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Experiments depict that the physico-mechanical response of miniature devices is microstructure-dependent. However, the classic continuum theory cannot correctly predict the microstructure-dependency. In this paper, the strain gradient theory is employed to examine the dynamic behavior and instability characteristics of miniature varactor with trapezoidal geometry. The governing equation of the varactor is obtained incorporating the effects of Coulomb force, van der Waals (vdW) attraction, squeeze film damping and structural damping. The influences of microstructure on the dynamic instability of equilibrium points are studied by plotting the phase portrait and bifurcation diagrams. It is found that increase in the microstructure parameter enhances the torsional stability. In the presence of the applied voltage, the phase portrait shows the saddle-node bifurcation while for free-standing varactor a subcritical pitchfork bifurcation is observed.

It has been well-established that the physical performance of nano-devices might be affected by the microstructure. Herein, a 2 degree-of-freedom model based on modified couple stress elasticity is developed to incorporate the impact of microstructure in the torsion/bending coupled instability of rotational nano-scanner. The governing equation of the varactor is derived incorporating the effects of electrostatic Coulomb and corrected Casimir forces with the consideration of the finite conductivity of interacting surfaces. Effect of microstructure-dependency on the instability parameters are determined as a function of the microstructure parameter, bending/torsion coupling ratio, vacuum fluctuation parameter and geometrical dimensions. It is found that the bending/torsion coupling substantially affects the stable behavior of the scanners especially those with long rotational beam elements. Depending on the geometry and material characteristics, the presented model is able to simulate both hardening behavior (due to size phenomenon) and softening behavior (due to torsion/bending coupling) of the nano-mirror.

In this research, rubber based nanocomposites with presence of nanoparticle has been studied. Styrene butadiene rubber (SBR)/nanocopper (NC) composites were prepared using two-roll mill method. Transmission electron microscope (TEM) and scanning electron microscope (SEM) images showed proper dispersion of NC in the SBR matrix without substantial agglomeration of nanoparticles. To evaluate the curing properties of nanocomposite samples, swelling and cure rheometric tests were conducted. Moreover, the rheological studies were carried out over a range of shear rates. The effect of NC particles was examined on the thermal behavior of the SBR using thermal gravimetric analysis (TGA). Furthermore, tensile tests were employed to investigate the capability of nanoparticles to enhance mechanical behavior of the compounds. The results showed enhancement in tensile properties with incorporation of NC to SBR matrix. Moreover, addition of NC increased shear viscosity and curing time of SBR composites. Keywords: Nanocopper, Rubber, Curing behavior, Rheological properties, Thermal stability, Tensile characteristics

For modeling the electromechanical behavior of nano-bridge structures with slender narrow-width beam elements, not only the simultaneous effects of surface layer and size dependency should be taken into account but also corrected force models should be considered. In this paper, the instability of a narrow-width nano-bridge is studied based on strain gradient theory and Gurtin–Murdoch surface elasticity. The mid-plane stretching is incorporated in the governing equation as well as corrected force distribution. Using Rayleigh–Ritz method, a parametric analysis is conducted to examine the impacts of surface layer, size dependence, dispersion forces and structural damping on static and dynamic instability voltage of the nano-bridge.

It has been well-established that the physical performance of nano-devices might be affected by the microstructure. Herein, a 2-degree-of-freedom model based on the modified couple stress elasticity is developed to incorporate the impact of microstructure in the torsion/bending coupled instability of rotational nano-electromechanical mirror. The governing equation of the mirror is derived incorporating the effects of electrostatic Coulomb and corrected Casimir forces with the consideration of the finite conductivity of interacting surfaces. Effect of microstructure-dependency on the instability parameters are determined as a function of the microstructure parameter, bending/torsion coupling ratio, vacuum fluctuation parameter and geometrical dimensions. It is found that the bending/torsion coupling substantially affects the stable behavior of the mirrors especially those with long rotational beam elements. Depending on the geometry and material characteristics, the presented model is able to simulate both hardening behavior (due to microstructure) and softening behavior (due to torsion/bending coupling) of the nano-mirror.

Nano-scale beams might not be considered uniform isotropic since the energy of the surface layer and microstructure of the bulk part highly affect the mechanical characteristics of the beams. Herein, the impact of the energy of the surface layer and the microstructure of the bulk on the mechanical stability of beam-type nanotweezers are investigated. A new bilayer continuum model has been developed which incorporates the strain energy of the surface atoms as well as the microstructure-dependent strain energy of the bulk material. The recently-developed consistent couple stress elasticity (CCSE) in combination with the Gurtin-Murdoch surface theory is applied to derive the governing equation. The nonlinear governing equation was solved using numerical generalized differential quadrature (GDQ). Effects of various parameters including characteristics of the surface layer, microstructure of the bulk and external forces on the static and dynamic stability threshold of the nanostructure are demonstrated.

A nonlinear beam formulation is presented based on the Gurtin-Murdoch surface elasticity and the modified couple stress theory. The developed model theoretically takes into account coupled effects of the energy of surface layer and microstructures sizedependency. The mid-plane stretching of a beam is incorporated using von-Karman nonlinear strains. Hamilton’s principle is used to determine the nonlinear governing equation of motion and the corresponding boundary conditions. As a case study, pull-in instability of an electromechanical nano-bridge structure is studied using the proposed formulation. The nonlinear governing equation is solved by the analytical reduced order method (ROM) as well as the numerical solution. Effects of various parameters including surface layer, size dependency, dispersion forces, and structural damping on the pullin parameters of the nano-bridges are discussed. Comparison of the results with the literature reveals capability of the present model in demonstrating the impact of nanoscale phenomena on the pull-in threshold of the nano-bridges.

Surface energy and size phenomenon can play significant roles in physical performance of nano-electromechanical systems. Herein, the static and dynamic pull-in behavior of nano-tweezers and nano-switch fabricated from conductive cylindrical nano-wires is studied. The Gurtin–Murdoch surface elasticity in combination with the couple stress theory is employed to incorporate the coupled effects of surface energy and microstructure dependency (size phenomenon). Using Green–Lagrange strain, the higher-order surface stress components are incorporated in the nonlinear governing equation. The effect of gas damping is considered in the model as well as structural damping. The governing equation is solved using the reduced order method. The effects of various parameters on the static and dynamic pull-in parameters, phase plans and stability threshold of the nano-structures are demonstrated.

The U-shaped and double-sided nanostructures are promising for developing miniature angular speed sensors. While the electromechanical instability of conventional beam-type nanostructures has been extensively addressed in the literature, few researchers have investigated this phenomenon in the double-sided and U-shaped sensors. In this regard, the present work demonstrates the effect of the centrifugal force on the pull-in performance of the double-sided and U-shaped sensors fabricated from cylindrical nanowire and operated in the van der Waals (vdW) regime. Based on the modified couple stress theory, the size-dependent constitutive equations of the sensors are derived. The governing equations are solved by two different approaches, i.e. the analytic Duan–Adomian method and the numerical differential quadrature method. The influences of the vdW and centrifugal forces, geometric parameters and the size phenomenon on the pull-in parameters are demonstrated.

Herein, the dynamic pull-in instability of cantilever nanoactuator fabricated from conductive cylindrical nanowire with circular cross-section is studied under the presence of Casimir force. The Gurtin–Murdoch surface elasticity in combination with the couple stress theory is employed to incorporate the coupled effects of surface energy and size phenomenon. Using Green–Lagrange strain, the higher order surface stress components are incorporated in the governing equation. The Dirichlet mode is considered and an asymptotic solution, based on the path integral approach, is applied to consider the effect of the Casimir attraction. Furthermore, the influence of structural damping is considered in the model. The nonlinear governing equation is solved using analytical reduced order method (ROM). The effects of various parameters on the dynamic pull-in parameters, phase planes and stability threshold of the actuator are demonstrated.

Finite conductivity, surface energy and nonlocal effect can influence the electromechanical performance of micro/nano-electromechanical systems (MEMS/NEMS). However, these factors are yet ignored on stability analysis of MEMS/NEMS fabricated from functionally graded materials (FGM). In this paper, dynamic stability of double-sided NEMS fabricated from non-symmetric FGM is investigated incorporating finite conductivity, surface energy and nonlocal effect. The Gurtin–Murdoch model and Eringen's elasticity are employed to consider the surface energy and nonlocal effect, respectively. Effect of finite conductivity of FGM on electrostatic and Casimir attractions is incorporated via relative permittivity and plasma frequency of the material. The stability analysis of the nanostructure is conducted by plotting time history and phase portraits. Moreover, bifurcation analysis is conducted to investigate the stability of the fixed points of the nano-structure. The validity of the proposed model is examined by comparing the results of the present study with those reported in the literature. The impact of various parameters i.e. finite conductivity, nonlocal parameter, surface stresses and material characteristics on the dynamic instability of the NEMS are addressed.

While vinyl-terminated butadiene acrylonitrile is frequently used for the toughening of vinylester and polyester, very limited research has been conducted on modification of epoxy with this oligomer. Herein, the effect of vinyl-terminated butadiene acrylonitrile addition to epoxy in bulk and glass reinforced composite is systematically investigated. Thermo-physical behavior and mechanical characteristics of the samples are determined. To interpret the test results, the void content of reinforced samples is measured and fracture surface of the specimens is investigated. It is found that vinyl-terminated butadiene acrylonitrile improves the toughness with slight negligible effects on other characteristics. Incorporation of 15 phr of vinyl-terminated butadiene acrylonitrile increases the KIC of epoxy from 0.6 MPa to 2.3 MPam0.5. Similarly, addition of 15 phr vinyl-terminated butadiene acrylonitrile leads to 67% enhancement in the interlaminar fracture toughness of composites. The toughening mechanism...

While few studies have been conducted on modeling the pull-in instability of cantilever nanoelectromechanical systems (NEMS) in electrolyte media, no researchers has investigated this phenomenon in double-clamped NEMS. Herein, the pull-in instability of the NEMS bridge immersed in ionic liquid electrolyte media is explored for the first time considering the size effect and dispersion forces. The strain gradient elasticity in conjunction with von-Karman strain is employed to incorporate the effect of size-dependency and beam stretching in the structural model. The presence of electrochemical force field and nano-scale attractions i.e. Casimir and van der Waals forces is incorporated in the model considering the presence of the liquid media. To solve the nonlinear constitutive equation of the system two different methods including the differential transformation method (DTM) and numerical solution are employed. The proposed model is validated by comparing with the results presented in literature. Impacts of various parameters i.e. the size dependency, dispersion forces, electrolyte ion concentration and potential ratio on the instability characteristics of the NEMS bridge are discussed. The results of present theory are compared with those predicted by the classic continuum theory as well as the modified couple stress theory.

Finite conductivity, size dependency and surface layer are known as crucial factors that affect the electromechanical response and pull-in instability of micro/nano-electromechanical systems (MEMS/NEMS). However, these effects are yet ignored on modeling the instability of MEMS/NEMS fabricated from functionally graded materials (FGMs). Herein, a modified continuum model is developed to incorporate these effects on the dynamic behavior and electromechanical instability of double-sided FGM NEMS bridge. Using Gurtin–Murdoch model in conjunction with nonlocal Eringen elasticity, the governing equations of the nano-bridges is derived considering the surface layer and size dependency. The Coulomb and Casimir forces are incorporated in the governing equation considering the corrections due to the finite conductivity of FGM (relative permittivity and plasma frequency). The validity of the proposed method is elucidated by comparing the results obtained from the present study with the results reported in the literature. The stability analysis of the nanostructure is conducted by plotting time history and phase portraits. The effects of various parameters including finite conductivity, nonlocal parameter, surface stresses and material characteristics on the dynamic pull-in behavior of the nano-bridges are studied. The obtained results imply a significant impact of the abovementioned corrections on the instability threshold and pull-in parameters of the FGM nanobridge.

In this paper, the electromechanical response and instability of the nanobridge immersed in ionic electrolyte media is investigated. The electrochemical force field is determined using double-layer theory and linearized Poisson–Boltzmann equation. The presence of dispersion forces, i.e., Casimir and van der Waals attractions are incorporated considering the correction due to the presence of liquid media between the interacting surfaces (three-layer model). The strain gradient elasticity is employed to model the size-dependent structural behavior of the nanobridge. To solve the nonlinear constitutive equation of the system, three approaches, e.g., the Rayleigh–Ritz method, Lumped parameter model and the numerical solution method are employed. Impacts of the dispersion forces and size effect on the instability characteristics as well as the effects of ion concentration and potential ratio are discussed.

Experimental observations reveal that the physical response of nano-structures is size-dependent. Herein, modified strain gradient theory in conjunction with Euler–Bernoulli beam theory have been used for mathematical modeling of the size dependent instability of nanostructures. Effect of van der Waals intermolecular force has been included in the mathematical model. Two common beam-type systems including double-clamped nano-bridge and clamped-free nano-cantilever have been investigated. Three approaches including using an approximated differential transformation method (DTM), applying iterative numerical method and developing a simple lumped parameter model have been employed to solve the governing equations of the systems. The obtained results have been compared with those obtained via numerical method. Furthermore a lumped parameter model has been developed to simply explain the physical performance of the systems without mathematical complexity. The pull-in parameters of the nanostructures as basic design parameters have been calculated. Effect of the size dependency on the pull-in performance has been discussed for both nano-structures.

Toughening of epoxy with different types of modifiers produces a bimodal blend that might show better fracture resistance in comparison with single-modified ones. In this research, bimodal epoxy formulations including mixtures of glass microsphere and silica nanoparticles are explored for possible synergistic toughening. The influence of composition on the glass transition temperature ( Tg), tensile characteristics, and fracture toughness ( KIC) is investigated. Interestingly, a synergism in fracture toughness is observed when mixtures of modifiers were incorporated. For the fixed overall modifier content, KIC is higher when the volume fraction of glass microsphere is lesser than the volume fraction of nanosilica. Fractographs reveal that glass microsphere increases the toughness of epoxy matrix by crack pinning/bridging mechanisms. On the other hand, nanosilica enhances the toughness by increase in plastic deformation via shear banding/particle debonding. Interestingly, the origin of synergistic toughness in bimodal epoxies is the interaction between the glass microsphere and crack-tip damage zone which provides comprehensive stable crack deflection within the nanosilica-toughened epoxy. This mechanism results in a mixed-mode crack growth that reduces the strain energy release rate in bimodal formulations.

In this research the effects of surface quality and loading history on fatigue behavior of CO 2 laser-machined poly(methyl methacrylate) are investigated. It is found that polishing of laser-machined surfaces can enhance the tensile strength up to 5% while significantly elongating the fatigue life of specimens. Microscopic observations reveal that defects on lateral laser machined surface and heat affected zone lead to a reduction of fatigue life by a factor of ten compared to samples with polished lateral surfaces. In the second part of this research it is observed that the loading sequence has a definite effect on the remaining life of poly(methyl methacrylate). Finally the ability of several cumulative damage formulations for modeling the fatigue life in multi-stage loading is evaluated, out of which Manson–Halford and Otani–Song models outperformed the others.

Herein, the fracture toughness of ternary epoxy systems containing nanosilica and hollow glass microspheres (HGMS) is investigated. The experimental measurements reveal synergistic fracture toughness in some hybrid compositions: The incorporation of 10 phr of HGMS and nanosilica alone modify the fracture toughness of epoxy by 39% and 91%, respectively. However, use of 10 phr hybrid modifier can enhance the fracture toughness of the resin up to 120%. Observations reveal different toughening mechanisms for the blends i.e., plastic deformation for silica nanoparticles and crack bifurcation for HGMS. Both of these toughening mechanisms additively contribute to the synergism in ternary epoxies.

It is well-recognized that the electromechanical response of a nanostructure is affected by its element size. In the present article, the size dependent stability behavior and nanotweezers fabricated from nanowires are investigated by modified couple stress elasticity (MCSE). The governing equation of the nanotweezers is obtained by taking into account the presence of Coulomb and intermolecular attractions. To solve the equation, four techniques, i.e., the modified variational iteration method (MVIM), the monotonic iteration method (MIM), the MAPLE numerical solver, and a lumped model, are used. The variations of the arm displacement of the tweezers versus direct current (DC) voltage are obtained. The instability parameters, i.e., pull-in voltage and deflection of the system, are computed. The results show that size-dependency will affect the stability of the nanotweezers significantly if the diameter of the nanowire is of the order of the length scale. The impact of intermolecular attraction on the size-dependent stability of the system is discussed.

Experimental observations reveal that the physical response of nanostructures is size-dependent. Herein, modified couple stress theory has been used to study the effect of intermolecular van der Waals force on the size dependent pull-in of nanobridges and nanocantilevers. Three approaches including using differential transformation method, applying numerical method and developing a simple lumped parameter model have been employed to solve the governing equation of the systems. The pull-in parameters i.e. critical tip deflection and instability voltage of the nanostructures have been determined. Effect of the van der Waals attraction and the size dependency and the importance of coupling between them on the pull-in performance have been discussed.

ZnO nanoparticles are synthesized and applied as ethanol gas sensors. In some cases, the sensitivity and response time of these particles are shown to be higher than that has been reported in the literature. It has been investigated that the most possible reason for this higher gas sensing performance can be attributed to the quantity of the activity coefficient of its initial components. However, other effects such as pH and thermal decomposition are of importance as well. Specific ion interaction (SIT) model is applied to derive the mean activity coefficient values of the additives used in synthesis of ZnO nanoparticles.

It has been well established that the physical performance of nanodevices might be affected by the microstructure. Herein, a two-degree-of-freedom model base on the modified couple stress theory is developed to incorporate the impact of microstructure in the torsion/bending coupled instability of rotational nanoscanner. Effect of microstructure dependency on the instability parameters is determined as a function of the microstructure parameter, bending/torsion coupling ratio, van der Waals force parameter and geometrical dimensions. It is found that the bending/torsion coupling substantially affects the stable behavior of the scanners especially those with long rotational beam elements. Impact of microstructure on instability voltage of the nanoscanner depends on coupling ratio and the conquering bending mode over torsion mode. This effect is more highlighted for higher values of coupling ratio. Depending on the geometry and material characteristics, the presented model is able to simulate both hardening behavior (due to microstructure) and softening behavior (due to torsion/bending coupling) of the nanoscanners.

The fracture behavior of a hybrid-rubber-modified epoxy system was investigated. The modified epoxy included amine-terminated butadiene acrylonitrile (ATBN) rubber and recycled tire particles as fine and coarse modifiers, respectively. The results of the fracture toughness (KIC) measurement of the blends revealed synergistic toughening in the hybrid system when 7.5-phr small particles (ATBN) and 2.5-phr large particles (recycled tire) were incorporated. Transmission optical micrographs showed different toughening mechanisms for the blends; fine ATBN particles increased the toughness by increasing the size of the damage zone and respective plastic deformation in the vicinity of the crack tip. However, in the case of hybrid resin, coarse recycled rubber particles acted as large stress concentrators and resulted in the branching of the original crack tip. Mode mixity at the branch tips led to synergistic KIC in the hybrid system. It seemed that the ductility of the matrix played an effective role in the nature of the crack-tip damage zone in the hybrid epoxies. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Effect of loading rate on toughness characteristics of hybrid rubber-modified epoxy was investigated. Epoxy was modified by amine-terminated butadiene acrylonitrile (ATBN) and recycled tire. Samples were tested at various loading rates of 1–1000 mm/min. Fracture toughness measurements revealed synergistic toughening in hybrid system at low loading rates (1–10 mm/min); hybrid system exhibited higher fracture toughness value in comparison with the ATBN-modified resin with same modifier content. However, synergistic toughening was eliminated by increasing the loading rate. At higher loading rates (10–1000), the fracture toughness of hybrid system decreased gradually to the level lower than that of ATBN-modified epoxy. Fractography of the damage zones showed the toughening mechanisms of ATBN-modified system was less affected by increasing the loading rate compared to that of hybrid system. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Herein, the deflection and instability of a freestanding carbon nanotube (CNT) probe/sensor in the vicinity of the graphene layers are investigated. A nanoscale continuum model is employed to obtain nonlinear constitutive equation of freestanding CNT. The van der Waals attraction is computed from the simplified Lennard-Jones potential for two common engineering cases including large and small number of graphene layers. Four approaches, i.e., approximated function, homotopy perturbation method, lumped parameter model and numerical analysis, are employed to solve the governing equation of CNT. The intermolecular force-induced deflection, minimum initial gap and stable length of freestanding CNT are determined. The analytical results agree well with the numerical solutions.

In this work, solvent blending in combination with extruding are applied to provide polystyrene/silica nanocomposite specimens. Transmission electron microscope (TEM) and scanning electron microscope (SEM) show same nanoparticle dispersion in PS matrix in low to high filler loadings. Differential scanning calorimetry (DSC), dynamic mechanical thermal analyzer (DMTA) and thermogravimetric analyzer (TGA) were used to study the thermophysical characteristic of the nanocomposites in solid state. In addition, the melt state rheological behavior of the samples was investigated under constant and zero shear rates. Interestingly, different behaviors were detected in nanocomposites in low and high nanoparticle loadings. In addition, rheological characteristics of molten polymer are dramatically affected in samples with low nanosilica concentration while stabilized in high filler loadings.

There is limited research on the effect of silica on the mechanical properties of polystyrene. For this lack of information, this study has focused on the fracture mechanism and mechanical properties of Polystyrene/silica nanocomposite. Transmission electron microscopy showed proper dispersion of nanoparticles in PS matrix in both low and high filler loadings. Scanning electron microscopy, TOM micrography, and non-contact surface profiler were used to study the fracture surface and fracture mechanism characteristics of the nanocomposite. It seems that the debonding mechanism is an important mechanism in toughening of Polystyrene/silica nanocomposites. In addition, mechanical behavior of the samples was investigated. Tensile, flexural, and compressive strength and also impact and plain-strain fracture toughness of nanocomposite samples showed different behaviors in low and high nanoparticle loadings and interestingly, we found an optimum value less than 2% for nanoparticle loading in which we observed the highest improvement in mechanical properties of the nanocomposite.

In this paper, tensile strain energy absorption of two different hybrid modified epoxies has been systematically investigated. In one system, epoxy has been modified by amine-terminated butadiene acrylonitrile (ATBN) and hollow glass spheres as fine and coarse modifiers, respectively. The other hybrid epoxy has been modified by the combination of ATBN and recycled Tire particles. The results of fracture toughness measurement of blends revealed synergistic toughening for both hybrid systems in some formulations. However, no evidence of synergism is observed in tensile test of hybrid samples. Scanning electron microscope (SEM), transmission optical microscope (TOM) and finite element (FEM) simulation were utilized to study deformation mechanisms of hybrid systems in tensile test. It is found that coarse particles induce stress concentration in hybrid samples. This produces non-uniform strain localized regions which lead to fracture of hybrid samples at lower tensile loading and energy absorption levels.

Modeling the buckling of multi walled carbon nanotube (MWCNT) probes/actuators in the vicinity of thin and thick graphite has been carried out for the first time via two analytical approximation methods as well as a numerical one. A hybrid nano-scale continuum model based on Lennard-Jones potential is applied to simulate the intermolecular force-induced deflection of MWCNT. The critical values of MWCNT tip deflection and MWCNT-graphite attraction at the onset of the instability are computed. In addition, minimum nanotube-graphite initial gap and stable length of freestanding CNT are determined as basic parameters for engineering applications and nano-devices design. The stable length of MWCNT is determined as a function of its geometrical and material characteristics, initial gap and number of graphene layers.

In this paper, a power series solution is used to study effect of intermolecular forces on the instability of multi walled carbon nanotube (MWCNT) probes/actuators in the vicinity of thin and thick graphite. A hybrid nano-scale continuum model based on Lennard-Jones potential is applied to simulate the intermolecular force-induced deflection of MWCNT. A power series method basis on symbolic computation is introduced to obtain an analytical solution for the distributed parameter model. The critical values of MWCNT tip deflection and MWCNT-graphite attraction at the onset of the instability are computed. Minimum nanotube-graphite initial gap and stable length of freestanding carbon nanotube are determined as basic parameters for engineering applications and nano-devices design. The stable length of MWCNT is determined as a function of its geometrical and material characteristics, initial gap and number of graphene layers. The obtained results are compared with the Adomian decomposition and Green's function as well as numerical results. The obtained results in compare with numerical results represent remarkable accuracy rather than the Adomian decomposition method and Green's function.

Spinning is frequently used for manufacturing axisymmetric shapes where press tooling might not be justified on grounds of size and production volumes. Spinning also has the possibility of producing parts that could not be deep drawn. The temperature effect is significant on the metal forming processes; for the quality of products and the tools life are extremely affected by it. In this paper, a three-dimensional explicit finite-element analysis (FEA) is employed to simulate the spinning process of an aluminium circular sheet and also we investigate the thermal effects on the conventional spinning process by the explicit finite element method. The energy terms in this study include the plastic strain energy, the frictional sliding energy, and the heat transfer energy. The various mesh types are examined in the simulations. The main benefit of the proposed model will save the tremendous costs in the die designing and the experimental works. The parameters and techniques using in the numerical model are helpful for the design of forming process and the coupling thermal-mechanical analysis.

Fatigue behavior of quasi-isotropic carbon/epoxy laminates were studied, using six stacking sequences. Experimental results show that there is a significant stacking sequence effect on monotonic and cyclic performance of laminates. Though by changing the stacking sequence the difference between failure strength of the strongest and weakest lay-ups reaches only to ∼ 12%, the difference between fatigue lives (cycles to failure) increased ∼ 1000% in some loading regions. In addition, both linear and sigmoidal functions were examined to model the fatigue life behavior.

The effect of loading rate on toughening mechanisms of rubber-modified epoxy resin was investigated using amine-terminated butadiene acrylonitrile (ATBN) rubber. Rubber-modified samples were tested at various loading rates in the range of 1–1000 mm/min. At low loading rates, modified resin exhibited a high fracture toughness, which decreased by increasing the rate of loading. At higher loading rate a transition region was observed, where the fracture toughness did not change noticeably with increasing the loading rate. Beyond the transition region, the resin fracture toughness dropped dramatically to the level of unmodified epoxy. Fractography of the damage zone showed different toughening mechanisms for the various loading rate regions, i.e., shear yielding and crack branching. It is believed that the interaction between these two toughening mechanisms, each dominating in one extreme loading rate condition, is responsible for the intermediate transition region, where toughness is virtually independent fr...

Rubber modification of hoop filament wound epoxy composites was investigated using amine-terminated butadiene acrylonitrile (ATBN) and carboxyl-terminated butadiene acrylonitrile (CTBN) oligomers. Both mechanical and physical properties of modified composites were investigated and the morphology of the samples was studied via scanning electron microscopy. Results indicated that, when added to the epoxy matrix, ATBN effected a superior increase in toughness, yet a more severe decline in compressive and flexural strength, compared to CTBN. In contrast, in the case of composite samples, incorporation of ATBN led to more favorable mechanical properties, from the viewpoint of both toughness and strength, relative to CTBN. This curious observation was generally attributed to the different amounts of rubber–epoxy precipitation and fiber–matrix adhesion, as well as various degrees of cavitation.

Toughness improvement of an epoxy resin and respective hoop wound composite were investigated systematically using amine-terminated butadiene acrylonitrile (ATBN) liquid rubber. Rubber modification improves fracture toughness of epoxy resin with slight reduction in the glass transition temperature (Tg), flexural and compressive properties of resin. Impact resistance of composite is improved by rubber modification similar to modified resin. Interlaminar shear strength (ILSS), compressive modulus and strength, and flexural strength of composite decreased slightly with rubber modification. To interpret the data, the void content of composite samples was determined and the damaged surfaces of fractured samples were investigated. The findings explained the performance of rubber modified specimens in comparison with the unmodified one. In addition, a simple formulation was developed to predict the impact strength of modified composite samples.

Surface energy and size phenomenon can play significant roles in physical performance of nano-electromechanical systems. Herein, the static and dynamic pull-in behavior of nano-tweezers and nano-switch fabricated from conductive cylindrical nano-wires is studied. The Gurtin–Murdoch surface elasticity in combination with the couple stress theory is employed to incorporate the coupled effects of surface energy and microstructure dependency (size phenomenon). Using Green–Lagrange strain, the higher-order surface stress components are incorporated in the nonlinear governing equation. The effect of gas damping is considered in the model as well as structural damping. The governing equation is solved using the reduced order method. The effects of various parameters on the static and dynamic pull-in parameters, phase plans and stability threshold of the nano-structures are demonstrated.

Nanoscale beams might not be considered uniform isotropic since the energy of the surface layer and microstructure of the bulk material highly affect the mechanical characteristics of the beams. Herein, the simultaneous effects of energy of the surface and microstructure of the bulk on the dynamic response and stability of beam-type electromechanical nanocantilevers are investigated. A bilayer model has been developed which incorporates the strain energy of the surface atoms as well as the microstructure-dependent strain energy of the bulk. The Gurtin–Murdoch surface elasticity in conjunction with the modified couple stress theory (MCST) is applied to derive the governing equation. Since the classical assumption for zero normal surface stresses is not consistent with the surface equilibrium assumption in Gurtin–Murdoch elasticity, the presence of normal surface stresses is incorporated. The von Karman nonlinear strain is employed to derive the governing equation. The presence of gas rarefaction at various Knudsen numbers is considered as well as the edge effect on the distribution of Coulomb and dispersion forces. The mode shapes of the nanobeam are determined as a function of the surface and microstructure parameter and the nonlinear governing equation is solved using Galerkin method. The dynamic response, phase plane and stability threshold of the nanocantilever are discussed.

The physico-mechanical behavior of nanoscale devices might be microstructure dependent. However, the classical continuum theory cannot correctly predict the microstructure dependency. In this paper, the strain gradient theory is employed to examine the instability characteristics of a nanoscanner with circular geometry. The governing equation of the scanner is derived incorporating the Coulomb and van der Waals (vdW) forces. The influences of applied voltage, squeeze damping and microstructure parameters on the dynamic instability of equilibrium points are studied by plotting the phase portrait and bifurcation diagrams. In the presence of the applied voltage, the phase portrait shows the saddle-node bifurcation while for freestanding scanner a subcritical pitchfork bifurcation is observed. It is concluded that the microstructure parameter enhances the torsional stability.

Herein, the impact of surface layer on the stability of nanoscale tweezers and cantilevers fabricated from nanowires with cylindrical cross section is studied. A modified continuum based on the Gurtin–Murdoch surface elasticity is applied for incorporating the presence of surface layer. Considering the cylindrical geometry of the nanowire, the presence of the Coulomb attraction and dispersion forces are incorporated in the derived formulations. Three different approaches, i.e. numerical differential quadrature method (DQM), an approximated homotopy perturbation method (HPM) and developing lumped parameter model (LPM) have been employed to solve the governing equations. The impact of surface layer on the instability of the system is demonstrated.

Surface effect often plays a significant role in the pull-in performance of nano-electromechanical systems (NEMS) but limited works have been conducted for taking this effect into account. Herein, the influence of surface effect has been investigated on instability behavior of cantilever nano-actuator in the presence of van der Waals force (vdW). Three different methods, i.e. an analytical modified Adomian decomposition (MAD), Lumped parameter model (LPM) and numerical solution have been applied to solve the governing equation of the system. The results demonstrate that surface effect reduces the pull-in voltage of the system. Moreover, surface energy causes the cantilever nano-actuator with the assigned parameter to deflect as a softer structure. It is found that while surface effect becomes important for low values of the cantilever nano-actuator thickness, vdW attraction is significant for low initial gap values. Surprisingly, the increase in the initial gap, enhances the contribution of surface effect in pull-in instability of the system while reduces the contribution of vdW attraction. Furthermore, the minimum initial gap and the detachment length of the cantilever nano-actuator that does not stick to the substrate due to vdW force and surface effect has been approximated. A good agreement has been observed between the values of instability parameters predicted via these three methods. Whilst compared to the instability voltage predicted by numerical solution, the pull-in voltage obtained by MAD series and LPM method is overestimated and underestimated, respectively.

In this work, we have analyzed two synthesis procedures through experimental characterizations, where one provides a main temperature region for the control of the shape and size of ZnO nano-particles in comparison to the other. We have found that the complexing agent has a significant role in showing such a control region. This effect might also improve the fabrication and properties of other interesting and applicable nano-structures.

It is well recognized that size-e ect often plays a signi cant role in the mechanical performance of nano-structures. Herein, strain gradient continuum elasticity is employed to investigate the size dependent pull-in instability of the cantilever nanoactuators immersed in ionic liquid electrolyte. The presence of dispersion forces, i.e. Casimir and van der Waals eld, is considered in the theoretical model as well as the double-layer electrochemical attraction. To solve the non-linear constitutive equation of the system, two approaches, i.e. the Rayleigh Ritz Method (RRM) and the numerical solution method, are employed. Impact of the size dependency and dispersion forces on the instability characteristics are discussed as well as the e ect of ion concentration in liquid.

Stability analysis and modeling the electromechanical response of nanotweezers is crucial for reliable design and manufacturing of these nano-devices. Herein, a modified model is developed for static and dynamic stability analysis of nanotweezers with low width to thickness ration (narrow width). The surface elasticity in conjunction with the strain gradient theory is employed to consider the coupled effects of scale dependency i.e. size-dependency of material characteristics and surface layer. The nonlinear governing equation was solved using analytical Rayleigh-Ritz method (RRM). The influence of various parameters including scale dependency, surface stresses, damping parameter and dispersion forces on the stability of the tweezers is addressed. Furthermore the maximum length and the minimum gap of the tweezers are computed which are of the important design parameters.