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Thin films that exhibit spatially heterogeneous swelling often buckle into the third dimension to minimize stress. These effects, in turn, offer a promising strategy to fabricate complex three-dimensional structures from two-dimensional sheets. Here we employ surface topography as a new means to guide buckling of swollen polymer bilayer films and thereby control the morphology of resulting three-dimensional objects. Topographic patterns are created on poly(dimethylsiloxane) (PDMS) films selectively coated with a thin layer of non-swelling parylene on different sides of the patterned films. After swelling in an organic solvent, various structures are formed, including half-pipes, helical tubules, and ribbons. We demonstrate these effects and introduce a simple geometric model that qualitatively captures the relationship between surface topography and the resulting swollen film morphologies. The model's limitations are also examined.

A finite element model was developed to predict the deformation, temperature history, carbon diffusion, phase fraction, and hardness during the carburizing heat treatment of automotive annulus gear ring, initially made of a medium carbon steel. Carburizing gas with a constant carbon potential for entire surfaces of the gear was assumed. The temperature and pressure driven carbon diffusion was solved by the finite element simulation based on Fick's law. Both the diffusional and displacive phase transformations during the heat treatment were modeled incorporating the carbon concentration inside the gear. The constitutive equation of the transformation plasticity was incorporated into the finite element model. Strains due to the phase transformation, transformation plasticity, and thermal expansion/contraction were calculated by the finite element model. The prediction accuracy for the phase evolution, hardness distribution, and dimensional change of the gear ring was verified with the measurement data.

We investigate the static and dynamic (phononic) elastic behavior of fractal-cut materials. These materials are novel in the sense that they deform by rotation of “rigid” units rather than by straining these units, can be fabricated by exploiting a simple cutting paradigm, and have properties that can be manipulated by control of the cut pattern and its hierarchy. We show that variation of fractal-cut level and cut pattern can be exploited to manipulate the symmetry of the elastic constant tensor, the elastic limit of deformation, and, therefore, the elastic response. By studying phonon behavior, we demonstrate how some cut symmetries naturally open acoustic band gaps. Several of the important features of the band structure can be directly related to the static elastic properties. Based upon our phonon calculations, we predict the acoustic transmission spectrum of an example fractal-cut structure and validated it through 3D printing and sound attenuation experiments.

Tapered nanopillar structures of different cross-sectional geometries including cone-, pencil-like, and stepwise are prepared from anodized aluminum oxide templates. The shape effect on the adhesion strength is investigated in experiments and simulation. Cone-shaped nanopillars are highly bendable under load and can recover after unloading, thus, warranting high adhesion strength, 34 N cm(-2) . The pencil-like and stepwise nano-pillars are, however, easily fractured and are not recoverable under the same conditions.

A three-dimensional (3D) thermo-mechanical model is developed considering the phase transformation occurring during the friction stir welding (FSW) of steel, and the simulated result is compared with both the measured temperature distribution during FSW and the microstructural changes after FSW. The austenite grain size (AGS) decreases significantly because of the frictional heat and severe plastic deformation generated during FSW, and the decreased AGS accelerates the diffusional phase transformation during FSW. The ferrite phase, one of the diffusional phases, is developed mainly in mild steel, whereas the bainite phase transformation occurs significantly in high-strength steel with large hardenability. Additionally, transformation-induced heat is observed mainly in the stir zone during FSW. The measured temperature distribution and phase fraction agree fairly well with the predicted data.

Metal-based cellular materials with periodic structures are currently the preferred choice as bone/cartilage implants under load-bearing conditions due to their controlled pore interconnectivity and porosities. This report presents a new methodology for the structural analysis of periodic cellular materials using X-ray microtomography and dual-level finite element modeling (FEM). A three-dimensional (3D) structure of periodic titanium foam produced using selective laser melting (SLM) is obtained using an X-ray microtomography. A dual-level FEM based on the 3D structure is used to simulate the deformation behavior of titanium foam with a regular structure under uniaxial compression, and the computed results are compared directly with the interrupted uniaxial compression experiments performed on the deformed 3D structures. The deformation behaviors of simplified structures with cylindrical and hexahedral struts are simulated, and the computed results demonstrate that unavoidable defects in the actual structure affect the mechanical stability significantly. Additionally, buckling-induced deformation behavior is analyzed by introducing an imperfection in the actual and simplified structures. The effects of certain selected process variables, such as internal angle and diameter, are also examined through a series of process simulations.

By prescribing asymmetric ligaments with different arrangements in elastomeric porous membranes of pre-twisted kagome lattices, the buckling instability is avoided, allowing for smooth and homogenous structural reconfiguration in a deterministic fashion. The stress-strain behaviors and negative Poisson's ratios can be tuned by the pre-twisting angles.

A smart window is fabricated from a composite consisting of elastomeric poly(dimethylsiloxane) embedded with a thin layer of quasi-amorphous silica nanoparticles. The smart window can be switched from the initial highly transparent state to opaqueness and displays angle-independent structural color via mechanical stretching. The switchable optical property can be fully recovered after 1000 stretching/releasing cycles.

Through a combined density functional theory and in situ scanning electron microscopy study, we provide evidence of the ultrafast chemical lithiation of a single crystalline Si nanowire which is brought into direct contact with Li metal in the absence of an applied external electric field. Unlike the previous in situ lithiation results, the ultra-fast lithiation process in this study is purely driven by the concentration gradient and is found to be limited by Li diffusion through the pristine/lithiated Si phase boundary. The experimental and calculated lithiation speeds are in excellent agreement at around 1 μm s−1, corresponding to a high Li diffusivity value of about 10−9 cm2 s−1. The improved understanding of lithiation kinetics may contribute to the design of higher-power Si-based anodes.

Significance Most materials can be stretched to a small degree, depending on their elastic limits and failure properties. For most materials the maximum elastic dilatation is very small, implying that the macroscopic shapes to which an elastic body can be deformed is severely limited. The present work addresses the simple modification of any material via hierarchical cut patterns to allow for extremely large strains and shape changes and a large range of macroscopic shapes. This is an important step in the development of shape-programmable materials. We provide the mathematical foundation, simulation results, and experimental demonstrations of the concept of fractal cut. This approach effectively broadens the design space for engineered materials for applications ranging from flexible/stretchable devices and photonic materials to bioscaffolds.

We demonstrate the design and fabrication of tilted micropillar arrays on wrinkled elastomeric poly(dimethylsiloxane) as a reversibly switchable optical window. Upon re-stretching the as-prepared (opaque) film to the original pre-strain, the grating color is restored and ∼ 30% transmittance is recovered. Further stretching beyond the pre-strain makes the film more transparent. This process is fully reversible and repeatable for many cycles.

The evolution of damage due to mechanical fatigue in thin metal films on flexible substrates was investigated by in situ electrical resistance measurements. A tensile fatigue load was applied to the metal films by subjecting a single edge of the curved samples to repeated linear motion. The change in the resistance of the metal films was monitored in situ. Upon the nucleation of a fatigue-induced crack, the electrical resistance of the metal film began to increase. The resistance subsequently continued to increase with crack propagation. Therefore, in situ electrical resistance measurements can be used to identify the fatigue-induced crack nucleation cycle. The number of cycles required for crack nucleation decreased with the increase in the fatigue-stressed area of the samples. This behavior is attributed to an increase in the crack nucleation probability with increasing sample size. The amount of strain applied also modified the number of cycles required for crack nucleation according to the Coffin–Manson relationship. The increase in the electrical resistivity with respect to the number of fatigue cycles can be accurately predicted when the fatigue cycle is normalized by the nucleation cycle. This indicates that the fatigue lifetime is determined by crack nucleation and not by crack propagation.

Ultrafine cellular microstructures around alumina particles in a low-carbon steel were observed, which survived even after cyclic austenitization. This indicates that their formation is closely related to internal stress because of a structural heterogeneity during phase transformation rather than to externally applied forces or deformation. Thermo-elasto-plastic finite element analysis confirmed the evolution of a large hydrostatic pressure around an alumina particle due to thermal mismatch during cooling. Therefore, the fine cellular microstructure might be generated as a result of the hydrostatic pressure, which retards the phase transformation around the particle during cooling. In addition, we observed microstructural similarity with the same steel processed under an ultra-high pressure, which was the evidence for the role of the delay in the transformation caused by the hydrostatic pressure.

Highly localized dislocations in GaN/ZnO hetero-nanostructures are generated from the residual strain field by lattice mismatches at two interfaces: between the substrate and hetero-nanostructures, and between the ZnO core and GaN shell. The local strain field is measured using tranmission electron microscopy, and the relationship between the nanostructure morphology and the highly localized dislocations is analyzed by a finite element method.

Steels with microstructural bands show dissimilar dilatometric behavior depending on the direction monitored. This behavior was simulated using finite-element analysis combining thermal, elasticity, conventional plasticity and transformation plasticity. The results suggest that transformation plasticity plays a major role in generating the characteristic dilatometric behavior derived from dimensional non-isotropy during transformation.

It is well known that nonisotropic volume changes in dilatometry were observed during the phase transformation in steel. In this study, a finite element (FE) model incorporating the transformation plasticity was adopted to describe the nonisotropic dilatometric behavior during the phase transformation in steel. An implicit numerical solution procedure to calculate the deformation during the dilatometric experiment was incorporated into the general purpose implicit FE program. The nonisotropic dilatometric behavior could be successfully reproduced by using the FE simulation considering the transformation plasticity. The transformation plasticity was caused by the small amount of stress that naturally developed in the specimen during the dilatometric experiment. In conventional low carbon steel, the stress in the specimen mainly forms due to the very small external force supplied to support it during the dilatometric experiment. As regards ultralow carbon steel, whose phase transformation occurs within an extraordinarily narrow temperature range, the inhomogeneous phase transformation due to the temperature deviation in the specimen was mainly responsible for the stress field in the specimen during the dilatometric experiment.

Asymmetric contraction during natural cooling is often observed after the coiling of hot-rolled steel, which has significantly large hardenability due to high content of carbon or other alloying elements. In this study, a finite element (FE) model incorporating transformation plasticity was used to analyze the thermo-mechanical and metallurgical behavior of hot-rolled steel during the phase transformation after coiling. The transformation plasticity was caused by a small amount of stress that develops naturally in a hot-rolled coil due to the gravity. The asymmetric contraction behavior of the coil was reproduced successfully using the FE simulation by considering the transformation plasticity. The effect of some selected process variables on the asymmetric contraction was examined through a series of process simulations.

A 50 kg-grade cold rolled dual phase (DP) steel was developed based on the orthogonal design method. The intercritical annealing, aging and galvanizing temperatures during the heat treatment were considered as controllable factors and three levels for each of these factors were selected. The optimal experimental conditions were acquired by the analysis of variance (ANOVA) on the ultimate tensile strength (UTS) and the total elongation of the DP steel. A dilatometry analysis was performed to measure the phase transformation kinetics of the DP steel and to verify the statistically analyzed results. These metallurgical observations explained well the statistically analyzed results.

In general, plastic strain occurs over a certain stress, called yield stress. However, it has been reported that the permanent strain could happen during boundary migrating even under the extremely slight externally applied stress. In this study, we performed dilatometry experiments under the various compressive stresses and measured the amount of recrystallization and growth induced permanent strain. A new empirical constitutive equation was suggested to describe the recrystallization and growth induced plasticity. This equation was verified by comparing the calculated values with dilatometric experimental data under the various compressive stresses.

A finite element model was developed to simulate the deformation, temperature and phase transformation behavior in high carbon steels. The heat capacity of each phase and the heat evolution due to phase transformation were obtained from the thermodynamic analysis of S45C, 50CrV4 and SK85 steels. Phase transformation kinetics of the steels were derived from continuous cooling experiments. An additivity technique was applied to a modified Johnson-Mehl-Avrami equation to analyze continuous cooling curve. To predict the strain due to TRansformation Induced Plasticity (TRIP), a variant selection model for diffusionless transformation and an accelerative creep model for diffusional transformation were adopted. In order to calculate the deformation behavior, the elastic strain, the volumetric strain due to thermal contraction and phase transformation, the plastic strain and the TRIP strain were taken into account. Using the finite element model developed in this study, the temperature-phase-deformation behavior of the high carbon steels was calculated.

We use a regular arrangement of kirigami elements to demonstrate an inverse design paradigm for folding a flat surface into complex target configurations. We first present a scheme using arrays of disclination defect pairs on the dual to the honeycomb lattice; by arranging these defect pairs properly with respect to each other and choosing an appropriate fold pattern a target stepped surface can be designed. We then present a more general method that specifies a fixed lattice of kirigami cuts to be performed on a flat sheet. This single "pluripotent" lattice of cuts permits a wide variety of target surfaces to be programmed into the sheet by changing the folding directions., Comment: 9 pages, whole megillah

In this paper we explore and develop a simple set of rules that apply to cutting, pasting, and folding honeycomb lattices. We consider origami-like structures that are extinsically flat away from zero-dimensional sources of Gaussian curvature and one-dimensional sources of mean curvature, and our cutting and pasting rules maintain the intrinsic bond lengths on both the lattice and its dual lattice. We find that a small set of rules is allowed providing a framework for exploring and building kirigami -- folding, cutting, and pasting the edges of paper., Comment: 5 pages, 5 figures

Light-emitting diodes (LEDs) become an attractive alternative to conventional light sources due to high efficiency and long lifetime. However, different material properties between GaN and sapphire cause several problems such as high defect density in GaN, serious wafer bowing, particularly in large-area wafers, and poor light extraction of GaN-based LEDs. Here, we suggest a new growth strategy for high efficiency LEDs by incorporating silica hollow nanospheres (S-HNS). In this strategy, S-HNSs were introduced as a monolayer on a sapphire substrate and the subsequent growth of GaN by metalorganic chemical vapor deposition results in improved crystal quality due to nano-scale lateral epitaxial overgrowth. Moreover, well-defined voids embedded at the GaN/sapphire interface help scatter lights effectively for improved light extraction, and reduce wafer bowing due to partial alleviation of compressive stress in GaN. The incorporation of S-HNS into LEDs is thus quite advantageous in achieving high efficiency LEDs for solid-state lighting.

Design and fabrication of reliable electrodes is one of the most important challenges in flexible devices, which undergo repeated deformation. In conventional approaches, mechanical and electrical properties of continuous metal films degrade gradually because of the fatigue damage. The designed incorporation of nanoholes into Cu electrodes can enhance the reliability. In this study, the electrode shows extremely low electrical resistance change during bending fatigue because the nanoholes suppress crack initiation by preventing protrusion formation and damage propagation by crack tip blunting. This concept provides a key guideline for developing fatigue-free flexible electrodes.

An implicit finite element model was developed to analyze the deformation behavior of low carbon steel during phase transformation. The finite element model was coupled hierarchically with a phase field model that could simulate the kinetics and micro-structural evolution during the austenite-to-ferrite transformation of low carbon steel. Thermo-elastic-plastic constitutive equations for each phase were adopted to confirm the transformation plasticity due to the weaker phase yielding that was proposed by Greenwood and Johnson. From the simulations under various possible plastic properties of each phase, a more quantitative understanding of the origin of transformation plasticity was attempted by a comparison with the experimental observation.

The functional properties of periodic cellular solids such as photonic and phononic crystals, nanocrystal superlattices and foams may be tuned by an applied inhomogeneous mechanical strain. A fundamental methodology to analyse the structure of periodic cellular materials is presented here and is compared directly with indentation experiments on three-dimensional microframed polymer photonic crystals. The application of single-continuum-scale finite-element modelling (FEM) was impossible due to the numerous cells involved and the intricate continuum geometry within each cell. However, a method of dual-scale FEM was implemented to provide stress and displacement values on both scales by applying an upper scale continuum FEM with reference to the lower scale continuum FEM to provide coarse-grained stress?strain relationships. Architecture and orientation dependences of the periodic porous materials on the macro-/microscopic responses were investigated under different loading conditions. Our study revealed a computational tool for exploring elastic strain engineering of photonic crystals and, more broadly, may help the design of metamaterials with mechanical controllability.

Stepwise cross-sectional crystallographic measurement was performed on the stress-induced void (SIV) of copper interconnect and it was possible to investigate the three-dimensional crystallographic structures near the submicron scale void. The void mainly happened at a triple junction of grain boundaries and there were at least two grains with high biaxial elastic modulus near the triple junction. The preferred site for the SIV could well be understood by considering the elastic anisotropy of each grain and the grain boundary type.