Showing papers by "Satish C. Sharma published in 2010"

Vibration signature analysis of single walled carbon nanotube based nanomechanical sensors

Abstract: In the present paper, the simulation of the mechanical responses of individual carbon nanotubes treated as thin shells with thickness has been done using FEM. The resonant frequencies of the fixed free and the bridged SWCNT have been investigated. This analysis explores the resonant frequency shift of SWCNTs caused by the changes in the size of CNT in terms of length as well as the masses. The results showed the sensitivity of the single walled carbon nanotubes to different masses and different lengths. The results indicate that the mass sensitivity of carbon nanotube nanobalances can reach 10−21 g and the mass sensitivity increases when smaller size nanotube resonators are used in mass sensors. The vibration signature exhibits super-harmonic and sub-harmonic response with different level of mass. In order to explore the suitability of the SWCNT as a mass detector device, the simulation results of the resonant frequency of fixed free SWCNT are compared to the published experimental data. It is shown that the FEM simulation results are in good agreement with the experimental data and hence the current modelling approach is suitable as a coupled-field design tool for the development of SWCNT-based NEMS applications.

A Study of Worn Hybrid Journal Bearing System With Different Recess Shapes Under Turbulent Regime

Abstract: The objective of the present paper is to study analytically the influence of wear on the performance of a capillary-compensated, four-pocket, hybrid journal bearing system operating in a turbulent regime by considering various geometric shapes of recess. The present study deals with bearings having four different geometric shapes of recess, i.e., square, circular, elliptical, and triangular recessed bearings. The wear on the bearing surface is modeled using Dufrane's abrasive wear model. The Reynolds equation based on Constantinescu's turbulent lubrication theory has been solved using finite element method along with a restrictor flow equation as a constraint together with appropriate boundary conditions. The numerically simulated results have been presented for a wide range of nondimensional external loads, wear depth parameters, and Reynolds numbers. The numerically simulated results suggest that the combined influence of wear, turbulence, and geometric shape of recess significantly affects the bearing performance. It has been observed that a triangular recessed bearing provides a greater value of minimum fluid film thickness when operating in a turbulent regime. It is also noticed that direct fluid film stiffness coefficients get reduced significantly when bearings operate in a turbulent regime compared with a laminar regime. Further, it is observed that from the viewpoint of fluid film stiffness, a square recessed bearing is found to be most suitable when operating in a turbulent regime.

Analysis of Crack Propagation in Fixed-Free Single-Walled Carbon Nanotube Under Tensile Loading Using XFEM

Abstract: Fracture mechanics at the nanoscale level is a very complex phenomenon, whereas the macroscale fracture mechanics approach can be employed for nanoscale to simulate the effect of fracture in single-walled carbon nanotubes (SWCNTs). In this study, an extended finite element method is used to simulate crack propagation in carbon nanotubes. The concept of the model is based on the assumption that carbon nanotubes, when loaded, behave like space frame structures. The nanostructure is analyzed using the finite element method, and the modified Morse interatomic potential is used to simulate the nonlinear force field of the C‐C bonds. The model has been applied to single-walled zigzag, armchair, and chiral nanotubes subjected to axial tension. The contour integral method is used for the calculation of the J-integral and stress intensity factors (SIFs) at various crack locations and dimensions of nanotubes under tensile loading. A comparative study of results shows the behavior of cracks in carbon nanotubes. It is observed that for the smaller length of nanotube, as the diameter increased, the stress intensity factor is linearly varied while for the longer nanotube, the variation in stress intensity factor is nonlinear. It is also observed that as the crack is oriented closer to the loading end, the stress intensity factor shows higher sensitivity to smaller lengths, which indicates more chances for crack propagation and carbon nanotube breakage. The SIF is found to vary nonlinearly with the diameter of the SWCNT. Also, it is found that the predicted crack evolution, failure stresses, and failure strains of the nanotubes correlate very well with molecular mechanics simulations from literature. DOI: 10.1115/1.4002417

Evaluation of the Mechanical Properties of CNT Based Composites Using Hexagonal RVE

Abstract: Carbon nanotubes (CNTs) possess extremely high stiffness, strength, and resilience, and may provide ultimate reinforcing materials for the development of nanocomposites In this paper, the effective material properties of CNT-based composites are evaluated based on the continuum mechanics using a hexagonal representative volume element (RVE) Numerical equations are used to extract the effective material properties from numerical solutions for the hexagonal RVEs under axial loading case An extended rule of mixtures for estimating effective Young’s modulus in the axial direction of the RVE is applied It has been observed that the addition of the CNTs in a matrix at volume fractions of only about 36%, the stiffness of the composite is increased by 33% for long CNT at Et/Em=10, whereas not much improvement in stiffness has been noticed in the case of short CNTS at Et/Em=10 Effectiveness of composites is evaluated in terms of various dimensions such as thickness, diameter, and length of CNT These results suggest that short CNTs in a matrix may not be as effective as long CNTs in reinforcing a composite The simulation results are consistent with the experimental ones reported in literature Also, the comparative evaluation of all three types of RVEs is presented here

Study of two‐lobe four recessed hybrid journal bearing

Abstract: Purpose – In recent years, researchers have focused a great deal of attention on multirecess hybrid journal bearing systems. The non‐circular journal bearings are widely used in industry on account of their better stability, simplicity, efficiency and low cost. The purpose of this paper is to present a theoretical investigation into the performance of a two‐lobe multirecess hybrid journal bearing system.Design/methodology/approach – The Reynold's equation governing the lubricant flow in the clearance space between the journal and bearing together with restrictor flow equations has been solved using finite element method. The bearing static and dynamic performance characteristics have been presented for the various values of the offset factors (0.75, 1, 1.25 and 1.50) for the hybrid mode of operation of the journal bearing system compensated by capillary and orifice restrictors for the commonly used bearing operating and geometric parameters. The offset of the journal has been accounted for by defining a n...

Influence of pocket‐size on misaligned hole‐entry journal bearing

Abstract: Purpose – Hole‐entry hybrid journal bearings are widely used in many applications owing to their favourable characteristics. Ever increasing technological developments demand much improved performance from these class of bearings operating under the most stringent, exact and precise conditions. Therefore, it becomes imperative that the hole‐entry journal bearings be designed on the basis of more accurately predicted bearing characteristics data. The purpose of this paper is to describe a theoretical study to demonstrate the combined influence of the effect of pocket size at the outlet of supply holes and the journal misalignment on the performance of an orifice compensated hole‐entry hybrid journal bearing system.Design/methodology/approach – Finite element method is used to solve the Reynolds equation governing the flow of an incompressible lubricant in the clearance space between the journal and bearing together with equation of flow through an orifice. The journal misalignment has been accounted for by...

Rolling Element Bearing Fault Diagnosis Using Complex Gaussian Wavelet

Abstract: This paper is focused on fault diagnosis of bearings due to localized defects i.e. spall on the bearing components, which is essential to the design of high performance rotor bearing system. The methodology proposed in this paper for fault diagnosis of rolling element bearings, utilizes autocorrelation of raw vibration signals to reduce the dimension of vibration signals with minimal loss of significant frequency content. Dimension of vibration signal is reduced to 10% with negligible loss of information. To extract most appropriate features from auto-correlated vibration signals and for effective classification of faults, vibration signals are decomposed using complex Gaussian wavelet. Total 150 signals of healthy and defective bearings at rotor speeds 250, 500, 1000, 1500 and 2000 rpm with three loading conditions are considered. 1-D continuous wavelet coefficients of these samples are calculated at the seventh level of decomposition (27 scales for each sample). Maximum Energy to Shannon Entropy ration criterion is used to determine scale corresponding to characteristic defect frequency. Statistical features are extracted from the wavelet coefficients corresponding to selected scales. Finally, bearing faults are classified using Support Vector Machine (SVM) method. The test results show that the SVM can be used efficiently for bearing fault classification. It is also observed that classification accuracy is improved by using autocorrelation.© 2010 ASME