AbstractAlthough it is generally agreed that microscale material structural features of gas shale dictate their mechanical and transport properties, which are key for efficient resource extraction,...
AbstractRate effects are dominant in many fracture processes including the separation of mineralized collagen fibrils under tension, the failure of bulk metallic glasses under compression or the na...
In this paper, the fracture process of nanoscale diamond is analyzed using atomistic simulations and fracture toughness obtained using four different continuum fracture-mechanics theories. In particular, the authors have used (1) the Griffith's energy release rate (Irwin modified), (2) crack-tip-opening displacement (CTOD) method, (3) Irwin's K-based method, and (4) the modified crack closure method. Three different nanosized cracks have been considered: 5a(0), 7a(0), and 9a(0), where a(0) = 0.357 nm as diamond's lattice constant. For applying the CTOD and the Griffith's methods, molecular dynamics (MD) simulation is sufficient to obtain fracture toughness values. In addition to MD simulation, the other two methods need supplementary finite-element analysis. The authors evaluated fracture toughness of diamond in terms of critical stress-intensity factors (K-IC) and critical energy release rate (G(IC)) using the four methods and found consistent fracture toughness values (approximately 8.85 MPa.m(0.5)) for diamond regardless of methods and crack lengths considered. (c) 2017 American Society of Civil Engineers.
On the basis of Reissner's mixed variational theorem (RMVT), rather than the principle of virtual displacement (PVD), the authors presented a nonlocal Timoshenko beam theory (TBT) for the geometrically nonlinear static analysis of multiwalled carbon nanotubes (MWCNT) embedded in an elastic medium. The embedded MWCNT was subjected to mechanical loads on its outer-most surface, with combinations of free, simply supported, and clamped edge conditions. The van der Waals interaction between any pair of walls constituting the MWCNT was considered, and the interaction between the MWCNT and its surrounding medium was simulated using the Pasternak-type foundation model. In the formulation, the governing equations of a typical wall and the associated boundary conditions were derived, in which von Karman geometrical nonlinearity was considered. Eringen's nonlocal elasticity theory was used to account for the small-length scale effect. The deformations induced in the embedded MWCNT were obtained using the differential quadrature method and a direct iteration approach. In the numerical examples, solutions of the RMVT-based nonlocal TBT converged rapidly, and the convergent solutions of its linear counterpart closely agreed with the analytical and numerical solutions of the PVD-based nonlocal beam theories available in the literature. (C) 2017 American Society of Civil Engineers.
An experimental study was conducted to detect the onset of damage in epoxy embedded carbon nanotubes (CNTs) using a novel combination of in situ electrical resistance and acoustic emission measurements. A simple fabrication process was employed to fabricate epoxy embedded CNTs, and experiments were conducted under quasi-static tensile loading conditions. In situ electrical measurements were made using a high-resolution four circumferential ring probe measurement system. Acoustic emissions were captured simultaneously using sensors mounted within the gauge length area. Both electrical and acoustic responses were later correlated with stress-strain response. The experimental data show that electrical resistance increases monotonically with axial deformation until about maximum tensile stress and later decreases remarkably before failure. Although acoustic counts are less in number at the initial stages of tensile loading, later the count increases significantly after a specimen reaches peak tensile stress owing to the generation and propagation of microcracks from agglomerated CNTs. (c) 2017 American Society of Civil Engineers.
Statistical mechanics has provided powerful techniques to measure mechanical properties of materials at the nanoscale and paved the way for bottom-up computational materials design. The introduction of such techniques in civil engineering applications, namely construction and geotechnical materials, remains limited to the elastic and fracture properties. This paper presents an atomistic approach to calculate the nanoscale cohesion, friction angle, and hardness. This method is based on the application of biaxial external deformation, or stress, in the weakest crystallographic direction in the material. The onset of the failure is characterized by investigating the unloading paths from several points on the stress-strain curve. Such calculations of the failure stress along different deformation paths provide multiple failure Mohr circles in the normal-shear stress space, which is found to provide a failure envelope akin to the Mohr-Coulomb failure criterion that is widely used for the plastic analysis of granular geomaterials. The failure envelope characterizes the nanoscale cohesion and friction angle, which in conjunction with continuum mechanics can be utilized to estimate the nanoscale hardness of layered materials. Application of this method to tobermorite and Na-montmorillonite crystals yields values that are close to the experimental measurements obtained using nanoindentation and atomic force microscopy techniques. (C) 2017 American Society of Civil Engineers.
Porous materials are essential for ion-electrospray propulsion systems (iEPS). These are miniaturized devices designed to provide mobility to small satellites after they have been launched into orbit; they also can be used as ion sources for applications in microtechnology and nanotechnology. This study investigates the use of xerogels as a family of materials for electrospray applications due to their large surface-area: volume ratio and exceptional pore-size uniformity. In particular, laser-ablated carbon xerogel substrates have been selected for initial synthesis and characterization. Preliminary data on emitted ionic current versus operating voltage curves will be presented for arrays of tips 300 mu m tall and 20 mu m wide spaced on a 450-mu m hexagonal lattice. Aerogels and xerogels present an opportunity to attain more-controllable emitter shapes than previous materials such as porous glass or metals. Improved emitter shape control and pore structure uniformity should have a positive impact on device lifetime and performance. (c) 2017 American Society of Civil Engineers.
Lattice approaches have emerged as a powerful tool to capture the effective mechanical behavior of heterogeneous materials using harmonic interactions inspired from beam-type stretch and rotational interactions between a discrete number of mass points. In this paper, the lattice element method (LEM) is reformulated within the conceptual framework of empirical force fields employed at the lattice scale. Within this framework, because classical harmonic formulations are but a Taylor expansion of nonharmonic potential expressions, they can be used to model both the linear and the nonlinear response of discretized material systems. Specifically, closed-form calibration procedures for such interaction potentials are derived for both the isotropic and the transverse isotropic elastic cases on cubic lattices, in the form of linear relations between effective elasticity properties and energy parameters that define the interactions. The relevance of the approach is shown by an application to the classical Griffith crack problem. In particular, it is shown that continuum-scale quantities of linear-elastic fracture mechanics, such as stress intensity factors (SIFs), are well captured by the method, which by its very discrete nature removes geometric discontinuities that provoke stress singularities in the continuum case. With its strengths and limitations thus defined, the proposed LEM is well suited for the study of multiphase materials whose microtextural information is obtained by, e.g., X-ray micro-computed tomography. (c) 2017 American Society of Civil Engineers.
Cement is one of the most consumed materials in the world. The cement industry is responsible for a large portion of global carbon dioxide emissions. Cement production and therefore carbon dioxide emissions can be decreased by increasing the durability and enhancing the mechanical properties of cement-based materials. On the other hand, an important weakness of concrete is its weak tensile properties, which are the main reasons for its failure and low durability. Therefore, over the past 30 years, many studies have focused on improving tensile properties using a variety of physical and chemical methods. One of the most successful attempts is to use polymer fibers in the structure of concrete to obtain a composite with high tensile strength and ductility. However, a thorough understanding of the mechanical behavior of fiber-reinforced concrete requires knowledge of fiber cement interfaces at the nano scale. In this study, a combination of experimental and molecular dynamics (MD) techniques is used to study the nanostructure of fiber cement interfaces. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) analysis are used to obtain a better understanding of the C-S-H fiber (in cement chemistry notation, C = CaO, S = SiO2, and H = H2O) interfaces. The results show that the C/S ratio changes in the interface of cement and polymeric fibers and is largely affected by the functional group of the polymers. The results are then used to propose a more realistic molecular dynamics model for C-S-H in the vicinity of the three most used polymeric fibers: polypropylene, polyvinyl alcohol, and nylon-6. The full atomistic simulations show that the molecular structure of C-S-H at the interface depends on the properties of the polymer functional group. The adhesion energy between the polymeric fibers and the relevant C-S-H structure is then computed using atomistic simulations. The adhesion energy between C-S-H and polymers increases with the polarity of the fiber. The mechanical response of cement paste with added polymeric fibers is then experimentally studied using the split-cylinder test. The experimental results further show that the adhesion energies between the fibers and cement increase as a function of the polarity of the fibers.(C) 2017 American Society of Civil Engineers.
Moisture effects on asphalt before and after oxidative aging are investigated in this paper with the molecular dynamics (MD) simulation method. Density, bulk modulus, and zero shear viscosity changes of unoxidized and oxidized asphalt under different moisture contents are compared. The simulations were conducted at 25°C with 0% and 1%, to 10% moisture inclusion incremented by 2.5%. Simulation results showed that the density, bulk modulus, and zero shear viscosity of oxidized asphalt were higher than those of the unoxidized asphalt before any moisture inclusion. These results indicate that hardening happens in asphalt during oxidation. However, after moisture inclusion, the bulk modulus and the zero shear viscosity of unoxidized and oxidized asphalt decreased with an increase in moisture content. Laboratory validation of zero shear viscosity for the unoxidized asphalt showed a result consistent with MD simulation. The moisture effect on density change was not significant for unoxidized or oxidized asphalt, but the density fluctuations of oxidized asphalt were higher than for the unoxidized asphalt. Moreover, moisture affects the bulk modulus and zero shear viscosity of oxidized asphalt more negatively, compared with the unoxidized asphalt. Specifically, the bulk modulus and zero shear viscosity of oxidized asphalt decreased faster than the unoxidized asphalt with moisture inclusion and became lower than the unoxidized asphalt after 5% moisture inclusion. This result indicates that oxidized asphalt is more susceptible to moisture damage.
Recent studies have showed that microindentation techniques allow assessing the logarithmic creep rate of a cement paste in good correlation with the long-term creep rates measured by compressive tests at macroscopic scale. After having applied microindentation techniques to characterize the effect of relative humidity (RH) on both the creep and relaxation behavior of a cement paste, the objective of this work is to analyze the duality between creep and relaxation curves by means of the analytical models which are currently employed in open literature. First, large grids of creep and relaxation microindentation tests were carried out on a cement paste sample in hygral equilibrium at different levels of RH. Thus, the results were modeled by a viscoelastic model by considering different creep functions (logarithmic and power-law) and corrective terms for initial plasticity under loading. The presented results provide new insights to understand the duality between creep and relaxation rates of a cement paste measured at micrometer scale, especially considering the possible plastic effect. (C) 2017 American Society of Civil Engineers.
Customary micromechanics models for the poroelasticity, creep, and strength of concrete restrict the domain affected by the hydration reaction to the cement paste volume, considering the latter as a thermodynamically closed system with respect to the (chemically inert) aggregate. Accordingly, the famous Powers hydration model appears to be a natural choice for the determination of clinker, cement, water, and aggregate volume fractions entering such micromechanical models. The situation changes once internal curing occurs, i.e., once part of the water present is absorbed initially by the aggregate, and then is sucked back to the cement paste during the hydration reaction. This paper develops an extended hydration model for this case, introducing water uptake capacity of the aggregate and paste void-filling extent as additional quantities. Based on constant values for just these two new quantities, and on previously determined creep properties of cement pastes as functions of an effective water: cement mass ratio (i.e., that associated with the cement paste domain rather than with the entire concrete volume), a series of ultrashort-term creep tests on different mortars and concretes can be very satisfactorily predicted by a standard microviscoelastic mathematical model. This further extends the applicability range of micromechanics modeling in cement and concrete research. (C) 2017 American Society of Civil Engineers.
A new methodology is proposed for investigating compressive failure behavior of cement paste at the micrometer scale. Micropillar geometries are fabricated by focused ion-beam milling on potential calcium-silicate-hydrate (C-S-H) locations identified through energy dispersive spectroscopy (EDS) spot analysis. Uniaxial compression testing of these pillars is performed using nanoindentation equipment. The compressive strength of C-S-H aggregates (225-606 MPa) measured from microcompression tests is found to be consistent with values from multiscale damage and molecular dynamic models. From posttest images, two primary deformation mechanisms at failure were identified; axial splitting and plastic collapse of the entire sample were observed. (C) 2017 American Society of Civil Engineers.
Metal-nanocomposites are drawing attention of the composites community due to improvements in stiffness, strength, crack-bridging ability, and resistance to creep and fracture. The analysis of nanocomposites involves studies at multiple length scales due to the small length of the reinforcement. This paper conducts a detailed study on the mechanical behavior of a metal nanocomposite (Al-BNNT)-made of an aluminum (Al) matrix reinforced with boron nitride nanotubes (BNNTs)-under compressive and shear loadings. First a representative volume element (RVE) is modeled and analyzed using molecular dynamics (MD) simulation. Then the elastic properties are derived for a specially orthotropic lamina using a hierarchical multiscale scheme in conjunction. This result is further extended to derive elastic and shear moduli of bulk nanocomposites with aligned and randomly oriented reinforcement. The result shows excellent agreement with previous experimental observations. The bounds of elastic moduli using Voigt and Reuss formulations diverge with an increase in volume fraction of reinforcement-unlike typical composites, in which these two bounds first diverge and then eventually converge. This anomaly is attributed to the weakness of nanotubes in the radial direction. However, most elastic properties are found to be improved by the reinforcement, especially by double-walled nanotubes. Depending on the type of loading, nanocomposite exhibits failure at the matrix, interface, or nanotubes. This reveals the importance of considering all three loading cases when modeling a nanocomposite. (C) 2017 American Society of Civil Engineers.
Biomaterials selection and design, and mechanical properties evolution during degradation and tissue regeneration play a critical role in the successful design of nanocomposite scaffolds for bone tissue regeneration. A new multiscale mechanics-based in silico approach is developed to provide a robust predictive methodology for nanocomposite scaffolds. Scaffolds are fabricated using amino acid-modified nanoclay with biomineralized hydroxyapatite (in situ HAPclay) and polycaprolactone (PCL). Steered molecular dynamics (SMD) simulations of the molecular models of HAPclay and the PCL composite provide a mechanical response of the material and the nature of the molecular interactions among constituents. The mechanical responses obtained from SMD are incorporated into a finite element (FE) model of a PCL/in situ HAPclay scaffold with its microstructure obtained from microcomputed tomography images. The model is validated using experimental results. The stress-strain response from multiscale models and experiments shows good agreement with the consideration of wall porosity correction. The multiscale models incorporate damage mechanics-based degradation and healing behavior to capture the evolution of the mechanical properties as the scaffolds degrade and human osteoblasts grow and proliferate inside the scaffolds. The novel multiscale models provide a robust prediction of the mechanical properties evolution in the scaffolds over the time evolution of cell growth proliferation and tissue formation. (C) 2017 American Society of Civil Engineers.
In this work, the effect of surface elasticity on the effective elastic properties of nanoporous gold is studied. To this end, a theoretical framework for surface elasticity effects in submicron-sized solids is implemented as a user-defined finite element subroutine. This allows the use of the theory in large-scale engineering problems. The theory suggests a zero-thickness surface accommodating unique energetic properties and surface tension. For the example of ball-and-stick diamond cubic unit cell structures for nanoporous gold, it is shown that incorporation of surface excess elasticity and surface tension allows prediction of the size effect associated with the change of the surface area-to-volume ratio by capturing, e.g., the increase in the effective Young's modulus and decrease in the effective Poisson's ratio with decreasing ligament diameter, a phenomenon that is not accessible to classical continuum elasticity approaches.
In this paper, conventional nonlocal constitutive equations are examined in the three-dimensional buckling problem of rectangular nanoplates. Those constitutive equations that are frequently used in mechanical analysis of nanostructures are studied in detail. It is shown that both integral and equivalent differential forms of the nonlocal constitutive equations have been originally derived based on interior points of a subbody (e.g., a nanostructure) where there is no surface effect and the cohesive zone is axisymmetric. So, they cannot model the small-scale effects accurately at all points of a nanostructure. It is shown that because the cohesive zone is no longer axisymmetric at the boundary layer, some additional loads are implicitly exerted by using those nonlocal constitutive equations. To show the effect of common nonlocal constitutive equations, buckling problem of simply supported rectangular nanoplates is solved analytically on the basis of the three-dimensional nonlocal elasticity theory. It is shown that because of such additional loads, the critical buckling load is obtained equal to zero, which is physically incorrect. Finally, some techniques are proposed to overcome some limitations of the conventional nonlocal constitutive equations. (c) 2017 American Society of Civil Engineers.
This study examines the static stability of a heavy axially compressed nanorod. The rod is free at one end, while the other end is rigidly connected to a circular disk positioned on an elastic half space. The critical values of load parameters, i.e., axial force and specific weight, for which the nanorod loses stability are determined by using the Euler method, which found that there is critical value of the length scale parameter. For values greater than the critical value, there is no possibility to apply adjacent equilibrium method for stability analysis. Postcritical shape of the rod is determined by numerical integration of the corresponding system of equations. Also discussed is the relation between integral and differential form of the Eringen model. (c) 2017 American Society of Civil Engineers.
Nanoscale structural heterogeneities were recently revealed in computational and experimental studies of calcium silicate hydrates in hardened cement pastes. In this work their consequences for the mechanics are analyzed by computing local pressures in model samples at different overall densities, corresponding to different initial water-to-cement ratios. The correlations between pore size distributions, local density, local cohesive energy, and local pressure clearly show how in these materials structural heterogeneities may be the origin of significant mechanical heterogeneities. The results indicate that even at high density pressure, heterogeneities develop during the densification of cement hydrates and result in the coexistence of regions of high positive pressure with regions of negative pressure, in spite of the overall mechanical stability of the samples. Furthermore, the regions of negative pressure, prone to mechanical instabilities and local plastic processes, tend to be localized close to the surface of large mesopores and hence to be more significant at higher initial water content.(C) 2017 American Society of Civil Engineers.
