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.

Improved Corrosion Protection of Magnesium Alloys AZ31B and AZ91 by Cold-Sprayed Aluminum Coatings

Abstract: Magnesium (Mg) alloys have a high strength/weight ratio, high dimensional stability, good machinability, and the ability to be recycled. However, their poor corrosion resistance in humid environments limits their usage for exterior aerospace components. This study aims to improve the corrosion resistance of two Mg alloys (AZ31B and AZ91) by using aluminum coatings. The latter have been deposited by a low pressure and temperature cold spray process. An aluminum powder (60 wt%) with a particle size ranging between 1 and 8 µm and nickel powder (40 wt%) with a particle size of about 70 µm were blended and used as feedstock powder. The coating thickness was about 240 μm. Its densification was achieved by the in-situ hammering effect of the nickel particles. The shot-peening effect also resulted in an enhanced coating hardness. The microstructure, mechanical properties, and corrosion resistance of the coatings have been investigated. They showed that the aluminum had a face centered cubic structure. Potentiodynamic polarization tests were performed along with a combination of materials characterization techniques to assess the corrosion resistance of the coatings when immersed in a 3.5 wt% NaCl solution for long durations. The results revealed that the corrosion resistance increased with the immersion time because of the formation of a protective oxide layer on the surface. These results were supported by elemental and structural analyses. This study shows that cold-sprayed aluminum coatings are a promising candidate for enhancing the corrosion resistance of AZ31B and AZ91Mg alloys compared to other thermal spray processes.

An approximate factorization scheme for elliptic grid generation with control functions

Abstract: An Alternating Direction Implicit (ADI), Approximate Factorization (AF) scheme is presented here for the solution of the two-dimensional elliptic partial differential equations, with control functions as source terms, used for grid generation. This scheme requires significantly less computational effort than a Successive Over Relaxation (SOR) scheme. The dependence of the choice of the acceleration parameter on the rate of convergence of the AF scheme has been studied. As an example, grids generated by this method are shown, along with a comparison of the convergence history for the present AF and SOR schemes.

Multiple laminar-turbulent transition cycles around a swept leading edge

Abstract: Certain interesting flow features involving multiple transition/relaminarization cycles on the leading edge of a swept wing at low speeds are reported here. The wing geometry tested had a circular nose and a leading edge sweep of 60°. Tests were made at a chord Reynolds number of 1.3 × 106 with model incidence α varied in the range of 3°−18° in discrete steps. Measurements made included wing chord-wise surface pressure distributions and wall shear stress fluctuations (using hot-film gages) within about 10 % of the chord in the leading edge zone. Results at α = 16° and 18° showed that several (often incomplete) transition cycles between laminar-like and turbulent-like flows occurred. These rather surprising results are attributable chiefly to the fact that the Launder acceleration parameter K (appropriately modified for swept wings) can exceed a critical range more than once along the contour of the airfoil in the leading edge region. Each such crossing results in a relaminarization followed by direct retransition to turbulence as K drops to sufficiently low values. It is further shown that the extent of each observed transition zone (of either type) is consistent with earlier data acquired in more detailed studies of direct transition and relaminarization. Swept leading edge boundary layers therefore pose strong challenges to numerical modelling.

Decomposition Behavior of Kevlar 49 Fibers: Part II. At T values < Td:

Abstract: The residual effects of thermal aging of Kevlar 49 fibers in the temperature range 150--450xB0;C have been analyzed. Thermal aging introduces crystallographic as well as macro-structural changes. Weight lossesand deterioration in tensile properties were also observed. Theorder in which the deterioration in crystallinity, weightand tensile strength occur has been identified. Mastercurves for predicting the time neededfor 50% deterioration at various temperatures and the corresponding activation energy have been estimated. The role of the parameter. tcum(T),the cumulative exposure to any temperature T, on thermally induced effects has been unambiguously established. In particular, the influence of the T- tClIm(T) effect oncrystallographic parameters has been observed for the first time.

Mesh distortion immunity of finite elements and the best-fit paradigm

Abstract: It has been known for some time that distorted finite elements produce relatively (and, sometimes, dramatically) poor results. This has been related to the completeness condition. In this paper, we investigate this issue and propose that the abstract mathematical viewpoint represented by the completeness condition is actually a statement of the physical need for a finite element computation to recover accurate stresses in the metric space. This follows from the projection theorem describing finite element analysis which shows that the stresses computed by the displacement finite element procedure are abest approximation of the true stresses at an element as well as global level. The simplest possible element is used to elucidate the principles.