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At least 1,729 records · Page 96
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Energetic proton and helium fluxes associated with interplanetary shocks and their relation to the solar wind composition

The He/H ratios in particle events associated with interplanetary travelling shocks are investigated. It is found that immediately after the arrival of the shock, the about 40 keV/nucl He/H flux ratio of energetic particles of equal velocity or equal energy per charge is correlated with the He/H density ratio in the solar wind. The ambient proton flux from the parent solar particle event appears to have a significant effect on the spectral shape of the shock-associated particle spectrum above 200 keV. These observations are consistent with a first-order Fermi acceleration of these particles in which the shock-heated solar wind is the principal contributor to the shock-associated particle population.

Tan, L. C.↗
NASA NTRS1986

Characterization of crystalline LARC-TPI powder

LARC-TPI, a linear aromatic polyimide, developed by NASA Langley Research Center in the late 1970's had thermoplastic character, but its melt-flow properties were much lower than those of conventional thermoplastics. An imidized version of the LARC-TPI molding powder was submitted to NASA-Langley for evaluation. The initial broad melt of this material occurs in the 270 C range and reorders to higher melting forms, depending on the physical and thermal treatments. Rheological characterization of this polymer shows it to have a very low melt viscosity, which increases as the melted specie transforms to other crystalline forms with higher melting temperatures.

Burks, Harold D.↗
NASA NTRS1987

Optimal cure cycle design of a resin-fiber composite laminate

Fibers reinforced composites are used in many applications. The composite parts and structures are often manufactured by curing the prepreg or unmolded material. The magnitudes and durations of the cure temperature and the cure pressure applied during the cure process have significant consequences on the performance of the finished product. The goal of this study is to exploit the potential of applying the optimization technique to the cure cycle design. The press molding process of a polyester is used as an example. Various optimization formulations for the cure cycle design are investigated. Recommendations are given for further research in computerizing the cure cycle design.

Hou, Jean W.↗
NASA NTRS1987

Analysis of cracked laminates with holes using the boundary force method

The boundary force method is an indirect boundary element technique for the analysis of composite laminates with holes and cracks which uses the orthotropic elasticity solution for a concentrated horizontal and vertical force applied at a given point in a cracked infinite sheet. The orthotropic formulation of the method is verified by comparison with accepted solutions for a counter-crack specimen that is subjected to uniaxial tension. Parametric studies are conducted for two configurations for which no orthotropic solutions are available: cracks emanating from a circular hole, and a four-hole cracked specimen; both are loaded in uniaxial tension.

Tan, P. W.↗
NASA NTRS1986

A synchronization technique for optical PPM signals

A technique for maintaining synchronization between optical PPM (pulse-position modulation) pulses and a receiver clock by means of a delay-tracking loop is described and analyzed. The tracking loop is driven by a doubly stochastic Poisson process that contains information about the location of the desired slot boundaries. The slot boundaries are subject to slowly varying random delays that are ultimately tracked by the loop. The concept of fractional rms delay error is introduced to quantify the effects of signal and background induced shot noise on the performance of the delay-tracking loop.

Vilnrotter, V. A.↗
NASA NTRS1987

A DSC study on crystalline LaRC TPI powder - A new version with higher initial molecular weight

A new version of crystalline Langley Research Center Thermoplastic Polyimide (LaRC-TPI) imidized powder, which possesses a higher initial molecular weight (MW), has been prepared and characterized. The scheme used for cyclodehydration during the synthesis of this material is described. Evidence of a higher initial MW for the subject LaRC-TPI is supported by both measurements of the inherent viscosity and the DSC thermograms. An initial melting peak at 295 C is noted in the freshly-synthesized sample. The powder can be recrystallized at any elevated temperatures below 340 C, and a single crystalline endothemic peak is always observed after various thermal histories. The heat of fusion for the fresh sample, as represented by the area under the melting peak in the directional scanning calorimeter thermogram, is about 3.9 cal/gm. Comparisons of thermal properties for the subject material are made to those exhibited by the commercial LaRC-TPI powder.

Hou, Tan-Hung↗
NASA NTRS1987

Analysis and interpretation of satellite fragmentation data

The velocity perturbations of the fragments of a satellite can shed valuable information regarding the nature and intensity of the fragmentation. A feasibility study on calculating the velocity perturbations from existing equations was carried out by analyzing 23 major documented fragmentation events. It was found that whereas the calculated values of the radial components of the velocity change were often unusually high, those in the two other orthogonal directions were mostly reasonable. Since the uncertainties in the radial component necessarily translate into uncertainties in the total velocity change, it is suggested that alternative expressions for the radial component of velocity be sought for the purpose of determining the cause of the fragmentation from the total velocity change. The calculated variances in the velocity perturbations in the two directions orthogonal to the radial vector indicate that they have the smallest values for collision induced breakups and the largest values for low-intensity explosion induced breakups. The corresponding variances for high-intensity explosion induced breakups generally have values intermediate between those of the two extreme categories. A three-dimensional plot of the variances in the two orthogonal velocity perturbations and the plane change angle shows a clear separation between the three major types of breakups. This information is used to reclassify a number of satellite fragmentation events of unknown category.

Tan, Arjun↗
NASA NTRS1988

Model of Jovian F region ionosphere

To date, seven electron density profiles of the Jovian ionosphere have been furnished by the radio occultation experiments aboard the Pioneer and Voyager space probes. The data correspond to various localities (latitudes and longitudes) and times (dawn and dusk) and phases of sunspot cycle (high and low). This renders comparative studies difficult. Nevertheless, the possibility of existence of diurnal variation, equatorial anomaly, and auroral particle precipitation in the Jovian ionosphere have been put forth. The grand magnitude and depth of the equatorial anomaly, in particular, is a matter of great interest and speculation. Correct interpretations of the data and the physical processes in the complex Jovian atmospheric environment will remain a major task for the Aeronomer for decades to come. Model studies of a Jovian ionosphere created by solar EUV radiation and subjected to model ExB drifts showed that equatorial anomaly similar to that in the terrestrial ionosphere can indeed be produced in the Jovian ionosphere. However, owing to the difference in size and rotation period of the two planets and the ionic compositions, much larger drift velocities are required to produce a comparable anomaly in the Jovian atmosphere.

Tan, A.↗
NASA NTRS1988

An improved boundary force method for analyzing cracked anisotropic materials

The Boundary Force Method (BFM), a form of indirect boundary element method, is used to analyze composite laminates with cracks. The BFM uses the orthotropic elasticity solution for a concentrated horizontal and vertical force and a moment applied at a point in a cracked, infinite sheet as the fundamental solution. The necessary stress functions for this fundamental solution were formulated using the complex variable theory of orthotropic elasticity. The current method is an improvement over a previous method using only forces and no moment. The improved method was verified by comparing it to accepted solutions for a finite-width, center-crack specimen subjected to uniaxial tension. Four graphite/epoxy laminates were used: (0 + or - 45/90)sub s, (0), (+ or - 45)sub s, and (+ or - 30)sub s. The BFM results agreed well with accepted solutions. Convergence studies showed that with the addition of the moment in the fundamental solution, the number of boundary elements required for a converged solution was significantly reduced. Parametric studies were done for two configurations for which no orthotropic solutions are currently available; a single edge crack and an inclined single edge crack.

Tan, Paul W.↗
NASA NTRS1987

Possible role of brain stem respiratory neurons in mediating vomiting during space motion sickness

The object of this study was to determine if brain stem expiratory neurons control abdominal muscle activity during vomiting. The activity of 27 ventral respiratory group expiratory neurons, which are known to be of primary importance for control of abdominal muscle activity during respiration, was recorded. It is concluded that abdominal muscle activity during vomiting must be controlled not only by some brain stem expiratory neurons but also by other input(s).

Miller, A. D.↗
NASA NTRS1986

Reduced modeling of flexible structures for decentralized control

Based upon the modified finite element-transfer matrix method, this paper presents a technique for reduced modeling of flexible structures for decentralized control. The modeling decisions are carried out at (finite-) element level, and are dictated by control objectives. A simply supported beam with two sets of actuators and sensors (linear force actuator and linear position and velocity sensors) is considered for illustration. In this case, it is conjectured that the decentrally controlled closed loop system is guaranteed to be at least marginally stable.

Yousuff, A.↗
NASA NTRS1987

Expiratory muscle control during vomiting - Role of brain stem expiratory neurons

The neural mechanisms controlling the muscles involved during vomiting were examined using decerebrated cats. In one experiment, the activity of the ventral respiratory group (VRG) expiratory (E) neurons was recorded during induced 'fictive vomiting' (i.e., a series of bursts of coactivation of abdominal and phrenic nerves that would be expected to produce expulsion in unparalyzed animals) and vomiting. In a second, abdominal muscle electromyographic and nerve activity were compared before and after sectioning the axons of descending VRG E neurons as they cross the midline between C1 and the obex (the procedure that is known to abolish expiratory modulation of internal intercostal muscle activity). The results of the study indicate that the abdominal muscles are controlled differently during respiration and vomiting.

Miller, A. D.↗
NASA NTRS1987

Control of abdominal and expiratory intercostal muscle activity during vomiting - Role of ventral respiratory group expiratory neurons

The role of ventral respiratory group (VRG) expiratory (E) neurons in the control of abdominal and internal intercostal muscle activity during vomiting was investigated in cats. Two series of experiments were performed: in one, the activity of VRG E neurons was recorded during fictive vomiting in cats that were decerebrated, paralyzed, and artificially ventilated; in the second, the abdominal muscle activity during vomiting was compared before and after sectioning the axons of descending VRG E neurons in decerebrate spontaneously breathing cats. The results show that about two-thirds of VRG E neurons that project at least as far caudally as the lower thoracic cord contribute to internal intercostal muscle activity during vomiting. The remaining VRG E neurons contribute to abdominal muscle activation. As shown by severing the axons of the VRG E neurons, other, as yet unidenified, inputs (either descending from the brain stem or arising from spinal reflexes) can also produce abdominal muscle activation.

Miller, Alan D.↗
NASA NTRS1986

Innervation of periesophageal region of cat's diaphragm - Implication for studies of control of vomiting

The extent of the region of the diaphragm around the esophagus that displays greatly reduced activity during the expulsive phase of vomiting was determined from electromyographic studies in cats to be about 0.75-1.0 cm from the esophagus. Horseradish peroxidase injected into this region retrogradely labeled motoneurons throughout most of the rostral-caudal extent of the phrenic nucleus, with the exception of caudal C6 and rostral C7. This widespread intermingling of motoneurons that innervate the region of reduced activity with other phrenic motoneurons creates a difficulty for needed follow-up studies of diaphragmatic control during vomiting.

Tan, L. K.↗
NASA NTRS1987

Excimer laser system for atmospheric remote sensing of ozone

A high-power narrow-linewidth XeCl excimer laser system developed for use by NASA in the remote sensing of atmospheric ozone is described. The laser system is designed for incorporation in a DIAL lidar utilizing stimulated Raman generation for the reference wavelength and sophisticated data averaging techniques. The laser output has a linewidth of 0.002 nm and a beam divergence of 0.15 mrad (FWHM). The laser was operated over a six-hour period with a constant average power of 18 W and a wavelength stable to within + or - 0.0006 nm.

Tan, K. O.↗
NASA NTRS1988

A virtual crack-closure technique for calculating stress intensity factors for cracked three dimensional bodies

A three-dimensional virtual crack-closure technique is presented which calculates the strain energy release rates and the stress intensity factors using only nodal forces and displacements from a standard finite element analysis. The technique is an extension of the Rybicki-Kanninen (1977) method, and it assumes that any continuous function can be approximated by a finite number of straight line segments. Results obtained by the method for surface cracked plates with and without notches agree favorably with previous results.

Shivakumar, K. N.↗
NASA NTRS1988

Downstream energetic proton and alpha particles during quasi-parallel interplanetary shock events

This paper considers the energetic particle populations in the downstream region of three quasi-parallel interplanetary shock events, which was explored using the ISEE 3 Ultra Low Energy Charge Analyzer sensor, which unambiguously identifies protons and alpha particles using the electrostatic deflection versus residual energy technique. The downstream particles were found to exhibit anisotropies due largely to convection in the solar wind. The spectral indices of the proton and the alpha-particle distribution functions were found to be remarkably constant during the downstream period, being generally insensitive to changes in particle flux levels, magnetic field direction, and solar wind densities. In two of the three events, the proton and the alpha spectra were the same throughout the entire downstream period, supporting the prediction of diffusive shock acceleration theory.

Tan, L. C.↗
NASA NTRS1988

Boundary force method for analyzing two-dimensional cracked plates

The Boundary Force Method (BFM) was formulated for the two-dimensional stress analysis of complex crack configurations. In this method, only the boundaries of the region of interest are modeled. The boundaries are divided into a finite number of straight-line segments, and at the center of each segment, concentrated forces and a moment are applied. This set of unknown forces and moments is calculated to satisfy the prescribed boundary conditions of the problem. The elasticity solution for the stress distribution due to concentrated forces and a moment applied at an arbitrary point in a cracked infinite plate are used as the fundamental solution. Thus, the crack need not be modeled as part of the boundary. The formulation of the BFM is described and the accuracy of the method is established by analyzing several crack configurations for which accepted stress-intensity factor solutions are known. The crack configurations investigated include mode I and mixed mode (mode I and II) problems. The results obtained are, in general, within + or - 0.5 percent of accurate numerical solutions. The versatility of the method is demonstrated through the analysis of complex crack configurations for which limited or no solutions are known.

Tan, Paul W.↗