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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 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1973

Liquid metal embrittlement

Crack propagation is discussed for metals with liquid metal in the crack space. The change in electrochemical potential of an electron in a metal due to changes in stress level along the crack surface was investigated along with the change in local chemistry, and interfacial energy due to atomic redistribution in the liquid. Coupled elastic-elastrostatic equations, stress effects on electron energy states, and crack propagation via surface roughening are discussed.

Tiller, W. A.↗
NASA NTRS1990

Scanning electron acoustic microscopy of indentation-induced cracks and residual stresses in ceramics

The ability of scanning electron acoustic microscopy (SEAM) to characterize ceramic materials is assessed. SEAM images of Vickers indentations in SiC whisker-reinforced alumina clearly reveal not only the radial cracks, the length of which can be used to estimate the fracture toughness of the material, but also reveal strong contrast, interpreted as arising from the combined effects of lateral cracks and the residual stress field left in the SiC whisker-reinforced alumina by the indenter. The strong contrast is removed after the material is heat treated at 1000 C to relieve the residual stresses around the indentations. A comparison of these observations with SEAM and reflected polarized light observations of Vickers indentations in soda-lime glass both before and after heat treatment confirms the interpretation of the strong contrast.

Cantrell, John H.↗
NASA NTRS1972

Influence of oxygen-vacancy complex /A center/ on piezoresistance of n-type silicon.

Changes in both magnitude and temperature dependence of the piezoresistance of electron-irradiated n-type silicon, induced by the latter's oxygen-vacancy complex (A center), are shown to be due to the fact that the presence of the A center causes the total conduction-band electron concentration to change with an applied stress. This change in electron concentration leads to an additional piezoresistance contribution that is expected to be important in certain many-valley semiconductors. This offers the possibility of tailoring the thermal variations of semiconductor mechanical sensors to more desirable values over limited temperature ranges.

Littlejohn, M. A.↗
NASA NTRS1993

Some Observations of Solder Joint Failure Under Tensile-Compressive Stress

It has long been known that solder joints under mechanical stress are subject to failure. In early electronic systems, such failures were avoided primarily by avoiding the use of solder as a mechanical structural component. The rule was to first make sound wire connections that did not depend mechanically on solder, and only then to solder them. Careful design and miniaturization in modern electronic systems limits the mechanical stresses exerted on solder joints to values less than their yield points, and these joints have become integral parts of the mechanical structures. Unfortunately, while these joints are strong enough when new, they have proven vulnerable to fatigue failures as they age.Details of the fatigue process are poorly understood, making predictions of expected lifetimes difficult.

solder joints fatigue failure thermal cycling test↗
NASA NTRS1993

Thermal Cycling Fatigue in DIPs Mounted with Eutectic Tin-Lead Solder Joints in Stub and Gullwing Geometries

It has long been known that solder joints under mechanical stress are subject to failure. In early electronic systems, such failures were avoided primarily by avoiding the use of solder as a mechanical structural component. The rule was first to make sound wire connections that did not depend mechanically on solder, and only then to solder them. Careful design and miniaturization in modern electronic systems limits the mechanical stresses exerted on solder joints to values less than their yield points, and these joints have become integral parts of the mechanical structures. Unfortunately, while these joints are strong enough when new, they have proven vulnerable to fatigue failures as they age. Details of the fatigue process are poorly understood, making predictions of expected lifetimes difficult.

fatigue failure solder joints failure mechanisms s↗
NASA NTRS1966

Gravitational collapse and weak interactions.

Massive star behavior during final catastrophic evolution stages, stressing effect of electron type neutrino interactions, using numerical hydrodynamics coupled with energy transfer methods

HYDRODYNAMICS↗
NASA NTRS1977

Conductive spacecraft materials development program

The objectives of this program are to provide design criteria, techniques, materials, and test methods to ensure control of absolute and differential charging of spacecraft surfaces. The control of absolute and differential charging of spacecraft cannot be effected without the development of new and improved or modified materials or techniques that will provide electrical continuity over the surface of the spacecraft. The materials' photoemission, secondary emission, thermooptical, physical, and electrical properties in the space vacuum environment both in the presence and absence of electrical stress and ultraviolet, electron, and particulate radiation, are important to the achievement of charge control. The materials must be stable or have predictable response to exposure to the space environment for long periods of time. The materials of interest include conductive polymers, paints, transparent films and coatings as well as fabric coating interweaves.

Lehn, W. L.↗
NASA NTRS1992

A compositional classification scheme for meteoritic chondrules

A compositionally based classification scheme for chondrules is proposed that will help in systematizing the wealth of data available and disentangling the effects of nebular and subsequent processes. The classification is not by texture or the composition of a single phase, or a mixture of these two, but rather is a comprehensive, systematic approach which uses the composition of the two main chondrule components. This scheme is applicable to over 95 percent of the chondrules and is easily applied using an electron microprobe. It stresses the original diversity of the chondrules and the complex yet facile way in which they respond to parent-body metamorphism. Results using this classification scheme suggest that arguments against an important role of chondrules in determining the compositional trends of the chondrites have been premature.

Sears, Derek W. G.↗
NASA NTRS1998

Electroformed Nickel Mirrors for the Next Generation Space Telescope

This paper summarizes the work to date on a novel mirror fabrication technique being developed at the Marshall Space Flight Center for potential use on the Next Generation Space Telescope (NGST). This technique involves forming an extremely lightweight mirror by electroplating nickel and nickel based alloys onto a highly polished precision mandrel. The resulting mirror shell can then be backed up with or attached to a lightweight structure to produce a mirror element that is on the order of 15 kg/sq m areal density. Since the mirrors are fabricated from a mandrel (or master), subsequent mirrors can be made with very high economy; this technique is particularly suited to segmented mirrors schemes whereby large apertures are achieved through the deployment of smaller segments. Control of the electroplating process is the key element for producing high quality optics; bath chemistry and real time control of the plating current density yields uniform grained electroforms with zero residual stress. To accomplish this, a special electronic sensor was developed whereby the residual stress can be monitored as the nickel is electrolytically deposited. This information is used in a feedback loop to modulate current density which, in turn, directly governs the residual stress. Details pertaining to this and other aspects of the fabrication of a half meter mirror will be published along with test results and metrology data.

Redmon, John W.↗
NASA NTRS2021

Stress-Activated Electric Currents in Icy Planetary Bodies: H2O2-Doped H2O Ices

Planetary satellites such as the Jovian moon Europa and several Kuiper Belt objects (including Pluto) have surfaces consisting largely of H2O ice, which is brittle and behaves as a rock at the low temperatures that prevail in the outer Solar System. Several of those ice-crusted bodies show evidence of tectonic activity indicating high levels of stress. This paper reports on laboratory experiments with pure H2O ice and H2O2–doped H2O ices with H2O2 concentration levels comparable to those in the ices of Europa. Elongated rectangular ice blocks at T = −81 °C [192 K] were stressed at one end to test whether electric currents are generated and capable of flowing down the stress gradient. Pure H2O ice was found to not produce currents above the 10−10 A background level except for occasional transients in the 10−9–10−8 A range during fracture or rapid plastic deformation due to electrons, e’. By contrast, stressing H2O2-doped H2O ices consistently produced electric currents in the 10−7–10−5 A range, due to holes, h•, propagating from the stressed end to the unstressed end. The h• charge carriers are generated by the break-up of peroxy bonds of H2O2 molecules, leading to O−, equivalent to defect electrons or holes h• in the O2− matrix.

Water ice↗
NASA NTRS2012

Effect of Compressive Stresses on Leakage Currents in Microchip Tantalum Capacitors

Microchip tantalum capacitors are manufactured using new technologies that allow for production of small size capacitors (down to EIA case size 0402) with volumetric efficiency much greater than for regular chip capacitors. Due to a small size of the parts and leadless design they might be more sensitive to mechanical stresses that develop after soldering onto printed wiring boards (PWB) compared to standard chip capacitors. In this work, the effect of compressive stresses on leakage currents in capacitors has been investigated in the range of stresses up to 200 MPa. Significant, up to three orders of magnitude, variations of currents were observed after the stress exceeds a certain critical level that varied from 10 MPa to 180 MPa for capacitors used in this study. A stress-induced generation of electron traps in tantalum pentoxide dielectric is suggested to explain reversible variations of leakage currents in tantalum capacitors. Thermo-mechanical characteristics of microchip capacitors have been studied to estimate the level of stresses caused by assembly onto PWB and assess the risk of stress-related degradation and failures. Keywords: tantalum capacitors, leakage current, soldering, reliability, mechanical stress.

Teverovsky, Alexander↗