Stretchable Organometal‐Halide‐Perovskite Quantum‐Dot Light‐Emitting Diodes

Stretchable light‐emitting diodes (LEDs) and electroluminescent capacitors have been reported to potentially bring new opportunities to wearable electronics; however, these devices lack in efficiency and/or stretchability. Here, a stretchable organometal‐halide‐perovskite quantum‐dot LED with both high efficiency and mechanical compliancy is demonstrated. The hybrid device employs an ultrathin (<3 µm) LED structure conformed on a surface‐wrinkled elastomer substrate. Its luminescent efficiency is up to 9.2 cd A^?1, which is 70% higher than a control diode fabricated on the rigid indium tin oxide/glass substrate. Mechanical deformations up to 50% tensile strain do not induce significant loss of the electroluminescent property. The device can survive 1000 stretch–release cycles of 20% tensile strain with small fluctuations in electroluminescent performance.     

Out‐of‐Plane 3D‐Printed Microfibers Improve the Shear Properties of Hydrogel Composites

One challenge in biofabrication is to fabricate a matrix that is soft enough to elicit optimal cell behavior while possessing the strength required to withstand the mechanical load that the matrix is subjected to once implanted in the body. Here, melt electrowriting (MEW) is used to direct‐write poly(ε‐caprolactone) fibers “out‐of‐plane” by design. These out‐of‐plane fibers are specifically intended to stabilize an existing structure and subsequently improve the shear modulus of hydrogel–fiber composites. The stabilizing fibers (diameter = 13.3 ± 0.3 µm) are sinusoidally direct‐written over an existing MEW wall‐like structure (330 µm height). The printed constructs are embedded in different hydrogels (5, 10, and 15 wt% polyacrylamide; 65% poly(2‐hydroxyethyl methacrylate) (pHEMA)) and a frequency sweep test (0.05–500 rad s^?1, 0.01% strain, n = 5) is performed to measure the complex shear modulus. For the rheological measurements, stabilizing fibers are deposited with a radial‐architecture prior to embedding to correspond to the direction of the stabilizing fibers with the loading of the rheometer. Stabilizing fibers increase the complex shear modulus irrespective of the percentage of gel or crosslinking density. The capacity of MEW to produce well‐defined out‐of‐plane fibers and the ability to increase the shear properties of fiber‐reinforced hydrogel composites are highlighted.     

Diffusing‐Wave Spectroscopy Contribution to Strain Analysis

Abstract: We present a new full‐field strain measurement method based on diffusing‐wave spectroscopy. Our technique makes it possible to measure strains in the vicinity of the surface of highly light‐scattering materials. Its main feature is an extreme sensitivity: the range of deformations measured is 10^?5–10^?3. To validate the measurements, experimental results from several plane stress configurations are compared with theoretical and numerical calculations. Furthermore, we propose an extension of the method for non‐scattering materials.     

Self‐sensing damage in CNT infused epoxy panels with and without glass‐fibre reinforcement

The objective of the study is to utilise a material's inherent electrical conductivity as means of damage quantification and damage location detection. After determining the percolation threshold for a carbon nanotube (CNT)‐epoxy mixture, an optimum concentration was chosen to infuse it into glass‐fabric reinforced panels to make them electrically conductive. Two different multiwalled CNT‐epoxy composites were manufactured for this study: CNT enhanced epoxy resin and glass‐fabric reinforced CNT epoxy resin. Epoxy resin‐based glass‐fabric reinforced composite panels enhanced with carbon nanotubes were manufactured with embedded electrodes and then subjected to damages. Rectangular panels of various proportions were studied. Disks made out of copper foil were affixed to surfaces of CNT epoxy panel, whereas in glass‐fabric CNT epoxy specimen, total of 64 electrodes (grid of 8 × 8) were embedded inside the composite panel under the top layer of the 10‐ply fabric. The disks acted as electrodes, enabling voltage measurements using in‐line 4‐probe technique, which minimises contact resistance between the electrodes and the material. Two different configurations of electrode network were employed to scan voltage change in the entire composite panel. The networks included evenly spaced (3 in. for inner ones) electrodes that spanned the surface of the panel. To further investigate influence of electrodes distribution, finite element simulations were used to solve the electrical potential distribution in the panel for various damage sizes and location. Predamage and postdamage voltage field was used as gauge in sensing the damage and its extent for quantification. The finite element method simulation results matched the experimental data closely. The results indicate that there is a consistent behaviour that can be correlated to the size and location of the damage. As spacing between electrodes is increased, they become less responsive to smaller damages. Forty‐eight electrodes (out of 64) were actively used and were enough to confirm that the method can be used as an alternative to electrical tomography method where fewer (boundary) electrodes per area are employed but at a higher cost of computational cost. One important aspect of this study with embedded and distributed electrodes is the fact that the method can be applied to larger panels increasing its utility in practical applications.     

A non‐linear interface element for 3D mesoscale analysis of brick‐masonry structures

This paper presents a novel interface element for the geometric and material non‐linear analysis of unreinforced brick‐masonry structures. In the proposed modelling approach, the blocks are modelled using 3D continuum solid elements, whereas the mortar and brick–mortar interfaces are modelled by means of the 2D non‐linear interface element. This enables the representation of any 3D arrangement for brick‐masonry, accounting for the in‐plane stacking mode and the through‐thickness geometry, and importantly it allows the investigation of both the in‐plane and the out‐of‐plane responses of unreinforced masonry panels. A co‐rotational approach is employed for the interface element, which shifts the treatment of geometric non‐linearity to the level of discrete entities, and enables the consideration of material non‐linearity within a simplified local framework employing first‐order kinematics. In this respect, the internal interface forces are modelled by means of elasto‐plastic material laws based on work‐softening plasticity and employing multi‐surface plasticity concepts. Following the presentation of the interface element formulation details, several experimental–numerical comparisons are provided for the in‐plane and out‐of‐plane static behaviours of brick‐masonry panels. The favourable results achieved demonstrate the accuracy and the significant potential of using the developed interface element for the non‐linear analysis of brick‐masonry structures under extreme loading conditions. Copyright © 2010 John Wiley & Sons, Ltd.     

Efficient and accurate multilayer solid‐shell element: non‐linear materials at finite strain

We present in this paper an efficient and accurate low‐order solid‐shell element formulation for analyses of large deformable multilayer shell structures with non‐linear materials. The element has only displacement degrees of freedom (dofs), and an optimal number of enhancing assumed strain (EAS) parameters to pass the patch tests (both membrane and out‐of‐plane bending) and to remedy volumetric locking. Based on the mixed Fraeijs de Veubeke‐Hu‐Washizu (FHW) variational principle, the in‐plane and out‐of‐plane bending behaviours are improved and the locking associated with (nearly) incompressible materials is avoided via a new efficient enhancement of strain tensor. Shear locking and curvature thickness locking are resolved effectively by using the assumed natural strain (ANS) method. Two non‐linear 3‐D constitutive models (Mooney–Rivlin material and hyperelastoplastic material at finite strain) are applied directly without requiring the enforcement of the plane‐stress assumption. In particular, we give a simple derivation for the hyperelastoplastic model using spectral representations. In addition, the present element has a well‐defined lumped mass matrix, and provides double‐side contact surfaces for shell contact problems. With the dynamics referred to a fixed inertial frame, the present element can be used to analyse multilayer shell structures undergoing large overall motion. Numerical examples involving static analyses and implicit/explicit dynamic analyses of multilayer shell structures with both material and geometric non‐linearities are presented, and compared with existing results obtained from other shell elements and from a meshless method. It is shown that elements that did not pass the out‐of‐plane bending patch test could not provide accurate results, as compared to the present element formulation, which passed the out‐of‐plane bending patch test. The present element proves to be versatile and efficient in the modelling and analyses of general non‐linear composite multilayer shell structures. Copyright © 2005 John Wiley & Sons, Ltd.     

Finite‐element analysis of a combined fine‐blanking and extrusion process

This paper presents the characteristics of the combined fine‐blanking and extrusion process and gives a detailed analysis of the process with the finite‐element method. To carry out the simulation step by step and avoid the tendency to diverge in the calculations, the remeshing, tracing and golden section methods were developed and introduced into the finite‐element program. Different boundary conditions were used in the simulation; the mesh distortion, field of material flow, and the stress and strain distributions were obtained. From the simulated results, the deformation characteristics under different boundary conditions were revealed. An experiment was also carried out to verify the simulated results. A large strain analysis technique was chosen to determine the effective strain distribution based on the experiment. The effective strain distributions from the simulation are in accordance with the effective strain distributions and the hardness distributions from the experiment. Copyright © 2005 John Wiley & Sons, Ltd.     

Three‐dimensional analyses of unified characterization parameter of in‐plane and out‐of‐plane creep constraint

Based on extensive three‐dimensional finite element analyses, the unified characterization parameter A_c of in‐plane and out‐of‐plane creep constraint based on crack‐tip equivalent creep strain for three specimen geometries (C(T), SEN(T) and M(T)) were quantified for 316H steel at 550 °C and steady‐state creep. The distributions of the parameter A_c along crack fronts (specimen thickness) were calculated, and its capability and applicability for characterizing a wide range of in‐plane and out‐of‐plane creep constraints in different specimen geometries have been comparatively analysed with the constraint parameters based on crack‐tip stress fields (namely R*, h and T_Z). The results show that the parameter A_c in the centre region of all specimens appears uniform distribution and lower value (higher constraint), and in the region near free surface it shows protuberant distribution and higher value (lower constraint). The parameter A_c can simultaneously and effectively characterize a wide range of in‐plane and out‐of‐plane creep constraints, while the parameters R*, h and T_Z based on crack‐tip stress fields cannot achieve this. The different capabilities of these parameters for characterizing in‐plane and out‐of‐plane creep constraints originate from their underlying theories. The parameter A_c may be useful for accurately characterizing the overall constraint level composed of in‐plane and out‐of‐plane constraints in actual high‐temperature components, and it may be used in creep life assessments for improving accuracy.     

Characterisation of Indentation‐Induced Pattern Using Full‐Field Strain Measurement

Abstract: In this study, digital image correlation (DIC)‐based strain analysis software was successfully developed. Its strain resolution lies in the order of 2.3 × 10^?4–3.1 × 10^?4. Full‐strain field measurement was used to study indentation‐induced plastic patterns around the spherical indenter for a polycrystal and a single crystal of pure aluminium. During indentation, the pure aluminium specimen of the single crystal revealed a symmetric indentation pattern of von Mises strain. The piling‐up around the residual impression was successfully and directly characterised by examining the sign of strain ?_X and ?_Y in the X and Y directions. However, the inward, out‐of‐plane movement results in an error in calculating in‐plane strain referred to as a ‘distortion strain’ using two‐dimensional DIC.     

Two‐Photon Vertical‐Flow Lithography for Microtube Synthesis

Two‐photon vertical‐flow lithography is demonstrated for synthesis of complex‐shaped polymeric microtubes with a high aspect ratio (>100:1). This unique microfluidic approach provides rigorous control over the morphology and surface topology to generate thin‐walled (<1 µm) microtubes with a tunable diameter (1–400 µm) and pore size (1–20 µm). The interplay between fluid‐flow control and two‐photon lithography presents a generic high‐resolution method that will substantially contribute toward the future development of biocompatible scaffolds, stents, needles, nerve guides, membranes, and beyond.     

Hybrid‐stress six‐node prismatic elements

This paper presents three novel hybrid‐stress six‐node prismatic elements. Starting from the element displacement interpolation, the equilibrating non‐constant stress modes for the first element are identified and orthogonalized with respect to the constant stress modes for higher computational efficiency. For the second element, the non‐constant stress modes are non‐equilibrating and chosen for the sake of stabilizing the reduced‐integrated element. The first two elements are intended for three‐dimensional continuum analysis with both passing the patch test for three‐dimensional continuum elements. The third element is primarily intended for plate/shell analysis. Shear locking is alleviated by a new assumed strain scheme which preserves the element accuracy with respect to the twisting load. Furthermore, the Poisson's locking along the in‐plane and out‐of‐plane directions is overcome by using the hybrid‐stress modes of the first element. The third element passes the patch test for plate/shell elements. Unless the element assumes the right prismatic geometry, it fails the patch test for three‐dimensional continuum elements. It will be seen that all the proposed elements are markedly more accurate than the conventional fully integrated element. Copyright © 2004 John Wiley & Sons, Ltd.     

Effects of approximations in analyses of beams of open thin‐walled cross‐section—part II: 3‐D non‐linear behaviour

In a companion paper, the effects of approximations in the flexural‐torsional stability analysis of beams was studied, and it was shown that a second‐order rotation matrix was sufficiently accurate for a flexural‐torsional stability analysis. However, the second‐order rotation matrix is not necessarily accurate in formulating finite element model for a 3‐D non‐linear analysis of thin‐walled beams of open cross‐section. The approximations in the second‐order rotation matrix may introduce ‘self‐straining’ due to superimposed rigid‐body motions, which may lead to physically incorrect predictions of the 3‐D non‐linear behaviour of beams. In a 3‐D non‐linear elastic–plastic analysis, numerical integration over the cross‐section is usually used to check the yield criterion and to calculate the stress increments, the stress resultants, the elastic–plastic stress–strain matrix and the tangent modulus matrix. A scheme of the arrangement of sampling points over the cross‐section that is not consistent with the strain distributions may lead to incorrect predictions of the 3‐D non‐linear elastic–plastic behaviour of beams. This paper investigates the effects of approximations on the 3‐D non‐linear analysis of beams. It is found that a finite element model for 3‐D non‐linear analysis based on the second‐order rotation matrix leads to over‐stiff predictions of the flexural‐torsional buckling and postbuckling response and to an overestimate of the maximum load‐carrying capacities of beams in some cases. To perform a correct 3‐D non‐linear analysis of beams, an accurate model of the rotations must be used. A scheme of the arrangement of sampling points over the cross‐section that is consistent with both the longitudinal normal and shear strain distributions is needed to predict the correct 3‐D non‐linear elastic–plastic behaviour of beams. Copyright © 2001 John Wiley & Sons, Ltd.     

Three‐dimensional analyses of in‐plane and out‐of‐plane crack‐tip constraint characterization for fracture specimens

Three‐dimensional elastic–plastic finite element analyses have been conducted for 21 experimental specimens with different in‐plane and out‐of‐plane constraints in the literature. The distributions of five constraint parameters (namely T‐stress, Q, h, T_z and A_p) along crack fronts (specimen thickness) for the specimens were calculated. The capability and applicability of the parameters for characterizing in‐plane and out‐of‐plane crack‐tip constraints and establishing unified correlation with fracture toughness of a steel were investigated. The results show that the four constraint parameters (T‐stress, Q, h and T_z) based on crack‐tip stress fields are only sensitive to in‐plane or out‐of‐plane constraints. Therefore, the monotonic unified correlation curves with fracture toughness (toughness loci) cannot obtained by using them. The parameter A_p based on crack‐tip equivalent plastic strain is sensitive to both in‐plane and out‐of‐plane constraints, and may effectively characterize both of them. The monotonic unified correlation curves with fracture toughness can be obtained by using A_p. In structural integrity assessments, the correlation curves may be used in the failure assessment diagram (FAD) methodology for incorporating both in‐plane and out‐of‐plane constraint effects in structures for improving accuracy.     

Moiré Interferometry Assessement of Residual Stress Variation in Depth on a Shot Peened Surface

Abstract: The main goal of this work was the development of experimental techniques to measure in depth non‐uniform residual stresses, as alternative to the more conventional hole‐drilling method with strain gauges. The proposed experimental methodology is based on moiré interferometry. This high resolution field technique allows in‐plane displacement assessment without contact. Grating replication techniques were also developed to record high quality diffraction gratings onto the specimen’s surface. A laser interferometry setup was implemented to generate the master grating (virtual). The stress relaxation was promoted by blind hole‐drilling and the obtained fringe patterns were video recorded. Image processing techniques were applied to assess the in‐plane strain full‐field. A finite elements code (FEM), ANSYS^®, was used to simulate the stress relaxation process and to calculate the hole‐drilling calibration constants.     

Evaluation of a laboratory‐scale pressure differential (force/decay) system for non‐destructive leak detection of flexible and semi‐rigid packaging

This study evaluated the capabilities of a bench‐top non‐destructive pressure differential leak tester using 355 ml polyethylene terephthalate/ethylene vinyl alcohol/polypropylene (PET/EVOH/PP) trays. This evaluation was done by monitoring the equipment's force/decay responses to leaks, changes in the package headspace volume and differences in the seal strength of 986 sample trays. Leak detection evaluation was done using artificially created channel leaks (10–200 µm) in the sealing areas and pinholes (5–50 µm) in the lids of the polymeric trays. Seal strength evaluation included the ability of the equipment to identify non‐leaking but weak seals and the extent to which the pressure differential unit affected good seals during a normal test run. The results showed that the equipment had a detection limit of 40 µm for channel leaks 6 mm in length and 15 µm for pinholes. The results also showed that the pressure differential unit caused a 9% reduction in the seal strength of the tested packages. However, peel strength analysis and distribution testing showed that this reduction in seal strength did not compromise the integrity of 99% of the packages tested. Results showed that the equipment could also detect weak but non‐leaking seals that had potential to lose integrity during transportation and retail handling. The results of this study could be used to determine the capabilities and limitations of a non‐destructive pressure differential bench‐top leak testing device intended for food packaging quality control. Copyright © 2002 John Wiley & Sons, Ltd.     

Monitoring Process Variance Using an ARL‐unbiased EWMA‐p Control Chart

Control charts are effective tools for signal detection in manufacturing processes. As much of the data in industries come from processes having non‐normal or unknown distributions, the commonly used Shewhart variable control charts cannot be appropriately used, because they depend heavily on the normality assumption. The average run length (ARL) is generally used to measure the detection performance of a process when using a control chart, but it is biased for the monitoring statistic with an asymmetric distribution. That is, the ARL‐biased control chart leads to take longer to detect the shifts in parameter than to trigger a false alarm. To overcome this problem, we herein propose an ARL‐unbiased exponentially weighted moving average proportion (EWMA‐p) chart to monitor the process variance for process data with non‐normal or unknown distributions. We further explore the procedure to determine the control limits and to investigate the out‐of‐control variance detection performance of the ARL‐unbiased EWMA‐p chart. With a numerical example involving non‐normal service times from a bank branch in Taiwan, we illustrate the applications of the proposed ARL‐unbiased EWMA‐p chart and also compare the out‐of‐control detection performance of the ARL‐unbiased EWMA‐p chart, the arcsin transformed symmetric EWMA variance, and other existing variance charts. The proposed ARL‐unbiased EWMA‐p chart shows superior detection performance. Thus, we recommend the ARL‐unbiased EWMA‐p chart for process data with non‐normal or unknown distributions. Copyright © 2015 John Wiley & Sons, Ltd.     

On the Performance of Phase‐I Bivariate Dispersion Charts to Non‐Normality

A phase‐I study is generally used when population parameters are unknown. The performance of any phase‐II chart depends on the preciseness of the control limits obtained from the phase‐I analysis. The performance of phase‐I bivariate dispersion charts has mainly been investigated for bivariate normal distribution. However, this assumption is seldom fulfilled in reality. The current work develops and studies the performance of phase‐I |S| and |G| charts for monitoring the process dispersion of bivariate non‐normal distributions. The necessary control charting constants are determined for the bivariate non‐normal distributions at nominal false alarm probability (FAP₀). The performance of these charts is evaluated and compared in a situation when samples are generated by bivariate logistic, bivariate Laplace, bivariate exponential, or bivariate t₅ distribution. The analysis shows that the proper consideration to underlying bivariate distribution in the construction of phase‐I bivariate dispersion charts is very important to give a real picture of in or out of control process status. Copyright © 2016 John Wiley & Sons, Ltd.     

Application of calcium carbonate in resin transfer molding process: An experimental investigation

The resin transfer molding (RTM) process is used to manufacture advanced composite materials made of continuous glass or carbon fibers embedded in a thermoset polymer matrix. In this process, a fabric preform is prepared, and is then placed into a mold cavity. After the preform is compacted between the mold parts, thermoset polymer is transferred from an injection machine to the mold cavity through injection gate(s). Resin flows through the porous fabric, and eventually flows out through the ventilation port(s). After the resin cure process (cross‐linking of the polymer), the mold is opened and the part is removed. The objective of this study is to verify the application of calcium carbonate mixed in resin in the RTM process. Several rectilinear infiltration experiments were conducted using glass fiber mat molded in a RTM system with cavity dimensions of 320 × 150 × 3.6 mm, room temperature, maximum injection pressure 0.202 bar and different content of CaCO₃ (10 and 40%) and particle size (mesh opening 38 and 75 µm). The results show that the use of filled resin with CaCO₃ influences the preform impregnation during the RTM molding, changing the filling time and flow front position, however it is possible to make composite with a good quality and low cost.     

Wave propagation analysis in inhomogeneous piezo‐composite layer by the thin‐layer method

The thin‐layer method (TLM) is used to study the propagation of waves in inhomogeneous piezo‐composite layered media caused by mechanical loading and electrical excitation. The element is formulated in the time‐wavenumber domain, which drastically reduces the cost of computation compared to the finite element (FE) method. Fourier series are used for the spatial representation of the unknown variables. The material properties are allowed to vary in the depthwise direction only. Both linear and exponential variations of elastic and electrical properties are considered. Several numerical examples are presented, which bring out the characteristics of wave propagation in anisotropic and inhomogeneous layered media. The element is useful for modelling ultrasonic transducers (UT) and one such example is given to show the effect of electric actuation in a composite material and the difference in the responses elicited for various ply‐angles. Further, an ultrasonic transducer composed of functionally graded piezoelectric materials (FGPM) is modelled and the effect of gradation on mechanical response is demonstrated. The effect of anisotropy and inhomogeneity is shown in the normal modes for both displacement and electric potential. The element is further utilized to estimate the piezoelectric properties from the measured response using non‐linear optimization, a strategy that is referred to as the pulse propagation technique (PPT). Copyright © 2005 John Wiley & Sons, Ltd.     

Microscale Electro‐Hydrodynamic Cell Printing with High Viability

Cell printing has gained extensive attentions for the controlled fabrication of living cellular constructs in vitro. Various cell printing techniques are now being explored and developed for improved cell viability and printing resolution. Here an electro‐hydrodynamic cell printing strategy is developed with microscale resolution (<100 µm) and high cellular viability (>95%). Unlike the existing electro‐hydrodynamic cell jetting or printing explorations, insulating substrate is used to replace conventional semiconductive substrate as the collecting surface which significantly reduces the electrical current in the electro‐hydrodynamic printing process from milliamperes (>0.5 mA) to microamperes (<10 µA). Additionally, the nozzle‐to‐collector distance is fixed as small as 100 µm for better control over filament deposition. These features ensure high cellular viability and normal postproliferative capability of the electro‐hydrodynamically printed cells. The smallest width of the electro‐hydrodynamically printed hydrogel filament is 82.4 ± 14.3 µm by optimizing process parameters. Multiple hydrogels or multilayer cell‐laden constructs can be flexibly printed under cell‐friendly conditions. The printed cells in multilayer hydrogels kept alive and gradually spread during 7‐days culture in vitro. This exploration offers a novel and promising cell printing strategy which might benefit future biomedical innovations such as microscale tissue engineering, organ‐on‐a‐chip systems, and nanomedicine.