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Showing papers by "Vikram Sarabhai Space Centre published in 2001"


Book ChapterDOI
TL;DR: Cyanate esters (CEs) form a family of new generation thermosetting resins whose performance characteristics make them attractive competitors to many current commercial polymer materials for such applications.
Abstract: The search for advanced, high performance, high temperature resistant polymers is on the rise in view of the growing demand for polymer matrix composites that are to meet stringent functional requirements for use in the rapidly evolving high-tech area of aerospace. Cyanate esters (CEs) form a family of new generation thermosetting resins whose performance characteristics make them attractive competitors to many current commercial polymer materials for such applications. The chemistry and technology of CEs are relatively new and continue to evolve and enthuse researchers. The CEs are gifted with many attractive physical, electrical, thermal, and processing properties required of an ideal matrix resin. These properties are further tunable through backbone structure and by blending with other polymer systems. The structure-property correlation is quite well established. Several new monomers have been reported while some are commercially available. The synthesis of new monomers has come to a stage of stagnation and the present attention is on evolving new formulations and processing techniques. The blends with epoxy and bismaleimide have attracted a lot of research attention and achieved commercial success. While the latter is now known to form an IPN, the reaction mechanism with epoxy is still intriguing. Extensive research in blending with conventional and high performance thermoplastics has led to the generation of key information on morphological features and toughening mechanisms, to the extent that even simulation of morphology and property has now become possible. Despite the fact that the resin and its technology are nearly two decades old, the fundamental aspects related to curing, cure kinetics, reaction modeling, etc. still evince immense research interest and new hypotheses continue to emerge.

228 citations


Journal ArticleDOI
TL;DR: In this paper, the authors used a ten-channel Quartz Crystal Microbalance (qcm) Impactor for the period October 1998-December 1999 at the tropical coastal station Trivandrum (8.5°N, 77°E).

114 citations


Journal ArticleDOI
TL;DR: In this paper, the thermal degradation of natural rubber/polystyrene interpenetrating polymer networks (IPNs) was studied and the effects of blend ratio, crosslink level and initiating system were analyzed.

106 citations


Journal ArticleDOI
TL;DR: In this paper, the intralaminar mode I fracture toughness values for a M55J/M18 carbon/epoxy cross-ply laminate [0°/90°]15 (alternate 0 and 90° layers) and its constituent sub-laminates are theoretically evaluated on the basis of a modified crackclosure integral (MCCI) method corresponding to the fracture loads obtained by testing C(T) specimens.

84 citations


Journal ArticleDOI
TL;DR: The thermal and pyrolysis characteristics of four different types of addition-cure phenolic resins were compared as a function of their structure in this article, and it was shown that the maleimide-functional resins exhibited lowest thermal stability and those crosslinked via ethynyl phenyl azo groups were the most thermally stable systems.

78 citations


Journal ArticleDOI
TL;DR: An efficient shear-flexible three-noded curved beam element is proposed in this paper, where the shear flexibility is based on Timoshenko beam theory and the element has three degrees of freedom, viz., tangential displacement (u), radial displacement (w) and the section-rotation (θ).
Abstract: An efficient shear-flexible three-noded curved beam element is proposed herein. The shear flexibility is based on Timoshenko beam theory and the element has three degrees of freedom, viz., tangential displacement (u), radial displacement (w) and the section-rotation (θ). A quartic polynomial interpolation for flexural rotation ψ is assumed a priori. Making use of the physical composition of θ in terms of ψ and u, a novel way of deriving the polynomial interpolations for u and w is presented, by solving force-moment and moment-shear equilibrium equations simultaneously. The field interpolation for θ is then constructed from that of ψ and u. The procedure leads to high-order polynomial field interpolations which share some of the generalized degrees of freedom, by means of coefficients involving material and geometric properties of the element. When applied to a straight Euler–Bernoulli beam, all the coupled coefficients vanish and the formulation reduces to classical quintic-in-w and quadratic-in-u element, with u, w, and ∂w/∂x as degrees of freedom. The element is totally devoid of membrane and shear locking phenomena. The formulation presents an efficient utilization of the nine generalized degrees of freedom available for the polynomial interpolation of field variables for a three-noded curved beam element. Numerical examples on static and free vibration analyses demonstrate the efficacy and locking-free property of the element. Copyright © 2001 John Wiley & Sons, Ltd.

64 citations


Journal ArticleDOI
TL;DR: In this paper, Tris(2-allylphenoxy)triphenoxycyclotriphosphazene was reacted with three bismaleimides (BMI) via the Alder-ene reaction.

63 citations


Journal ArticleDOI
TL;DR: In this article, an addition-curable novolac resin (ABPF) was synthesized by the reaction of diallyl bisphenol A with formaldehyde using p-toluene sulfonic acid as the catalyst.
Abstract: A novel, addition-curable novolac resin (ABPF) was synthesized by the reaction of diallyl bisphenol A with formaldehyde using p-toluene sulfonic acid as the catalyst. The synthesis conditions were optimized to obtain soluble polymer of desirable molecular weight distribution which was characterized by FT-IR, NMR and SEC. ABPF was reactively blended with bisphenol A bismaleimide (BMIP) and cured through an Alder-ene reaction at high temperatures. The cure characteristics of BMIP–ABPF blend with a maleimide:allyl phenol stoichiometry of 1:1 were studied using FT-IR, DSC and DMA, which evidenced the multi-step cure reactions taking place in the system. Cure optimization was evaluated by DSC, DMA and adhesive property tests. The moderately crosslinked blend was conducive for achieving the optimum adhesive properties on aluminium substrates. Retention of the adhesive properties was greater than 100% at 150 °C. © 2001 Society of Chemical Industry

62 citations


Journal ArticleDOI
TL;DR: In this paper, the thermal behavior of NBR/poly(ethylene-co-vinyl acetate) (EVA) blends was studied by thermogravimetry, and the effects of blend ratio, different crosslinking systems (sulphur, peroxide and mixed), various fillers (silica, clay and carbon black) and filler loading on the thermal properties were evaluated.
Abstract: The thermal behaviour of nitrile rubber (NBR)/poly(ethylene-co-vinyl acetate) (EVA) blends was studied by thermogravimetry. The effects of blend ratio, different crosslinking systems (sulphur, peroxide and mixed), various fillers (silica, clay and carbon black) and filler loading on the thermal properties were evaluated. It was found that the initial decomposition temperature increased with the addition of NBR to EVA. Among the various crosslinking systems studied, the peroxide cured system showed the highest initial decomposition temperature. This is associated with the high bond dissociation energy of C–C linkages. The addition of fillers improved the thermal stability of the blend. The mass loss at different temperatures and activation energy of degradation were also studied. The thermal ageing of these blends was carried out at 50 and 100°C for 72 h. It was seen that the properties are not affected by the mild ageing condition. Also, the peroxide cured system was found to exhibit better retention in properties, than other crosslinking systems.

43 citations


Journal ArticleDOI
TL;DR: Rheo-kinetic studies on bulk polymerization reaction between hydroxyl-terminated polybutadiene (HTPB) and di-isocyanates such as toluene-di-iso-yanate (TDI), hexamethylene-dimethylene (HMDI), and isophorone-diisocynate (IPDI) were undertaken by following the buildup of viscosity of the reaction mixture during the cure reaction as discussed by the authors.
Abstract: Rheo-kinetic studies on bulk polymerization reaction between hydroxyl-terminated polybutadiene (HTPB) and di-isocyanates such as toluene-di-isocyanate (TDI), hexamethylene-di-isocyanate (HMDI), and isophorone-di-isocyanate (IPDI) were undertaken by following the buildup of viscosity of the reaction mixture during the cure reaction. Rheo-kinetic plots were obtained by plotting ln (viscosity) vs. time. The cure reaction was found to proceed in two stages with TDI and IPDI, and in a single stage with HMDI. The rate constants for the two stages k1 and k2 were determined from the rheo-kinetic plots. The rate constants in both the stages were found to increase with catalyst concentration and decrease with NCO/OH equivalent ratio (r-value). The ratio between the rate constants, k1/k2 also increased with catalyst concentration and r-value. The extent of cure reaction at the point of stage separation (xi) increased with catalyst concentration and r-value. Increase in temperature caused merger of stages. Arrhenus parameters for the uncatalyzed HTPB-isocyanate reactions were evaluated. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1869–1876, 2001

38 citations


Journal ArticleDOI
TL;DR: In this paper, the Williamson reaction of a novolac with propargyl bromide was used to synthesize phenolic resins, bearing varying extents of prop argyl ether groups (PN resins).
Abstract: Thermosetting phenolic resins, bearing varying extents of propargyl ether groups (PN resins), have been synthesized by the Williamson reaction of a novolac with propargyl bromide and the products characterized. The resin precursors were cured through Claisen rearrangement of the propargyl ether groups and thermal polymerization. The activation energy for thermal cure is substantially lower than that of model bispropargyl ether compounds but is quite independent of the degree of functionalization. The isothermal cure profile, extrapolated from non-isothermal DSC kinetics studies is consistent with the results from DMA studies. The mechanical properties of glass composites of the resins, of varying propargyl contents, reveal good consolidation of the interphases, evident from the initial gain in both interlaminar shear strength (ILSS) and flexural strength. The benefits of better resin–reinforcement interactions are not retained on crosslinking the resin further, wherein the composite fails by a combination of fibre debonding and brittle fracture of the matrix. Although the resins show better thermal stability than cured resoles, a higher extent of propargylation is detrimental for the thermal stability. Resins with moderate propargylation show good mechanical and thermal properties. © 2001 Society of Chemical Industry

Journal ArticleDOI
TL;DR: In this article, the effect of wheel speed on thickness and microhardness of RS ribbons was investigated using melt spinning technique and it was shown that the micro hardness of the ribbons increases with the heat treatment temperature upto 200°C and thereafter it starts decreasing.
Abstract: Applications of magnesium alloys in the aerospace industry are limited because of their poor mechanical properties, corrosion resistance and workability. Refinement of microstructure through rapid solidification processing is one of the highly potential approaches to overcome these limitations. In the present study, the technology of processing of rapidly solidified (RS) ribbons in Mg-9%Al-1%Zn-0.2%Mn alloy has been established using melt spinning technique. The effect of wheel speed on thickness and microhardness of the ribbons is presented. Microhardness is found to increase with the wheel speed. It is further observed that microhardness of the ribbons increases with the heat treatment temperature upto 200°C and thereafter it starts decreasing. Precipitation of the intermetallic phase Mg17Al12, at temperatures upto 200°C is found to prevent the grain growth and improve the properties of the ribbons. This in-turn reveals that the temperature for secondary processing of RS ribbons in Mg-9%Al-1%Zn-0.2%Mn alloy should not exceed 200°C in order to retain the benefits of rapid solidification processing.

Journal ArticleDOI
TL;DR: In this article, a two-stage copolymerization of phenol with N-(4-hydroxyphenyl)maleimide (HPM) and formaldehyde in the presence of an acid catalyst is presented.
Abstract: Phenolic resins bearing varying concentrations of phenyl maleimide functions were synthesized by copolymerizing phenol with N-(4-hydroxyphenyl)maleimide (HPM) and formaldehyde in the presence of an acid catalyst. The resins underwent a two-stage curing, through condensation of methylol groups and addition polymerization of maleimide groups. The cure characterization of the resin by dynamic mechanical analysis confirmed the two-stage cure and the dominance of maleimide polymerization over methylol condensation in the network buildup process. The kinetics of both cure reactions, studied by the Rogers method, substantiated the earlier proposed cure mechanism for each stage. Although the initial decomposition temperature of the cured resin was not significantly improved, enhancing the crosslink density through HPM improved thermal stability of the material in a higher temperature regime. The anaerobic char yield also increased proportional to the maleimide content. Isothermal pyrolysis and analysis of the char confirmed that pyrolysis occurs by loss of hydrocarbon and nitrogenous products. The resins serve as effective matrices in silica- and glass fabric–reinforced composites whose mechanical properties are optimum for moderately crosslinked resins, in which failure occurs through a combination of fiber debonding and resin fracture. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1664–1674, 2001

Journal ArticleDOI
TL;DR: In this article, Turbulent kinetic energy dissipation rates (e) and eddy diffusion coefficients (Kz) in the tropical mesosphere over Gadanki (13.5° N, 79.2° E), estimated from Doppler widths of MST radar echoes (vertical beam), observed over a 3-year period, show a seasonal variation with a dominant summer maximum.
Abstract: . Turbulent kinetic energy dissipation rates (e) and eddy diffusion coefficients (Kz) in the tropical mesosphere over Gadanki (13.5° N, 79.2° E), estimated from Doppler widths of MST radar echoes (vertical beam), observed over a 3-year period, show a seasonal variation with a dominant summer maximum. The observed seasonal variation of e and Kz in the mesosphere is only partially consistent with that of gravity wave activity inferred from mesospheric winds and temperatures measured by rockets for a period of 9 years at Trivandrum (8.5° N, 77° E) (which shows two equinox and one summer maxima) lying close to Gadanki. The summer maximum of mesospheric e and Kz values appears to be related to the enhanced gravity wave activity over the low-latitude Indian subcontinent during the southwest monsoon period (June – September). Both e and Kz in the mesosphere over Gadanki show an increase with an increase in height during all seasons. The absolute values of observed e and Kz in the mesosphere (above ~80 km) does not show significant differences from those reported for high latitudes. Comparison of observed Kz values during the winter above Gadanki with those over Arecibo (18.5° N, 66° W) shows that they are not significantly different from each other above the ~80 km altitude. Key words. Meteorology and atmospheric dynamics (middle atmosphere dynamics; tropical meteorology; wave and tides)

Journal ArticleDOI
TL;DR: In this article, new membranes based on styrene-butadiene rubber (SBR), natural rubber (NR) and SBR/NR blends were prepared for gas permeation studies.

Journal ArticleDOI
TL;DR: In this paper, a maleimide-functional phenolic resin was reactively blended with an allyl functional novolac in varying proportions, leading to enhanced toughness and improved resin-dominant mechanical properties of the resultant silica laminate composites.
Abstract: A maleimide-functional phenolic resin was reactively blended with an allyl-functional novolac in varying proportions. The two polymers were coreacted by an addition mechanism through Alder-ene and Wagner–Jauregg reactions to form a crosslinked network system. The cure characterization was done by differential scanning calorimetry and dynamic mechanical analysis. The system underwent a multistep curing process over a temperature range of 110–270°C. Although the cure profiles were independent of the composition, the presence of maleimide led to a reduced isothermal gel time of the blend. Increasing the allylphenol content decreased the crosslinking in the cured matrix, leading to enhanced toughness and improved resin-dominant mechanical properties of the resultant silica laminate composites. Changing the reinforcement from silica to glass resulted in further amelioration of the resin-reinforcement interaction, but the resin-dominant properties of the composite remained unaltered. Increasing the maleimide content resulted in enhanced thermal stability. Integrating both the reactive groups in a single polymer and its curing led to enhanced thermal stability and Tg, but to decreased mechanical properties of the laminate composites. This can be attributed to a brittle matrix resulting from enhanced crosslinking facilitated by interaction of the reactive groups located on the polymer of an identical backbone structure. The cured polymers showed a Tg in the range of 170–190°C. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 737–749, 2001

Journal ArticleDOI
TL;DR: Phenyl ethynyl functional addition curable phenolic resins were synthesized by reacting a mixture of phenol and 3-(phenylethynyl) phenol (PEP) with formaldehyde in presence of an acid catalyst as discussed by the authors.
Abstract: Phenyl ethynyl functional addition curable phenolic resins were synthesised by reacting a mixture of phenol and 3-(phenylethynyl) phenol (PEP) with formaldehyde in presence of an acid catalyst. Relatively narrow molar-mass distributed polymers were obtained in good yield. The presence of PEP led to reduced molar-mass and narrow distribution of the copolymers. The resin underwent thermal curing at around 250–275°C, much lower than the cure temperature of PEP. The thermal stability and anaerobic char residue of the cured resins increased with increase in ethynyl-content and these properties were more than those of resol resin. These addition cure phenolics provided an overall increase in char of about 70% vis-a-vis resol resin when compared on the basis of uncured resins.

Journal ArticleDOI
TL;DR: In this article, the authors used a bistatic CW lidar at Trivandrum for the last one decade to investigate the role of ABL micro-meteorological processes in controlling the altitude distribution and size spectrum.
Abstract: Characteristics of aerosols in the Atmospheric Boundary Layer (ABL) obtained from a bistatic CW lidar at Trivandrum for the last one decade are used to investigate the role of ABL micro-meteorological processes in controlling the altitude distribution and size spectrum. The altitude structure of number density shows three distinct zones depending on the prevailing boundary layer feature; viz, the well-mixed region, entertainment region and upper mixing region. In the lower altitudes vertical mixing is very strong (the well-mixed region) the upper limit of which is defined as aerosol-mixing height, is closely associated with the low level inversion. The aerosol mixing height generally lies in the range 150 to 400 m showing a strong dependence on the vertical eddy mixing processes in ABL. Above this altitude, the number density decreases almost exponentially with increase in altitude with a scale height of 0.5–1.5 km. The aerosol mixing height is closely associated with the height of the Thermal Internal Boundary Layer (TIBL). Sea-spray aerosols generated as a result of the interaction of surface wind with sea surface forms an important component of mixing region aerosols at this location. This component shows a non-linear dependence on wind speed. On an average, depending on the season, the mixing region contributes about 10–30% of the columnar aerosol optical depth (AOD) at 0.5Μm wavelength. A long term increasing trend (∼ 2.8% per year) is observed in mixing region AOD from 1989 to 1997. A study on the development of the aerosols in the nocturnal mixing region shows that the convectively driven daytime altitude structure continues to persist for about 4–5 hrs. after the sunset and thereafter the altitude structure is governed by vertical structure of horizontal wind. Stratified aerosol layers associated with stratified turbulence is very common during the late night hours.

Journal ArticleDOI
TL;DR: In this paper, a phenolic resins bearing methylol and phenyl ethynyl functions and curing by both condensation and addition mechanisms were synthesized by the reaction of 3-(phenyl eynyl) phenol (PEP) with formaldehyde under alkaline conditions.
Abstract: Novel phenolic resins bearing methylol and phenyl ethynyl functions and curing by both condensation and addition mechanisms were synthesized by the reaction of 3-(phenyl ethynyl) phenol (PEP) with formaldehyde under alkaline conditions. Resins with varying relative concentration of the two functional groups were synthesized and characterized. The resins underwent a two-stage cure, confirmed by both DSC and DMA analyses. The low-temperature cure due to methylol condensation led to early gelation of the system. The ultimate curing through addition reaction of phenylethynyl group required heating at 275°C. The cured resins exhibited better thermal stability and anaerobic char yield in comparison to a conventional resole. The thermal stability and char-yielding property showed a diminishing trend with enhanced methylol substitution. Resin with F/P ratio less than unity offered excellent thermal stability and anaerobic char yield. The thermal degradation of the cured resins occurred in two kinetic steps. Methylene groups favored the initial degradation, whereas the higher temperature carbonization process was independent of the network structure. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 3371–3377, 2001

Journal ArticleDOI
TL;DR: In this paper, the imido phenolic triazines exhibited inferior thermal properties in comparison to a conventional phenolic-triazine resin, and the presence of triazine crosslinks was found to disfavour the char forming reaction of the otherwise high-char yielding maleimide-novolac resins.

Journal ArticleDOI
TL;DR: In this article, Rapid radio soundings were made over Ahmedabad, a low latitude station during the period 16-20 November 1998 to study the sporadic-E layer associated with the Leonid shower activity using the KEL Aerospace digital ionosonde.
Abstract: . Rapid radio soundings were made over Ahmedabad, a low latitude station during the period 16–20 November 1998 to study the sporadic-E layer associated with the Leonid shower activity using the KEL Aerospace digital ionosonde. Hourly ionograms for the period 11 November to 24 November were also examined during the years from 1994 to 1998. A distinct increase in sporadic-E layer occurrence is noticed on 17, 18 and 19 November from 1996 to 1998. The diurnal variations of f0Es and fbEs also show significantly enhanced values for the morning hours of 18 and 19 November 1998. The ionograms clearly show strong sporadic-E reflections at times of peak shower activity with multiple traces in the altitude range of 100–140 km in few ionograms. Sporadic-E layers with multiple structures in altitude are also seen in some of the ionograms (quarter hourly) at Thumba, situated near the magnetic equator. Few of ionograms recorded at Kodaikanal, another equatorial station, also show sporadic- E reflections in spite of the transmitter power being significantly lower. These new results highlighting the effect of intense meteor showers in the equatorial and low latitude E-region are presented. Key words. Ionosphere (equatorial ionosphere) – Radio science (ionospheric physics)

Journal ArticleDOI
TL;DR: In this article, a comparison of the degradation rate of polystyrene peroxide (PSP) initiated photopolymerization of vinyl monomers has been carried out using benzoyl peroxide or di-tert-butyl peroxide as a reference photoinitiator with radiations of wavelength P 300 nm.

Journal ArticleDOI
TL;DR: The mechanism of adhesive bonding of rubber to metal using an interlayer of bonding agent (adhesive) is discussed with respect to various physical and chemical events such as adsorption at the metal surface, chemical crosslinking within the adhesive, interdiffusion, and formation of interpenetrating networks at the rubber-adhesive interface as mentioned in this paper.
Abstract: The mechanism of adhesive bonding of rubber to metal using an interlayer of bonding agent (adhesive) is discussed with respect to various physical and chemical events such as adsorption at the metal surface, chemical crosslinking within the adhesive, interdiffusion, and formation of interpenetrating networks at the rubber–adhesive interface. An investigation on the peel strength of a natural rubber (NR)–adhesive–metal joint, made by vulcanization bonding using nitrile–phenolic adhesive containing various concentrations of toluene diisocyanate–nitrosophenol (TDI–NOP) adduct, is presented. A single-coat adhesive, consisting of a p-cresol phenol formaldehyde resin, nitrile rubber (NBR), and vulcanizing agents in methyl ethyl ketone solvent, was selected for the study. Considerable improvement in the peel strength was obtained by the incorporation of TDI–NOP adduct into the nitrile–phenolic adhesive. The peel strength increases as the concentration of TDI–NOP adduct in the adhesive composition increases, then levels off with a transition from interfacial failure to cohesive tearing of rubber. The peel strength improvement is believed to be attributed to the interfacial reactions between the bonding agent and natural rubber, when TDI–NOP adduct is incorporated. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2597–2608, 2001

Journal ArticleDOI
TL;DR: In this article, the vertical flux of the horizontal momentum associated with the equatorial Kelvin and Rossby-gravity waves are estimated from the winds measured by the Indian MST radar located at Gadanki (13.5° N, 79.2° E) during September 1995 to August 1996 in the tropospheric and lower stratospheric regions for all four seasons.
Abstract: . The vertical flux of the horizontal momentum associated with the equatorial Kelvin and Rossby-gravity waves are estimated from the winds measured by the Indian MST radar located at Gadanki (13.5° N, 79.2° E) during September 1995 to August 1996 in the tropospheric and lower stratospheric regions for all four seasons. The present study shows that momentum flux values are greater during equinox seasons than solstices, with values near the tropopause level being 16 × 10-3, 7.4 × 10-3, 27 × 10-3 and 5.5 × 10-3 m2 s-2 for Kelvin waves and 5.5 × 10-3, 3.5 × 10-3, 6.7 × 10-3 and 2.1 × 10-3 m2 s-2 for RG waves during autumnal equinox, winter, vernal equinox and summer seasons, respectively. Using these momentum flux values near the tropopause level, acceleration of the mean flow in the stratosphere up to a 29 km height were computed following Plumb (1984), by considering the wave-meanflow interaction and the deposition of the momentum through the radiative dissipation of the waves. A comparison of the estimated mean-flow acceleration in the stratosphere compares well, except at a few height levels, with the observed mean-flow accelerations in the stratosphere derived from the radiosonde data from a nearby station. Key words. Meteorology and atmosphenic dynamics (tropical meteorology; waves and tides)

Journal ArticleDOI
TL;DR: In this article, an attempt is made to simulate the diurnal tidal amplitudes and phases in the lower atmosphere over Gadanki using classical tidal theory by incorporating diurnal heat sources, namely, solar radiation absorption by water vapour, planetary boundary layer (PBL) heat flux, latent heat release in deep convective clouds and short wave solar radiation absorbing by clouds.
Abstract: . Diurnal tidal components in horizontal winds measured by MST radar in the troposphere and lower stratosphere over a tropical station Gadanki (13.5° N, 79.2° E) are presented for the autumn equinox, winter, vernal equinox and summer seasons. For this purpose radar data obtained over many diurnal cycles from September 1995 to August 1996 are used. The results obtained show that although the seasonal variation of the diurnal tidal amplitudes in zonal and meridional winds is not strong, vertical phase propagation characteristics show significant seasonal variation. An attempt is made to simulate the diurnal tidal amplitudes and phases in the lower atmosphere over Gadanki using classical tidal theory by incorporating diurnal heat sources, namely, solar radiation absorption by water vapour, planetary boundary layer (PBL) heat flux, latent heat release in deep convective clouds and short wave solar radiation absorption by clouds. A comparison of the simulated amplitudes and phases with the observed ones shows that agreement between the two is quite good for the equinox seasons, especially the vertical structure of the phases of the meridional wind components. Key words. Meteorology and atmospheric dynamics (tropical meteorology; waves and tides)

Journal ArticleDOI
TL;DR: In this paper, the Zinc octoate/nonylphenol catalysed thermal cure of bisphenol A dicyanate (BACY) was investigated using non-isothermal differential scanning calorimetry (DSC).
Abstract: The kinetics of the Zinc octoate/nonylphenol catalysed thermal cure of bisphenol A dicyanate (BACY) was investigated using non-isothermal differential scanning calorimetry (DSC). The kinetic parame...

Journal ArticleDOI
TL;DR: The influence of Io on the Jovian magnetosphere and its radio emissions has been studied for more than three decades as discussed by the authors, and the first direct evidence of the IFT footprint was obtained in a near-infrared image of Jupiter's H 3 + emissions at 3.4 μm in 1992.

Journal ArticleDOI
TL;DR: In this article, the authors improved the stress fracture criteria proposed by Whitney-Nuismer known as the point stress criterion and the average stress criterion (ASC) to predict the strength of composite laminates containing holes and slits.
Abstract: This paper improves the stress fracture criteria proposed by Whitney–Nuismer known as the point stress criterion (PSC) and the average stress criterion (ASC), to predict the strength of composite laminates containing holes and slits. A simple relation is used for the characteristic length to improve the accuracy while evaluating the notched strength of laminates. The applicability of the proposed approach is examined by considering the notched strength data on carbon/epoxy and boron/aluminum laminates. The analytical results obtained in the present study correlate well with the test results of different notched configurations. This study confirms that the present modification in the stress fracture criteria can be used for notched strength evaluation of composite laminates.

Journal ArticleDOI
TL;DR: In this article, local burning rates were measured at different radial locations across the web during static tests of center-perforated solid propellant rocket motor grains, showing a higher burning rate in the midweb region and a lower burning rate toward the port and motor case.
Abstract: With the use of the ultrasonic pulse-echo technique, local burningrates aremeasured at different radial locations across the web during static tests of center-perforated solid propellant rocket motor grains. The measurement shows a higher burning rate in the midweb region and a lower burning rate toward the port and motor case. The measured motor pressure history could be very well simulated by using the measured local burning rates. Thus, direct evidence is provided that the variationof the burning rate across the web is the cause of themidweb anomaly in chamber pressure widely reported in solid motors with center-perforated grains.

Journal ArticleDOI
TL;DR: An AISI 304L austenitic stainless steel tank used for chemical storage showed cracks during the post weld qualification programme as mentioned in this paper, which revealed that the cracking was due to a shift in the solidification mode from primary ferrite to primary austenite, and residual stress introduced during rolling and forming of material as well as additional contractional strain during welding under fixtured condition, are additional factors which caused cracking.