Monon Mahboob

An Improved Model for Magnetorheological Fluid-Based Actuators and Sensors:

The resistance to flow of magnetorheological (MR) fluids is greatly increased by the application of a magnetic field. At present, most devices exploiting this MR fluid behavior are pistons executing straight-line motion. The MR fluids in these devices are subjected to shear flow, and are modeled by either the Bingham plastic or Herschel-Buckley models, both 1D, phenomenological continuum-level forms relating strain rate, magnetic field magnitude, and stress magnitude, and fit to continuum-level empirical measurements. We employ a multiscale model of the MR fluid introduced in an earlier paper, which integrates nanoscale behavior over a mesoscale volume to deduce continuum properties. This approach replaces many of the phenomenological features of the Bingham plastic and Herschel-Buckley models with first principles, and isolates those few phenomenological features that remain into a single scalar term. The model is compared to the Bingham plastic and Herschel-Buckley models, assessing each model’s ability to capture the experimentally measured mechanical response of a particular MR fluid-based damper to specified magnetic fields. The result of this comparison is that, our model possesses the flexibility to best match the measured behavior of the MR fluid device observed in our experiments, with fewer required experimental measurements.

A constitutive model for characterization of shear and extensional rheology and flow induced orientation of carbon nanofiber/polystyrene melt composites

We characterize the transient shear and uniaxial extensional rheology of melt phase polystyrene/carbon nanofiber composites and measure orientation development during each flow type. Experimental measurements show an increase in the viscosity of the composite with increasing carbon nanofiber concentration under both types of flow. We present a microstructurally based constitutive model to predict the experimentally observed rheological behavior and evolution of nanofiber orientation. Experimental measurements were performed to determine the average orientation of the carbon nanofibers at various points in time during shear and extensional flows. We demonstrate that the constitutive model predicts the transient shear and extensional viscosities of polystyrene/carbon nanofiber composites and the orientation evolution of nanofibers during flow.

Characterization of the thermal expansion properties of graphene using molecular dynamics simulations

In the present study, the temperature-dependent coefficient of thermal expansion (CTE) of a graphene sheet (GS) is determined using molecular dynamics (MD) simulations. Our simulations show that the CTE of a GS (i) varies non-linearly with temperature, (ii) is negative over a temperature range of 0–500 K and (iii) differs by no more than 9% in the armchair and zigzag directions. We find good agreement between our MD results and recent experimental data. The present study also investigates the effect of missing atoms (vacancy defects) on the CTE of a GS. In our MD simulations of a 4.9 nm × 4.9 nm GS, we find that the presence of two vacant atoms (about 1.56% by volume) increases the negative CTE by as much as 40%. Correlations between the CTE and the number of missing atoms have been developed based on MD simulation results for a perfect GS and a GS with 1.56% defects by volume. Predictions of the CTE of a defective GS from the correlations compare favourably with MD simulations at 3.13% defects by volume.

Effect of temperature and geometric parameters on elastic properties of tungsten nanowire: A molecular dynamics study

Tungsten is a promising material and has potential use as battery anode. Tungsten nanowires are gaining attention from researchers all over the world for this wide field of application. In this paper, we investigated effect of temperature and geometric parameters (diameter and aspect ratio) on elastic properties of Tungsten nanowire. Aspect ratios (length to diameter ratio) considered are 8:1, 10:1, and 12:1 while diameter of the nanowire is varied from 1-4 nm. For 2 nm diameter sample (aspect ratio 10:1), temperature is varied (10K ~ 1500K) to observe elastic behavior of Tungsten nanowire under uniaxial tensile loading. EAM potential is used for molecular dynamic simulation. We applied constant strain rate of 109 s−1 to deform the nanowire. Elastic behavior is expressed through stress vs. strain plot. We also investigated the fracture mechanism of tungsten nanowire and radial distribution function. Investigation suggests peculiar behavior of Tungsten nanowire in nano-scale with double peaks in stress vs....

Analysis of mechanical and micro-structural properties of trabecular bones with statistical review of osteoporosis among females in Bangladesh

Osteoporosis is one of the most notable causes of bone fracture and approximately 200 million women are affected by osteoporosis worldwide. According to a report of International Osteoporosis Foundation, at least one in five men and one in three women in Bangladesh suffer a bone fracture due to osteoporosis in their remaining lifetime. Limited data is available in the open literature on osteoporosis among women in Bangladesh and more so on the micro-structural analysis of osteoporosis. The present study particularly aims at identifying the trend of osteoporosis of women of in Bangladesh, with special emphasis on the mechanical micro-structural analysis of female bones. A comprehensive statistical analysis has been conducted by collecting data of 250 Bangladeshi females with age ranging from 20 to 80 years. Trabecular bones of Tibia of female have been analysed with computed tomography for micro-structural assessment. To characterize several parameters of trabecular bones, the computed tomography scans have been analysed and parameters like bone volume fraction, fractal dimension etc. have been evaluated. A correlation between structural properties of the bones has been formed by multiple regression analysis. It shows a positive correlation between fractal dimension and bone volume fraction of the specimens.

Measurement of 3-D Orientation and 3-D Orientation Tensors in Polymer/Nanofiber Composites

We present a novel method to compute the three dimensional orientation tensors of carbon nanofibers in CNF/polymer composites. Performance properties of nanofiber composites are significantly affected by fiber orientation patterns. So an accurate description of the fiber orientation is necessary to validate models relating processing technique and properties of the final composites. Orientation tensors and probability distribution functions (PDF) are commonly used to describe orientation patterns. But physical dimensions of nanofibers and the imaging technique (Transmission Electron Microscopy, TEM) make all currently available methods of image analysis useless to accurately computing these descriptions in 3-dimensions. In this study TEM sections are cut with specific thickness and angle that result in majority of the fibers being cut at both ends. Fiber projection dimensions measured from this special section provide the complete three dimensional orientation of each fiber. This method is benchmarked by using simulated 3D samples in AutoCAD.Copyright