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Mohamed Shaat

Publications and source records attributed to Mohamed Shaat.

13 recordsLinked to original sources
arXiv2021

Mechanical activity and odd elasticity of passive, 2D chiral metamaterials

We demonstrate a general route to making active, odd elastic solids from passive chiral elements that can act as sources of mechanical work by violating static equilibrium without internal sources of energy or momentum. We further demonstrate that by starting from a discrete, Newtonian mechanics viewpoint of the chiral unit cell, we can develop the continuum field equations for isotropic 2D chiral metamaterials that reveal odd elasticity, while elucidating the structure-property relationships underpinning the chiral, elastic moduli that enable the mechanical activity of the chiral metamaterials. By demonstrating the energy gain of chiral metamaterials as they undergo quasistatic deformation cycles, we show a new route to designing active, non-reciprocal mechanical metamaterials that can operate at zero frequency.

physics.app-ph↗
arXiv2020

Hybrid continuum-molecular modeling of fluid slip flow

Experiments on fluid systems in micro-/nano-scale solid conveyors have shown a violation of the no-slip assumption that have been adopted by the classical fluid mechanics. To correct this mechanics for the fluid slip, various approaches have been proposed to determine the slip boundary conditions. However, these approaches have revealed contradictory results for a variety of systems, and a debate on the mechanisms and the conditions of the fluid slip/no-slip past solid surfaces is sustained for a long time. In this paper, we establish the hybrid continuum-molecular modeling (HCMM) as a general approach of modeling the fluid slip flow under the influence of excess fluid-solid molecular interactions. This modeling approach postulates that fluids flow over solid surfaces with/without slip depending on the difference between the applied impulse on the fluid and a drag due to the excess fluid-solid molecular interactions. In the HCMM, the Navier-Stokes equations are corrected for the excess fluid-solid interactions. Measures of the fluid-solid interactions are incorporated into the fluid viscosity. We demonstrate that the correction of the fluid mechanics by the slip boundary conditions is not an accurate approach, as the fluid-solid interactions would impact the fluid internally. To show the effectiveness of the proposed HCMM, it is implemented for water flow in nanotubes. The HCMM is validated by an extensive comparison with over 90 cases of experiments and molecular dynamics simulations of different fluid systems. We foresee that the hybrid continuum-molecular modeling of the fluid slip flow will find many important implementations in fluid mechanics.

cond-mat.soft↗
arXiv2020

Topological Mechanics of Micromorphic Metamaterials

The topological mechanics is a perfect tool that can bridge the gap between the quantum and Newtonian physics and mechanics of materials. It requires discrete models of the material with analogies with the topological characteristics of quantum matter. Despite bridging this gap using continuous models of matter would seem challenging, we demonstrate here this possibility. We demonstrate the bridging of the quantum-continuum mechanics gap by studying the topological mechanics of metamaterials based on the micromorphic theory. A general micromorphic model of metamaterials is developed and used to study their topological characteristics. The conditions of the band-gaps, band localization, and band inversion are defined. Whereas closing the band-gap of topological insulator is obviously due to band localization, we demonstrate a mechanism of closing the band-gap with no band localization. In addition, despite the fact that the band inversion of many topological insulators cannot be completed without closing the gap, we demonstrate a case of optical band inversion with no gap closing. We foresee that the exceptional topological characteristics of metamaterials explored here will help in the design of advanced topological insulators, and open new venues of the implementations of topological insulators in mechanical applications.

physics.app-ph↗
arXiv2020

Nonreciprocal Elasticity

Nonreciprocity has been introduced to various fields to realize asymmetric, nonlinear, and/or time non-revisal physical systems. By virtue of the Maxwell-Betti reciprocal theorem, breaking the time-reversal symmetry of dynamic mechanical systems is only possible using nonlinear materials. Nonetheless, nonlinear materials should be accompanied by geometrical asymmetries to achieve nonreciprocity in static systems. Here, we further investigate this and demonstrate a novel nonreciprocal elasticity concept. We show that the nonreciprocity of static mechanical systems can be achieved only and only if the material exhibits nonreciprocal elasticity. We experimentally demonstrate linear and nonlinear materials with nonreciprocal elasticities. By means of topological mechanics, we demonstrate that the mechanical nonreciprocity requires nonreciprocal elasticity no matter what the material is linear or nonlinear elastic. We show that linear materials with nonreciprocal elasticity can realize nonreciprocal-topological systems. The nonreciprocal elasticity developed here will open new venues of the design of mechanical systems with effective nonreciprocity.

physics.class-ph↗
arXiv2018

Iterative Nonlocal Residual Elasticity

Motivated by the existing complications of finding solutions of Eringen nonlocal model, an alternative model is developed here. The new formulation of the nonlocal elasticity is centered upon expressing the dynamic equilibrium requirements based on a nonlocal residual stress field. This new nonlocal elasticity is explained from the lattice mechanics and continuum mechanics points of view. Boundary value problems obtained based on the new nonlocal elasticity are solved following a proposed iterative procedure. This iterative procedure is centered upon correcting the solution of the classical field problem for the nonlocal residual field of the elastic domain. Convergence analyses are presented to show the convergence of the iterative procedure to the solution of the nonlocal field problem. The iterative procedure is an integrated part of the proposed nonlocal elasticity. Therefore, the newly developed nonlocal elasticity is give the name iterative nonlocal residual elasticity.

physics.app-ph↗
arXiv2018

An iterative nonlocal residual constitutive model for nonlocal elasticity

Recently, it was claimed that the two-phase local/nonlocal constitutive models give well-posed nonlocal field problems and eliminates the ill-posedness of the fully nonlocal constitutive models. In this study, it is demonstrated that, both, the fully nonlocal and the two-phase local/nonlocal constitutive models secrete ill-posed nonlocal boundary value problems. Moreover, it is revealed that all Eringen integral and differential nonlocal constitutive models secrete unsolvable nonlocal boundary value problems. In this study, it is demonstrated that solutions of nonlocal elasticity problems are exist, and Eringen constitutive model cannot determine these solutions. To overcome the limitations of Eringen constitutive models, novel integral and differential iterative nonlocal residual constitutive models are proposed. Using these two constitutive models, the sum of the nonlocal residual field at a point is iteratively formed. Then, this nonlocal residual is imposed to the local boundary value problem. Thus, the nonlocal elasticity is obtained in the form of a local boundary value problem with an imposed nonlocal residual field. Using any of these constitutive models, a solution is guaranteed for a nonlocal field problem. To show the effectiveness of the proposed constitutive models, the nonlocal field problems of beams with different natural boundary conditions are considered. The results of the proposed integral and differential constitutive models are identical and feasible.

physics.app-ph↗
arXiv2018

Influence of surface integrity on geometry and dynamics of functionally graded nanobeams

In this study effects of surface integrity on the mechanics of functionally graded (FG) nanobeams are investigated. This study reports the changes in the geometry and dynamics of FG nanobeams because of changes in their surface textures and/or surface mechanical properties. A new model for FG nanobeams with engineering surfaces is developed. This engineering surface is considered as a different material phase with a surface texture (waviness and roughness). The initial curvatures of cantilever, simple supported, and clamped-clamped FG nanobeams due to surface residual stresses are determined. Moreover, their natural frequencies and mode shapes are derived depending on surface integrity. The initial curvatures of FG beams are obtained increasing with an increase in the slope of the surface texture and/or a decrease in the heights of the surface roughness. Moreover, it is observed that the natural frequencies of FG beams may decrease or increase due surface integrity depending on the boundary conditions. Thus, as a first prospect, the surface roughness allows the vibration energy to propagation over the beam length and hence its natural frequency decreases resulting in a zero-frequency mode. As for the other prospect, surface roughness inhibits the propagation of the vibration energy through the beam length leading to a mode localization. It is revealed that a mode localization is accompanied with an increase in the natural frequency of the nanobeam. The proposed surface integrity model for FG nanobeams is compared with Gurtin-Murdoch surface elasticity model. The results demonstrate that the surface integrity model is preferred over the former model where it accounts for, both, surface texture and surface mechanical properties effects. However, Gurtin-Murdoch model assumes smooth surfaces of nanobeams which leads to under/overestimations of their mechanics.

physics.app-ph↗
arXiv2018

Mode localization phenomenon in microbeams due to surface roughness

This is the first study on the mode localization phenomenon in microbeams due to surface roughness. A new model for microbeams with rough surfaces is developed. The natural frequencies and mode shapes of cantilever, simple supported, and clamped-clamped microbeams are determined depending on the beam surface roughness. A parametric study is presented demonstrating two prospects: surface roughness may lead to a zero-frequency mode or a mode localization. As for the first prospect, it is demonstrated that surface roughness may add more softness to a specific mode of vibration and reduce its natural frequency causing a rigid-body mode. As for the second prospect, surface roughness may inhibit the propagation of vibration energy throughout the beam length leading to a mode localization. It is revealed that a mode localization is accompanied with an increase in the natural frequency of the microbeam. It is revealed that the description of the beam vibration according to one of these two prospects depends on the beam size, parameters of the surface roughness, and the boundary conditions.

physics.app-ph↗
arXiv2017

A reduced micromorphic model for multiscale materials and its applications in wave propagation

In this study, a reduced micromorphic model for multiscale materials is developed. In the context of this model, multiscale materials are modeled with deformable microstructures. The deformation energy is formed depending on microstrain and macroscopic strain residual fields. The constitutive equations according to the reduced micromorphic model only depend on eight material coefficients for linear elastic materials. These material coefficients are related to the material micro/macro-stifnesses and the material microstructural features. The wave dispersions in multiscale materials are then derived according to the reduced micromorphic model. It is revealed that this model can reflect nine dispersion curves (three acoustic modes and six optics) for a two-scale material. To demonstrate the effectiveness of the proposed model, the wave propagation characteristics, the band structure, and the absolute bandgap features of phononic materials are investigated. It is demonstrated that the reduced micromorphic model can effectively reflect the increase in the bandgap width with the increase in the filling factor in a composite phononic material with square lattices.

physics.app-ph↗
arXiv2017

On performance of nano-resonators produced by magnetron sputtering deposition

Magnetron sputtering is a perfect technique for processing nanomaterials for engineering and medical applications. A material can be processed with specific mechanical properties, microstructure, and surface texture by controlling the parameters of magnetron sputtering. Therefore, studies should be conducted on investigating the processing conditions on the performance of nanomaterials processed by magnetron sputtering. In this study, effects of the processing force on performance of micro/nano-resonators produced by magnetron sputtering are revealed. The processing force is defined as the ratio of the sputtering power-to-the substrates traveling velocity. By comparing the substrate traveling velocity to the deposition rate of the sputtered particles, relations are derived for the thickness and surface texture evolutions with the processing force. The coefficients of these relations are experimentally determined for mechanical resonators made of FeNiCr alloy. Then, the variations of the natural frequencies of these resonators with the processing force of magnetron sputtering and the deposition rate of the sputtered particles are depicted. It is demonstrated that special considerations should be given for the effects of the processing conditions when design mechanical resonators produced by magnetron sputtering.

physics.app-ph↗
arXiv2017

Correction of local-linear elasticity for nonlocal residuals: Application to Euler-Bernoulli beams

Complications exist when solving the field equation in the nonlocal field. This has been attributed to the complexity of deriving explicit forms of the nonlocal boundary conditions. Thus, the paradoxes in the existing solutions of the nonlocal field equation have been revealed in recent studies. In the present study, a new methodology is proposed to easily determine the elastic nonlocal fields from their local counterparts without solving the field equation. This methodology depends on the iterative-nonlocal residual approach in which the sum of the nonlocal fields is treaded as a residual field. Thus, in this study the corrections of the local-linear elastic fields for the nonlocal residuals in materials are presented. These corrections are formed based on the general nonlocal theory. In the context of the general nonlocal theory, two distinct nonlocal parameters are introduced to form the constitutive equations of isotropic-linear elastic continua. In this study, it is demonstrated that the general nonlocal theory outperforms Eringen-nonlocal theory in accounting for the impacts of the material Poisson ratio on its mechanics. To demonstrate the effectiveness of the proposed approach, the corrections of the local static bending, vibration, and buckling characteristics of Euler-Bernoulli beams are derived. Via these corrections, bending, vibration, and buckling behaviors of simple-supported nonlocal Euler-Bernoulli beams are determined without solving the beams equation of motion.

physics.app-ph↗
arXiv2017

Infeasibility of the nonlocal strain gradient theory for applied Physics

In this communication, the feasibility of the nonlocal strain gradient theory for fields of applied mechanics is investigated. It is demonstrated that the nonlocal strain gradient theory is physically incorrect. It is proved that each of the nonlocal theory and the strain gradient theory can model, both, hardening and softening behaviors of materials. Moreover, it is proved that the nonlocal theory and the strain gradient theory describe the same physical phenomena, and hence the strain gradient theory cannot be merged with the nonlocal theory in a unified model. This short communication comments on a series of papers in which the nonlocal strain gradient theory was utilized in different areas of applied mechanics.

physics.app-ph↗