A size-dependent inhomogeneous beam model, which accounts for the through-length power-law variation of a two-constituent axially functionally graded (FG) material, is deduced in the framework of the ...nonlocal strain gradient theory and the Euler–Bernoulli beam theory. By employing the Hamilton principle, the equations of motion and boundary conditions for size-dependent axially FG beams are deduced. A material length scale parameter and a nonlocal parameter are introduced in the axially FG beam model to consider the significance of strain gradient stress field and nonlocal elastic stress field, respectively. The bending, buckling and vibration problems of axially FG beams are solved by a generalized differential quadrature method. The influences of power-law variation and size-dependent parameters on the bending, buckling and vibration behaviors of axially FG beams are investigated. The mechanical behaviors can be affected by the through-length grading of the FG material and therefore may be controlled by choosing appropriate values of the power-law index. When considering concentrated and uniformly distributed loads, the maximum deflection decreases with increasing length scale parameter. The axially FG beam may exert a stiffness-softening effect or a stiffness-hardening effect on the critical buckling force and the natural frequencies depending on the values of the two size-dependent parameters.
•SCC induced by machined surface residual stress in boiling MgCl2 was tested.•SCC micro-crack density was evaluated as a function of machined residual stress.•A strong correlation between residual ...stress and SCC micro-crack density was observed.•There exists a critical residual stress for SCC micro-crack initiation.
The effect of machining-induced surface residual stress on the stress corrosion cracking (SCC) initiation in 316 stainless steel was investigated in boiling magnesium chloride solution. The crack density was used to evaluate the SCC initiation and propagation at different residual stress levels. The results showed a strong correlation between the residual stress and the resultant micro-crack density. When the residual stress reached a critical value, the micro-crack density increased significantly in the very early phase, and the critical stress is 190MPa for 316 stainless steel. Additionally, the cracking behavior could be correlated with the machining effects on the surface layer.
Hybrid structure of graphene sheets supported by carbon nanotubes (CNTs) sustains unique properties of both graphene and CNTs, which enables the utilization of advantages of the two novel materials. ...In this work, the capability of three-dimensional pillared graphene structure used as nanomechanical sensors is investigated by performing molecular dynamics simulations. The obtained results demonstrate that: (a) the mass sensitivity of the pillared graphene structure is ultrahigh and can reach at least 1 yg (10
g) with a mass responsivity 0.34 GHz · yg
; (b) the sizes of pillared graphene structure, particularly the distance between carbon nanotube pillars, have a significant effect on the sensing performance; (c) an analytical expression can be derived to detect the deposited mass from the resonant frequency of the pillared graphene structure. The performed analyses might be significant to future design and application of pillared graphene based sensors with high sensitivity and large detecting area.
In this paper, the damping capacity and mechanical strength of Ni-coated carbon nanotube (CNT) reinforced copper-matrix nanocomposites (Ni-coated CNT/CMNc) and single-crystal copper are investigated ...using molecular dynamics (MD). It is found that the mechanical strength of copper can be significantly improved by the embedded Ni-coated CNT. However, a relatively higher dissipation rate is observed for the Ni-coated CNT/CMNc compared with single-crystal copper. To have a better understanding of the augmented dissipation rate for Ni-coated CNT/CMNc, the effects of oscillation frequency and temperatures on the quality factor (Q factor) are explored. The simulation results show that the Q factor decreases with the increase in angular frequency or temperature for both single-crystal copper and Ni-coated CNT/CMNc. In addition, a weaker frequency and temperature dependence is obtained for the case of Ni-coated CNT/CMNc compared with single-crystal copper. Furthermore, by tracing the source of dissipated energy, we demonstrate that the distorted Cu lattice structure caused by the attraction of Ni is the dominant factor for the high damping rate of Ni-coated CNT/CMNc.
Display omitted
•The mechanical strength of single-crystal copper can be greatly enhanced by Ni-coated carbon nanotube.•Ni-coated carbon nanotube reinforced copper-matrix nanocomposites show a larger dissipate rate.•Distortion of copper lattice caused by the attraction of Ni is responsible for the high dissipate rate.•Quality factor decreases with the increase of angular frequency and temperature.
With the increasing demand on structural toughness and energy absorption capacity, the addition of fiber to concrete has become an issue of significance. This paper studies the bond behavior of ...Basalt FRP (BFRP) minibar with straight, twisted and crimped geometric characteristics. The failure modes, pullout curves and bond strength were compared, and their interface degradation, damage evolution and failure mechanism between single fiber and concrete matrix were analyzed. Additionally, commercial steel fibers and polypropylene fibers were compared with the BFRP minibars to clarify the effect of fiber type on failure morphology, bond strength and interface properties. The results showed that the pulling out process for straight BFRP minibars, twisted BFRP minibars and crimped BFRP minibars was constituted by the elastic bonding stage, the partial de‐bonding stage, the complete de‐bonding stage and the pullout of fiber. The bond strength was composed of chemical adhesion and mechanical bond, where the mechanical bond play important role after the initial failure of chemical adhesion. The bond strength of crimped BFRP minibar, the hooked steel fiber, the indented polypropylene fibers and the twisted BFRP minibar were enhanced by 358%, 324%, 115%, and 54.5% compared to straight BFRP minibars. The interface bonding strengths improve with the increase of matrix strength. Dense microstructures of matrix improve the bond strength. The bond‐slip constitutive model was proposed for crimped, twisted and straight BFRP minibars with regression coefficient of 0.978, 0.891, and 0.992, respectively.
Highlights
Influence of fiber geometry and concrete strength on the pullout behavior
Interface degradation, damage evolution and failure mechanism between minibar and concrete
Bond‐slip constitutive model of straight, twisted and crimped BFRP minibars
Evolution of bond strength and pullout energy.
In this study, the ultra‐low porous auxetic piezoelectric composite made of lead zirconate titanate (PZT) and hollow polyvinylidine difluoride (PVDF) is first proposed by combining “additive” and ...“subtractive” approaches. The electromechanical properties of the auxetic piezoelectric composite are investigated by finite element analysis. It is found that the ultra‐low porosity in the composite may lead to great extensibility along with the excellent figures of merit obtained in conventional highly porous piezoelectric ceramics. The small amount of additional PVDF can greatly tune the electromechanical properties of composite and effectively reduce the maximum stress of PZT. To elucidate the role of Poisson’s ratio on the electromechanical properties of piezoelectric materials, the relationship expressions among some electromechanical coefficients are derived. Moreover, the semi‐empirical expression of “longitudinal” compliance coefficients and a simple strategy for predicting some electromechanical coefficients are presented to instruct design and application of the ultra‐low porous auxetic piezoelectric materials.
The recently synthesized novel 1D diamond nanothreads (DNTs) have been reported to possess excellent mechanical property and may act as extraordinary new reinforcements for polymer-matrix ...nanocomposites. By performing a series of molecular dynamics simulations of cross-linked epoxy nanocomposites, the presence of DNTs are found to provide a remarkable improvement in the mechanical property of DNTs/epoxy nanocomposites, yielding an increase of ∼33% in tensile modulus compared with neat epoxy resin. Interestingly, a moderate aggregation of DNTs enables better enhancement in the tensile modulus, which contradicts to common notion that aggregation of nanofillers usually causes a severe deterioration on the mechanical property of nanocomposites. This encouraging reinforcing mechanism mainly stems from two aspects: 1) aggregated DNTs cause a less deterioration to the tensile modulus of cross-linked epoxy network than uniformly dispersed DNTs, and 2) aggregated DNTs enable a higher reinforcing efficiency along the loading direction owing to their straight structure arising from their relatively high bending rigidity. Moreover, the mechanical property of DNTs/epoxy nanocomposites can be further enhanced through the DNTs' functionalization because of improved load transfer capability across DNT/matrix interface. Our results suggest that DNTs are truly attractive reinforcement and may provide valuable design guidelines for high performance nanocomposites applications.
Size-dependent nonlinear Euler-Bernoulli and Timoshenko beam models, which account for the through-thickness power-law variation of two-constituent functionally graded (FG) materials, are derived to ...investigate the nonlinear bending and free vibration behaviors in the framework of the nonlocal strain gradient theory. The nonlinearity due to the stretching effect of the mid-plane of the FG beam is the source of nonlinearity of the considered bending and free vibration problems. The size-dependent equations of motion and boundary conditions are derived by employing the Hamilton’s principle. The beam models contain material length scale and nonlocal parameters to consider the effects of both inter-atomic long-range force and microstructure deformation mechanism. In the case of hinged-hinged boundary conditions, the analytical solutions for the nonlinear bending deflection and free vibration frequencies of nonlocal strain gradient Euler-Bernoulli and Timoshenko beams are deduced. The influences of the through-thickness power-law variation of a two-constituent FG material and size-dependent parameters on nonlinear bending deflection and free vibration frequencies are investigated. Due to the intrinsic stiffening effect brought by the stretching effect of the mid-plane of the beam, the nonlinear bending deflections are smaller than their linear counterparts under the action of the same force, while the nonlinear vibration frequencies are higher than their linear counterparts for the same amplitude of the nonlinear oscillator. The nonlinear bending deflections and free vibration frequencies can be affected significantly by the through-thickness grading of FG materials in the beam. When the nonlocal parameter is smaller than the material characteristic parameter, the nonlinear FG beam reveals a stiffness-hardening effect. When the material characteristic parameter is smaller than the nonlocal parameter, the FG beam reveals a stiffness-softening effect.
This paper focuses on the buckling behaviors of a micro-scaled bi-directional functionally graded (FG) beam with a rectangular cross-section, which is now widely used in fabricating components of ...micro-nano-electro-mechanical systems (MEMS/NEMS) with a wide range of aspect ratios. Based on the modified couple stress theory and the principle of minimum potential energy, the governing equations and boundary conditions for a micro-structure-dependent beam theory are derived. The present beam theory incorporates different kinds of higher-order shear assumptions as well as the two familiar beam theories, namely, the Euler-Bernoulli and Timoshenko beam theories. A numerical solution procedure, based on a generalized differential quadrature method (GDQM), is used to calculate the results of the bi-directional FG beams. The effects of the two exponential FG indexes, the higher-order shear deformations, the length scale parameter, the geometric dimensions, and the different boundary conditions on the critical buckling loads are studied in detail, by assuming that Young’s modulus obeys an exponential distribution function in both length and thickness directions. To reach the desired critical buckling load, the appropriate exponential FG indexes and geometric shape of micro-beams can be designed according to the proposed theory.
We present a practical computational framework for the coarse-graining of cross-linked epoxies by developing a machine-learning technique, which integrates molecular dynamics simulations with ...artificial neural network (ANN) assisted particle swarm optimization (PSO) algorithm. Key features of the framework include two aspects: (1) determining the bonded interactions via the iterative Boltzmann inversion method to emulate the local structures of the epoxies and, (2) optimizing the nonbonded interaction potentials through the machine-learning approach to reproduce the mechanical properties. Such machine-learning based technique is computationally efficient in searching for the optimal solution of nonbonded potential parameters and enables the CG model to become transferable within a wide range of cross-linking degrees. This is mainly attributed to the fact that ANN can give good predictions based on training database obtained from CG simulations and thus greatly accelerates the PSO algorithm in achieving the optimal solution. On the basis of the DOC-transferable CG model, the cohesive interaction strength is phenomenologically adjusted to preserve the temperature-dependent properties. The CG model allows the mechanical properties of cross-linked epoxies to be predicted with reasonable accuracy over wide ranges of cross-linking degrees and temperature. The proposed framework will become highly beneficial to the design of high performance epoxy-matrix nanocomposites.
Display omitted
•A practical computational framework for coarse-graining cross-linked epoxy is proposed.•Machine-learning technique is computationally efficient in searching for the optimal solution of nonbonded potential parameters of the CG model.•The temperature-dependent properties of epoxy are preserved by phenomenologically adjusting the cohesive interaction strength.•The proposed CG model for epoxy allows the mechanical properties to be reasonable predicted over wide ranges of cross-linking degrees and temperatures.
PDF
