Showing papers by "Vikram Sarabhai Space Centre published in 2013"

Trends in aerosol optical depth over Indian region: Potential causes and impact indicators

Abstract: The first regional synthesis of long-term (back to similar to 25 years at some stations) primary data (from direct measurement) on aerosol optical depth from the ARFINET (network of aerosol observatories established under the Aerosol Radiative Forcing over India (ARFI) project of Indian Space Research Organization over Indian subcontinent) have revealed a statistically significant increasing trend with a significant seasonal variability. Examining the current values of turbidity coefficients with those reported similar to 50 years ago reveals the phenomenal nature of the increase in aerosol loading. Seasonally, the rate of increase is consistently high during the dry months (December to March) over the entire region whereas the trends are rather inconsistent and weak during the premonsoon (April to May) and summer monsoon period (June to September). The trends in the spectral variation of aerosol optical depth (AOD) reveal the significance of anthropogenic activities on the increasing trend in AOD. Examining these with climate variables such as seasonal and regional rainfall, it is seen that the dry season depicts a decreasing trend in the total number of rainy days over the Indian region. The insignificant trend in AOD observed over the Indo-Gangetic Plain, a regional hot spot of aerosols, during the premonsoon and summer monsoon season is mainly attributed to the competing effects of dust transport and wet removal of aerosols by the monsoon rain. Contributions of different aerosol chemical species to the total dust, simulated using Goddard Chemistry Aerosol Radiation and Transport model over the ARFINET stations, showed an increasing trend for all the anthropogenic components and a decreasing trend for dust, consistent with the inference deduced from trend in Angstrom exponent.

Progress in shape memory epoxy resins

Abstract: This review analyses the progress in the field of shape memory epoxy resins (SMEPs). Partial crystallisation and vitrification are the basis of shape memory effect in SMEPs. Several synthetic approaches for SMEPs, their composites and foams have been reviewed. Strategically incorporated thermally reversible segments induce the shape memory effect in epoxy resins. By varying the nature and concentration of shape memory segments, wide range of shape memory properties and transition temperatures (shape memory temperatures) can be achieved. Triple shape memory, self-healability and electroactive capability are some of the additional features that can be created in SMEPs. Among the thermoset resins, shape memory epoxies are the most attractive systems because of the ease of processability, composite forming properties and dimensional stability. Shape memory epoxy polymers that can be processed into elastic memory composites are candidate materials in the processing of many smart engineering systems. In this background, a review consolidating the progress in SMEP has contemporary relevance. The present article takes a stock of the trend in SMEP with a view to assess the direction of future initiatives in this area. It is concluded that there is tremendous scope for research leading to technological evolution in the field of SMEP.

Processing of copper-carbon nanotube composites by vacuum hot pressing technique

Abstract: Copper (Cu) matrix composites reinforced with 0.2, 5 and 10 vol% single wall carbon nanotubes (SWCNT) and 5 and 10 vol% multi-wall carbon nanotubes (MWCNT) were processed by high energy milling of pure copper powder with carbon nanotubes (CNTs) and subsequent consolidation by vacuum hot pressing. Microstructural observations of the sintered composites revealed equiaxed twinned microstructure for 0.2 vol% SWCNT composite and elongated grain structure, with CNT layers in between, in composites having higher CNTs content. The porosity in the composites was associated with CNT layers. The distribution of CNTs was found to be different in axial and transverse directions. Significant improvement in hardness of Cu–SWCNT composite was observed with increase in CNTs content. Whereas, in case of MWCNT composite, hardness reduced for 10 vol% CNT composites. Compression strength of the SWCNT samples was found to be higher than the MWCNT reinforced samples.

Development of polyimide–nanosilica filled EPDM based light rocket motor insulator compound: Influence of polyimide–nanosilica loading on thermal, ablation, and mechanical properties

Abstract: Thermal protection materials are necessary to protect structural components of launch vehicles during lift-off of launching system. The present study deals with the development of a novel thermally protected light rocket motor insulator compound (RMIC) of polyimide–silica filled EPDM nanocomposites. The insulation compound prepared for the studies comprised of aromatic polyimide and nanosilica particles. The addition of these materials in rocket insulator compound enhanced the multifunctional thermal and insulation characteristics. EPDM when grafted with maleic anhydride, contributed polarity in the non-polar EPDM matrix. Nanosilica contributes specifically better erosion resistance. SEM and TEM micrograph of EPDM nanocomposites exhibits good dispersion of nanosilica in polyimide–EPDM matrix. Nanocomposite formation was characterised by FTIR. Density, co-efficient of thermal expansion, thermal conductivity, ablation rate, specific heat, maximum thermal degradation, char yield and mechanical properties of the RMIC have been measured. This developmental study may find wide scope for commercial exploitation.

Processing and characterization of Al–Cu–Li alloy AA2195 undergoing scale up production through the vacuum induction melting technique

Abstract: The inherent properties of lithium, such as high reactivity and toxicity, relatively low density, low melting point, along with its high cost requires a special technological approach to cast Al–Cu–Li alloy AA2195 as compared to the conventional Direct Chill (DC) casting of aluminum alloys. This paper describes the processing requirements for melting and casting of 200 kg of Al–Cu–Li alloy in a Vacuum Induction Melting (VIM) furnace under dynamic inert atmosphere. The as-cast billets have been homogenized to remove microsegregation as well as to avoid incipient melting, and subsequently subjected for secondary metal processing operations viz., forging and rolling. The product in the form of 4 mm thick sheets was subjected to various heat treatments in T8 (Solution Treatment+WQ+CW+Aging) condition. Mechanical properties were evaluated at room temperature and were correlated with microstructures of the sheets processed under different conditions using transmission electron microscopy (TEM).

Remote energetic neutral atom imaging of electric potential over a lunar magnetic anomaly

Abstract: The formation of electric potential over lunar magnetized regions is essential for understanding fundamental lunar science, for understanding the lunar environment, and for planning human explorati ...

On the Prediction of Strength from Hardness for Copper Alloys

Abstract: Hardness and strength values of over 55 copper alloys strengthened by solid solution strengthening, precipitation hardening, cold working, and dispersion strengthening were compiled. The yield strength (YS) and ultimate tensile strength (UTS) values of the copper alloys examined ranged between 50 to 1300 MPa and 200 to 1400 MPa, respectively. The compiled values were classified based on strain-hardening potential an indirect method to understand the effect of strain-hardening characteristics. Least squares regression analysis was employed to establish correlations between strength and Vickers hardness values. Strain-hardening potential showed a significant effect on the correlations. In all the cases, a linear relation was obtained for both YS and UTS with hardness for the entire range of values under analysis. Simple empirical equations were proposed to estimate the strength using bulk hardness. The proposed correlations obtained for the entire range of values were verified with experimental values. A good agreement was observed between experimental and predicted values.

Effect of temperature and strain rate on the deformation behavior and microstructure of a homogenized AZ31 magnesium alloy

Abstract: The plastic deformation behavior and dynamic recrystallization (DRX) in homogenized AZ31 Mg alloy was investigated in uniaxial compression in the temperature range between 150 and 400 °C with strain rates ranging from 10 −3 to 10 2 s −1 . Twinning was found to contribute significantly during the early stages of deformation. The onset of twinning was examined in detail by recourse to the examination of the appearance of first local maxima before peak strain in the stress–strain responses and the second derivative of stress with strain. High strain hardening rate was observed immediately after the onset of twinning and was found to increase with the Zener–Hollomon parameter. DRX was observed at temperatures above 250 °C whereas deformation at lower temperatures (

Contrasting aerosol characteristics and radiative forcing over Hyderabad, India due to seasonal mesoscale and synoptic‐scale processes

Abstract: Regular measurements of spectral Aerosol Optical Depth (AOD) at ten wavelengths, obtained from multi-wavelength radiometer (MWR) under cloudless conditions in the outskirts of the tropical urban region of Hyderabad, India for the period January 2008 to December 2009, are examined In general, high AOD with a coarse-mode abundance is seen during the pre-monsoon (March to May) and summer monsoon (June to September) with flat AOD spectra and low Angstrom wavelength exponent (α), while in post-monsoon (October–November) and winter (December–February) seasons, fine-mode dominance and steep AOD spectra are the basic features The aerosol columnar size distribution (CSD) retrieved from the spectral AOD using King's inversion showed bimodal size distributions for all the seasons, except for the monsoon, with an accumulation-mode radius at 012–025 µm and a coarse-mode one at 086–130 µm On the other hand, the CSD during the monsoon follows the power law for fine mode and the unimodal distribution for coarse mode The fine-mode aerosols during post-monsoon and winter appear to be associated with air masses from continental India, while the coarse-mode particles during pre-monsoon and monsoon with air masses originating from west Asia and western India The single-scattering albedo (SSA) calculated using the OPAC model varied from 083 ± 005 in winter to 091 ± 001 during the monsoon, indicating significant absorption by aerosols due to larger black carbon mixing ratio in winter, whereas a significant contribution of sea-salt in the monsoon season leads to higher SSAs Aerosol radiative forcing (ARF) calculated using SBDART shows pronounced monthly variability at the surface, top of atmosphere (TOA) and within the atmosphere due to large variations in AOD and SSA In general, larger negative ARF values at the surface (−65 to −80 W m−2) and TOA (∼−17 W m−2) are observed during the pre-monsoon and early monsoon, while the atmospheric heating is higher (∼50–70 W m−2) during January-April resulting in heating rates of ∼16–20 K day−1 Copyright © 2012 Royal Meteorological Society

Recent developments in modeling of heat transfer during TIG welding-a review

Abstract: This paper is an attempt to list the recent developments in the area of arc welding heat transfer simulation. Fusion welding modeling is a broad area where a number of research groups were spending their efforts to get solutions for both research and industrial problems. Starting from fundamentals of arc physics, heat transfer, microstructure models, thermal stress, and modern techniques like pattern recognition comes into picture while considering the complete solution of welding-related problems. These areas are developing almost independently and there are only few efforts to couple them together as computational welding mechanics, which includes the computational fluid mechanics, magneto hydrodynamics, thermo mechanical problems, and computational material science. Here, an effort is made to list down major developments in this area and to plot the present state of research on the TIG welding heat transfer modeling by giving priority to last few years of research.

Microstructure and Properties of a High-Strength Cu-Ni-Si-Co-Zr Alloy

Abstract: A high-strength Cu-Ni-Si alloy was developed with the additions of Co and Zr. The aging curve for the alloy was generated using hardness. Electron microscopy studies were conducted to analyze the phases in the alloy. Two types of phases, one of copper matrix and the other of Ni-Si-Co-Zr intermetallic phase, could be identified using scanning electron microscopy. Transmission electron microscopy studies confirmed the presence of two types of precipitates in solution-treated and aged (STA) condition, i.e., Ni2Si and Co2Si. Mechanical properties and electrical conductivity were evaluated in solution-treated (ST) and STA conditions. Aging of the ST samples at 500 °C for 3 h has shown an increase of 72 and 15% in yield strength (YS) and electrical conductivity, respectively. This increase in YS and conductivity on aging is primarily attributed to the formation of fine Ni2Si and Co2Si precipitates.

Energetic neutral atom imaging of the lunar surface

Abstract: [1] Since the Moon is not shielded by a global magnetic field or by an atmosphere, solar wind plasma impinges onto the lunar surface almost unhindered. Until recently, it was assumed that almost all of the impinging solar wind ions are absorbed by the surface. However, recent Interstellar Boundary Explorer, Chandrayaan-1, and Kaguya observations showed that the interaction process between the solar wind ions and the lunar surface is more complex than previously assumed. In contrast to previous assumptions, a large fraction of the impinging solar wind ions is backscattered as energetic neutral atoms. Using the complete Chandrayaan-1 Energetic Neutral Analyzer data set, we compute a global solar wind reflection ratio of 0.16 ± 0.05 from the lunar surface. Since these backscattered neutral particles are not affected by any electric or magnetic fields, each particle's point of origin on the lunar surface can be determined in a straight-forward manner allowing us to create energetic neutral atom maps of the lunar surface. The energetic neutral atom measurements recorded by the Chandrayaan-1 Energetic Neutral Analyzer cover ∼89% of the lunar surface, whereby the lunar farside is almost completely covered. We analyzed all available energetic neutral atom measurements recorded by the Chandrayaan-1 Energetic Neutral Analyzer to create the first global energetic neutral hydrogen maps of the lunar surface.

CloudSat observations of cloud-type distribution over the Indian summer monsoon region

Abstract: . The three-dimensional distribution of various cloud types over the Indian summer monsoon (ISM) region using five years (2006–2010) of CloudSat observations during June-July-August-September months is discussed for the first time. As the radiative properties, latent heat released and microphysical properties of clouds differ largely depending on the cloud type, it becomes important to know what types of clouds occur over which region. In this regard, the present analysis establishes the three-dimensional distribution of frequency of occurrence of stratus (St), stratocumulus (Sc), nimbostratus (Ns), cumulus (Cu), altocumulus (Ac), altostratus (As), cirrus (Ci) and deep convective (DC) clouds over the ISM region. The results show that the various cloud types preferentially occur over some regions of the ISM, which are consistent during all the years of observations. It is found that the DC clouds frequently occur over northeast of Bay of Bengal (BoB), Ci clouds over a wide region of south BoB–Indian peninsula–equatorial Indian Ocean, and Sc clouds over the north Arabian Sea. Ac clouds preferentially occur over land, and a large amount of As clouds are found over BoB. The occurrence of both St and Ns clouds over the study region is much lower than all other cloud types.The interannual variability of all these clouds including their vertical distribution is discussed. It is envisaged that the present study opens up possibilities to quantify the feedback of individual cloud type in the maintenance of the ISM through radiative forcing and latent heat release.

Remote Energetic Neutral Atom Imaging of Electric Potential Over a Lunar Magnetic Anomaly

Abstract: The formation of electric potential over lunar magnetized regions is essential for understanding fundamental lunar science, for understanding the lunar environment, and for planning human exploration on the Moon. A large positive electric potential was predicted and detected from single point measurements. Here, we demonstrate a remote imaging technique of electric potential mapping at the lunar surface, making use of a new concept involving hydrogen neutral atoms derived from solar wind. We apply the technique to a lunar magnetized region using an existing dataset of the neutral atom energy spectrometer SARA/CENA on Chandrayaan-1. Electrostatic potential larger than +135 V inside the Gerasimovic anomaly is confirmed. This structure is found spreading all over the magnetized region. The widely spread electric potential can influence the local plasma and dust environment near the magnetic anomaly.

Glycidyl Azide Polymer Crosslinked Through Triazoles by Click Chemistry: Curing, Mechanical and Thermal Properties

Abstract: Glycidyl azide polymer (GAP) was cured through click chemistry by reaction of the azide group with bispropargyl succinate (BPS) through a 1,3-dipolar cycloaddition reaction to form 1,2,3-triazole network. The properties of GAP-based triazole networks are compared with the urethane cured GAP-systems. The glass transition temperature (T-g), tensile strength, and modulus of the system increased with crosslink density, controlled by the azide to propargyl ratio. The triazole incorporation has a higher T-g in comparison to the GAP-urethane system (T-g-20 degrees C) and the networks exhibit biphasic transitions at 61 and 88 degrees C. The triazole curing was studied using Differential Scanning Calorimetry (DSC) and the related kinetic parameters were helpful for predicting the cure profile at a given temperature. Density functional theory (DFT)-based theoretical calculations implied marginal preference for 1,5-addition over 1,4-addition for the cycloaddition between azide and propargyl group. Thermogravimetic analysis (TG) showed better thermal stability for the GAP-triazole and the mechanism of decomposition was elucidated using pyrolysis GC-MS studies. The higher heat of exothermic decomposition of triazole adduct (418kJmol(-1)) against that of azide (317kJmol(-1)) and better mechanical properties of the GAP-triazole renders it a better propellant binder than the GAP-urethane system.

Synthesis and characterization of shape memory epoxy-anhydride system

Abstract: Thermoset polymers exhibiting shape memory properties were derived on reacting digycidyl ether of bisphenol A (DGEBA) with pyromellitic dianhydride (PMDA) in presence of a shape memory polymer precursor i.e. carboxyl telechelic poly(tetramethylene oxide) (PTAC) that was synthesized. Cure characteristics of the blends were investigated by differential scanning calorimetry and FTIR. Blends with varying proportions of DGEBA/PTAC/PMDA were studied for their flexural, dynamic mechanical and shape memory properties. The flexural strength and modulus decreased with increase in concentration of PTAC. The storage modulus and the transition temperatures (Ttrans) of the systems also showed a similar trend. These polymers showed good shape memory properties. PTAC increased the modulus ratio (Eg/Er) which in turn increased the extent of shape recovery with a concomitant diminution in the shape recovery time. An increase in temperature results in faster recovery with an increase in the extent of shape recovery. The epoxy-anhydride system possesses adequate thermal, mechanical and shape memory characteristics for possible use in smart actuator systems.

Microstructure and Mechanical Properties of Cu-Ag-Zr Alloy

Abstract: The Cu-3Ag-0.5Zr alloy was produced by vacuum induction melting and subsequently processed through hot forging and rolling. Detailed microstructural characterization of solution-treated (ST) specimen shows three types of phases: Cu matrix, zirconium-rich phase, and Cu-Ag-Zr intermetallic phase. Transmission electron microscopy studies together with energy-dispersive x-ray spectroscopy analysis established the presence of Zr-rich large particles in the ST condition. Aging at 450 °C for 4.5 h after solution treatment resulted in the formation of uniformly distributed fine spherical silver precipitates with an average diameter of 9.0 ± 2.0 nm. Consequently, room temperature yield strength (YS) and ultimate tensile strength (UTS) of the aged specimen increased by 110% and 15%, respectively, compared to those of 120 and 290 MPa of the ST specimen. At elevated temperature, the YS decreased to 146 and 100 MPa at 540 and 640 °C, respectively, for the aged sample. On the contrary, the YS increased to 140 MPa at 540 °C, and thereafter a decrease was observed with a value of 105 MPa at 640 °C for the ST sample. This decrease in YS at higher temperatures is attributed to coarsening of precipitates and dissolution of the precipitates, whereas an increase in YS is attributed to in-situ aging of the samples.

A discussion on the methods of extracting gravity wave perturbations from space-based measurements

Abstract: [1] The disparities in satellite-based observations of global gravity wave activity are discussed in terms of methods used to extract the gravity wave perturbations from background and the sensitivity of the given satellite to the gravity wave spectrum. The temperature measurements from TIMED/SABER are used to obtain the global gravity wave maps in terms of their potential energies by employing two widely used methods to extract the gravity wave perturbations viz. (1) removal of 0–6 zonal wavenumber large-scale waves and (2) high pass filter with cut-off vertical wavelength at 10 km. The present study for the first time employed these two different methods on the same satellite observations to investigate the sensitivity of global gravity wave patterns and their magnitudes to the methods used to extract them. The results showed significant differences in the gravity wave potential energy magnitudes estimated by employing these two methods. Further, employing the first method on COSMIC-measured temperature profiles, the global gravity wave pattern is estimated and the same is compared with that obtained using SABER observations. This comparison substantiated the assertion that using the same method to extract the gravity wave perturbations from different satellite observations yields the similar global gravity wave pattern. The present study thus provided very useful insights into the observed discrepancies among current global gravity wave patterns and it is envisaged that this is a step forward in unifying the existing methods to extract gravity wave parameters using space-based observations.