Showing papers by "Vikram Sarabhai Space Centre published in 1996"
TL;DR: In this article, the influence of fiber loading, fiber length, and fiber treatment on the theology of composites was investigated in the temperature range of 125 to 145°C and shear rate of 1.6.4 to 5468 s−1.
168 citations
TL;DR: In this paper, the thermal behavior of pineapple leaf fiber reinforced polyethylene composites was studied by thermogravimetric and dynamic mechanical thermal analysis, and the effects of fibre loading and surface modification on the thermal properties were evaluated.
Abstract: The thermal behaviour of pineapple leaf fibre (PALF) reinforced polyethylene composites was studied by thermogravimetric and dynamic mechanical thermal analysis. Fibre treatment was carried out using isocyanate, silane and peroxide to improve the interfacial adhesion between fibre and matrix. The effects of fibre loading and surface modification on the thermal properties were evaluated. It was found that at high temperature PALF degrades before the polyethylene matrix. The storage modulus increased with increase of fibre loading and decreased with increase of temperature. The treated fibre composites impart better properties compared to untreated system. Tan δ showed a distinct peak at low temperature ascribed to the glass transition temperature of polyethylene but no peak was observed for PALF fibre. The relative damping increased with fibre loading. Cole-Cole analysis was made to understand the phase behaviour of the composite samples.
165 citations
TL;DR: In this paper, the authors used the alpha-model developed by Marsh et al. to model the network properties of copolyurethane systems of varying compositions based on hydroxyterminated polybutadiene and poly(12-hydroxy stearic acid-co-TMP) ester polyol (PEP).
Abstract: The modeling of network properties of copolyurethane systems of varying compositions based on hydroxy-terminated polybutadiene and poly(12-hydroxy stearic acid-co-TMP) ester polyol (PEP), a copolyester polyol derived from 12-hydroxy stearic acid and trimethylol propane, crosslinked by different triols, and cured by various isocyanate compounds, has been carried out by the alpha-model developed by Marsh et al. The network parameters such as crosslink density, effective chain length, sol content, and mechanical properties of these systems have been evaluated. The functionality distribution of PEP has been derived from Si-29 nuclear magnetic resonance spectroscopy of the silyl derivative of PEP and chemical analysis. The extent of cure reaction (p) has been estimated by the use of an empirical relationship of the type: p = a - b(A(3) x F-s), where A(3) is the mole fraction of the trifunctional component of the OH group and F-s is the fraction of secondary hydroxyls in the system. A method for evaluating the constants a and b is presented. The calculated network parameters by the alpha-model are found to be in good agreement with the observed values. Excellent linear correlations between the calculated network parameters and the mechanical properties of the urethane elastomers confirm the utility of the approach presented here in arriving at formulations with desired properties.
63 citations
TL;DR: In this article, an eight-node isoparametric plate finite element with 5 d.f, per node, was developed to investigate the buckling and post-buckling of moderately thick laminated plates.
59 citations
TL;DR: In this paper, the influence of penetrant size on the sorption behavior of SBR has been examined and the thermodynamic constants such as standard entropy, standard enthalpy and first-order kinetic rate constant have been evaluated.
56 citations
TL;DR: From the temporal spectra of vertical wind obtained from MST radar observations, the Brunt-Vaisala frequency was identified and the altitude profile of temperature in troposphere and lower stratosphere was derived.
Abstract: From the temporal spectra of vertical wind obtained from MST radar observations, the Brunt-Vaisala frequency is identified. Altitude profile of temperature in troposphere and lower stratosphere is derived from the altitude profile of the Brunt-Vaisala frequency thus obtained.
49 citations
TL;DR: In this paper, the uniqueness of the solution of the title problem has been examined through a simple mathematical procedure, and the solution has been shown to be unique in a number of applications.
Abstract: The uniqueness of the solution of the title problem has been examined through a simple mathematical procedure.
39 citations
TL;DR: A coupled chemistry-transport model was developed to study the production and loss of neutral atomic carbon and oxygen in the inner coma (≤10 4 km) of Comet Halley.
37 citations
TL;DR: In this paper, a detailed experimental study on flame spread over non-uniform ports of solid propellant rockets has been carried out and it has been shown conclusively that under certain conditions of step location, secondary ignition may occur far downstream of the step.
Abstract: A detailed experimental study on flame spread over non-uniform ports of solid propellant rockets has been carried out. An idealised. 2-dimensional laboratory motor was used for the experimental study with the aid of cinephotography. Freshly prepared rectangular HTPB propellant with backward facing step was used as the specimenfor this study. It has been shown conclusively that under certain conditions of step location. step height and port height which govern the velocity of gases at the step by the partially ignited propellant surface. secondary ignition may occur far downstream of the step. This is very likely to be within the recirculatingflow region. The secondary ignition gives rise to two additional flame fronts one of which spreads backwardat a relatively lower velocity,presumably due to the low reverse velocities present around the separation zone. This phenomenon is likely to play an important role in the ignition transient of solid propellant rockets with non-uniform ports.
33 citations
32 citations
TL;DR: In this paper, the free vibration and stability properties of uniform beams and columns on nonlinear elastic foundations were evaluated using a finite element formulation and numerical results were presented for various end boundary conditions with varying foundation parameters.
TL;DR: In this paper, the methode selon Ozawa amelioree par l'equation d'approximation a deux termes pour la fonction p(x) (d'apres VSSC) a donne des resultats tout a fait comparables a ceux obtenus par la methode Kissinger.
Abstract: Des etudes TG-DSC realisees dans une atmosphere d'air ou d'azote sur des polybutadienes a terminaison hydroxy (HTPB) ont montre trois transitions qui ont entraine (1) un pic DSC exotherme et une augmentation de la masse lors de la TG a 170°C-240°C, seulement dans l'air, (2) un pic exotherme et une faible perte de masse dans l'air ainsi que dans l'azote, causes par la depolarisation, la cyclisation et la reticulation, et (3) une perte de masse due a la pyrolyse dans l'azote ou la combustion dans l'air, ce qui se traduit respectivement par un pic endotherme et un pic exotherme aigu dans les deux atmospheres. L'analyse FTIR des produits isoles lors de la TG a l'issue de l'augmentation de masse revele que de l'oxygene a ete ajoute a la structure butadiene. Les parametres d'activation d'Arrhenius (E et A) ont ete calcules pour l'addition exotherme d'oxygene. La methode selon Ozawa amelioree par l'equation d'approximation a deux termes pour la fonction p(x) (d'apres VSSC) a donne des resultats tout a fait comparables a ceux obtenus par la methode Kissinger.
TL;DR: In this paper, cyclotriphosphazene-containing trihydroxy and tetrahydroxy compounds were synthesized, characterized and examined as crosslinking agents (CLA) in polyurethane systems constituted independently with polyether polyols such as polyethylene glycol (PEG), polyoxytetramethylene (POTMG) and polybutadiene polyol (HTPB) and reacted with tolylene diisocyanate.
TL;DR: In this article, the postweld reaging of AA 2219 weldments in T62 condition enhances both strength and ductility in Indian launch vehicle programs and is the material proposed for the construction of tanks for liquid cryogenic fuels like liquid oxygen and liquid hydrogen.
Abstract: AA 2219 aluminium alloy is the material proposed for the construction of tanks for liquid cryogenic fuels like liquid oxygen and liquid hydrogen, in Indian launch vehicle programmes. To meet the material requirement, ISRO has developed the alloy AA 2219 indigenously on an industrial scale. Process development and structure-property correlation of auto TIG weldments of AA 2219 Al alloy reported in this paper establishes that postweld reaging of AA 2219 weldments in T62 condition enhances both strength and ductility.
TL;DR: In this paper, the authors investigated the time evolution of the perturbations in aerosol spectral optical depths associated with the Mt. Pinatubo volcanic eruption of June 1991 using a ground-based multiwavelength solar radiometer at the tropical station, Trivandrum.
TL;DR: In this paper, the stability characteristics of laminated plates subjected to various types of in-plane loadings were examined, and a rectangular four node finite element, having fourteen degrees of freedom per node, based on a simple higher-order shear deformation theory was developed.
TL;DR: In this article, a developpe une correlation generalisee pour la vitesse de deformation de la contrainte de cisaillement sur la base des parametres rheologiques tels que the contraintes d'ecoulement, l'indice de consistance et l'Indice de pseudoelasticite, which sont respectivement independant of la fraction de volume solide, de la duree de vulcanisation t and de the temperature; le coefficient de correlation avec les valeurs determinees
Abstract: Les propergols solides a base de polybutadienes a terminaison hydroxy (HTPB) sont devenus les precurseurs parmi les propergols dans le monde moderne des moteurs de fusees. En raison de leur taux de remplissage eleve, de tels propergols ont des proprietes rheologiques tres complexes. On a deja etudie et publie des rapports sur l'effet de la substance de remplissage, de la taille des particules de l'oxydant et de la teneur en Al, ainsi que sur l'effet de la temperature, la cinetique du melange et le temps de cycle lors du melange. Dans la presente etude, on a developpe une correlation generalisee pour la vitesse de deformation de la contrainte de cisaillement sur la base des parametres rheologiques tels que la contrainte d'ecoulement, l'indice de consistance et l'indice de pseudoelasticite, qui sont respectivement independants de la fraction de volume solide, de la duree de vulcanisation t et de la temperature; le coefficient de correlation avec les valeurs determinees experimentalement est de 0,94. Un terme communement utilise dans l'industrie des propergols, la duree de conservation, a egalement ete defini et depouille pour le systeme de propergol HTPB.
TL;DR: In this paper, the effect of prevulcanization on the rheological behavior of natural rubber (NR), styrene butadiene rubber (SBR) latices, and their blends was studied with special reference to shear rate, blend ratio, vulcanizing systems, and accelerator systems.
Abstract: The effect of prevulcanization on the rheological behavior of natural rubber (NR), styrene butadiene rubber (SBR) latices, and their blends was studied with special reference to shear rate, blend ratio, vulcanizing systems, prevulcanization time, and accelerator systems. The NR latex showed a sharp increase in viscosity with increase in prevulcanization time due to high extent of crosslinking. However, SBR latex showed marginal effect on viscosity with prevulcanization time due to its low dry rubber content and low degree of unsaturation. Blends showed variations in viscosity according to the change in composition. The use of a single accelerator was found to have marked influence on the viscosity of the blends compared with a combination of accelerators. Swelling experiments were carried out in order to determine the crosslink density of the blends. The viscosity changes have been correlated with the crosslinking density of the latices and their blends. © 1996 John Wiley & Sons, Inc.
TL;DR: In this article, four different grades of hydroxy terminated polybutadiene (HTPB) polymers were studied for their processability characteristics and the activation energies were found to be 20 and 40 kJ mol −1, respectively, for the pre-gel and post-gel stages of the reaction catalysed by DBTDL.
TL;DR: In this article, the vapour dynamics of working fluid is considered in the numerical analysis of the heat pipe, and a two-phase analysis is carried out for the heat pipeline, where the compressible flow equations for vapour-phase interaction with water particle phase are solved by finite volume technique.
Abstract: This paper presents numerical simulation of the physical phenomena in heat pipe. The vapour dynamics of working fluid is considered in the numerical analysis of the heat pipe. A two-phase analysis is carried out for the heat pipe. The compressible flow equations for vapour-phase interaction with water particle phase are solved by a finite volume technique. A three stage Runge-Kutta time-stepping method is employed to solve vapour dynamics. Damping term is added to stablize the numerical scheme. An example is worked out to study the two phase flow in the heat pipe.
TL;DR: In this paper, a semianalytical finite element approach to investigate the asymmetric buckling behavior of tapered orthotropic laminated annular plates is presented and the prebuckling state of stress in the plate utilized to derive the geometric stiffness matrix is obtained analytically using a power series method.
TL;DR: In this paper, a general finite element formulation is employed to evaluate the postbuckling behavior of linearly tapered moderately thick isotropic circular plates subjected to thermal load, and numerical results in terms of linear critical load and thermal load ratios in the post-buckling range for different thicknesses and taper parameter values are presented.
TL;DR: In this paper, the nonlinear boundary layer equations of the title problem are solved numerically for obtaining the coefficient of skin-friction, and it is found that the magnitude of the normal surface velocity gradient increases with the magnetic interaction parameter.
Abstract: The nonlinear boundary layer equations of the title problem are solved numerically for obtaining the coefficient of skin-friction. It is found that the magnitude of the normal surface velocity gradient increases with the magnetic interaction parameter.
TL;DR: In this article, a high strength low alloy steel, having composition similar to AFNOR 15CDV6 steel but with higher carbon content, has been developed indigenously following the Electroslag Refining (ESR) process.
TL;DR: In this article, simultaneous polyurethane-poly(methyl methacrylate) (PU-PMMA) interpenetrating polymer networks (IPNs) were synthesized with the PMMA polymerization initiated at room temperature.
Abstract: Simultaneous polyurethane-poly(methyl methacrylate) (PU-PMMA) interpenetrating polymer networks (IPNs) were synthesized with the PMMA polymerization initiated at room temperature. Transparent IPNs with better miscibility and synergism of mechanical properties were obtained. Dynamic mechanical analysis data indicated that up to 30% PMMA can be incorporated into PU networks without substantial phase separation. The PU-PMMA 90/10 IPNs elicit less than 2% hemolysis, suggesting that these materials could be used as blood contacting materials.
TL;DR: In this paper, the nighttime equatorial F -region zonal electric field E yF has been deduced from h ′ F data at Trivandrum, a magnetic equatorial station in the Indian longitude sector.
TL;DR: In this paper, the buckling load parameters for simply supported and clamped circular plates are obtained for different locations, lengths and extents of axisymmetric damage, and the results are expected to be useful as design data.
TL;DR: In this article, the nonsimilar boundary layer equations of the title problem are solved using a reliable numerical scheme for obtaining the coefficient of skin-friction, and it is found that the normal surface velocity gradient increases with the magnetic interaction parameter.
Abstract: The nonsimilar boundary layer equations of the title problem are solved using a reliable numerical scheme for obtaining the coefficient ofskin-friction. It is found that the normal surface velocity gradient increases with the magnetic interaction parameter.
TL;DR: In this article, a finite element method was used for thermal buckling analysis of simply supported and clamped circular plates with varying locations, magnitudes and lengths of damage, where the damages were characterized as degradation in material properties locally.
TL;DR: In this article, the activation energy and preexponential factor A of the charge have been calculated from TG and DSC curves using different approaches developed by Coats-Redfern and Kissinger.
Abstract: Boron-potassium nitrate pyrotechnic composition can be converted into a hot wire-sensitive initiator formulation by the addition of an extra fuel. viz. lead thiocyanate. The ignition temperature of this composition depends on the percentage of thiocyanate in the mix and follows a binomial fit. The kinetic parameters. viz. activation energy E and pre-exponential factor A of the charge have been calculated from TG and DSC curves using different approaches developed by Coats-Redfern and Kissinger. Ignition delays measured from isothermal TG runs were found to yield equally good values of E and A. A comparison of these values for the tricomponent system' with those of the bicomponent systems as well as of the ingredients suggests that the starting reaction in this formulation is the reaction between lead thiocyanate and potassium nitrate which energises the main reaction between boron and potassium nitrate. leading to ignition.