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At least 865 records · Page 48

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NASA NTRS1991

GPS measurements of deformation associated with the 1987 Superstition Hills earthquake: Evidence for conjugate faulting

Large station displacements observed from Imperial Valley Global Positioning System (GPS) campaigns are attributed to the November 24, 1987 Superstition Hills earthquake sequence. Thirty sites from a 42 station GPS network established in 1986 were reoccupied during 1988 and/or 1990. Displacements at three sites within 3 kilometers of the surface rupture approach 0.5 m. Eight additional stations within 20 km of the seismic zone are displaced at least 10 cm. This is the first occurrence of a large earthquake (M(sub S) 6.6) within a preexisting GPS network. Best-fitting uniform slip models of rectangular dislocations in an elastic half-space indicate 130 + or - 8 cm right-lateral displacement along the northwest-trending Superstition Hills fault and 30 + or - 10 cm left-lateral displacement along the conjugate northeast-trending Elmore Ranch fault. The geodetic moments are 9.4 x 10(exp 25) dyne-cm and 2.3 x 10(exp 25) dyne-cm for the Superstition Hills and Elmore Ranch faults, respectively, consistent with teleseismic source parameters. The data also suggest the post seismic slip along the Superstition Hills fault is concentrated at shallow depths. Distributed slip solutions using Singular Value Decomposition indicate near uniform displacement along the Elmore Ranch fault and concentrated slip to the northwest and southeast along the Superstition Hills fault. A significant component of non-seismic displacement is observed across the Imperial Valley, which is attributed in part to interseismic plate-boundary deformation.

Larsen, Shawn↗

NASA NTRS1990

Extension of transonic flow computational concepts in the analysis of cavitated bearings

An analogy between the mathematical modeling of transonic potential flow and the flow in a cavitating bearing is described. Based on the similarities, characteristics of the cavitated region and jump conditions across the film reformation and rupture fronts are developed using the method of weak solutions. The mathematical analogy is extended by utilizing a few computational concepts of transonic flow to numerically model the cavitating bearing. Methods of shock fitting and shock capturing are discussed. Various procedures used in transonic flow computations are adapted to bearing cavitation applications, for example, type differencing, grid transformation, an approximate factorization technique, and Newton's iteration method. These concepts have proved to be successful and have vastly improved the efficiency of numerical modeling of cavitated bearings.

Vijayaraghavan, D.↗

NASA NTRS1990

Drop coalescence studies

The objective of this experimental study is to understand the detailed mechanics of the coalescence of liquid drops. The experiments are being conducted in an immiscible acoustic levitator with degassed water as the host medium. Typically, a quasineutrally buoyant drop of silicon oil mixed with bromobenzene is levitated close to the velocity node of the levitator. A second drop of the same liquid is introduced, and as it slowly seeks levitation position, the drops coalesce. Coalescence is delayed until the host film between drops is completely drained. Following coalescence, the excess surface energy in the coalesced drop is dissipated through shape oscillations. The final events of film rupture followed by drop coalescence are rapid and are photographically studied with high-speed video (1000 fps). The laser-induced fluorescence technique is used to visualize the dynamics of host film drainage. The details of the coalescence mechanics are presented.

Anikumar, A. V.↗

NASA NTRS1991

Fully articulated four-point-bend loading fixture

A fully articulated four-point bend loading fixture for Modulus of Rupture (MOR) and fracture toughness specimens utilizes an upper loading plate in combination with a lower loading plate. The lower plate has a pair of spring loaded ball bearings which seat in V-shaped grooves located in the upper plate. The ball bearings are carried in the arms of the lower plate. A load is applied to the specimen through steel rollers, one large roller and one smaller roller each located on both the upper and lower plates. The large rollers have needle roller bearings which enable a single loading roller to rotate relative to the plate to which it is attached.

Calomino, Anthony M.↗

NASA NTRS1991

Destructive physical analysis results of Ni/H2 cells cycled in LEO regime

Six 48-Ah individual pressure vessel (IPV) Ni/H2 cells containing 26 and 31 percent KOH electrolyte were life cycle tested in low Earth orbit. All three cells containing 31 percent KOH failed (3729, 4165, and 11,355 cycles), while those with 26 percent KOH were cycled over 14,000 times in the continuing test. Destructive physical analysis (DPA) of the failed cells included visual inspections, measurements of electrode thickness, scanning electron microscopy, chemical analysis, and measurements of nickel electrode capacity in an electrolyte flooded cell. The cycling failure was due to a decrease of nickel electrode capacity. As possible causes of the capacity decrease, researchers observed electrode expansion, rupture, and corrosion of the nickel electrode substrate, active material redistribution, and accumulation of electrochemically undischargeable active material with cycling.

Lim, Hong S.↗

NASA NTRS1991

Terrestrial geophysics in the SeaRISE project

Some areas of research in the SeaRISE project are briefly discussed. They are as follows: (1) Radar Sounding serves multiple purposes. The most general and obvious is mapping ice thickness and the surface and bedrock topography of the ice sheet. (2) The purpose of Seismic Shooting, in addition to water depth measurements on floating ice, is to provide information about the internal physical characteristics of the ice sheet, the rock beneath it, and the interface between the two. (3) Passive Seismic monitoring of microearthquakes can be used to study brittle fracture within the ice or the rock beneath it. Common parameters available from these studies are fault location, orientation, and displacement, as well as the size of the rupture area, stress drop, and energy released. (4) There is a large contrast in Electrical Resistivity between ice or permafrost on the one hand and liquid water or wet rock on the other hand. Thus, electrical resistivity profiles have the ability of revealing the depth to the melting point, whether it is found at the base of the ice or in the subglacial rock. (5) Gravity anomalies, especially combined with seismic measurements, are an effective tool for determining deeper crustal structure. Anomalies averaged over extensive areas are useful also for their potential to reveal isostatic imbalance, which is a measure of average glacial change over the last several hundred years.

Bentley, C. R.↗

NASA NTRS1990

The mechanics of the South Iceland Seismic Zone

The mechanics of the South Iceland Seismic Zone is examined by means of boundary element modeling. The differences between the observed geometry of the major tectonically active features and that of a geometrically orthogonal ridge-transform system (which assumes that the upper part of the earth's crust is an elastic plate containing vertical cuts) are examined. It is suggested that north-south faults can accommodate transform deformation only if the faults are longer or more numerous than those observed so far. This is considered reasonable because earthquake surface rupture lengths are commonly less than the inferred fault length at depth. The South Iceland Seismic Zone is subject to sequences of large earthquake every 45-112 years. In comparing the seismic moment release derived from earthquake magnitudes with that predicted by the models, it is confirmed that the system of north-south faults can act as a transform fault.

Hackman, M. Christine↗

NASA NTRS1990

Extension of transonic flow computational concepts in the analysis of cavitated bearings

Vijayaraghavan, D.↗

NASA NTRS1991

Slow crack growth in glass in combined mode I and mode II loading

Slow crack growth in soda-lime glass under combined mode I and mode II loading was investigated in precracked disk specimens in which pure mode I, pure mode II, and various combinations of mode I and mode II were achieved by loading in diametral compression at selected angles with respect to symmetric radial cracks. It is shown that slow crack growth under these conditions can be described by a simple exponential relationship with elastic strain energy release rate as the effective crack-driving force parameter. It is possible to interpret this equation in terms of theoretical models that treat subcritical crack growth as a thermally activated bond-rupture process with an activation energy dependent on the environment, and the elastic energy release rate as the crack-driving force parameter.

Shetty, D. K.↗

NASA NTRS1989

Cryogenic mechanical properties of low density superplastically formable Al-Li alloys

The aerospace industry is considering the use of low density, superplastically formable (SPF) materials, such as Al-Li alloys in cryogenic tankage. SPF modifications of alloys 8090, 2090, and 2090+In were tested for strength and Kahn tear toughness. The results were compared to those of similar tests of 2219-T87, an alloy currently used in cryogenic tankage, and 2090-T81, a recently studied Al-Li alloy with exceptional cryogenic properties (1-9). With decreasing temperature, all materials showed an increase in strength, while most materials showed an increase in elongation and decrease in Kahn toughness. The indium addition to 2090 increased alloy strength, but did not improve the strength-toughness combination. The fracture mode was predominantly intergranular along small, recrystallized grains, with some transgranular fracture, some ductile rupture, and some delamination on large, unrecrystallized grains.

Verzasconi, S. L.↗

NASA NTRS1991

Pressure vessel burst test program. II

The current status is disucssed of a program to study the characteristics of blast waves and fragmentation generated by ruptured gas-filled pressure vessels. Current methods for assessing vessel safety and burst parameters are briefly reviewed, and pneumatic burst testing operations and testing results are examined. A comparison is made with current methods for burst assessment. It is tentatively concluded that, at close distances, vessel burst overpressures are less than those of high-explosive (HE) blasts with equivalent energy and are greater than HE far from the vessel. The impulse appears to be the same for both vessel bursts and equivalent energy HE blasts. The functional relationship between shock velocity and overpressure ratio appears to be the same for vessel bursts as for HE blasts. The initial shock overpressure appears to be much less than vessel pressure and may be found using the one-dimensional shock tube equation.

Cain, Maurice R.↗

NASA NTRS1991

Mechanical properties of a porous mullite material

Modulus of rupture specimens were used to determine crack growth parameters of a porous mullite material. Strength testing was performed in ambient and moist environments. The power law crack growth rate parameters n and 1n B in 50 percent relative humidity were found to be 44.98 and 0.94, respectively. The inert strength, fracture toughness, and elastic modulus were also determined and found to be 19 MPa, 055 MPa(m) exp 1/2, and 11.6 GPa, respectively.

Viens, Michael J.↗

NASA NTRS1990

Atmospheric degradation mechanisms of hydrogen containing chlorofluorocarbons (HCFC) and fluorocarbons (HFC)

The current knowledge of atmospheric degradation of hydrogen containing chlorofluorocarbons (HCFC 22 (CHClF2), HCFC 123 (CHCl2CF3), HCFC 124 (CHClFCF3), HCFC 141b (CFCl2CH3), HCFC 142b (CF2ClCH3)) and fluorocarbons (HFC 125 (CHF2CF3), HFC 134a (CH2FCF3), HFC 152a (CHF2CH3)) is assessed. Except for the initiation reaction by OH radicals, there are virtually no experimental data available concerning the subsequent oxidative breakdown of these molecules. However, from an analogy to the degradation mechanisms of simple alkanes, some useful guidelines as to the expected intermediates and final products can be derived. A noteable exception from this analogy, however, appears for the oxi-radicals. Here, halogen substitution induces new reaction types (C-Cl and C-C bond ruptures) which are unknown to the unsubstituted analogues and which modify the nature of the expected carbonyl products. Based on an evaluation of these processes using estimated bond strength data, the following simplified rules with regards to the chlorine content of the HCFC's may be deduced: (1) HCFC's containing one chlorine atom such as 22 and 142b seem to release their chlorine content essentially instantaneous with the initial attack on the parent by OH radicals, and for HCFC 124, such release is apparently prevented; (2) HCFC's such as 123 and 141b with two chlorine atoms are expected to release only one of these instantaneously; and the second chlorine atom may be stored in potentially long-lived carbonyl compounds such as CF3CClO or CClFO.

Zellner, Reinhard↗

NASA NTRS1991

Redesign of flight space shuttle main engine nozzle G-15 seal area based on the thermal analysis and flow models

The main objective is to understand the present sealing area response to environmental conditions of the supersonic turbulent combustion gases which has led to both stress rupture cracks and hot gas leakage. A review of the existing thermal analysis and flow modeling has been performed of the present design and seal alternatives have been suggested to possibly reduce or eliminate the hot gas flow recirculation problem and the associated seal cracking.

Mcdonald, Gary H.↗

NASA NTRS1986

Helium jet dispersion to atmosphere

On the event of loss of vacuum guard of superinsulated helium dewar, high rate of heat transfer into the tank occurs. The rapid boiling of liquid helium causes the burst disk to rupture at four atmospheres and consequently the helium passes to the atmosphere through vent lines. The gaseous helium forms a vertical buoyant jet as it exits the vent line into a stagnant environment. Characterization of the gaseous jet is achieved by detailed analysis of the axial and radial dependence of the flow parameters.

Khan, Hasna J.↗

NASA NTRS1991

Frequency effects on the stability of a journal bearing for periodic loading

The stability of a journal bearing is numerically predicted when a unidirectional periodic external load is applied. The analysis is performed using a cavitation algorithm, which mimics the Jakobsson-Floberg and Olsson (JFO) theory by accounting for the mass balance through the complete bearing. Hence, the history of the film is taken into consideration. The loading pattern is taken to be sinusoidal and the frequency of the load cycle is varied. The results are compared with the predictions using Reynolds boundary conditions for both film rupture and reformation. With such comparisons, the need for accurately predicting the cavitation regions for complex loading patterns is clearly demonstrated. For a particular frequency of loading, the effects of mass, amplitude of load variation and frequency of journal speed are also investigated. The journal trajectories, transient variations in fluid film forces, net surface velocity and minimum film thickness, and pressure profiles are also presented.

Vijayaraghavan, D.↗

NASA NTRS1991

Long term integrity for space station power systems

A study was made of the High Temperature Design Codes ASME N47, British R5, and the French RCC-MR Rules. It is concluded that all these codes provide a good basis of design for space application. The new British R5 is the most complete since it deals with the problem of defects. The ASME N47 was subjected longer to practical application and scrutiny. A draft code is introduced, and a proposed draft for high temperature design in which attempts were made to identify gaps and improvements is suggested. The design is limited by creep characteristics. In these circumstances, life is strongly affected by the selected value of the factor of safety. The factor of safety of primary loads adopted in the codes is 1.5. Maybe a lower value of 1.25 is permissible for use in space. Long term creep rupture data for HAYNES 188 is deficient and it is suggested that extrapolation methods be investigated.

Leckie, F. A.↗

NASA NTRS1992

Fault tree analysis: NiH2 aerospace cells for LEO mission

The Fault Tree Analysis (FTA) is one of several reliability analyses or assessments applied to battery cells to be utilized in typical Electric Power Subsystems for spacecraft in low Earth orbit missions. FTA is generally the process of reviewing and analytically examining a system or equipment in such a way as to emphasize the lower level fault occurrences which directly or indirectly contribute to the major fault or top level event. This qualitative FTA addresses the potential of occurrence for five specific top level events: hydrogen leakage through either discrete leakage paths or through pressure vessel rupture; and four distinct modes of performance degradation - high charge voltage, suppressed discharge voltage, loss of capacity, and high pressure.

Klein, Glenn C.↗