Shaker A. Meguid

Modeling electrical conductivities of nanocomposites with aligned carbon nanotubes

We have developed an improved three-dimensional (3D) percolation model to investigate the effect of the alignment of carbon nanotubes (CNTs) on the electrical conductivity of nanocomposites. In this model, both intrinsic and contact resistances are considered, and a new method of resistor network recognition that employs periodically connective paths is developed. This method leads to a reduction in the size effect of the representative cuboid in our Monte Carlo simulations. With this new technique, we were able to effectively analyze the effects of the CNT alignment upon the electrical conductivity of nanocomposites. Our model predicted that the peak value of the conductivity occurs for partially aligned rather than perfectly aligned CNTs. It has also identified the value of the peak and the corresponding alignment for different volume fractions of CNTs. Our model works well for both multi-wall CNTs (MWCNTs) and single-wall CNTs (SWCNTs), and the numerical results show a quantitative agreement with existing experimental observations.

Differences in osseointegration rate due to implant surface geometry can be explained by local tissue strains

Experimental evidence indicates that the surface geometry of bone‐interfacing implants influences the nature and rate of tissues formed around implants. In a previously reported animal model study, we showed that non‐functional, press‐fitted porous‐surfaced implants placed in rabbit femoral condyle sites osseointegrated more rapidly than plasma‐sprayed implants. We hypothesized that the accelerated osseointegration observed with the porous‐surfaced design was the result of this design providing a local mechanical environment that was more favourable for bone formation. In the present study, we tested this hypothesis using finite element analysis and homogenization methods to predict the local strains in the pre‐mineralized tissues formed around porous‐surfaced and plasma‐sprayed implants. We found that, for loading perpendicular to the implant interface, the porous surface structure provided a large region that experienced low distortional and volumetric strains, whereas the plasma‐sprayed implant provided little local strain protection to the healing tissue. The strain protected region, which was within the pores of the sintered porous surface layer, corresponded to the region where the difference in the amount of mineralization between the two implant designs was the greatest. Low distortional and volumetric strains are believed to favour osteogenesis, and therefore the model results provide initial support for the hypothesis that the porous‐surfaced geometry provides a local mechanical environment that favours more rapid bone formation in certain situations.

Mechanical regulation of localized and appositional bone formation around bone-interfacing implants.

The local mechanical environment around bone-interfacing implants determines, in large part, whether bone formation leading to functional osseointegration will occur. Previous attempts to relate local peri-implant tissue strains to tissue formation have not accounted for implant surface geometry, which has been shown to influence early tissue healing in vivo. Furthermore, the process by which mechanically regulated peri-implant bone formation occurs has not been considered previously. In the current study, we used a unit cell approach and the finite element method to predict the local tissue strains around porous-surfaced and plasma-sprayed implants, and compared the predictions to patterns of bone formation reported in earlier in vivo experiments. Based on the finite element predictions, we determined that appositional bone formation occurred when the magnitudes of the strain components at the tissue-host bone interface were <8%. Localized, de novo bone formation occurred when the distortional tissue strains were less than approximately 3%. Based on these threshold tissue strains, we propose a mechanoregulatory model to relate local tissue strains to the process of peri-implant bone formation. The mechanoregulatory model is novel in that it predicts both appositional and localized bone formation and its predictions are dependent on implant surface geometry. The model provides initial criteria with which the osseointegration potential of bone-interfacing implants may be evaluated, particularly under conditions of immediate or early loading.

Analysis of a central crack normal to a piezoelectric-orthotropic interface

This study is concerned with the general solution of the field intensity factors and energy release rate for a Griffith crack normal to the interface between a rectangular piezoelectric ceramic and two same rectangular orthotropic materials of finite length under combined in-plane electrical and anti-plane mechanical loading. Both electrically continuous and impermeable crack surface conditions are considered. Employing Fourier transforms and Fourier series, the problem is reduced to dual integral forms. The solution to the resulting expressions is expressed in terms of Fredholm integral equation of the second kind. Analytical solutions are provided for a number of cases to study the influence of material combinations, geometry, crack surface boundary and loading conditions. The work also examines the influence of the crack boundary conditions on the results of the stress intensity factors and the energy release rate.

Experimental and theoretical evaluation of a novel shock absorber for an electrically powered vehicle

Abstract The increasingly stringent pollution laws in North America and Europe have recently stimulated interest in the development of a new generation of electrically powered vehicles. Typically, a large number of conventional lead-acid batteries are utilized in order to ensure adequate performance as measured by vehicle speed and range. The electric vehicle design relating to this article uses a conventional General Motors G-Van as the base vehicle. The absence of an up-front internal combustion engine and the presence of a large battery tray greatly alter this design for surviving crashes. In view of these changes, a novel shock absorber utilizing a symmetric stepped circular thin-walled tube has been designed in order to provide the battery tray with the necessary compliance, thus delaying the effect of its added mass upon impact and reducing the severity of any collision. Two aspects of the work were accordingly examined in the evaluation of the current shock absorber. The first utilized plastic hinge and finite element analyses to predict the collapse loads and the level of energy absorbed, while the second utilized quasi-static axial crush tests to verify the theoretical predictions. The results indicate that the present stepped design is capable of minimizing the severity of the collision of an electrically powered vehicle.

Efficient multi-level modeling technique for determining effective board drop reliability of PCB assembly

In this paper, we develop a new and computationally efficient multi-level approach to investigate board level drop reliability of printed circuit board (PCB) assembly. The approach is composed of two levels of finite element (FE) simulations: solder joint level and board level. Initially, static simulations of the solder joint level were used to obtain the homogenized property of the solder-underfill interconnection. This was followed by explicit FE simulations of the board assembly. The results of the proposed multi-level approach were compared with commonly adopted FE analysis and good correspondence is revealed between the two. Through drop test simulations that involved fifteen Integrated Circuit (IC) packages, as per the standard JESD22-B111 of Joint Electron Device Engineering Council (JEDEC), the critical board locations and interconnection in each location were identified and analyzed. The results reveal that peak stresses occur at the corner of the central package. They also show that the interconnection stresses result mainly from the dynamic bending of the PCB.

Photoelastic analysis of the singular stress field in a bimaterial wedge

This paper presents an experimental investigation of the singular stress field near the vertex of a bimaterial wedge using a digital photoelastic technique. Special attention is given to the casting of bimaterial wedge specimens and analysis technique for extracting stress intensity factors from photoelastic samples. Different bimaterial wedge specimens are made of two different photoelastic materials bonded through a special casting procedure and loaded in simple tension. A new multiple-parameter method is developed to obtain the stress intensity factor reliably from the isochromatic fringe patterns and the series representation of the stress field at the vertex of the wedge. Experimental results are compared with finite element predictions, and good agreement is observed.

Atomistic Modelling of Nanoindentation of Multilayered Graphene-Reinforced Nanocomposites

The force-displacement curves, obtained from a nanoindentation experiment, are generally analysed using continuum contact mechanics models. However, the applicability of these models at the nanoscale is questionable due to several inherited nanoscale phenomena, e.g., discreteness, quantum manifestations, and scale effects. Atomistic simulations such as molecular dynamics could provide better insight into the contact mechanics of nanoscale systems. In this chapter, we present a comprehensive molecular dynamics simulations of the contact behaviour of multilayered graphene-reinforced composite systems. Three aspects of the work were considered. The first was concerned with the force-displacement curves resulting from nanoindentation of a polyethylene matrix reinforced by multilayered graphene sheets. The second is concerned with the associated deformation patterns as well as the atomic adhesion associated with the retraction stage of the indenter. The third is concerned with the reinforcement mechanism and fracture behaviour associated with the increase in the number of graphene sheets and their spatial locations within the composite. The results of our work reveal: (a) strong interlayer interaction of graphene results in higher indentation resistance, (b) indentation resistance of a single-layer graphene-coated polyethylene is about 13-fold of the indentation resistance of pure polyethylene, (c) strong atomic adhesion between the indenter and the graphene prevails at the nanoscale, and (d) the proper choice of interlayer separation is critical in achieving the best performance of multilayered graphene-reinforced nanocomposites.

Multiscale Modeling of Nanoreinforced Composites

In this chapter, we present different multiscale modeling techniques to determine the elastic and interfacial properties of carbon nanotube (CNT)-reinforced polymer composites. The elastic properties of CNT-reinforced composite (hereinafter the “nanocomposite”) are obtained in a two-step approach. First, at the nanoscale level, molecular dynamics (MD) and atomistic-based continuum (ABC) techniques are used to determine the effective elastic properties of a representative volume element (RVE) that is comprised of a nanofiller and its immediate surrounding. Second, at the microscale level, several micromechanics models and hybrid Monte Carlo finite-element (FE) simulations are used to determine the bulk properties of nanocomposite. The interfacial properties are determined through pullout test using MD and ABC techniques. The effect of length, diameter, agglomeration, waviness, defects, and orientation of CNTs on the elastic and interfacial properties of nanocomposites is also investigated. The development of multiscale modeling and the proper selection of simulation parameters are discussed in detail. The results of several studies are presented and compared to show the inherited limitations in each technique.

Effects of wavy sidewall on vortex breakdown in an enclosed cylindrical chamber with a rotating end wall

The effects of the wavy sidewall on flow behavior in an enclosed cylindrical chamber with a rotating end wall were investigated by a numerical model based on the steady, axisymmetric Navier–Stokes equations. The wavy sidewall influences vortex breakdown by either enhancing or reducing the swirling velocity near the top axis, depending on its orientation, amplitude, and period. When the period is small, the orientation of the wavy sidewall varies the volume of the top region of the chamber, which affects the swirling velocity there, thus influencing the occurrence of vortex breakdown. The volume-variation effect is enhanced with an increase in the amplitude, while reduced with an increase in the period. Moreover, an increase in the amplitude or period increases the length of the wavy sidewall, which enhances the dissipation of the fluid angular momentum along it. Thus, the swirling velocity near the top axis region is decreased and vortex breakdown is delayed. The resultant effect of the wavy sidewall is d...