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This paper presents an improved numerical model for the reliability analysis of corrosion initiation due to the combined action of carbonation and chloride ingress. The coupled mechanism of carbon dioxide, chloride ions, heat and moisture is considered in the model. The probability of corrosion initiation is evaluated by means of a single limit state function in which the critical concentration of chloride ions depends on the degree of carbonation. In addition, the coupled diffusion coefficients are interpolated linearly depending on the degree of carbonation in order to simplify the structure of the model without compromising accuracy.

We report the fabrication of a stretchable array of active-matrix (AM) electrochromic devices (ECDs). The ECD is made of poly(3-methylthiophene) and Prussian blue electrodes with a mixed gel electrolyte of acetonitrile, poly(methyl methacrylate), propylene carbonate, and lithium perchlorate. The ECD displays can express red, green, or blue (RGB) depending on the applied voltage and has a high stability in air. It exhibits low power consumption levels of 331 and 378 μW cm−2 (10 s duration) at −1.0 and 1.0 V, respectively, and a high coloration efficiency of 201.6 cm2 C−1 at 1.0 V. A 4 × 4 AM ECD array is integrated on a deformable Ecoflex substrate via a dry transfer method using patterned liquid metal interconnections. The display shows a mechanical stability under bending and biaxial stretching by 30%. Finite element method analysis of the strain distribution also confirms that strain is not applied to the ECDs, guaranteeing stable operation of the array under deformation. Furthermore, a stretchable 6 × 6 AM ECD array for higher resolution display also exhibits mechanical stability under 30% biaxial stretching. This work demonstrates the high potential of our stretchable ECD array in low power-consuming displays for stretchable electronics, wearable devices, and electronic skin.

This study investigates the implications of carbon nanomaterial type and amount on the electrical properties of cement paste. For this, five different nanomaterials, i.e., carbon nanotube (CNT), carbon fiber (CF), graphite nanofiber (GNF), graphene (G), and graphene oxide (GO), and two different volume fractions of 0.5 and 1% were considered. In addition, the self-sensing capacity of the cement composites with nanomaterials was evaluated under cyclic compressive loads. Test results indicate that the conductivity of plain cement paste was improved by adding carbon nanomaterials. In most cases, the conductivity of the composites was reduced by an increase in curing age and a decrease in nanomaterial amount, except for CF. The composites with CNTs exhibited the best self-sensing capacity regardless of volume fraction (vf), and the order of self-sensing capacity of the composites at a vf of 1% was CNT > GO ≈ GNF > G. The composites with 0.5 and 1 vol% CFs were determined to be not appropriate for a sensor measuring compressive behaviors. The gauge factor of the composites incorporating 1 vol% CNTs was obtained as 77.2–95.5.

A safe and reliable railway operation requires an organic and systematic approach to railway maintenance. Despite the importance of timely and valid track maintenance and applicability of inspected data to the optimum track management process, inspected wear data inspected by a railway inspection system in Korea have not been utilized for decision making of maintenance scenario, but just accumulated. Moreover, the process of inspecting wear data includes some uncertainties, probabilistic-based models have more reasonable application in field. This can be accomplished by developing probabilistic-based stochastic model considering uncertainties for the prediction of rail wear using inspected data. This paper reports on the development and verification of a probabilistic forecasting model for rail wear progress. This developed forecasting model utilizes the particle filter method concept based on Bayesian theory and real inspected wear data of Seoul Metro are applied to verify the model.

This study investigated the effects of carbon fiber (CF) length, electrode spacing, and probe configuration on the electrical conductivity of cement composites. Accordingly, 57 different types of samples were prepared, considering three different CF lengths, five different CF contents, three different electrode spacings, and two different probe configurations. This research found that the influence of CF length on the electrical resistivity of cement composite depends electrode spacing. For the cement composite with wide electrode spacing of 40 mm, its resistivity decreased as increasing CF length as in the previous study. However, when the electrode spacing is 10 mm, which is narrow (10 mm), the resistivity of the cement composite rather increased with increasing CF length. The results implied that when an electrode is designed for the cement composite incorporating CF, the CF length should be short compared to the electrode spacing. The percolation threshold of CF measured by the two-probe configuration was 2% or more. This is higher than that measured by the four-probe configuration (1%). At a lower CF content than 2%, the two-probe configuration gave higher resistivity of the cement composite than the four-probe configuration. However, the difference coming from the different probe configurations was marginal as increasing the CF content.

A chemo-damage model is proposed to predict the expansion caused by the alkali silica reaction (ASR). The model covers the formation of the pre-expansion gel driven by alkali and the swelling of the gel driven by water. The swelling capacity of the ASR gel is quantified by the sodium to calcium ratio in the pore solution. The bound alkali in the gel recycled by calcium is also considered in this model. Both external alkali supply and internal alkali released from aggregates are included. Several sets of experimental data are compared with the simulation results for the verification of the model.

In this paper, the phantom-node method is developed for cracked plates and shells simulated by continuum mechanics based 4-node quadrilateral shell elements using the mixed interpolation of tensorial components (MITC) technique for the shear-locking removal. The proposed method permits an arbitrary crack to cut through elements. The discontinuity across the crack is described by continuous displacement fields of overlapping elements that are constructed by duplicating homologous nodes and realized on the real parts of the cracked elements. New kinematical constraints for the nodal displacements of the overlapping triple elements are derived to represent the displacement fields of the partially cracked elements. The interaction domain integral of cracked shell structures is formulated to extract the fracture parameters. Several numerical benchmark cracked plates and shells are analyzed to evaluate the accuracy and robustness of the proposed method.

The aim of this paper is to report on the fabrication of a stretchable array of high-performance solid-state micro-supercapacitors (MSCs), which can be charged with integrated, commercial Si-based solar cells (SCs). This would facilitate the powering of an integrated strain sensor. The planar MSCs comprised electrodes of potentiostatically deposited polypyrrole, on spray-coated multi-walled carbon nanotube film, and a gel-type electrolyte of LiCl/polyvinyl alcohol with a redox additive of 1-methyl-3-propylimidazolium iodide. The fabricated MSC achieved an areal capacitance of 5.17 mF cm−2. After 5000 charge/discharge cycles, the MSC retained 80% of their initial capacitance. A strain sensor was fabricated utilizing a composite film of fragmentized graphene foam and polydimethylsiloxane. Such fabricated twelve parallel connected MSCs, a strain sensor, and SCs were integrated on a single deformable polymer substrate with embedded stiff platforms of negative epoxy series resist (SU-8) via long serpentine interconnections of polyimide encapsulated Ti/Pt metal film for mechanical stability under stretching. After 1000 repetitive biaxial stretching/releasing cycles by 30%, no noticeable change was observed in the charge/discharge behavior of the MSC array. Furthermore, both the photo-charge/discharge characteristics and electrochemical performance remained stable. When the whole integrated system was attached to the wrist, the integrated strain sensor could detect both externally applied strain and the arterial pulse using the energy stored in the MSCs from the SCs.

We predict the macroscopic tensile strength and fracture toughness of fully exfoliated nano silicate clay epoxy composites accounting for the interphase behavior between the polymeric matrix and clay reinforcement. A phase field approach is employed to model fracture in the matrix and the interphase zone of the polymeric nanocomposites (PNCs) while the stiff clay platelets are considered as linear elastic material. The effect of the interphase zones, e.g. thickness and mechanical properties (Young’s modulus and strain energy release rate) on the tensile strength, and fracture parameters of the composite is studied in detail. The dissipation energy due to fracture in the PNCs is extracted for different thicknesses and properties of the interphase zones. We show through numerical experiments that the interphase thickness has the most influence on the tensile strength while the critical strain energy release rate of the interphase zones affects the dissipation energy depending on the interphase zone thickness.

Display devices that can visually present various bio-signals are a core future technology required for skin attachable wearable electronics. Quantum dot light emitting diodes (QD-LEDs), which can be used as high performance display devices, exhibit many advantages such as a narrow bandwidth, high color purity, high environmental stability, easy and fast processing, and a high brightness at low voltage. In this study, we report on the fabrication of a stretchable QD-LED array for the visual display of body movement and skin temperature signals from skin-attached sensors. After forming an array of a rigid NOA63 islands, an array of CdSe/ZnS QD-LEDs is fabricated on top of that island array and electrically connected using liquid metal Galinstan interconnections. Finally, the array of QD-LEDs on the NOA63 islands is transferred onto a stretchable elastomer substrate of Ecoflex/PDMS. Of particular importance, for stable performance of the QD-LEDs under deformation, in replacement of the conventionally used ITO, a flexible and highly transparent electrode of Au grid/Ethylene glycol doped PEDOT:PSS is used. Such a design architecture can minimize the strain applied to the QD-LEDs by concentrating the strain onto the soft elastomer film substrate and the Galinstan interconnections. As a result, the fabricated QD-LED array exhibits stable operation under both 50% uniaxial and 30% biaxial strains. After attachment of the stretchable QD-LED array onto skin, the extent of knee bending, and changes in skin temperature are displayed as changes in the pattern of the QD-LED array. This work demonstrates the potential application of our stretchable array of QD-LEDs for easy and daily monitoring of health conditions through a visual display.

In the strain-based design, pipeline steels are required to satisfy not only the stress capacity but also the strain capacity. Thus, considerable efforts have been made to meet the various requirements of the pipe-material properties that might undergo large deformation during the installation stage in various hostile environments. An adequate compressive strain capacity can help effectively avoid local buckling. Hence, highly deformable steel becomes crucial in achieving the required strain capacity. This study proposes a nonlinear finite element procedure based on the commercial software ABAQUS combined with the User-defined Material Module (UMAT), which is created using Fortran language. The Gurson–Tvergaard–Needleman (GTN) model is implemented in UMAT to fully consider the nonlinear behavior of an API X100 pipe subjected to compressive loading. The numerical simulation results of the full-scale bending test were compared with the experimental results to verify the nonlinear finite element procedure equipped with the GTN model. The simulation results are in good agreement with the experimental results. The parametric studies reveal the effects of geometric imperfections and material properties on the compressive strain limit.

Computational modeling of geological materials is challenging. Firstly, they are heterogeneous with numerous uncertainties in the input parameters and secondly, the computational cost of modeling geological structures is time consuming due to the large and different length scales involved. In this article, we propose an efficient computational method for heterogeneous geological materials based on goal oriented error estimation and adaptive mesh refinement. Instead of estimating the error in a specific norm, the proposed novel error estimation approach which is called dual-weighted residual error estimation, approximates the error with respect to the quantity of interest. The dual-weighted residual error estimation is a dual-based scheme which requires an adjoint problem. The adjoint or dual problem is described by defining the quantity of interest in a functional form. Then by solving the primal and dual problems, errors in terms of the specified quantities are calculated. In many applications in engineering geology, the material is heterogeneous. In such cases, the material properties can be regarded as random fields. This variety of material properties leads to non-uniform distributions of the solution gradients, e.g. stresses. Therefore, it is vital to apply a reliable error estimation approach to be able to do efficiently the mesh-adaptivity procedure with regard to varying material parameters with pre-defined correlation lengths. Hence, the proposed error estimator is extended by accounting for a random field model to describe the material properties. Local estimated errors are exploited in order to accomplish the mesh adaptivity procedure. The goal-oriented mesh adaptivity controls the local errors in terms of the prescribed quantities. Both refinement and coarsening processes are applied to raise the efficiency. The performance of the proposed computational approach is demonstrated for several examples.

This paper presents a comprehensive model for the prediction of the process of steel reinforcement corrosion and concrete damage in two dimensions. The ingress of chloride ions, carbonation, electrochemical reaction and corrosion-induced mechanical damage are coupled in this model. The evolution of the active zone on the reinforcement is determined automatically. Using this model, we can predict the non-uniform distribution of the corrosion product, the expansive pressure, and the growth of corrosion-induced cracks.

The growing demand for compact energy storage devices may be met through the use of thin-film microbatteries, which generally rely on charge storage in thin or conformal layers. A promising technique for creating thin-film electrodes is layer-by-layer (LbL) assembly, based on the alternating adsorption of oppositely charged species to a surface to form a nanostructured electrode. Thin-film energy storage devices must have a high energy density within a limited space, so new electrode structures, materials, and assembly methods are important. To this end, both two-dimensional MXenes and polyaniline nanofibers (PNFs) have shown promising energy storage properties. Here, we report on the LbL assembly of positively charged PNFs and negatively charged Ti

This study aims to investigate the grindability and performance of glass-blended cements depending on the grinding method and glass type. Two grinding methods, intergrinding and separate grinding, and two types of waste glass, liquid crystal display glass (LCDG) and soda-lime glass (SLG), were considered. To study grindability, the milling characteristics of the coarse waste glass and fine glass-blended mixtures were investigated. The four types of glass-blended cements produced by the two grinding methods and the two types of waste glass were evaluated on particle size distribution, chemical composition, heat flow of hydration, hydration behavior, strength development and mitigation in alkali–silica reaction (ASR) expansion. It was found that the intergrinding method was more cost-efficient in producing glass-blended cements than the separate grinding method. Intergrinding of the glass-blended mixtures resulted in finer cement and coarser glass fractions. The interground glass-blended cements showed faster hydration reactivity but slightly lower strength and mitigation in ASR expansion than did separately ground glass-blended cements.

In this study, the strength and alkali-silica reaction (ASR) of concrete by incorporating pretreated soda-lime glass particles as fine aggregates were investigated. The glass aggregates were pretreated with three different solutions: Ca(OH)2, NaOH, and a mixture of Ca(OH)2 and NaOH. The glass aggregates were pretreated for 1, 4, and 7 days to investigate the time-dependent effect. Results show that the ASR was mitigated in all pretreatments. The effect was more noticeable with pretreatment in a mixed solution. The effect increased with an increase in pretreatment time. The ASR expansion was reduced by approximately 50% by pretreating the glass aggregate in the mixed solution at 80 ° C for 7 days. Moreover, pretreatment of the glass aggregate was beneficial for strength improvement. For example, by pretreating the glass aggregate in the Ca(OH)2 solution at 80 ° C for 7 days, the 28-day compressive strength demonstrated an approximate 6% increment compared with the specimen with non-pretreated glass aggregates.

In this paper, the combined effect of domain size, lattice orientation and crack length on the mechanical properties of Graphene, namely the yield strength and strain, are studied extensively based on molecular dynamics simulations. Numerical predictions are compared with the continuum- based laws of size effect and multifractal scaling. The yield strength is found to vary with the specimen size as approximate to L-1/3, which is in agreement with the multifractal scaling law, and with the inverse square of the initial crack length as approximate to a(0)(1/2), according to the Griffith's energy criterion for fracture. (C) 2016 Elsevier B.V. All rights reserved.

This paper presents a comprehensive probabilistic model for the corrosion initiation of steel reinforcement due to the combined action of carbonation and chloride ingress in reinforced concrete structures. The coupled mechanism of the transport of carbon dioxide, chloride ions, heat and moisture is taken into account in this model. A minimum number of random variables were chosen from a sensitivity analysis to reduce the computational cost. The corrosion is assumed to be initiated when either the concentration of the free chloride ion exceeds its threshold value or the pH of the pore solution decreases to below 11. According to this model, the probability of the corrosion initiation in the above-mentioned combined action of carbonation and chloride ingress is almost two times higher than the case in which the two mechanisms are considered separately. The Kolmogorov-Smirnov test indicates that the time of corrosion initiation approximately follows a log-normal distribution in this combined action.

The residual strength of components can be abruptly reduced due to multiple site damage (MSD). In general, the fatigue and fracture performance of MSD contains a significant number of uncertainties. Major uncertainties can be characterized by initial flaw, material variability and crack growth rates, among other factors. To cope with uncertain random variables, some probabilistic methods can be considered. However, these seldom obtain efficient and reliable results because of the complexities included in computations of fatigue and fracture mechanics, and probabilistic approaches. To overcome these difficulties in the life-cycle reliability analysis of MSD, the Gaussian process (GP) response surface model has been assembled with one of the recent multiple crack analysis tools, XFEM, in this study. The assembled GP-XFEM method represents a convenient way to obtain the response surface and sensitivity factors of multiple crack propagation in a structure (or a component) under a complex environment with computational efficiency. The accuracy and advantages of the proposed method were verified by a number of experimental results and numerical examples.

The combined effect of carbonation and chloride ingress in concrete is studied in this paper. Based on the change of the pore structure and the chemical equilibrium, a comprehensive model is proposed for this problem. A coupled simulation of the transports of carbon dioxide, chloride ions, heat and moisture is carried out. Several sets of experimental data were compared with the prediction by the numerical model developed in this paper, for its verification. Parametric study shows that the differences between the combined mechanism and the independent mechanisms are significant in many aspects.

The waste glass sludge is a waste produced in the glass industry. It is in a dust form and disposed with water. In the disposal process, various cohesive agents are incorporated in order to precipitate the glass particles efficiently. In this paper, we investigate the pozzolanic reaction of the waste glass sludge incorporating precipitation additives experimentally. The consumption of calcium hydroxide, the setting time and the compressive strength and the pore structure were tested for two different types of the waste glass sludge depending on whether precipitation additives were used. It was found that the waste glass sludge incorporating the precipitation additives had a higher pozzolanic potential than the reference waste glass sludge without precipitation additives.

We report on the fabrication of a fractal-designed transparent and stretchable (TS) microsupercapacitor (MSC) as a skin-attachable energy storage device for driving an integrated strain sensor (SS). The MSC is designed to have a fractal structure with long and narrow patterns to ensure high transparency and stretchability. MnO2/Carbon nanotubes (CNTs) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide/poly(methyl methacrylate) ([BMIM][TFSI]/PMMA) are used as electrodes and a stretchable electrolyte for the MSC, respectively. The fabricated MSC exhibits high transparency of 79% and an areal capacitance of 12.6 mF cm−2 at a current density of 5 mA cm−2. The TS MSC shows excellent mechanical stability in that it can maintain stable electrochemical performance even after 2000 repeated stretching cycles up to 30%. The TS SS is facilely prepared via spray coating of Ag nanowires (NWs) and subsequent curing of PDMS, where the transparency and the electrical resistance are controlled by adjusting the deposited volume of Ag NWs. Wrist bending and a wrist pulse are detected by the SS driven with the stored energy of the MSC. This work demonstrates the possible application of our TS MSC to future skin-attachable electronics as an integrated energy storage device.

Clastic sandstone is widely distributed in oil and gas reservoirs, its internal structure has many micro-defects. Under different stress environments of burial depth, significant damage evolution and plastic deformation easily occur. A series of triaxial compression tests were performed to study the coupled elastoplastic damage mechanical behavior of clastic sandstone samples at different burial depths ranging from 581.28 m to 979.82 m. Results reveal that the stress-strain responses of clastic sandstone samples exhibit significant nonlinear and softening characteristics. The mechanical behavior is due to the coupling of plastic deformation and mechanical damage. Plastic and damage internal variables cause damage stiffness degradation and plastic flow. Considering the coupling of elastoplastic damage in the loading process, an elastoplastic damage coupling model is proposed to study the mechanical behavior of different burial depth clastic sandstones. The model can effectively describe the mechanical behavior of clastic sandstone, such as the volume compression and dilatancy transformation, plastic hardening and damage softening, which are in good agreement with the experimental results. Furthermore, the mechanical behavior of the clastic sandstone shows a dependency on the confining pressure and burial depth. The load-bearing capacity and the ability to resist deformation of the clastic sandstone are improved as the confining pressure and burial depth increase. Relevant results can provide reliable basis for the safe exploitation of oil and gas engineering.

Cement production is a highly energy-intensive and environmentally polluting process, however, despite the detrimental effects, the production rates are increasing annually to meet the ever-rising demand. Hence, it is imperative to look out for materials that can be used in place of cement for promoting green construction and sustainability. On the other hand, glass, post-consumption, is dumped in landfills instead of recycling due to factors such as high recycling cost, cheap imports and lack of awareness to name a few. Since glass is a non-biodegradable material, the practice of dumping waste glass in the landfills is environmentally unsustainable. Recently it has been revealed that glass at particular fineness exhibits pozzolanic characteristics. The focus of this study is to investigate the pozzolanic attribute of a coarse glass powder (≤150 µm) used as a cement replacement; the substitution levels employed were 15% and 30% by weight of cement. In addition to the basic compressive strength testing, durability tests such as drying shrinkage, resistance to chloride ion penetration and heat of hydration were evaluated. The test results indicate that 30% is the ideal replacement level, providing a strength activity index (SAI) of 117%, a 32% higher resistance to chloride ion permeability and a 24% reduction in heat from hydration.

The speed of vehicles on highways has increased due to improved driving environments. However, this increased speed has become one of the main reasons for collisions between vehicles and ro...

We report on the fabrication of a stretchable patterned graphene gas sensor driven by integrated micro-supercapacitor (MSC) array on the same deformable substrate. The integrated MSCs consist of polyaniline-wrapped multi-walled carbon nanotube electrodes and an ionogel electrolyte of poly(ethylene glycol)diacrylate and 1-ethyl-3-methylimidazoliumbis (trifluoromethylsulfonyl) imide. The deformable soft Ecoflex substrate was designed to suppress the applied strain on the gas sensor and MSCs via insertion of stiff platforms of SU-8 underneath and serpentine electrical interconnections of polymer-encapsulated long and narrow Au thin film. The fabricated MSC array demonstrated stable electrochemical performance under a uniaxial strain of 50% and a biaxial strain of 40%, maintaining its initial characteristics even after 1000 cycles of repetitive uniaxial and biaxial stretching. Furthermore, the patterned-graphene sensor detected NO 2 gas for longer than 50 min via integration with MSCs using the serpentine interconnections even under uniaxial stretching by 50%. This work suggests fundamental progress towards the development of stretchable environmental sensor system which can be driven by integrated energy storage devices without external long wire connections to power source.

A stretchable multisensor system is successfully demonstrated with an integrated energy-storage device, an array of microsupercapacitors that can be repeatedly charged via a wireless radio-frequency power receiver on the same stretchable polymer substrate. The integrated devices are interconnected by a liquid-metal interconnection and operate stably without noticeable performance degradation under strain due to the skin attachment, and a uniaxial strain up to 50%.

A stretchable polyaniline nanofiber temperature sensor array with an active matrix consisting of single-walled carbon nanotube thin-film transistors is demonstrated. The integrated temperature sensor array gives mechanical stability under biaxial stretching of 30%, and the resultant spatial temperature mapping does not show any mechanical or electrical degradation.

An algorithm to solve the inverse problem of detecting inclusion interfaces in a piezoelectric structure is proposed. The material interfaces are implicitly represented by level sets which are identified by applying regularization using total variation penalty terms. The inverse problem is solved iteratively and the extended finite element method is used for the analysis of the structure in each iteration. The formulation is presented for three-dimensional structures and inclusions made of different materials are detected by using multiple level sets. The results obtained prove that the iterative procedure proposed can determine the location and approximate shape of material sub-domains in the presence of higher noise levels.

The paper presents a multiscale method for crack propagation. The coarse region is modelled by the differential reproducing kernel particle method. Fracture in the coarse scale region is modelled with the Phantom node method. A molecular statics approach is employed in the fine scale where crack propagation is modelled naturally by breaking of bonds. The triangular lattice corresponds to the lattice structure of the (111) plane of an FCC crystal in the fine scale region. The Lennard-Jones potential is used to model the atom-atom interactions. The coupling between the coarse scale and fine scale is realized through ghost atoms. The ghost atom positions are interpolated from the coarse scale solution and enforced as boundary conditions on the fine scale. The fine scale region is adaptively refined and coarsened as the crack propagates. The centro symmetry parameter is used to detect the crack tip location. The method is implemented in two dimensions. The results are compared to pure atomistic simulations and show excellent agreement. (C) 2014 Elsevier B. V. All rights reserved.

Cement-based sensors with hybrid conductive fillers using both carbon fibers (CFs) and multi-walled carbon nanotubes (MWCNTs) were experimentally investigated in this study. The self-sensing capacities of cement-based composites with only CFs or MWCNTs were found based on preliminary tests. The results showed that the percolation thresholds of CFs and MWCNTs were 0.5–1.0 vol.% and 1.0 vol.%, respectively. Based on these results, the feasibility of self-sensing composites with four different amounts of CFs and MWCNTs was considered under cyclic compression loads. When the amount of incorporated CFs increased and the amount of incorporated MWCNTs decreased, the self-sensing capacity of the composites was reduced. It was concluded that cement-based composites containing both 0.1 vol.% CFs and 0.5 vol.% MWCNTs could be an alternative to cement-based composites with 1.0 vol.% MWCNTs in order to achieve equivalent self-sensing performance at half the price. The gauge factor (GF) for that composite was 160.3 with an R-square of 0.9274 in loading stages I and II, which was similar to the GF of 166.6 for the composite with 1.0 vol.% MWCNTs.

This study examined the electrical and self-sensing capacities of ultra-high-performance fiber-reinforced concrete (UHPFRC) with and without carbon nanotubes (CNTs). For this, the effects of steel fiber content, orientation, and pore water content on the electrical and piezoresistive properties of UHPFRC without CNTs were first evaluated. Then, the effect of CNT content on the self-sensing capacities of UHPFRC under compression and flexure was investigated. Test results indicated that higher steel fiber content, better fiber orientation, and higher amount of pore water led to higher electrical conductivity of UHPFRC. The effects of fiber orientation and drying condition on the electrical conductivity became minor as sufficiently high amount of steel fibers, 3% by volume, was added. Including only steel fibers did not impart UHPFRC with piezoresistive properties. Addition of CNTs substantially improved the electrical conductivity of UHPFRC. Under compression, UHPFRC with a CNT content of 0.3% or greater had a self-sensing ability that was activated by the formation of cracks, and better sensing capacity was achieved by including greater amount of CNTs. Furthermore, the pre-peak flexural behavior of UHPFRC was precisely simulated with a fractional change in resistivity when 0.3% CNTs were incorporated. The pre-cracking self-sensing capacity of UHPFRC with CNTs was more effective under tensile stress state than under compressive stress state.

Networks of silver nanowires (Ag NWs) have been considered as promising materials for stretchable and transparent conductors. Despite various improvements of their optoelectronic and electromechanical properties over the past few years, Ag NW networks with a sufficient stretchability in multiple directions that is essential for the accommodation of the multidirectional strains of human movement have seldom been reported. For this paper, biaxially stretchable, transparent conductors were developed based on 2D mass-spring networks of wavy Ag NWs. Inspired by the traditional papermaking process, the 2D wavy networks were produced by floating Ag NW networks on the surface of water and subsequently applying biaxial compression to them. It was demonstrated that this floating-compression process can reduce the friction between the Ag NW-water interfaces, providing a uniform and isotropic in-plane waviness for the networks without buckling or cracking. The resulting Ag NW networks that were transferred onto elastomeric substrates successfully acted as conductors with an excellent transparency, conductivity, and electromechanical stability under a biaxial strain of 30%. The strain sensors that are based on the prepared conductors demonstrated a great potential for the enhanced performances of future wearable devices.

Recently, there was a collision accident involving vehicle–concrete median barrier in South Korea, and unfortunately, passengers on the opposite direction road were killed by the flying broken pieces of concrete generated by the collision. Primarily after this accident, we felt the need for developing an improved concrete median barrier up to level of SB6 impact severity in order to minimize the amount of broken pieces of concrete and any possibility of traffic accident casualty under the impact loading of truck. Accordingly, in this study, several designs of concrete median barriers have been examined, and a preliminary study has been conducted for developing and verifying appropriate collision model. First, type of vehicle was selected based on impact analysis on rigid wall. Then, the effects of element size and other key parameters on the capacity of the concrete median barrier under impact were studied. It was found that the key parameters for controlling behaviors of the median barrier under impact loading were contact option, threshold value, and mesh and boundary conditions. Furthermore, as a parametric study, effect of geometry and amount of wire-mesh or steel rebar in concrete median barrier on impact resistances of median barrier for reducing the collision debris were investigated. The amount of volume loss after the collision of truck was compared for various reinforcement ratios.

The effect of lattice orientation and crack length on the mechanical properties of Graphene are studied based on molecular dynamics simulations. Bond breaking and crack initiation in an initial edge crack model with 13 different crack lengths, in 10 different lattice orientations of Graphene are examined. In all the lattice orientations, three recurrent fracture patterns are reported. The influence of the lattice orientation and crack length on yield stress and yield strain of Graphene is also investigated. The arm-chair fracture pattern is observed to possess the lowest yield properties. A sudden decrease in yield stress and yield strain can be noticed for crack sizes < 10 nm. However, for larger crack sizes, a linear decrease in yield stress is observed, whereas a constant yield strain of 0.05 is noticed. Therefore, the yield strain of 0.05 can be considered as a critical strain value below which Graphene does not show failure. This information can be utilized as a lower bound for the design of nano-devices for various strain sensor applications. Furthermore, the yield data will be useful while developing the Graphene coating on Silicon surface in order to enhance the mechanical and electrical characteristics of solar cells and to arrest the growth of micro-cracks in Silicon cells.

We report the fabrication of an encapsulated, high-performance, stretchable array of stacked planar micro-supercapacitors (MSCs) as a wearable energy storage device for waterproof applications. A pair of planar all-solid-state MSCs with spray-coated multiwalled carbon nanotube electrodes and a drop-cast UV-patternable ion-gel electrolyte was fabricated on a polyethylene terephthalate film using serial connection to increase the operation voltage of the MSC. Additionally, multiple MSCs could be vertically stacked with parallel connections to increase both the total capacitance and the areal capacitance owing to the use of a solid-state patterned electrolyte. The overall device of five parallel-connected stacked MSCs, a microlight-emitting diode (μ-LED), and a switch was encapsulated in thin Ecoflex film so that the capacitance remained at 82% of its initial value even after 4 d in water; the μ-LED was lit without noticeable decrease in brightness under deformation including bending and stretching. Furthermore, an Ecoflex encapsulated oximeter wound around a finger was operated using the stored energy of the MSC array attached to the hand (even in water) to give information on arterial pulse rate and oxygen saturation in the blood. This study suggests potential applications of our encapsulated MSC array in wearable energy storage devices especially in water.

We predict macroscopic fracture related material parameters of fully exfoliated clay/epoxy nanocomposites based on their fine scale features. Fracture is modeled by a phase field approach which is implemented as user subroutines UEL and UMAT in the commercial finite element software Abaqus. The phase field model replaces the sharp discontinuities with a scalar damage field representing the diffuse crack topology through controlling the amount of diffusion by a regularization parameter. Two different constitutive models for the matrix and the clay platelets are used; the nonlinear coupled system consisting of the equilibrium equation and a diffusion-type equation governing the phase field evolution are solved via a Newton–Raphson approach. In order to predict the tensile strength and fracture toughness of the clay/epoxy composites we evaluated the J integral for different specimens with varying cracks. The effect of different geometry and material parameters, such as the clay weight ratio (wt.%) and the aspect ratio of clay platelets are studied.

In this study, a three-dimensional (3D) non-ordinary state-based peridynamics (NOSB-PD) formulation for thermomechanical brittle and ductile fracture is presented. The Johnson–Cook (JC) constitutive and damage model is used to taken into account plastic hardening, thermal softening and fracture. The formulation is validated by considering two benchmark examples: 1) The Taylor-bar impact and 2) the Kalthoff–Winkler tests. The results show good agreements between the numerical simulations and the experimental results.

SUMMARY A method for modelling the growth of multiple cracks in linear elastic media is presented. Both homogeneous and inhomogeneous materials are considered. The method uses the extended finite element method for arbitrary discontinuities and does not require remeshing as the cracks grow; the method also treats the junction of cracks. The crack geometries are arbitrary with respect to the mesh and are described by vector level sets. The overall response of the structure is obtained until complete failure. A stability analysis of competitive cracks tips is performed. The method is applied to bodies in plane strain or plane stress and to unit cells with 2‐10 growing cracks (although the method does not limit the number of cracks). It is shown to be efficient and accurate for crack coalescence and percolation problems. Copyright ! 2004 John Wiley & Sons, Ltd.

Body-attachable sensors can be applied to electronic skin (e-skin) as well as safety forewarning and health monitoring systems. However, achieving facile fabrication of high-performance, cost-effective sensors with mechanical stability in response to deformation due to body movement is challenging. Herein we report the material design, fabrication and characteristics of a skin-like stretchable array of multi-functional (MF) sensors based on a single sensing material of polyurethane foam coated with multi-walled carbon nanotube/polyaniline nanocomposite, which enables simultaneous detection of body temperature, wrist pulse and ammonia gas. These sensors exhibit high sensitivity, fast response and excellent durability. Furthermore, the fabricated sensor array shows stable performance under biaxial stretching up to 50% and attachment to skin owing to the use of direct-printed Galinstan liquid metal interconnections. This work proposes a facile method for fabrication of high-performance, stretchable MF sensors via appropriate selection of sensor design and functional materials that are applicable to e-skin and health monitoring systems. Combining stretchy foams with conductive nanomaterials enables real-time temperature, pulse, and gas exposure sensing from a patient's skin. Interactive ‘e-skin’ devices need to resolve different stimuli into clear electronic signals while attached to a deformable substrate. To satisfy these challenging constraints, Jeong Sook Ha and colleagues at Korea University coated polyurethane foams with a nanocomposite made from multiwalled carbon nanotubes and polyaniline. The porous microstructure of this hybrid material imparted mechanical flexibility and high responsiveness to electrical resistance changes and output voltage induced by pressure and heat variations, respectively, while the polyaniline component made it selective to toxic ammonia gas. A prototype device in which the foam is sandwiched between thin-film electrodes and which uses an adhesive gel to attach to the skin showed a rapid response and an excellent durability when attached to the fingertips and the wrist. A skin-like stretchable multi-functional sensor array based on a combination of PU foam and MWCNT-PANI nanocomposite is demonstrated to simultaneously detect pressure, temperature and ammonia gas with high sensitivity. The integrated sensor array with printed liquid metal interconnections operates stably under applied strain due to its attachment to the skin and under a biaxial strain up to 50%.

In order to investigate the effects of fiber length, placement method, and test method on the biaxial flexural behaviors of ultra-high-performance fiber-reinforced concrete (UHPFRC), several UHPFRC panels with two different fiber lengths (Lf of 13 and 19.5 mm) were fabricated using two different placement methods (placing concrete at the center and the edge) and were then tested by two different test methods (ASTM C1550-12a and a novel biaxial flexure test (BFT)). Image analysis was also performed to quantitatively investigate the fiber distribution characteristics according to the fiber length and placement method and to thoroughly analyze the experimental results. The first cracking strength and corresponding toughness were found to be insignificantly influenced by the fiber length, placement method, and test method, but the panels with longer fibers and with concrete placed at the center (in the maximum moment region) were found to have higher biaxial flexural strength, deflection capacity, and toughness after a deflection of 2.5 mm (d2.5). The panels tested by the BFT method showed lower flexural strength, more cracks with a random distribution, and higher deviation in flexural performances than those tested by ASTM C1550. These test results were verified by evaluating the fiber distribution characteristics (i.e., the number of fibers per unit area, fiber orientation, and fiber dispersion) at localized crack surfaces by using the image analysis technique.