Showing papers by "Vikram Sarabhai Space Centre published in 1990"

Room temperature curing of CTBN-toughened epoxy adhesive with elevated temperature service capability

Abstract: The adhesive joint strengths at room temperature (25°C) and at 120°C for an epoxy resin cured at room temperature with tris (N,N-dimethylaminomethyl) phenol and containing varying amounts of a carboxyl-terminated polybutadiene-acrylonitrile copolymer (CTBN) were investigated and reported. It was observed that as CTBN content increased, lap shear strength and T-peel strength increased significantly and passed through a maximum at about 10 parts CTBN content per 100 parts epoxy resin and then decreased. To explain the observed results, the effect of CTBN content on the bulk tensile strength, impact energy, glass transition temperature (ETg), and morphology were also evaluated. SEM observations showed microphase separation between CTBN and epoxy matrix for the composition containing 10 parts CTBN per 100 parts epoxy resin, and this composition exhibited maximum bulk tensile strength and impact energy. The enhancement of bulk mechanical properties, hence the adhesive joint strength, was attributed to the higher toughness produced by the dispersed rubber particles. At higher CTBN content, the rubber phase became continuous, and the system exhibited a fall in mechanical properties, hence in adhesive joint strength, which was attributed to the flexibilization effect. The glass transition temperature of the cured epoxy resin was little influenced by CTBN incorporation up to 10 parts per 100 epoxy resin. Higher levels of CTBN content due to the flexibilization effect, decreased the ETg, hence the elevated temperature capability. The results of this study demonstrate that epoxy resins containing low levels of CTBN, cured at room temperature with tris (N,N-dimethylaminomethyl) phenol can be used to formulate structural adhesives for use in elevated temperature environments.

Polyurethane‐polyacrylamide IPNs. I. Synthesis and characterization

Abstract: Hydrophobic-hydrophilic interpenetrating polymer networks (IPNs) of polyurethane and polyacrylamide have been synthesized. The IPNs have been characterized by IR, thermal, and mechanical studies. SEM studies indicate phase mixing of some IPNs. Solvent resistance and surface hydrophilicity as evidenced by contact angle measurements suggest that these IPNs could be used for biomedical applications.

Thermal degradation of functionally terminated polybutadienes: Pyrolysis gas chromatography and thermogravimetric studies

Abstract: Thermal degradation of functionally terminated liquid polybutadiene polymers, carboxy-terminated polybutadiene (CTPB), hydroxy-terminated polybutadiene (HTPB), and poly(butadieneacrylonitrile-acrylic acid) terpolymer (PBAN) and their cured analogues were studied by pyrolysis gas chromatography in the temperature range 500–850°C. It was found that CTPB, HTPB, and PBAN could be distinguished from one another by the variation in the composition of their pyrolyzates. The differences in the product composition were rationalized by the relative importance of unzipping and hydrogen transfer reactions in the mechanisms of their degradation. The presence of the OH group was found to favor hydrogen transfer reactions whereas the COOH group has only marginal effect. The cyclization reactions were found to predominate in the thermal degradation of cured PBAN and the pyrolysis results were substantiated by thermogravimetric studies at several heating rates. The activation energy for the decomposition of cured PBAN was estimated to be 259 ± 16 kJ mol−1 by Ozawa's method.

GPC studies on the cationic polymerization of cardanol initiated by borontrifluoridediethyletherate

Abstract: The cationic polymerization of cardanol using borontrifluoridediethyletherate as initiator has been investigated by gel permeation chromatographic techniques. The molecular weight and molecular weight distribution of the polymer at different temperatures and various initiator concentrations were studied, and the polymerization conditions have been optimized as 140°C with an initiator concentration of 1%. The reaction was found to follow first-order kinetics with respect to the monomer. The activation energy and rate constants for the system have also been evaluated. The polymer has been characterized by IR, 1H and 13C-NMR spectra.

On the convergence of the Baba and Dorea random optimization methods

Abstract: The Baba and Dorea global minimization methods have been applied to two physical problems. The first one is that of finding the global minimum of the transformer design function of six variables subject to constraints. The second one is the problem of fitting the orbit of a satellite using a set of observations. The latter problem is reduced to that of finding the global minimum of the sum of the squares of the differences between the observed values of the azimuth, elevation, and range at certain intervals of time from the epoch and the computed values of the azimuth, elevation, and range at the same intervals of time. Baba and Dorea established theoretically that the random optimization methods converge to the global minimum with probability one. The numerical experiments carried out for the above two problems show that convergence is very slow for the first problem and is even slower for the second problem. In both cases, it has not been possible to reach the global minimum if the search domains of the variables are wide, even after a very large number of function evaluations.

Metallographic investigations of the heat-affected zone II/parent metal interface cracking in 18Ni maraging steel welded structures

Abstract: During the fabrication of a large diameter pressure vessel out of 18 Ni maraging steel by manual TIG welding, microcracks were noticed at the heat-affected zone (HAZ)/parent metal interface. The location of these cracks was very different from those reported at the fusion zone/HAZ I interface due to “constitutional liquation”. Extensive optical metallography, scanning electron microscopy and energy dispersive X-ray analyses were carried out to identify the cause for the occurrence of these cracks. It is inferred from the experimental results that the microsegregation of titanium and nickel due to repeated thermal cycling during multipass welding led to the formation of TiC/Ti(CN) and stable austenite film on the grain boundaries. Under severe thermal stresses developed during welding, microvoids generated at the interface of TiC/Ti(CN) inclusions and austenite and further propagated intergranularly due to the premature failure of the austenite films.

Periodic orbits of the third kind in the restricted three-body problem with oblateness

Abstract: By use of the secular perturbing potential due to oblateness, the existence of periodic orbits of the third kind is established through Poincare's method of analytic continuation using Delaunay's canonical variables and other three sets of canonical variables which are linear combinations of Delaunay's variables, in the three-dimensional restricted three-body problem when the more massive primary is an oblate spheroid with its equatorial plane coincident with the plane of motion. For two sets of the canonical variables, the singularities are found at the inclinationsi=68 . ° 5833,111 . ° 4167, while for the other two sets of the canonical variables, the singularities are ati=55 . ° 3854, 103 . ° 575.

Magnetospheric substorm‐related electric fields in the ionosphere: Discrepancy of an observation with model predictions

Abstract: Theoretical models of disturbance electric fields in the middle and low latitude ionosphere have successfully explained several observed characteristics of the above electric fields. The models generally predict a large westward electric field in the 2300 – 0400 LT sector; but a recent study of a major F-region disturbance at mid-latitudes revealed a large east-ward electric field in the midnight-to-morning sector, during a magnetospheric substorm. This discrepancy indicates that processes additional to those considered in the models so far are playing an important role during substorms. One possible process is the generation of large, secondary ionospheric electric fields in and around the auroral zone due to the interplay of the spatially varying and time-varying enhanced conductivities, currents and electric fields during substorms. A second process may be the generation of large secondary electric fields in the inner magnetosphere due to the large spatial gradients of particle drifts during the substorm-time unsteady convection. During substorms, such secondary electric fields may be dominant, in comparison with the effects of the large scale magnetospheric convection electric field and field-aligned currents treated successfully in the models.

Concept of an interlaced phased array for beam switching

Abstract: A novel concept is described for feeding and phasing a large linear array of N antenna elements using only three or five feed points and phase shifters and still achieving beam switching. The idea consists of drastically reducing the number of input points by interlacing a small numer of serially fed subarrays which are suitably phased. This so-called interlaced phased array (IPA) concept was tested using an array of 15 four-element Yagi antennas with a spacing equal to 0.8 wavelengths and found feasible. Some of the distinct advantages of the IPA in comparison with a conventional system of beam switching are reduced power loss, reduced phasing errors, reduced cost, increased reliability resulting from greatly reduced number of phase shifters, and better symmetry of off-zenith beams. >

Thermal decomposition of viscose rayon in the presence of inorganic additives. A kinetic study

Abstract: Viscose rayon, like many other cellulosic materials, has a thermal decomposition reaction. The rate of change of weight loss of this material is very high at a narrow range of temperatures. When viscose rayon is impregnated with inorganic additives, there is a shift in the temperature of maximum rate of weight loss (Tmax) towards the lower side of the temperature. This effect, due to the addition of ammonium chloride, calcium chloride, and a mixture of calcium chloride and ammonium chloride, was studied in the temperature range 200–400°C through thermogravimetry and the changes observed in the values of activation energy, order of reaction, and char yield are presented in this paper.

Vertical structure and characteristics of 23-60 day (zonal) oscillations over the tropical latitudes during the winter months of 1986 - Results of equatorial wave campaign-II

Abstract: The equatorial wave campaign-II which formed a part of the Indian Middle Atmosphere Programme (IMAP), was conducted from SHAR (13.7°N, 80.2°E) from 15 January to 28 February 1986. Winds were measured from ground to 60 km by means of high altitude balloon and a meteorological rocket (RH-200), once everyday, for 45 days. The frequencies of the oscillations in the deviations of the east-west component of the winds from its mean at each height with one kilometer interval were obtained by the maximum entropy (ME) method and phases/amplitudes of these frequencies were determined by the least squares technique on the wind variation time series. The ME method has the inherent advantage of providing periodicities up to 1.5 times the data length.

Effect of molecular weight on non-isothermal decomposition kinetics of hydroxyl terminated polybutadiene

Abstract: Hydroxyl terminated polybutadiene prepared by free radical polymerization was fractionated by a solvent-nonsolvent precipitation method. The fractions were characterized by gel permeation chromatography for their molecular weight averages (¯Mn,¯Mw and¯Mz) and dispersities. The kinetic parameters, viz., energy of activationE and preexponential factorA for the thermal decomposition of the fractions were computed from their TG data, using four nonisothermal integral equations. Quantitative correlations between the kinetic constants and the molecular weight parameters were derived for the first time for HTPB as:E (or InA)=k1−k2/¯Mn (or ¯Mw or ¯Mz) and the trend is explained on the basis of the kinetic compensation effect.

A rapid non‐aqueous titrimetric estimation of ammonium perchlorate in pyrotechnic compositions

Abstract: Pour l'analyse, le reactif utilise est l'hydroxyde de tetrabutylammonium et le solvant utilise est le DMF, le DMSO ou la pyridine; l'analyse se fait en utilisant le bleu de thymol comme indicateur ou une methode potentiometrique

Thermal Management of Electronic Packages for Space Applications

Abstract: This paper describes the state of the art for thermal mathematical modeling of electronic packages during transient operation. The methods for calculating thermal contact conductance, view factors, and heat transfer coefficients are reviewed, and an algorithm for computer software is provided. Monte Carlo treatment of the data uncertainties is explained. The computer algorithm uniquely incorporates the subroutines for calculating the thermal contact conductance, absorption factor, and statistical representation of the thermal parameters for Monte Carlo analysis.

Erosion of nozzle throat inserts and silica-phenolic ablative liner morphology

Abstract: A requalificatio n tes t progra m fo r 250-k g thrus t nozzl e throa t insert s (NTI ) fo r th e Augmente d Satellit e Launc h Vehicl e missio n wa s necessitate d du e t o 6 s o f extr a burnin g duratio n compare d t o th e Satellit e Launc h Vehicl e mission . I t ha s bee n observe d tha t th e erosio n o f NT I wa s closel y relate d t o chambe r pressure , cumulativ e burnin g time , an d th e erosio n o f silica-phenolic-base d ablativ e line r morphology . Thi s pape r present s a n analysi s o f th e erosio n result s o f autotranspiration-coole d NT I an d silica-phenoli c ablativ e liner s use d i n th e combustio n chamber.

The effect of hall currents on the magneto hydro dynamic (MHD) boundary layer flow over a flat plate

Abstract: Studies are made on laminar compressible magneto hydro dynamic (MHD) boundary layer flow over a flat-plate with Hall and ionslip currents. Viscous drag, magnetic drag and heat transfer coefficients are determined for the values of interaction parameter, electromagnetic load parameter, Hall parameter and ionslip current parameter. It is found that the overall drag (magnetic plus viscous) coefficient increases and the heat transfer coefficient decreases with interaction parameter.

Stability of tapered cantilever columns subjected to a tip concentrated subtangential follower force

Abstract: A simple and accurate numerical scheme is used for the determination of critical loads of tapered cantilever columns subjected to a tip concentrated subtangential follower force. Three types of variation of taper are considered and the critical loads are presented for various values of the subtangential parameter.

Quality Aspects in Silicon Power Device Manufacturing

Abstract: Quality problems encountered in the manufacture of silicon power devices are investigated with the perspectives of material quality and processes involved. The importance of in-process quality control is illustrated with certain studies on different process steps.