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Transport properties of p-type Ca3-xLnxCo4O9-Ag (Ln = Lu, Yb; 0.1≤x≤0.2) oxides
No full textThermoelectric transport properties of p-type Ca3-xLnxCo4O9/yAg oxides (Ln = Lu & Yb; 0.1 ≤ x ≤ 0.2; 0.05 ≤ y ≤ 0.1) synthesized by sol-gel methodology were investigated in this paper. The structural analyses (SEM, XRD and TEM) confirmed the presence of two phases, viz, Ca3xLnxCo4O9 and Ag-metallic phases. The contribution of rare earth doping in one hand and presence of Ag as secondary phase on the other hand were studied. The resistivity measurements indicated the reduction of electrical resistance at the grain boundary leading to an overall decrease in electrical resistivity with increasing Ag-concentration. The enhancement of Seebeck coefficient is attributed to the substitution of Ln3+ at Ca2+ sites that in turn reduces hole concentration through formation Co3+ for charge concentration counter balance in Ca3-xLnxCo4O9/yAg matrix. The tuning of electrical transport properties through Ca3-xLnxCo4O9 and Ag-metallic bi-phasic formation resulted high power factor of 582 μW m−1 K−2 for Ca2.8Ln0.2Co4O9/0.05Ag and 548 μW m−1 K−2 for Ca2.8Yb0.2Co4O9/0.05Ag at 950 K highlighting its potential application on small scale energy harvesting to power sensor and wireless sensor network where requirement of power is in the milliwatt range
Estimation of Residual Life and Failure Mechanism of Cracked Aircraft Wing Skin
No full textMany failures in aircraft structures are due to fatigue cracks initiating and developing from fastener holes at which there are large stress concentrations. In a typical wing skin, in the zone of riveted joint of rib/skin, the combination of high stress concentration could potentially lead to the appearance of the crack initiation and then crack growth under cyclic loading. Stress Intensity Factor (SIF) solutions are required for the assessment of fracture strength and residual fatigue life for defects in structures. In this context, many research works focused on evaluating the residual life of various cracked aircraft structures but only a few works have been done on light transport aircraft wing skin. The material used for wing skin is AL 2024-T351. A computational model for estimating the residual fatigue life of cracked wing skin is proposed. The complete computation procedure for the crack propagation analysis using low-cycle fatigue material properties is illustrated with the damaged wing skin. Initially, stress concentration effects at the cracked wing skin rivet holes are determined by applying analytical and numerical methods. Further, residual life and the failure mechanism in the cracked rivet holes of the wing skin are estimated. The wing skin with two cracked rivet holes for a pitch of 26 mm was analyzed using MSC NASTRAN/PATRAN for different crack lengths using MVCCI (Modified Virtual Crack Closure Integral) method by which strain energy release rate as well as stress intensity factors are calculated for different crack lengths, and fatigue crack growth life for progressive cracks for different R ratios is computed using a MATLAB program. Comparisons of the stress intensity factors estimated by FE analysis were in good agreement with the analytical solutions. Further, using SIF solutions, the residual life was estimated for the cracks emanating from the two rivet holes using crack growth models. The work also investigates the first failure mechanism out of two competing mechanisms of failure; Failure due to fracture or Failure due to plastic collapse at the net section between two advancing crack tips of the rivet holes of the wing skin. It was observed that the wing skin with crack rivet holes would fail by plastic collapse due to net section yielding. Further, the study can be extended to a multi-axial stress condition
Electrospinning of cellulose acetate nanofiber membrane using methyl ethyl ketone and N, N-Dimethylacetamide as solvents.
No full textCellulose acetate (CA) nanofiber membrane was prepared by electrospinning method using solvent mixtures of methyl ethyl ketone (MEK) and N, N, - dimethylacetamide (DMAc) in different ratios (2:1, 1:1, 1:2) and also different concentration of CA (7–19%). MEK was selected in place of acetone due to its high boiling point, thereby, minimises the evaporation loss of the solvent enabling the longer duration of electrospinning. The morphology of electrospun nanofibers was observed by Scanning Electron Microscope (SEM). It was observed that cylindrical fibers formed at higher concentration of polymer with increase of DMAc. Fiber diameters were in the range of 40–500 nm with large diameters formed at higher polymer concentration. Contact angle measurement revealed that membranes have good wetting property. The water flux measurements of membranes were carried out under gravity. A water flux of 10,197 Lm−2h−1 was measured initially and was reduced subsequently to 365-200 Lm−2h−1. The membranes could be reused up to four times without rupture. The above experiments suggest that MEK and DMAc could be an alternate solvent system in addition to other systems
Effect of Atomization Quality on Lean Blow-Out Limits and Acoustic Oscillations in a Swirl Stabilized Burner
No full textThe present experimental work highlights the influence of atomization quality on lean blow-out (LBO) limits and acoustic oscillations in a swirl stabilized burner with simplex atomizer. With decrease in the initial spray droplet diameter, the LBO limit shifts toward lower equivalence ratios. Reduction in droplet size also strongly influences the mode of LBO from diffusion flame to premixed lifted flame. Correlations have been developed for the LBO limit, involving mainly the time scales for evaporation, reaction and residence times for the fuel drops, as well as the gas flow. Delay in evaporation causes vapor accumulation before combustion and hence it influences both acoustic oscillations and LBO limit. The frequency of acoustic oscillations locks-in with the quarter wave frequency of the combustor duct for all initial droplet diameters considered. The amplitude of acoustic oscillations decreases with decrease in the initial droplet size
Machining response and damage evolution of amorphous carbon coated tools in high-speed micromilling of Ti-6Al-4V
No full textMicromilling process is widely used to create complex 3D miniature products due to its flexibility and its ability to process difficult-to-cut material like Titanium alloys. High rotational speeds are used to overcome the limited flexural stiffness of the tool but the cutting zone temperatures rise due to the high rotational speeds. In addition to this, friction between the tool work-piece and tool chip also plays a major role in the temperature rise. The friction and temperature affect the cutting forces, tool life and stability of the process. To reduce the friction and heat generation, nanostructured solid lubricant coatings can be used. This study is focused on characterizing the effect of amorphous carbon (WC/a-C) coating on the micromachining response during high-speed micromilling of Ti-6Al-4V. A decrease in cutting forces for coated tools is observed for lower feed. A comprehensive tool coating damage assessment has been done in terms of debonding area on flank and rake faces. An increase in debonding area has been observed with lengths of cut but at a feed/flute of 4 μm, tool breakage occurs after a machining length of 60mm
Radar Absorbing Structures Using Frequency Selective Surfaces: Trends and Perspectives
No full textMicrowave radar absorbers are widely used in the strategic sector and wireless communication systems to reduce the radar cross-section of a target and electromagnetic interferences, respectively. For airborne stealth platforms, it is desired to have wide bandwidth RAS with minimum thickness and adequate structural rigidity. However, the classical RAS structures are thicker with a narrowband of absorption. So the demand of thin and broadband absorber for a modern stealth platform can be accomplished by designing metallic or resistive frequency selective surfaces (FSSs) based radar absorbing structures (RAS). In view of this, a technological assessment on frequency selective surface (FSS)-based radar absorbing structure are presented in this paper, which includes historical review on radar absorber development, design techniques, physical models, and optimization techniques. Ultra-wideband absorbers with essentially thin structures can be realized by optimizing FSS and dielectric parameters. The genetic algorithm (GA) is identified as one of the effective searching algorithms among several numerical algorithms, which are discussed in detail for single and multi-layered FSS–RAS. The fabrication techniques of resistive FSS by printing the periodic pattern using resistivity-controlled ink is also addressed. For proof-of-the concept, a prototype of cross-dipole FSS based RAS is fabricated and measured
A Comparative Analytical study of Standard Composite Double Cantilever Beam using Virtual Crack Closure Technique and Cohesive Zone Modelling method.
No full textDouble cantilever beam specimens made up of Carbon Fibre-Reinforced Polymer laminate (CFRP) and Glass Fibre-Reinforced Polymer (GFRP) laminate were modelled, analysed and their fracture parameters were evaluated using Abaqus-Standard. For the quasi-isotropic specimens, load-displacement responses were computed by varying tensile loads using Virtual Crack Closure Technique (VCCT) and Cohesive Zone Modelling (CZM) method. From the results, it was seenthat modelling effort and computational-time required were quite higher in CZM method. Onset of delamination for both specimens were studied using VCCT after computing energy release rates. Also, the delamination pattern onset was found comparable in both the materials with delay of initiation in GFRP laminate specimen
On the Recurrence Signatures of Flapping Wings Exposed to Gusty Simple Shear Flow.
No full textThe primary aim of the research reported in this paper was to understand the effect of change of gusty simple shear inflow’s gradient on the force and moment patterns of a flapping wing in the 3D reference frame. A wing undergoing one degree of freedom asymmetric flapping and rectangular planform shape was considered. The gradient of the gusty simple shear inflow profile, Vgrad, was varied from –10 to +10 in steps of 5 and corresponding vertical and horizontal forces and moment about the flapping axis were computed. Time series of these forces and moment were used to plot the global recurrence plots and were compared. Quantitative analysis of the findings was carried out by the windowed recurrence quantification analysis of the force and moment patterns. Eight recurrence parameters, viz. recurrence rate, determinism, laminarity, trapping time, ratio, entropy, maximum line and trend were calculated and compared. Numerical investigations revealed that negative gusty shear gradient induced a considerable increase in vertical force and moment and marginally decreased the horizontal forces. Positive gusty shear gradient induced a marginal increase in horizontal forces but caused a substantial decrement in vertical force and moment
Thermo-mechanical analysis and estimation of turbine blade tip clearance of a small gas turbine engine under transient operating conditions
No full textTurbine blade tip clearance is one of the significant factors that influence turbine efficiency, Specific Fuel Consumption (SFC), Exhaust Gas Temperature (EGT), and emissions. Controlling these parameters in a small gas turbine engine (SGT) is a challenging task due to small blade height and viscous working environment. SGT are subjected to high-temperature gradients at the combustor outlet, which affects the turbine blade tip clearance. This paper presents the thermo-mechanical analysis of a typical SGT engine to study the blade tip clearance influenced by the deformation of turbine stage components (turbine rotor, nozzle guide vane (NGV) with integral blade shroud) during transient phases. ANSYS Workbench is used to perform transient thermal and structural analyses. The structural analysis is performed taking the material properties to be temperature-dependent. The SGT engine under consideration operates at a design speed of 45,000 rpm. Initially, steady-state thermal analysis and static structural analysis were carried out to understand the structural behaviour of the system under a thermal and centrifugal loading environment. Since different components of the engine assembly operate at different temperatures, the effects of convection and conduction at the interfaces influence the radial clearances between the static and rotating parts of the engine. A one-way coupled transient thermal-structural analysis was performed on a three-dimensional model to capture the actual behaviour of the tip clearance during transient operating conditions. Significant growth of blade and rotor was observed relative to the casing resulting in minimal clearances during these transient operations. Hence, it is important to estimate desired cold clearance, considering transient phenomena, to avoid mechanical blade rub with the shroud. It is observed that high-temperature gradients contribute primarily to the stresses and radial displacement of the rotor compared to centrifugal effects. The turbine rotor takes more time (t = 600 s) to reach steady-state temperatures compared to NGV (t = 120 s) due to the solid mass of the disc. The location and magnitude of maximum and minimum equivalent stress changes with time in NGV and rotor, and they experience maximum stress at the initial time steps compared to steady-state
