National Aerospace Laboratories

About: National Aerospace Laboratories is a facility organization based out in Bengaluru, India. It is known for research contribution in the topics: Coating & Corrosion. The organization has 1838 authors who have published 2349 publications receiving 36888 citations.

Microstructural evolution and aging behavior of cryorolled Al–4Zn–2Mg alloy

Abstract: The effect of cryorolling on microstructural evolution and aging characteristics of an Al–4Zn–2Mg alloy is investigated using differential scanning calorimetry (DSC), transmission electron microscopy (TEM), electron back scattered diffraction (EBSD) and hardness measurements. The Al–4Zn–2Mg alloy in solutionized condition was cryorolled to achieve 70–90% deformation. The cryorolled samples were short annealed followed by three different aging treatments viz., natural aging, peak aging and over aging. The EBSD results suggest the presence of ultrafine grains in the cryorolled alloy. DSC thermograms and aging studies revealed that cryorolling enhances the aging kinetics of the alloy. The two-step peak aging treatment indicated that the degree of peak hardness is directly proportional to the extent of deformation and the time to attain it varied inversely with respect to the amount of deformation. TEM studies of cryorolled and peak aged (CRPA) samples showed large number of uniformly distributed, finer and closely spaced η′ strengthening precipitates when compared to bigger precipitates in coarse grain peak aged (CGPA) alloy. Variations in precipitate distribution and morphology between CRPA and CGPA alloys are attributed to the stored energy and high density of dislocations generated during cryorolling.

Viscous Drag Reduction Using Riblets on NACA 0012 Airfoil to Moderate Incidence

Abstract: Results of viscous drag reduction using riblets from 3M on a NACA 0012 airfoil model up to moderate angles of attack are presented. Measurements made consisted of model surface pressure distributions, mean velocity and streamwise turbulence intensity profiles in the boundary layer (just ahead of the trailing edge), and total airfoil drag for two riblet heights of 0.152 and 0.076 mm. Results show significantly higher skin friction drag reduction with incidence compared to plat plate flows; the reduction was as high as 16% at ? = 6 deg. Results of mean velocity profiles show that a larger contribution to drag reduction results from the suction side of the airfoil, indicating increased effectiveness of riblets in adverse pressure gradients. Examination of turbulence intensity profiles in the wall region indicates an appreciable reduction in the presence of riblets; correspondingly, the spectra show reduced energy levels at low frequencies.

Preparation and characterization of siliconized epoxy/bismaleimide (N,N′‐bismaleimido‐4,4′‐diphenyl methane) intercrosslinked matrices for engineering applications

Abstract: Novel hybrid intercrosslinked networks of hydroxyl-terminated polydimethylsiloxane-modified epoxy and bismaleimide matrix systems have been developed. Epoxy systems modified with 5, 10, and 15 wt % of hydroxyl-terminated polydimethylsiloxane (HTPDMS) were developed by using epoxy resin and hydroxyl-terminated polydimethylsiloxane with γ-aminopropyltriethoxysilane (γ-APS) as compatibilizer and dibutyltindilaurate as catalyst. The reaction between hydroxyl-terminated polydimethylsiloxane and epoxy resin was confirmed by IR spectral studies. The siliconized epoxy systems were further modified with 5, 10, and 15 wt % of bismaleimide (BMI). The matrices, in the form of castings, were characterized for their mechanical properties. Differential scanning calorimetry and thermogravimetric analysis of the matrix samples were also performed to determine the glass-transition temperature and thermal-degradation temperature of the systems. Data obtained from mechanical studies and thermal characterization indicate that the introduction of siloxane into epoxy improves the toughness and thermal stability of epoxy resin with reduction in strength and modulus values. Similarly the incorporation of bismaleimde into epoxy resin improved both tensile strength and thermal behavior of epoxy resin. However, the introduction of siloxane and bismaleimide into epoxy enhances both the mechanical and thermal properties according to their percentage content. Among the siliconized epoxy/bismaleimide intercrosslinked matrices, the epoxy matrix having 5% siloxane and 15% bismaleimide exhibited better mechanical and thermal properties than did matrices having other combinations. The resulting siliconized (5%) epoxy bismaleimide (15%) matrix can be used in the place of unmodified epoxy for the fabrication of aerospace and engineering composite components for better performance. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 38–46, 2001