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We studied the effects of important ternary elements, such as Cr, Nb, and V, on the plasticity of $$\upgamma $$ γ -TiAl crystals by calculating the point defect formation energy and the change in the generalized stacking fault energy (GSFE) surface from first-principles calculations. For all three elements, the point defect formation energies of the substitutional defects are lower in the Ti site than in the Al site, which implies that substitution on the Ti site is energetically more stable. We computed the GSFE surfaces with and without a substitutional solute and obtained the ideal critical resolved shear stress (ICRSS) of each partial slip. The change in the GSFE surface indicates that the substitution of Ti with Cr, Nb, or V results in an increase in the yield strength because the ICRSS of the superlattice intrinsic stacking fault (SISF) partial slip increases. Interestingly, we find that Cr substitution on an Al site could occur owing to the small difference between the substitutional defect formation energies of the Ti and Al sites. In that case, the reduction of ICRSSs of the SISF partial slip and twinning would lead to improved twinnability. We discuss the implications of the computational predictions by comparing them with experimental results in the literature.

Advancing and receding angles are physical quantities frequently measured to characterize the wetting properties of a rough surface. Thermodynamically, the advancing and receding angles are often interpreted as the maximum and minimum contact angles that can be formed by a droplet without losing its stability. Despite intensive research on wetting of rough surfaces, the gravitational effect on these angles has been overlooked because most studies have considered droplets smaller than the capillary length. In this study, however, by combining theoretical and numerical modeling, we show that the shape of a droplet smaller than the capillary length can be substantially modified by gravity under advancing and receding conditions. First, based on the Laplace pressure equation, we predict the shape of a two-dimensional Cassie-Baxter droplet on a textured surface with gravity at each pinning point. Then, the stability of the droplet is tested by examining the interference between the liquid surface and neighboring pillars and analyzing the free energy change upon depinning. Interestingly, it turns out that the apparent contact angles under advancing and receding conditions are not affected by gravity, while the overall shape of a droplet and the position of the pinning point are affected by gravity. In addition, the advancing and receding of the droplet with continuously increasing or decreasing volume are analyzed, and it is shown that the gravitational effect plays a key role in the movement of the droplet tip. Also, the gravitational effect on the degree of the stability of the droplet upon the external effect such as vibration is discussed. Finally, the theoretical predictions were validated against line tension-based front tracking modeling (LTM) that seamlessly captures the attachment and detachment between the liquid surface and the solid substrate. Our findings provide a deeper understanding on the advancing and receding phenomena of a droplet and essential insight into designing devices that utilize the wettability of rough surfaces.

The demand for display technology is expected to increase with the continuous spread of portable electronics and with the expected emergence of flexible, wearable, and transparent display devices. A touch screen is a critical component in display technology that enables user interface operations, and the future generation of touch screens, the so-called 3D touch screens, is expected to be able to detect multiple levels of pressure. To enable 3D touch screens, transparent pressure sensors with high linearity over a working range that encompasses the pressure range of human touch (10-100 kPa) are required. In this work, a transparent and linear capacitive pressure sensor is reported with a transmittance over 85% and high linearity (R2 = 0.995) over 5-100 kPa of pressure. To render the sensor transparent, a microstructured "hard" elastomer layer was filled in with a refractive index matching a "soft" elastomer layer, through which light scattering was minimized. High linearity was attained from the sensor's unique architecture that increases the effective area of the capacitor with applied pressure. These attributes render our sensor highly suitable for future 3D touch screen applications.

In this study, we investigate the piezoresistivity of metal nanowire networks embedded in a polymer, which has been widely used as a highly-stretchable strain sensor, by varying the orientation distribution of nanowires. For simplicity, we assume the affine transform of the nanowire network upon stretching and compute the effective conductivity of the nanowire network by recognizing the percolation network connecting low and high voltage boundaries. Orientation-dependence is then studied firstly by varying the range of polar angle and secondly by changing the degree of alignment along loading direction. Since nanowires are embedded in thin nanowire-polymer film in most stretchable sensor applications, instead of having random orientation distribution over full solid angle, nanowires have a limited range of polar angle distribution around 90° (when the surface normal vector of a thin film is given as the zenith direction). We show that the gauge factor (relative resistance change over applied mechanical strain) decreases as the polar angle distribution gets narrower. We then study the effect of nanowire alignment on the piezoresistive response, by assuming partial alignment distribution along the tensile direction. We find that a wide range of the gauge factor, from negative to positive, appears as the initial partial alignment angle varies, and explained such response by analyzing the relative electrical path change between a pair of nanowires. Our study deepens the understanding on the percolation-network based piezoresistive sensors and provides a guideline for designing a stretchable strain sensor with the desired gauge factor.

Reduction of CO2 to carbon monoxide (CO) is the starting point for producing valuable carbon chemicals and liquid fuels. To date, efficient CO2 electroreduction to CO has been accomplished predominantly by using the control of the structural parameters of a catalyst, including its nanostructure and surface morphology. In this study, the highly selective CO formation (∼90% of faradaic efficiency) at a low applied potential (−0.4 V vs. reverse hydrogen electrode) is reported by using the control of the local pH near the reaction site, which has been of less interest compared to other factors in prior studies. The local pH near the reaction site was controlled by using a confined cavity in the Au wrinkle film. Experimental and computational calculation results revealed that such enhancement is mainly related to the few-micrometer-amplitude confined space in the tens of nanometers thickness Au wrinkle film. Notably, the fabrication of the wrinkle catalyst film was extremely simple, permitting large-scale fabrication.

Hydrophobicity is abundant in nature and obtainable in industrial applications by roughening hydrophobic surfaces and engineering micropatterns. Classical wetting theory explains how surface roughness can enhance water repellency, assuming a droplet to have a flat bottom on top of micropatterned surfaces. However, in reality, a droplet can partially penetrate into micropatterns to form a round-bottom shape. Here, we systematically investigate the evolution of evaporating droplets on micropatterned surfaces with X-ray microscopy combined with three-dimensional finite element analyses and propose a theory that explains the wetting transition with gradually increasing penetration depth. We show that the penetrated state with a round bottom is inevitable for a droplet smaller than the micropattern-dependent critical size. Our finding reveals a more complete picture of hydrophobicity involving the partially penetrated state and its role in the wetting state transition and can be applied to understand the stability of water repellency of rough hydrophobic surfaces.

For the facile use of liquid metal composites (LMCs) for soft, stretchable and thermal systems, it is crucial to understand and predict the thermal conductivity of the composites as a function of liquid metal (LM) volume fraction and applied strain. In this study, we investigated the effective thermal conductivity of LMCs based on various mean-field homogenization frameworks including Eshelby, Mori-Tanaka, differential and double inclusion methods. The double inclusion model turned out to make the prediction closest to the experimental results in a wide range of LM volume fractions. Interestingly, we found that the theoretical models based on the assumption of ideal LM dispersion and zero interfacial resistance underestimated the thermal conductivity compared to the experimental results in a low volume fraction regime. By considering the accompanied variations in the LM inclusion's aspect ratios under a typical size distribution of inclusions (∼μm), the change of effective thermal conductivity was predicted under a uniaxial 300% tensile strain. Our study will deepen the understanding of the thermal properties of LMCs and support the designs of stretchable thermal interfaces and packaging with LMCs in the future.

To realize the potential of bioinspired fibrillar adhesive applications ranging from biomedical devices to robotic grippers, there has been a significant effort to improve their adhesive strength and understanding of the underlying adhesion and detachment mechanisms. These efforts include changes to the backing layer, which connects the roots of all of the pillars in the fibrillar adhesive. However, previous approaches such as thickness or elastic modulus changes are selectively advantageous to the adhesive strength depending on the substrate condition because of the trade-off between conformity to misaligned/rough surfaces and increased interfacial stress concentrations. In this work, we explore mechanical divisions (cuts) in the backing layer as a new approach to improve the adhesive strength without this trade-off. We combine experiments and finite element analysis (FEA) to study the effect of the divisions, which decouples the mechanical interaction between the pillars on the divided layers, and show that the adhesive strength can be improved regardless of the substrate condition. Tensile adhesion experiments show increased adhesive strength with cuts to a micropost array (150 μm diameter posts) by approximately 25% for 4 divisions. In situ imaging of pillar detachment shows a transition of the detachment process from a peel-like detachment to a random detachment sequence. FEA simulations of the detachment process suggest that the increased strength may originate from a simultaneous enhancement of the load distribution between the pillars and the compliance of the backing layer.

In order to design composites that mimic the remarkable balance of properties such as strength, toughness, and stiffness of staggered platelet structures in nature, it is crucial to understand their load transfer and failure mechanisms. Recently, we proposed an analytical model to predict the stress distribution within staggered platelet structures for a wide range of constituent materials’ moduli and geometric parameters in the elastic response regime. Here, based on the model, a fracture pattern diagram featuring three distinct mechanisms categorized according to the failure sequences of soft tip, soft shear zone, and hard platelet is constructed. The proposed fracture map is capable of capturing the transition of failure mechanisms observed in crack phase field simulations and draws parallels to mechanisms seen in experiments. Our study sheds light on the origin of failure mechanism transitions and enables rational designs of future staggered platelet composites with unprecedented properties.

Composites are ubiquitous throughout nature and often display both high strength and toughness, despite the use of simple base constituents. In the hopes of recreating the high-performance of natural composites, numerical methods such as finite element method (FEM) are often used to calculate the mechanical properties of composites. However, the vast design space of composites and computational cost of numerical methods limit the application of high-throughput computing for optimizing composite design, especially when considering the entire failure path. In this work, the authors leverage deep learning (DL) to predict material properties (stiffness, strength, and toughness) calculated by FEM, motivated by DL’s significantly faster inference speed. Results of this study demonstrate potential for DL to accelerate composite design optimization.

Molecular dynamics simulations were carried out to study the effect of the loading direction on the deformation behavior of the pearlite structure with a Bagaryatsky orientation relationship at the ferrite-cementite interface. We found excellent ductility in the ferrite and pearlite nanocomposites along the $$\left[ {\bar{1}10} \right]_{f} ||\left[ {001} \right]_{c}$$ loading direction, while a brittle behavior was observed along the $$\left[ {111} \right]_{f} ||\left[ {100} \right]_{c}$$ loading direction because of the reduced number of activated slip systems. Additionally, we reveal that the ductility is improved by either increasing the temperature or reducing the interlamellar spacing.

We systematically investigated the effect of tensile loading direction and additional elements (Cr and Nb) on the initial deformation mechanism in the γ phase of TiAl alloys, which is one of the crucial factors to decide room temperature ductility. First, we carefully chose two different TiAl alloys with limited and enhanced ductility. Second, by synthesizing the sample with a single γ phase via extracting γ composition in both TiAl alloys, the initial deformation mechanism of γ phase is confirmed depending on alloy composition as well as loading direction using an in situ transmission electron microscopy. Third, using the first principle density functional theory calculation, we calculate the change in activation factors of deformation mechanism according to each additional element (Cr and Nb) in TiAl alloys. As a result, it can be understood that the deformation mechanism of the γ phase changes depending on the additional element as well as loading direction. In particular, by comparing the experimental and theoretical study, it was revealed that the activation difference of three deformation mechanisms in the γ phase decreases as Nb is added, which leads to the activation of all deformation mechanisms in the γ phase, and hence obtains the enhanced room temperature ductility in TiAl alloys. Our results provide an effective strategy for enhancing room temperature ductility of TiAl alloys via reduction of activation difference of deformation mechanisms in the γ phase of TiAl alloys, which will open a new era to use TiAl alloys as various structural parts with enhanced ductility.

A theoretical analysis of the sliding of a Cassie-Baxter droplet on a microstructured surface is conducted. The conventional theory based on the force balance has been frequently used to predict the sliding condition of the droplet; however, the sliding condition cannot be precisely determined because the theory requires the available ranges of the contact angles at the rear and front ends of the droplet. In this study, by calculating the droplet shape and examining the stability of a droplet at every possible pinning point, we propose a new theoretical model that can predict the sliding condition of a two-dimensional (2D) Cassie-Baxter droplet without any a priori measurement but using only the surface information. With the proposed theory, we answer two open questions in sliding research: (i) whether the sliding initiates with front end slip or rear end slip and (ii) whether the advancing and receding contact angles measured on the horizontal surface are comparable with the front and rear contact angles of the droplet at the onset of sliding. Additionally, a new droplet translation motion mechanism promoted by a cycle of condensation and evaporation is suggested, which can be further utilized for precise droplet transportation. Finally, the theoretical results are validated against the 2D line-tension-based front-tracking method (LTM), which can seamlessly capture the attachment and detachment between the droplet and the textured surface.

Nanocrystals can exist in multiply twinned structures like the icosahedron, or single crystalline structures like the cuboctahedron or Wulff-polyhedron. Structural transformation between these polymorphic structures can proceed through diffusion or displacive motion. Experimental studies on nanocrystal structural transformations have focused on high temperature diffusion mediated processes. Thus, there is limited experimental evidence of displacive motion mediated structural transformations. Here, we report the high-pressure structural transformation of 6 nm Au nanocrystals under nonhydrostatic pressure in a diamond anvil cell that is driven by displacive motion. In-situ X-ray diffraction and transmission electron microscopy were used to detect the transformation of multiply twinned nanocrystals into single crystalline nanocrystals. High-pressure single crystalline nanocrystals were recovered after unloading, however, the nanocrystals quickly reverted back to multiply twinned state after redispersion in toluene solvent. The dynamics of recovery was captured using transmission electron microscopy which showed that the recovery was governed by surface recrystallization and rapid twin boundary motion. We show that this transformation is energetically favorable by calculating the pressure-induced change in strain energy. Molecular dynamics simulations showed that defects nucleated from a region of high stress region in the interior of the nanocrystal, which make twin boundaries unstable. Deviatoric stress driven Mackay transformation and dislocation/disclination mediated detwinning are hypothesized as possible mechanisms of high-pressure structural transformation., 32 pages, 14 figures, and 1 movie (please open pdf with Adobe Acrobat Reader to see the embedded movie)

The robust and reliable mechanical characteristics of metal nanoparticle (NP) thin films on flexible substrates are important because they operate under tensile, bending, and twisting loads. Furthermore, in wearable printed electronics applications, salty solutions such as sweat and seawater can affect the mechanical reliabilities of devices. In this paper, we investigated the effect of sodium chloride (NaCl) solutions on silver (Ag) NP thin films on flexible polymer substrate. After exposure to NaCl solution of Ag NP thin film, we observed the aggregation behavior between Ag NPs and formation of larger pores in the film due to the removal of organic capping layer from the surface of Ag NPs. The average porosity and 5% deviation strains of Ag NP thin films on the polyimide substrate were dramatically increased and decreased from 2.99% to 9.64% and from 3.94% to 0.87%, respectively, after exposure to NaCl solution for 1 h. Also, we verified a drastic deterioration of the surface adhesion of the Ag NP thin film to the substrate by exposure to NaCl solution. We could observe crack propagation and delamination by in-situ scanning electron microscope imaging. In addition, passivation effect by a parylene layer for preventing the permeation of the saline solution was investigated.

Stress-strain curves are an important representation of a material's mechanical properties, from which important properties such as elastic modulus, strength, and toughness, are defined. However, generating stress-strain curves from numerical methods such as finite element method (FEM) is computationally intensive, especially when considering the entire failure path for a material. As a result, it is difficult to perform high throughput computational design of materials with large design spaces, especially when considering mechanical responses beyond the elastic limit. In this work, a combination of principal component analysis (PCA) and convolutional neural networks (CNN) are used to predict the entire stress-strain behavior of binary composites evaluated over the entire failure path, motivated by the significantly faster inference speed of empirical models. We show that PCA transforms the stress-strain curves into an effective latent space by visualizing the eigenbasis of PCA. Despite having a dataset of only 10-27% of possible microstructure configurations, the mean absolute error of the prediction is

Owing to the high strength-to-weight and stiffness-to-weight ratios, composite materials are today essential building blocks for a wide range of industrial applications. However, their complex microstructures make it difficult to predict their failure mechanisms and residual lives under varying external loads. In situ health monitoring systems have received much attention in recent years as promising solutions for the above-mentioned limitations of composite materials. Here, we suggest a coupled health monitoring system in which infrared thermography and electrical resistance measurements are simultaneously applied for diagnosing the damage state of composite samples during tensile testing. In addition, we build a multiphysics simulation framework to model the interplay between the physical phenomena occurring in the three damage stages, involving crack propagation, variation in the temperature profile, and electrical resistance. The coupled electro-thermal monitoring system allows the estimation of the “damage stress ( σ D )”, which represents the onset of a micro-damage and may be correlated to the fatigue strength of the material and the assessment of the damage evolution process in glass fiber-reinforced polymer composites under quasi-static tensile loading. The proposed simulation framework enables the simultaneous understanding of the multiple physical phenomena occurring in the composite at the instance of crack nucleation and propagation.

The crack phase field model has been well established and validated for a variety of complex crack propagation patterns within a homogeneous medium under either tensile or shear loading. However, relatively less attention has been paid to crack propagation under combined tensile and shear loading or crack propagation within composite materials made of two constituents with very different elastic moduli. In this work, we compare crack propagation under such circumstances modelled by two representative formulations, anisotropic and hybrid formulations, which have distinct stiffness degradation schemes upon crack propagation. We demonstrate that the hybrid formulation is more adequate for modeling crack propagation problems under combined loading because the residual stiffness of the damaged zone in the anisotropic formulation may lead to spurious crack growth and altered load–displacement response.

In the manufacturing process of a filler-reinforced composite, the fillers are partially aligned due to the shear flow in the drawing stage. Besides, various imperfections form at the interface between the matrix and the fillers, leading to debonding and slip under mechanical loading. There have been numerous micromechanics studies to predict effective moduli of the composites in the presence of partial alignment of fillers and interface imperfections. Here, we present an improved theory that overcomes two limitations in the existing micromechanics based approaches. First, we find that the interface damage tensor for axisymmetric ellipsoidal inhomogeneity developed to model the interfacial damage leads to the prediction of infinite or negative effective moduli. We show that these anomalies can be eliminated if correctly derived damage tensor is used. Second, we reveal that the previous theory on the effective moduli with axisymmetric filler orientation distribution fails because longitudinal and transverse moduli predictions do not converge in the limit of random orientation distribution. With appropriate corrections, we derive analytic expressions for the orientation average of arbitrary transversely isotropic 4th order tensor under general axisymmetric orientation distribution. We apply the improved method to compute the effective moduli of a polymer-carbon nanotube composite with non-uniform filler orientation and interface damage.

Lattice spring models (LSMs), being simple to implement, have been employed to simulate the elastic and fracture behaviors of materials. However, previous studies have mainly focused on the infinitesimal deformation range, and the applicability of LSMs for large deformations has not been thoroughly investigated. In this study, we systematically study the nonlinearity and anisotropy of elastic responses at a finite strain range as well as the fracture behaviors of two-dimensional triangular and square LSMs (tLSM and sLSM). We show that the mechanical responses of perfect lattices for both LSMs can be exactly predicted analytically. We also investigate the fracture behaviors of pre-cracked specimens under three-point bending, mode-I fracture, and shear fracture loading modes by varying the orientation of the crack with respect to the lattice. Our results on both perfect and pre-cracked lattices indicate that the mechanical properties of the sLSM is significantly more isotropic than the tLSM.

The load transfer mechanism in the unique staggered platelet structure of nacre has been intensively analyzed, and its structural features have been mimicked by various manufacturing methods such as 3D printing . However, due to the limited applicability of most analytical models and the limitations of 3D printers, it remains difficult to quantitatively predict and design the properties of nacre-inspired synthetic structures. In this study, we improve the analytic models to predict the effective elastic properties of various staggered platelet structures for a wide range of constituent materials’ elastic moduli and geometric parameters by carefully accounting for the volume average of elastic properties. We also systematically investigate the effect of the printing orientation and width of the composite structure fabricated by a 3D printer. We show that our improved analytic model accurately predicts the properties of a 3D printed composite when the manufacturing-condition-dependent material properties are used as input. Our study reveals the origin of discrepancy between theory and 3D printed structures and enables a rational design of nacre-inspired structures.

In the field of soft strain sensors, piezo-transmittance based strain sensors, which detect strains by optical transmittance change, have promising advantages of fast response, high sensitivity, long-term stability, and negligible effect from environmental factors. However, they feature low sensor-to-sensor and in-sensor uniformity as well as unpredictable response and high stiffness. This study exploits the gap control of an auxetic-patterned elastomer to develop a piezo-transmittance based strain sensor. Gap opening mechanism in the negative Poisson’s ratio metamaterial with rotating square structures makes the sensor free from these limitations; thus, achieving a designable response and low stiffness. In addition, high sensor-to-sensor ((root-mean-square deviation (RSD)

Using a multiscale modeling framework that combines molecular dynamics (MD) simulation and micromechanics-based homogenization method, the effective elastic properties of polymer-matrix nanocomposites are predicted for a wide range of interfacial bond stiffness (energy). We consider the homogenization method with two interface models, the interface spring model and interphase model, to account for the interfacial effect, and test the validity of both models by comparing them to the finite element analyses. We then calculate the bulk moduli of Nylon6 (matrix)–SiC (nanoparticle) nanocomposite using MD simulations by varying the interfacial bond stiffness and size of the SiC nanoparticle. By comparing the bulk moduli from MD simulations with the theoretical predictions from the homogenization method, we find the interfacial bond stiffness range within which each interfacial model is relevant. We also investigate and explain the effect of particle size on the effective material properties at different interfacial bond stiffnesses. The effect of interphase and interfacial imperfection (damage) observed from the MD simulation on the effective properties are analyzed. Finally, we predict the effective properties of the Nylon6-SiC nanocomposite by varying the level of SiC particle agglomeration and study how the effect of agglomeration changes over the interfacial bond stiffness.

At the continuum length scale, mechanical properties of metals show relatively weak orientation dependence; however, they exhibit strong anisotropic behaviors as the size of sample decreases to micron and nanometer length scales. In this study, three-dimensional dislocation dynamics (DD) simulations are performed to investigate the orientation-dependent plasticity in submicron face-centered cubic (FCC) micropillars subjected to torsion. Accommodating results from atomistic modeling, updated surface nucleation schemes in DD models have been developed for three orientations ([001], [101], and [111]), allowing investigation of the dislocation microstructure evolution and the corresponding anisotropic mechanical response upon torsional loading and unloading. The DD simulation results show that the coaxial and hexagonal networks formed in [101] and [111] oriented nanopillars, respectively, exhibited excellent plastic recovery, while the rectangular network formed in the [001] crystal orientation was more stable and did not experience as much plastic recovery.

By employing a mean-field homogenization theory to an anisotropic thermoelectric composite involving spherical fillers, we obtain analytical predictions of the effective Seebeck coefficient, effective electrical conductivity, and effective thermal conductivity in the presence of interfacial electrical and thermal resistances. First, we validate the thermoelectric Eshelby tensor and concentration tensor for an anisotropic matrix in the presence of interfacial resistance using finite element analysis (FEA) results and subsequently demonstrate that the predicted effective thermoelectric properties match well with the FEA up to a filler volume fraction of less than 20%. Based on the homogenization theory, we investigate the effect of filler size and filler properties on the effective thermoelectric properties of a composite. An interesting observation is that the porosity, which was known not to affect the effective Seebeck coefficient in isotropic materials, noticeably affects the effective Seebeck coefficient in anisotropic materials. Under strong anisotropy, there was a mismatch between the Seebeck coefficients of the porous medium obtained from numerical simulation and theoretical prediction. We discuss the origin of such phenomena in terms of the thermoelectric eddy current existing in an anisotropic medium: as the electric field is not conservative anymore, there is a circulating current in the porous anisotropic materials.

Nanoscale metal/graphene nanolayered composite is known to have ultrahigh strength as the graphene effectively blocks dislocations from penetrating through the metal/graphene interface. The same graphene interface, which has a strong sp2 bonding, can simultaneously serve as an effective interface for deflecting the fatigue cracks that are generated under cyclic bendings. In this study, Cu/Gr composite with repeat layer spacing of 100 nm was tested for bending fatigue at 1.6% and 3.1% strain up to 1,000,000 cycles that showed for the first time a 5-6 times enhancement in fatigue resistance compared to the conventional Cu thin film. Fatigue cracks that are generated within the Cu layer were stopped by the graphene interface, which are evidenced by cross-sectional scanning electron microscopy and transmission electron microscopy images. Molecular dynamics simulations for uniaxial tension of Cu/Gr showed limited accumulation of dislocations at the film/substrate interface, which makes the fatigue crack formation and propagation through thickness of the film difficult in this materials system.

We obtained an analytical solution for the effective thermal conductivity of composites composed of orthotropic matrices and spherical inhomogeneities with interfacial thermal resistance using a micromechanics-based homogenization. We derived the closed form of a modified Eshelby tensor as a function of the interfacial thermal resistance. We then predicted the heat flux of a single inhomogeneity in the infinite media based on the modified Eshelby tensor, which was validated against the numerical results obtained from the finite element analysis. Based on the modified Eshelby tensor and the localization tensor accounting for the interfacial resistance, we derived an analytical expression for the effective thermal conductivity tensor for the composites by a mean-field approach called the Mori-Tanaka method. Our analytical prediction matched very well with the effective thermal conductivity obtained from finite element analysis with up to 10% inhomogeneity volume fraction.

We present a molecular dynamics (MD) simulation study on the hypervelocity dynamics of shock compressed graphite -up to hundreds of gigapascals- and impacted multilayer graphene armours by employing the AIREBO-M potential. The Morse-type non-singular intermolecular interaction allows the usage of relatively large integration timesteps for simulating materials’ response at such high strain-rate. The MD simulation results are in good agreement with the shock Hugoniot curves and with graphite-to-diamond transition obtained from both density functional theory (DFT) and experiments available in literature. We then show that thermodynamic properties of graphite from MD calculations can be used as input for a reliable equation of state to be employed in continuum simulations. Finally, we find that the size-scaling of the hypervelocity impact properties of graphene armours matches well with the DFT results and theoretical predictions of earlier studies. Our results open a concrete possibility towards accurate and fast multiscale simulation from atomistic to continuum level of shock propagation, shock-induced phase transformation, and dynamic fracture in large or hierarchical carbon systems, such as graphene-based foams and nanocomposites.

As circuitry approaches single nanometer length scales, it is important to predict the stability of metals at these scales. The behavior of metals at larger scales can be predicted based on the behavior of dislocations, but it is unclear if dislocations can form and be sustained at single nanometer dimensions. Here, we report the formation of dislocations within individual 3.9 nm Au nanocrystals under nonhydrostatic pressure in a diamond anvil cell. We used a combination of x-ray diffraction, optical absorbance spectroscopy, and molecular dynamics simulation to characterize the defects that are formed, which were found to be surface-nucleated partial dislocations. These results indicate that dislocations are still active at single nanometer length scales and can lead to permanent plasticity., Comment: 33 pages, 12 figures

Staggered platelet composites found in nature, such as nacre, bone, and conch-shell, exhibit a remarkable combination of high toughness, strength, and/or stiffness, and have inspired the development of bio-inspired composites mimicking their characteristic features. However, those excellent mechanical properties are primarily observed under specific loading conditions due to their mechanical anisotropy, which originates from the aligned microstructures consisting of high aspect ratio inclusions. In this study, we combine numerical simulations and 3D-printing to define a design strategy of isotropic two-dimensional structural composites consisting of stiff and soft constituents that are arranged in square, triangular, and quasicrystal lattices. For relatively isotropic structures, the soft tile/stiff boundary configuration significantly outperforms the stiff tile/soft boundary configuration in terms of normalized toughness, strength, and stiffness with respect to the simple rule of mixture estimates for each, because the former provides more extrinsic toughening mechanisms and effectively lowers the stress concentration near the crack tip. The quasicrystal lattice offers the best isotropy in elastic response, while its absolute values of stiffness, strength, and toughness turn out to be similar or lower than those of triangular lattice composites due to more irregular stress distribution. In contrast, for the highly anisotropic staggered platelet structure, the stiff tile/soft boundary configuration significantly outperforms the inverted one, owing to its unique load-transfer mechanism, which relies primarily on the shear-lag effect.

Recently, multiscale modeling frameworks combining micromechanics-based homogenization methods and atomistic simulations have been widely applied to predict the effective stiffness of particulate-reinforced composites. Although most studies demonstrated that theoretical predictions incorporating interfacial damage are necessary to explain atomistic simulation results, the microscopic origin of the interfacial damage has not been systematically analyzed in terms of interatomic potential and interfacial structure. In this study, first, we conduct a series of particle simulations of two fictitious model crystalline composites with coherent interfaces: one has a two-dimensional triangular structure described by a bead-spring model and the other has a face-centered cubic structure described by the artificial Lennard-Jones potential. By comparing the simulation results with micromechanics theory, we obtain the interfacial bonding (damage) parameter used in the homogenization method in terms of parameters at the atomistic level. Second, we study the effects of the interfacial structures (coherent/incoherent) because of lattice or crystallographic orientation mismatch on the effective properties of composites. We obtain the elastic stiffness of Si(matrix)-Ge(nanoparticle) nanocomposites with different interfacial structures (coherent/incoherent structures) using atomistic simulations and observe that nanoparticle-size-dependency occurs only for the composite with incoherent interfaces. We propose a homogenization scheme considering the pre-stress (or residual stress) and interfacial imperfection, and explain the results from Si-Ge nanocomposite simulations.

The strength of pristine graphene and its grain boundaries (GBs) are mainly measured by nanoindentation with a spherical tip due to the difficulty of conducting uniaxial tensile tests. However, we recently showed that the fracture forces from the spherical indenter cannot be directly mapped onto the uniaxial strength. In this paper, employing a series of molecular dynamics simulations combined with a fracture mechanics analysis, we demonstrate that the fracture force from cylindrical indenters can be directly mapped onto the strength of graphene under uniaxial tension. Under indentation with cylindrical tips or uniaxial tension, the rupture of graphene sheets that have GBs with a low-tilt angle occurs simultaneously with the onset of crack nucleation at the GBs. On the contrary, when indented by a spherical indenter tip, the graphene sheets sustain the indentation loads until the crack size becomes comparable to the tip radius. Furthermore, the results show that estimating the strength with a cylindrical indenter is not very sensitive to the indentation site as well as angular misalignments that can be caused by human error or the limitations of the apparatus. Our work presents the feasibility of obtaining the tensile strength from nanoindentation experiments, which may suggest a new standard to measure the tensile strength of graphene and related two-dimensional materials.

We carry out molecular dynamics simulations of nanoindentation to investigate the effect of cementite size and temperature on the deformation behavior of nanocomposite pearlite composed of alternating ferrite and cementite layers. We find that, instead of the coherent transmission, dislocation propagates by forming a widespread plastic deformation in cementite layer. We also show that increasing temperature enhances the distribution of plastic strain in the ferrite layer, which reduces the stress acting on the cementite layer. Hence, thickening cementite layer or increasing temperature reduces the likelihood of dislocation propagation through the cementite layer. Our finding sheds a light on the mechanism of dislocation blocking by cementite layer in the pearlite.

The characteristics of the misfit dislocations at ferrite/cementite interfaces (FCIs) for various orientation relationships (ORs) have important implications for the mechanical behavior and the phase transformation of pearlitic steels; however, the detailed characteristics of these misfit dislocations have not been thoroughly elucidated to date. Using the extended atomically informed Frank–Bilby (xAIFB) method and atomistic simulation, we characterized the structures of misfit dislocations and calculated the interface energies of five ORs (Bagaryatsky, Isaichev, Pitsch–Petch, Near Bagaryatsky and Near Pitsch–Petch), respectively. Atomistic calculations of the interface energies of five ORs reveal that (1) the Isaichev OR has the lowest interface formation energy and (2) Near Bagaryatsky and Near Pitsch–Petch ORs are energetically more favorable than exact Bagaryatsky and Pitsch–Petch ORs in spite of small misorientation angle. The interface formation energy of each OR is qualitatively well explained by the structure and spacing of FCI dislocations, which demonstrate the importance of the characterization of misfit dislocations.

We report a flexible and wearable pressure sensor based on the giant piezocapacitive effect of a three-dimensional (3-D) microporous dielectric elastomer, which is capable of highly sensitive and stable pressure sensing over a large tactile pressure range. Due to the presence of micropores within the elastomeric dielectric layer, our piezocapacitive pressure sensor is highly deformable by even very small amounts of pressure, leading to a dramatic increase in its sensitivity. Moreover, the gradual closure of micropores under compression increases the effective dielectric constant, thereby further enhancing the sensitivity of the sensor. The 3-D microporous dielectric layer with serially stacked springs of elastomer bridges can cover a much wider pressure range than those of previously reported micro-/nanostructured sensing materials. We also investigate the applicability of our sensor to wearable pressure-sensing devices as an electronic pressure-sensing skin in robotic fingers as well as a bandage-type pressure-sensing device for pulse monitoring at the human wrist. Finally, we demonstrate a pressure sensor array pad for the recognition of spatially distributed pressure information on a plane. Our sensor, with its excellent pressure-sensing performance, marks the realization of a true tactile pressure sensor presenting highly sensitive responses to the entire tactile pressure range, from ultralow-force detection to high weights generated by human activity.

Kinking, a common anomaly in nanowire (NW) vapor-liquid-solid (VLS) growth, represents a sudden change of the wire's axial growth orientation. This study focuses on defect-free kinking during germanium NW VLS growth, after nucleation on a Ge (111) single crystal substrate, using Au-Ge catalyst liquid droplets of defined size. Statistical analysis of the fraction of kinked NWs reveals the dependence of kinking probability on the wire diameter and the growth temperature. The morphologies of kinked Ge NWs studied by electron microscopy show two distinct, defect-free, kinking modes, whose underlying mechanisms are explained with the help of 3D multiphase field simulations. Type I kinking, in which the growth axis changes from vertical [111] to ⟨110⟩, was observed in Ge NWs with a nominal diameter of ∼ 20 nm. This size coincides with a critical diameter at which a spontaneous transition from ⟨111⟩ to ⟨110⟩ growth occurs in the phase field simulations. Larger diameter NWs only exhibit Type II kinking, in which the growth axis changes from vertical [111] directly to an inclined ⟨111⟩ axis during the initial stages of wire growth. This is caused by an error in sidewall facet development, which produces a shrinkage in the area of the (111) growth facet with increasing NW length, causing an instability of the Au-Ge liquid droplet at the tip of the NW.

Using a series of molecular dynamics simulations, we study the plastic deformation of 110 -oriented single crystalline face-centered-cubic (FCC) metallic nanowires (NWs) under torsional loading for two representative materials with different stacking fault energies, i.e., gold (Au) and aluminum (Al). A torsional periodic boundary condition is employed to avoid artifacts from otherwise fixed regions at the ends. We analyze the correlation among the surface tension, dislocation structure, and applied torque for Au and Al NWs with different radii to understand the effect of stacking fault energy on the hardening behavior. We discover that, contrary to uniaxial loading, smaller NWs are weaker and less stiff in the elastic regime under torsion; subsequently, we explain the origin by using a continuum model and computing the resolved shear stress from the surface tension. Unlike the elastic response, smaller NWs indicate a higher plastic flow stress during plastic deformation. Finally, we demonstrate that the flow stress increases with the ratio of stacking fault width to NW radius due to the higher density of planar defects such as stacking faults and twin boundaries, which impede dislocation motion.

Fabric-based wearable electronics are becoming more important in the fourth industrial revolution (4IR) era due to their connectivity, wearability, comfort, and durability. Conventional fabric-based wearable electronics have been demonstrated by several researchers, but still need complex methods or additional supports to be fabricated and sewed in clothing. Herein, a cost-effective, high throughput, and strongly integrated fabric-based wearable piezoelectric energy harvester (fabric-WPEH) is demonstrated. The fabric-WPEH has a heterostructure of a ferroelectric polymer, poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF-TrFE)] and two conductive fabrics via simple fabrication of tape casting and hot pressing. Our fabrication process would enable the direct application of the unit device to general garments using hot pressing as graphic patches can be attached to the garments by heat press. Simulation and experimental analysis demonstrate fully bendable, compact and concave interfaces and a high piezoelectric d 33 coefficient (−32.0 pC N−1) of the P(VDF-TrFE) layer. The fabric-WPEH generates piezoelectric output signals from human motions (pressing, bending) and from quantitative force test machine pressing. Furthermore, a record high interfacial adhesion strength (22 N cm−1) between the P(VDF-TrFE) layer and fabric layers has been measured by surface and interfacial cutting analysis system (SAICAS) for the first time in the field of fabric-based wearable piezoelectric electronics.

We derive an analytical expression to predict the effective properties of a particulate-reinforced piezoelectric composite with interfacial imperfections using a micromechanics-based mean–field approach. We correctly derive the analytical formula of the modified Eshelby tensor, the modified concentration tensor, and the effective property equations based on the modified Mori–Tanaka method in the presence of interfacial imperfections. Our results are validated against finite element analyses (FEA) for the entire range of interfacial damage levels, from a perfect to a completely disconnected and insulated interface. For the facile evaluation of the nontrivial tensorial equations, we adopt the Mandel notation to perform tensor operations with 9 × 9 symmetric matrix operations. We apply the method to predict the effective properties of a representative piezoelectric composite consisting of polyvinylidene fluoride (PVDF) and SiC reinforcements.

Although the pearlitic steel is one of the most extensively studied materials, there are still questions unanswered about the interface in the lamellar structure. In particular, to deepen the understanding of the mechanical behavior of pearlitic steel with fine lamellar structure, it is essential to reveal the structure-property relationship of the ferrite/cementite interface. In this study, we analyzed the in-plane shear deformation of the ferrite/cementite interface using atomistic simulation combined with extended atomically informed Frank-Bilby method and disregistry analyses. In the atomistic simulation, we applied in-plane shear stress along twelve different directions to the ferrite/cementite bilayer for Isaichev, Near Bagaryatsky and Near Pitsch-Petch orientation relationship, respectively. The simulation results reveal that Isaichev and Near Bagaryatsky orientations show dislocation-mediated plasticity except two directions, while Near Pitsch-Petch orientation shows mode II (in-plane shear) fracture at the ferrite/cementite interface along all directions. Based on the extended atomically informed Frank-Bilby and disregistry analysis results, we conclude that the in-plane shear behavior of the ferrite/cementite interface is governed by the magnitude of Burgers vector and core-width of misfit dislocations.

Electronic skin are devices that mimic the functionalities of human skin, which require high sensitivity, large dynamic range, high spatial uniformity, low-cost and large-area processability, and the capacity to differentiate various external inputs. We herein introduce a versatile droplet-based microfluidic-assisted emulsion self-assembly process to generate three-dimensional microstructure-based high-performance capacitive and piezoresistive pressure sensors for electronic skin applications. Our technique can generate uniformly sized micropores that are self-assembled in an orderly close-packed manner over a large area, which results in high spatial uniformity. The size of the micropores can easily be tuned from 100 to 500 μm, through which sensitivity and dynamic range were controlled as high as 0.86 kPa-1 and up to 100 kPa. Our device can be printed on curvilinear surfaces and be molded into various shapes. We furthermore demonstrate that by simultaneously utilizing capacitive and piezoresistive pressure sensors, we can distinguish between pressure and temperature, or between pressure and proximity. These demonstrations make our process and sensors highly useful for a wide variety of electronic skin applications.

Micromechanics-based homogenization has been employed extensively to predict the effective properties of technologically important composites. In this review article, we address its application to various physical phenomena, including elasticity, thermal and electrical conduction, electric, and magnetic polarization, as well as multi-physics phenomena governed by coupled equations such as piezoelectricity and thermoelectricity. Especially, for this special issue, we introduce several research works published recently from our research group that consider the anisotropy of the matrix and interfacial imperfections in obtaining various effective physical properties. We begin with a brief review of the concept of the Eshelby tensor with regard to the elasticity and mean-field homogenization of the effective stiffness tensor of a composite with a perfect interface between the matrix and inclusions. We then discuss the extension of the theory in two aspects. First, we discuss the mathematical analogy among steady-state equations describing the aforementioned physical phenomena and explain how the Eshelby tensor can be used to obtain various effective properties. Afterwards, we describe how the anisotropy of the matrix and interfacial imperfections, which exist in actual composites, can be accounted for. In the last section, we provide a summary and outlook considering future challenges.

We obtained an analytical solution for the effective thermal conductivity of composites composed of orthotropic matrices and spherical inhomogeneities with interfacial thermal resistance using a micromechanics-based homogenization. We derived the closed form of a modified Eshelby tensor as a function of the interfacial thermal resistance. We then predicted the heat flux of a single inhomogeneity in the infinite media based on the modified Eshelby tensor, which was validated against the numerical results obtained from the finite element analysis. Based on the modified Eshelby tensor and the localization tensor accounting for the interfacial resistance, we derived an analytical expression for the effective thermal conductivity tensor for the composites by a mean-field approach called the Mori-Tanaka method. Our analytical prediction matched very well with the effective thermal conductivity obtained from finite element analysis with up to 10% inhomogeneity volume fraction.

Molecular dynamics simulations combined with quantitative atomic displacement analyses were performed to study the deformation behaviors of polycrystalline cementite (Fe3C). At low temperature and large grain size, dislocation glide acts as the preferred deformation mechanism. Due to the limited number of slip systems at low temperature, polycrystalline cementite breaks by forming voids at grain boundaries upon tensile loading. When the temperature rises or the grain size reduces, grain boundary sliding becomes the primary mechanism and plastic deformation is accommodated effectively.

A folding technique is reported to incorporate large-area monolayer graphene films in polymer composites for mechanical reinforcement. Compared with the classic stacking method, the folding strategy results in further stiffening, strengthening, and toughening of the composite. By using a water-air-interface-facilitated procedure, an A5-size 400 nm thin polycarbonate (PC) film is folded in half 10 times to a ≈0.4 mm thick material (1024 layers). A large PC/graphene film is also folded by the same process, resulting in a composite with graphene distributed uniformly. A three-point bending test is performed to study the mechanical performance of the composites. With a low volume fraction of graphene (0.085%), the Young's modulus, strength, and toughness modulus are enhanced in the folded composite by an average of 73.5%, 73.2%, and 59.1%, respectively, versus the pristine stacked polymer films, or 40.2%, 38.5%, and 37.3% versus the folded polymer film, proving a remarkable mechanical reinforcement from the combined folding and reinforcement of graphene. These results are rationalized with combined theoretical and computational analyses, which also allow the synergistic behavior between the reinforcement and folding to be quantified. The folding approach could be extended/applied to other 2D nanomaterials to design and make macroscale laminated composites with enhanced mechanical properties.

Despite the significance of surface effects on the deformation behaviours of small-scale metallic glasses, systematic investigations on surface states are lacking. In this work, by employing atomistic simulations, we characterise the distributions of local inhomogeneity near surfaces created by casting and cutting, along with internal distributions in pristine and irradiated bulk specimens, and investigate the effects of inhomogeneity on the mechanical properties. The cast surface shows enhanced yield strength and degrees of shear localisation, while the cut surface shows the opposite effects, although the fraction of vibrational soft spots, known to indicate low-energy barriers for local rearrangement, is high near both surfaces. Correspondingly, plastic deformation is initiated near the cut surface, but far from the cast surface. We reveal that improved local orientational symmetry promotes strengthening in cast surfaces and originates from the effectively lower quenching rate due to faster diffusion near the surface. However, a significant correlation among vibrational soft spots, local symmetries, and the degree of shear localisation is found for the pristine and irradiated bulk materials. Our findings reveal the sensitivity of the surface state to the surface preparation methods, and indicate that particular care must be taken when studying metallic glasses containing free surfaces.

Interest in soft actuators for next-generation electronic devices, such as wearable electronics, haptic feedback systems, rollable flexible displays, and soft robotics, is rapidly growing. However, for more practical applications in diverse electronic devices, soft actuators require multiple functionalities including anisotropic actuation in three-dimensional space, active tactile feedback, and controllable wettability. Herein, we report anisotropic dielectric elastomer actuators with uni- and bi-axially wrinkled carbon black electrodes that are formed through pre-streching and relaxation processes. The wrinkled dielectric elastomer actuator (WDEA) that shows directional actuation under electric fields is used to control the anisotropic wettability. The morphology changes of the electrode surfaces under various electric stimuli are investigated by measuring the contact angles of water droplets, and the results show that the controllable wettability has a broad range from 141° to 161° along the wrinkle direction. The present study successfully demonstrates that the WDEA under electrically controlled inputs can be used to modulate the uni- or bi-axially wrinkled electrode surfaces with continous roughness levels. The controllable wrinkled structures can play an important role in creating adaptable water repellency and tunable anisotropic wettability.

The interaction between a graphene edge and the underlying metal is investigated through the use of scanning tunneling microscopy (STM) and density functional theory (DFT) calculations and found to influence the geometrical structure of the graphene edge and its electronic properties. STM study reveals that graphene nanoislands grow on a Pt(1 1 1) surface with the considerable bending of the graphene at the edge arising from the strong graphene-edge–Pt-substrate interactions. Periodic ripples along the graphene edge due to both the strong interaction and the lattice mismatch with the underlying metal were seen. DFT calculations confirm such significant bending and also reproduce the periodic ripples along the graphene edge. The highly distorted edge geometry causes strain-induced pseudo-magnetic fields, which are manifested as Landau levels in the scanning tunneling spectroscopy. The electronic properties of the graphene edge are thus concluded to be strongly influenced by the curvature rather than the localized states along the zigzag edge as was previously predicted.

We investigate the ultimate strengths and failure types of asymmetric tilt grain boundaries (GBs) with various tilt angles with the aid of molecular dynamics simulations and analytic theory. The tensile strength of armchair-oriented graphene GBs shows a tendency to increase as the misorientation angle rises, while that of zigzag-oriented graphene GBs non-monotonically increases. The overall strength enhancement and weakening behaviors can be explained by a continuum stress analysis of pentagon–heptagon defects along GBs. Nevertheless, full atomistic analyses are required to predict the exact bonds from which mechanical failure initiate. Two different fracture types are observed in our studies; one with cracks growing along the GB and another with cracks growing away from the GB. Detailed understanding of the atomic arrangement along the GB, in addition to defect density, is necessary to ascertain the ultimate strength and rupture process of GBs.

Wearable strain sensors for human motion detection are being highlighted in various fields such as medical, entertainment and sports industry. In this paper, we propose a new type of stretchable strain sensor that can detect both tensile and compressive strains and can be fabricated by a very simple process. A silver nanoparticle (Ag NP) thin film patterned on the polydimethylsiloxane (PDMS) stamp by a single-step direct transfer process is used as the strain sensing material. The working principle is the change in the electrical resistance caused by the opening/closure of micro-cracks under mechanical deformation. The fabricated stretchable strain sensor shows highly sensitive and durable sensing performances in various tensile/compressive strains, long-term cyclic loading and relaxation tests. We demonstrate the applications of our stretchable strain sensors such as flexible pressure sensors and wearable human motion detection devices with high sensitivity, response speed and mechanical robustness.