of specimen sizes and laboratories showed that the results were not due to a heat-to-heat or lab-to-lab variation. The results to be presented here are those obtained at the original laboratory. Growth rates were measured using the electric potential drop technique at R values of 0.1, 0.7, and 0.9. Compact tension specimen sizes with planer dimensions of 25.4 mm, 50.8 mm, and 127 mm were used. Crack growth rates at threshold were generally below 2.5 X 10(exp -8) mm / cycle. Closure measurements were made on some of the specimens by a manual procedure using a clip gage. When the crack growth rate data for the specimens tested at R equal to 0.1 were plotted as a function of applied Delta K, the thresholds varied with specimen width. The larger the width, the higher the threshold. The thresholds varied from 6.5 MPa-m(exp 1/2) for the 25.4 mm specimen to 15.4 MPa-m(exp 1/2) for the 127 mm specimen. At R equal to 0.7, the 25.4 mm and 50.8 mm specimens had essentially the same threshold, about 2.9 MPa-m(exp 1/2)while the 127 mm specimen had a threshold of 4.5 MPa-m(exp 1/2). When plotted as a function of effective Delta K, the R equal to 0.1 data are essentially normalized. Various aspects of the test procedure will be discussed as well as the results of analysis of the data using some different closure models.

Garr, K. R.↗
NASA NTRS1998

A Size Effect on the Fatigue Crack Growth Rate Threshold of Alloy 718

Fatigue crack growth rate (FCGR) tests were conducted on Alloy 718 in the solution annealed and aged condition at room temperature. In each test, the FCGR threshold was measured using the decreasing (Delta)K method. Initial testing was at two facilities, one of which used C(T) specimens with W = 127 mm. Previous data at the other facility had been obtained with specimens with W = 50.8 mm. A comparison of test results at R = 0.1 showed that the threshold for the 127 mm specimen was considerably higher than that of the 50.8 mm specimen. A check showed that this difference was not due to a heat-to-heat or lab-to-lab variation. Additional tests were conducted on specimens with W = 25.4 mm and at other R values. Data for the various specimens is presented along with parameters usually used to describe threshold behavior.

Garr, K. R.↗
NASA NTRS2007

An Engineering Solution for Solving Mesh Size Effects in the Simulation of Delamination with Cohesive Zone Models

This paper presents a methodology to determine the parameters to be used in the constitutive equations of Cohesive Zone Models employed in the simulation of delamination in composite materials by means of decohesion finite elements. A closed-form expression is developed to define the stiffness of the cohesive layer. A novel procedure that allows the use of coarser meshes of decohesion elements in large-scale computations is also proposed. The procedure ensures that the energy dissipated by the fracture process is computed correctly. It is shown that coarse-meshed models defined using the approach proposed here yield the same results as the models with finer meshes normally used for the simulation of fracture processes.

FROM↗
NASA NTRSDate not supplied

Size Effects on Laser Powder Bed Fusion and Laser Powder Directed Energy Deposition GRCop-42 Alloy

Additive manufacturing (AM) plays a significant role in the fabrication of regeneratively cooled rocket engine combustion chambers, where internal channels are used to transport high pressure propellants to keep the engine from overheating. Cooling channels are designed with thin walls between the cooling and combustion sides, experiencing high thermal and mechanical stresses from large temperature gradients and internal pressures. Hence, selecting materials suited for this environment and maintaining a high-quality processing for this critical application is of utmost importance. GRCop-alloys (Cu-Cr-Nb) were developed as an ideal material for combustion chambers due to its high conductivity and high strength at temperature, being employed as a structural member for the hotwall with excellent heat transfer capabilities. GRCop-42 (Cu, 4 at% Cr, 2 at% Nb) has significantly matured using the Laser Powder Bed Fusion (L-PBF) process through microstructure, mechanical and thermophysical properties and evaluations using hot-fire testing. Additionally, Laser Powder Directed Energy Deposition (LP-DED) has demonstrated successful deposition and geometry samples using GRCop-42. While the maturation in properties has focused on bulk material properties from standard test specimens, these cannot be extrapolated to be used in thin walls, as different features (i.e., microstructure, porosity, and surface texture) have a greater effect on thin walls, especially on AM structures built using L-PBF and LP-DED. This presentation provides an overview of the effects introduced by reducing the thickness on the fabrication, microstructure, surface metrology and subsequent mechanical properties of GRCop-42 using L-PBF and LP-DED as well as future work planned for this research.

Gabriel Demeneghi↗
NASA NTRS1989

Asteroid collisional history - Effects on sizes and spins

The effects of asteroid collisional history on sizes and spins of present-day objects are discussed. Collisional evolution studies indicate that collisions have altered the spin-rates of small bodies, but that the largest asteroids may have retained their primordial rotation rates. Most asteroids larger than 100 km diam have probably been shattered, but have gravitationally recaptured their fragments to form a rubble-pile structure. Large angular momentum asteroids appear to have Maclaurian spheroidal or Jacobi-ellipsoid-like shapes; some of them may have fissioned into binaries. An integrated size and spin collisional evolution model is presented, with two critical parameters: one which determines the spin rates for small fragments resulting from a shattering collision, and the other determines the fraction of impact angular momentum that is retained by the target.

Davis, Donald R.↗