National Aerospace Laboratories
National Aerospace Laboratories Institutional RepositoryNot a member yet
7121 research outputs found
Sort by
Magnitude Square Coherence (MSC) Estimation via an ARMA Model Based on Analytic DCT and Group Delay.
No full textA new method of magnitude square coherence (MSC) estimation by an auto-regressive moving average (ARMA) model based on analytic discrete cosine transform (ADCT) and group delay (GD) property is proposed. The estimation is achieved by modeling the Welch-MSC derived from ADCT and the ARMA model realized by GD. The ADCT provides twice frequency resolution and reduced variance compared to those of conventional DFT. The proposed MSC estimate is superior to that based on MSC estimation using DFT, in terms of normalized sum of the sample mean square error (NSSMSE), maximum sample root-mean-square error and bias. The proposed method for the two examples involving two stationary stochastic processes reduces the NSSMSE by 60 and 30% over those of ARMA-MSC based on DFT. Further, the Welch-MSC based on ADCT itself reduces the performance indices over that based on DFT (significant for example-2). The proposed method is also applied to signals with sinusoids whose frequencies are located at off the DFT grid bin and closely spaced. The minimum variance distortionless response method of MSC estimation known for its frequency resolution fails to detect and resolve closely spaced off bin frequencies, whereas the proposed ADCT-based Welch-MSC and its ARMA detect and resolve those frequencies. The DFT-based Welch-MSC and its ARMA version though detect off the bin sinusoids, but due to their poor frequency resolution cannot resolve the closely spaced sinusoids. The average variance per frequency bin of ADCT-ARMA and DFT-ARMA is 0.0061 and 0.0108 (i.e., 43.52% reduction by ADCT), respectively
Electrochemical Behavior of Biomedical Titanium Alloys Coated with Diamond Carbon in Hanks’ Solution
Get PDFBiomedical implants in the knee and hip are frequent failures because of corrosion and stress on the joints. To solve this important problem, metal implants can be coated with diamond carbon, and this coating plays a critical role in providing an increased resistance to implants toward corrosion. In this study, we have employed diamond carbon coating over Ti-6Al-4V and Ti-13Nb-13Zr alloys using hot filament chemical vapor deposition method which is well-established coating process that significantly improves the resistance toward corrosion, wears and hardness. The diamond carbon-coated Ti-13Nb-13Zr alloy showed an increased microhardness in the range of 850 HV. Electrochemical impedance spectroscopy and polarization studies in SBF solution (simulated body fluid solution) were carried out to understand the in vitro behavior of uncoated as well as coated titanium alloys. The experimental results showed that the corrosion resistance of Ti-13Nb-13Zr alloy is relatively higher when compared with diamond carbon-coated Ti-6Al-4V alloys due to the presence of β phase in the Ti-13Nb-13Zr alloy. Electrochemical impedance results showed that the diamond carbon-coated alloys behave as an ideal capacitor in the body fluid solution. Moreover, the stability in mechanical properties during the corrosion process was maintained for diamond carbon-coated titanium alloys
Shape induced magnetic vortex state in hexagonal ordered cofe nanodot arrays using ultrathin alumina shadow mask
No full textThe magnetization reversal process of hexagonal ordered CoFe nanodot arrays was investigated as a function of nanodot thickness varying from 10 to 30 nm with fixed diameter. For this purpose, ordered CoFe nanodots with a diameter of 80 ± 4 nm were grown by sputtering using ultra-thin alumina mask. The vortex annihilation and the dynamic spin configuration in the ordered CoFe nanodots were analyzed by means of magnetic hysteresis loops in complement with the micromagnetic simulation studies. A highly pinched hysteresis loop observed at 20 nm thickness suggests the occurrence of vortex state in these nanodots. With increase in dot thickness from 10 to 30 nm, the estimated coercivity values tend to increase from 80 to 175 Oe, indicating irreversible change in the nucleation/annihilation field of vortex state. The measured magnetic properties were then corroborated with the change in the shape of the nanodots from disk to hemisphere through micromagnetic simulation
Microstructural damage based micromechanics model to predict stiffness reduction in damaged unidirectional composites.
No full textPrediction of the residual stiffness of the carbon fiber reinforced polymer composite, subjected to fatigue loading, can be performed using some of the phenomenological models. However, it is still a challenge to find the stiffness based on the known microstructural damage state (that was developed irrespective of the load history). In this work, two micromechanics-based models were developed to predict reduction in the stiffness of the damaged composite. Fiber crack density and interface debonding was used to define the microstructural damage state of the composite. These models account for the fiber crack density in the form of change in either geometry (equivalent ellipsoid model) or material property of the fiber (reduced stiffness model). The microstructural damage state in the unidirectional carbon fiber reinforced polymer composite, obtained from the on-axis tension–tension fatigue loading, was used to validate the models. The results from reduced fiber stiffness model were compared against experiment and finite element analysis for the given microstructural damage. The stiffness obtained using reduced fiber stiffness model was in good agreement with that obtained from the experiment. However, reduced fiber stiffness model underestimated reduction in stiffness compared to finite element analysis
Effect of Cr2AlC MAX phase addition on strengthening of Ni-Mo-Al alloy coating on piston ring: Tribological and twist-fatigue life assessment.
No full textThe aim of the present study is to investigate the effect of Cr2AlC additions on the tribological properties of Piston Ring coated with Ni-Mo-Al alloy. For this Cr2AlC MAX phase was blended with Ni-Mo-Al alloy powder in different proportions (10 wt%, 20 wt% and 50 wt%) and coated on the stainless steel piston ring by Air Plasma Spraying (APS). During coating Cr2AlC MAX reacts with air and forms Cr7C3 and Al2O3 phases. The in-situ formation of Cr7C3 and Al2O3 was observed to strengthen the alloy. 20 wt% Cr2AlC additions to the Ni-Mo-Al alloy yields a good combination of properties, such as improved adhesion, hardness and wear resistance. Composite coating was found to be stable during the exfoliation test of the coated ring. Coating life cycles were found to be nearly doubled by MAX phase addition, as assessed through the twist-fatigue study. These improved properties could be attributed to the finely distributed oxides (alumina) and chromium carbides (Cr7C3) within the coated layer, which were formed in-situ during plasma spraying
Estimation of static burst pressure in unflawed high pressure cylinders using nonlinear FEA
No full textA study has been carried out to predict the static burst pressure in closed thick-walled unflawed cylinders using finite element analysis (FEA). The failure of the cylinder purely by ductile fracture mode is considered in the analysis. Thick-walled cylinders of different diameter ratios (K=outer diameter/inner diameter, ranging from 1.5 to 6) have been investigated. The commercially available ANSYS FEA code has been used, for carrying out an implicit elasto-plastic analysis of the cylinders considering the geometric and material nonlinearities. The Arc-length algorithm available in ANSYS code has been employed to overcome the solution convergence problem, due to tangent singularity at limit state. The FEA predicted static burst pressure for thick-walled cylinders over the considered range of diameters ratios i.e., K=1.5 to 6, showed good agreement with Svensson burst model. The implication of using elastic-perfectly plastic model for the prediction of static burst pressure has also been investigated
Integral weaving of orthogonal 3D ‘T’ stiffeners based on pleat weaving concept
No full textThis paper details about the manufacturability of integrally woven ‘T’ stiffeners using orthogonal weaving technology based on the pleat formation concept. The development was carried out on a customized automatic take-up-at-will rapier weaving machine equipped with multi-beam warp let-off and electronic dobby. Bridging the gap between the base portions of the flange sections and locking the web sections together by using equally spaced intermittent extra warp yarns were the added innovations in the work apart from integral weaving of the ‘T’ stiffener. These innovations were achieved by suitably modifying the weave design. The development comprised a 5-part weave design for the ‘T’ stiffener and was carried out using 6K carbon tows. Calculations for X, Y, Z contents and overall fibre content by volume, from first principles of cloth construction approaches, for the flange and web sections of the ‘T’ profile, have also been included
A Brief Exposition on Brain-Computer Interface
Get PDFBrain-Computer Interface is a technology that records brain signals and translates them into useful commands to operate a drone or a wheelchair. Drones are used in various applications such as aerial operations, where pilot’s presence is impossible. The BCI can also be used for patients suffering from brain diseases who lose their body control and are unable to move to satisfy their basic needs. By taking advantage of BCI and drone technology, algorithms for Mind-Controlled Unmanned Aerial System can be developed. This paper deals with the classification of BCI & UAV, methodologies of BCI, the framework of BCI, neuro-imaging methods, BCI headset options, BCI platforms, electrode types & their placement, and the result of feature extraction technique (FFT) with 72.5% accuracy
High-pressure strengths of Ni3Al and Ni-Al-Cr
No full textHigh-pressure x-ray diffraction on Ni3Al, non-hydrostatically compressed at room temperature to 30 GPa in radial geometry can be interpreted in terms of a hydrostatic pressure–volume equation of state. We found the yield strength of Ni3Al to increase from about 0.2 to 2 GPa as pressure increases from 0 to 30 GPa. Yield-strength values determined from x-ray diffraction in axial geometry by including a Poisson ratio effect and pressure gradient methods are in good accord with each other. Our results indicate that the strengths of both pure and Cr-doped (7 at. %) Ni3Al increase with pressure, with the yield strength of Ni3Al slightly higher than Ni-Al-Cr alloy
Is the subharmonic threshold always lower than the fundamental one in plane Poiseuille flow.
Get PDFWe revisit the secondary instability of a Tollmein-Schlichting wave in plane Poiseuille flow at Re =
5000 and find that the fundamental threshold can be lower than the subharmonic one in the wavenumber
space, in contrast to the prevalent view in the literature that only instability of subharmonic
modes occurs at very low disturbance amplitudes and fundamental modes need a higher disturbance
amplitude to become unstable. This is because of overlooking the even class of fundamental modes,
which happen to produce the lower threshold at those wavenumbers. Published by AIP Publishing