Bending Situations Research Articles

Transversely isotropic constitutive properties modeling of tubular sandwich composite structures

Sandwich structures have been studied extensively for planar structures; however, the use of composite tubing, manufactured by pultrusion, in a bending situation where a core material can contribute to take shear stresses, can find many applications in modern structures made of composite materials. The objective of this article is to develop analytical solutions for axial effective modulus and major Poisson’s ratio of a pultruded unidirectional composite tubing filled with a core material. In this work, the unidirectional composite tubing and its core are considered to have transversely isotropic and isotropic properties, respectively. For the validation of the results, the obtained exact analytical solutions are reduced to a case where both the materials are isotropic and compared to the existing solutions for an isotropic material filling in an isotropic tube. Further validations of our exact analytical solutions for the transversely isotropic tubing and isotropic core are carried out employing a finite element analysis of the same structure, where the results show excellent agreements between the analytical solutions and the numerical results. Finally, a parametric study is conducted to investigate the variations in the effective properties of the two-phase composite cylinder based on the variations in the skin and/or core geometries and their material properties.

Bending effects of ZnO nanorod metal–semiconductor–metal photodetectors on flexible polyimide substrate

The authors report the fabrication and I-V characteristics of ZnO nanorod metal–semiconductor–metal photodetectors on flexible polyimide substrate. From field-emission scanning electron microscopy and X-ray diffraction spectrum, ZnO nanorods had a (0002) crystal orientation and a wurtzite hexagonal structure. During the I-V and response measurement, the flexible substrates were measured with (i.e., the radius of curvatures was 0.2 cm) and without bending. From I-V results, the dark current decreased, and the UV-to-visible rejection ratio increased slightly in bending situation. The decreasing tendency of the dark current under bending condition may be attributed to the increase of the Schottky barrier height.

Explicit Simulation of Forming Processes Using a Novel Solid-Shell Concept Based on Reduced Integration

The contribution deals with the simulation of sheet metal forming processes by means of a recently developed hexahedral solid-shell finite element. In contrast to this earlier work, we pursue here explicit integration. The element formulation has the following features. In order to avoid undesired effects of locking an enhanced assumed strain (EAS) concept using only one EAS degree-of-freedom has been implemented. In addition, by means of the assumed natural strain (ANS) method an excellent performance in bending situations is obtained. A key point of the element formulation is the construction of the hourglass stabilization by means of different Taylor expansions. This procedure leads to the important advantage that the sensitivity of the results with respect to mesh distortion is noticeably reduced. Further the hourglass stabilization is in this way designed that locking is eliminated and numerical stability guaranteed. The finite strain material model for plastic anisotropy and non-linear kinematic and isotropic hardening is motivated by a rheological model including Armstrong-Frederick kinematic hardening. The element formulation has been implemented into the academic code FEAP. Some standard benchmark examples are computed.

Flexible Nonvolatile Memory Thin-Film Transistor Using Ferroelectric Copolymer Gate Insulator and Oxide Semiconducting Channel

The flexible nonvolatile memory thin-film transistor (F-MTFT) is demonstrated. The gate stack is composed of ferroelectric poly(vinylidene fluoride-trifluoroethylene) gate insulator and ZnO semiconducting channel. All the processes are performed below 150 °C on a poly(ethylene naphthalate) substrate. The ferroelectric field-effect-driven memory window and the on/off ratio of the fabricated F-MTFT were 7.8 V and 108, respectively. These behaviors did not show so marked degradations at bending situations with a curvature radius of 0.97 cm and after repetitive bendings of 20,000 cycles. The programmed on/off ratio was initially 6.6 × 105 and retained to be approximately 130 after a lapse of 15,000 s.

Influence of mechanical bending and temperature on the threshold voltage instability of a-Si:H thin-film transistors under electrical stress

This paper studies the electrical characteristics of hydrogenated amorphous silicon thin-film transistors (a-Si:H TFTs) under flat and bending situations after AC/DC stress at different temperatures. Stress temperature was varied from 77 K to 400 K, and threshold voltage shifts were extracted to analyze degradation mechanisms. It was found that high temperature and mechanical bending played important roles under AC stress, with an enhanced stress effect resulting in a more serious degradation. This study also discusses the dependence between the accumulated sum of bias rising and falling time and the threshold voltage shifts under AC stress.

Bending characteristics of ferroelectric poly(vinylidene fluoride trifluoroethylene) capacitors fabricated on flexible polyethylene naphthalate substrate

Ferroelectric capacitors using poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] were fabricated on the plastic polyethylene naphthalate (PEN) substrate for the various applications of flexible-type nonvolatile memories. It was successfully confirmed that the Au/P(VDF-TrFE)/Au capacitors showed sound ferroelectric characteristics even when they were fabricated using a lithography-compatible patterning process at low temperature below 150 °C in the PEN. The behaviors of ferroelectric polarization and saturation as a function of the applied electric field were not so sensitive to the changes in bending curvature radius. However, because small variations in switching properties for the polarization reversal were also observed especially for higher frequency and lower voltage regions, suitable operation schemes should be designed for the flexible memory devices with lower voltage and higher speed operations even at bending situations.

Challenges in cross-sectional scanning tunneling microscopy on semiconductors

Cross sectional scanning tunneling microscopy (X-STM) has now become a well established method for the investigation of the structural and electronic properties of semiconductor nano-structures down to the scale of individual impurity atoms. Nevertheless, some aspects still remain challenging, for example in the sample preparation by cleavage as well as in the quantitative interpretation of the results. We present a brief overview of the techniques and geometries employed to cleave different semiconductors such as the lll-V materials, mainly GaAs, and the elementary semiconductors Si and Ge. Furthermore, we discuss the inevitable impact of the surface on the properties of the addressed impurities. This is mainly an issue when the surface reconstruction creates electronic surface states in the band gap. But also the unreconstructed GaAs(110) surface significantly modifies the symmetry of acceptor wave functions and the binding energy of donors and acceptors in the first few atomic layers. The impact of the tip will be addressed as a third quite important challenge, which is frequently neglected in the analysis of X-STM data. On surfaces with an unpinned Fermi level, the presence of the STM tip and the voltage applied to the tunneling contact significantly modifies the spectral positions of the observed electronic states and bands. Furthermore, different band bending situations open up qualitatively different tunneling paths to address individual electronic states in the sample. Detailed knowledge of the tunneling mechanism and of the tip properties, mainly apex radius and work function, is required in order to correctly extract the energetic levels from the tunneling spectroscopy data.

Organic/Inorganic Hybrid-Type Nonvolatile Memory Thin-Film Transistor on Plastic Substrate below 150°C

ABSTRACTAn organic/inorganic hybrid-type nonvolatile memory TFT was proposed as a core device for the future flexible electronics. The structural feature of this memory TFT was that a ferroelectric copolymer and an oxide semiconductor layers were employed as a gate insulator and an active channel, respectively. The memory TFT with the structure of Au/poly(vinylidene fluoride-trifluoroethylene)/Al2O3/ZnO/Ti/Au/Ti/poly(ethylene naphthalate) could be successfully fabricated at the process temperature of below 150°C. It was confirmed that the TFT well operated as a memory device even under the bending situations.

One-Dimensional Semiconductor Nanostructure Based Thin-Film Partial Composite Formed by Transfer Implantation for High-Performance Flexible and Printable Electronics at Low Temperature

Having high bending stability and effective gate coupling, the one-dimensional semiconductor nanostructures (ODSNs)-based thin-film partial composite was demonstrated, and its feasibility was confirmed through fabricating the Si NW thin-film partial composite on the poly(4-vinylphenol) (PVP) layer, obtaining uniform and high-performance flexible field-effect transistors (FETs). With the thin-film partial composite optimized by controlling the key steps consisting of the two-dimensional random dispersion on the hydrophilic substrate of ODSNs and the pressure-induced transfer implantation of them into the uncured thin dielectric polymer layer, the multinanowire (NW) FET devices were simply fabricated. As the NW density increases, the on-current of NW FETs increases linearly, implying that uniform NW distribution can be obtained with random directions over the entire region of the substrate despite the simplicity of the drop-casting method. The implantation of NWs by mechanical transfer printing onto the PVP layer enhanced the gate coupling and bending stability. As a result, the enhancements of the field-effect mobility and subthreshold swing and the stable device operation up to a 2.5 mm radius bending situation were achieved without an additional top passivation.

A mathematical model for describing the mechanical behaviour of root canal instruments

The purpose of this study was to establish a general mathematical model for describing the mechanical behaviour of root canal instruments by combining a theoretical analytical approach with a numerical finite-element method. Mathematical formulas representing the longitudinal (taper, helical angle and pitch) and cross-sectional configurations and area, the bending and torsional inertia, the curvature of the boundary point and the (geometry of) loading condition were derived. Torsional and bending stresses and the resultant deformation were expressed mathematically as a function of these geometric parameters, modulus of elasticity of the material and the applied load. As illustrations, three brands of NiTi endodontic files of different cross-sectional configurations (ProTaper, Hero 642, and Mani NRT) were analysed under pure torsion and pure bending situation by entering the model into a finite-element analysis package (ANSYS). Numerical results confirmed that mathematical models were a feasible method to analyse the mechanical properties and predict the stress and deformation for root canal instruments during root canal preparation. Mathematical and numerical model can be a suitable way to examine mechanical behaviours as a criterion of the instrument design and to predict the stress and strain experienced by the endodontic instruments during root canal preparation.

New twelve node serendipity quadrilateral plate bending element based on Mindlin-Reissner theory using Integrated Force Method

The Integrated Force Method (IFM) is a novel matrix formulation developed for analyzing the civil, mechanical and aerospace engineering structures. In this method all independent/internal forces are treated as unknown variables which are calculated by simultaneously imposing equations of equilibrium and compatibility conditions. This paper presents a new 12-node serendipity quadrilateral plate bending element MQP12 for the analysis of thin and thick plate problems using IFM. The Mindlin-Reissner plate theory has been employed in the formulation which accounts the effect of shear deformation. The performance of this new element with respect to accuracy and convergence is studied by analyzing many standard benchmark plate bending problems. The results of the new element MQP12 are compared with those of displacement-based 12-node plate bending elements available in the literature. The results are also compared with exact solutions. The new element MQP12 is free from shear locking and performs excellent for both thin and moderately thick plate bending situations.

New eight node serendipity quadrilateral plate bending element for thin and moderately thick plates using Integrated Force Method

A new 8-node serendipity quadrilateral plate bending element (MQP8) based on the Mindlin-Reissner theory for the analysis of thin and moderately thick plate bending problems using Integrated Force Method is presented in this paper. The performance of this new element (MQP8) is studied for accuracy and convergence by analyzing many standard benchmark plate bending problems. This new element MQP8 performs excellent in both thin and moderately thick plate bending situations. And also this element is free from spurious/zero energy modes and free from shear locking problem.

깊이가 깊은 사출 금형의 새로운 측벽 두께 설계에 관한 연구

Cavity in the mold is exposed to high pressure during injection molding operation. Injection molded articles with deep depth are often demanded as design variety increases. Subsequently mold becomes weak and deformation increases as the mold depth increases. Thus the injection molds for deep depth articles should be designed to hold out high pressure or stress concentration and large deformation. Through this study, equation for mold design was examined and suggested novel method to determine equation for mold design with deep depth. Novel equation developed in this study was modified from beam theory considering cantilever and two points bending situation while previous equation was modified from just cantilever bending situation. The validity of novel equation was verified through computer simulations for various mold side and wall thickness.

Connection of anisotropic conductivity to tip-induced space-charge layers in scanning tunneling spectroscopy ofp-doped GaAs

The electronic properties of shallow acceptors in $p$-doped GaAs{110} are investigated with scanning tunneling microscopy (STM) at low temperature. Shallow acceptors are known to exhibit distinct triangular contrasts in STM images for certain bias voltages. Spatially resolved $I(V)$ spectroscopy is performed to identify their energetic origin and behavior. A crucial parameter---the scanning tunneling microscope tip's work function---is determined experimentally. The voltage dependent potential configuration and band bending situation are derived. Ways to validate the calculations with the experiment are discussed. Differential conductivity maps reveal that the triangular contrasts are only observed with a depletion layer present under the STM tip. The tunnel process leading to the anisotropic contrasts calls for electrons to tunnel through vacuum gap and a finite region in the semiconductor.

Bilinear plate bending element for thin and moderately thick plates using Integrated Force Method

Using the Mindlin-Reissner plate theory, many quadrilateral plate bending elements have been developed so far to analyze thin and moderately thick plate problems via displacement based finite element method. Here new formulation has been made to analyze thin and moderately thick plate problems using force based finite element method called Integrated Force Method (IFM). The IFM is a novel matrix formulation developed in recent years for analyzing civil, mechanical and aerospace engineering structures. In this method all independent/internal forces are treated as unknown variables which are calculated by simultaneously imposing equations of equilibrium and compatibility conditions. In this paper the force based new bilinear quadrilateral plate bending element (MQP4) is proposed to analyze the thin and moderately thick plate bending problems using Integrated Force Method. The Mindlin-Reissner plate theory has been used in the formulation of this element which accounts the effect of shear deformation. Standard plate bending benchmark problems are analyzed using the proposed element MQP4 via Integrated Force Method to study its performance with respect to accuracy and convergence, and results are compared with those of displacement based 4-node quadrilateral plate bending finite elements available in the literature. The results are also compared with the exact solutions. The proposed element MQP4 is free from shear locking and works satisfactorily in both thin and moderately thick plate bending situations.

Prototype of a silicon nitride ceramic-based miniplate osteofixation system for the midface

The favorable properties of silicon nitride (Si3N4) ceramics, such as high mean strength level and fracture toughness, suggest biomedical use as an implant material. Minor reservations about the biocompatibility of Si3N4 ceramics were cleared up by previous in vitro and in vivo investigations. A Si3N4 prototype minifixation system was manufactured and implanted for osteosynthesis of artificial frontal bone defects in 3 minipigs. After 3 months, histological sections, computed tomography (CT) scans, and magnetic resonance imaging (MRI) scans were obtained. Finite element modeling (FEM) was used to simulate stresses and strains on Si3N4 miniplates and screws to calculate survival probabilities. Si3N4 miniplates and screws showed satisfying intraoperative workability. There was no implant loss, displacement, or fracture. Bone healing was complete in all animals. The formation of new bone was observed in direct contact to the implants. The implants showed no artifacts on CT and MRI scanning. FEM simulation confirmed the mechanical reliability of the screws, whereas simulated plate geometries regarding pullout forces at maximum load showed limited safety in a bending situation. Si3N4 ceramics show a good biocompatibility outcome both in vitro and in vivo. In ENT surgery, this ceramic may serve as a biomaterial for osteosynthesis (eg, of the midface including reconstruction the floor of the orbit and the skull base). To our knowledge, this is the first introduction of a ceramic-based miniplate-osteofixation system. Advantages compared with titanium are no risk of implantation to bone with mucosal attachment, no need for explantation, and no interference with radiologic imaging. Disadvantages include the impossibility of individual bending of the miniplates.

Three-point bend testing of HVOF AMDRY 9954 coating on Ti–6Al–4V alloy

HVOF coating of AMDRY 9954 powders onto Ti–6Al–4V alloy is considered. Three-point bending tests are carried out to examine the tensile-shear response of the coating under the bending load. The study is extended to include the heat treatment of the workpieces after the coating process prior to three-point bending tests. The morphological and microstructural analyses are carried out using scanning electron microscopy (SEM). Finite element method (FEM) is introduced to simulate the bending situation and predict the stress distribution in the workpieces. It is found that coating with small porosity and voids is achieved. Elongated cracks are developed in the coating in the region of deflection of the workpiece during the bending tests. This, in turn, results in partial attachment of the coat onto the base material without complete peeling off from the substrate surface due to stress relaxation at coating interface. However, the coating with severe crack formation and total peeling off from the substrate surface are also observed, which is more pronounced for heat treated workpieces.

Bending of circular-section bonded rubber blocks

Convenient exact closed-form expressions are derived for calculating the bending stiffness of and stresses within loaded cylindrical bonded rubber blocks of circular cross-section. The particular solutions for simple bending, cantilever loading and apparent shear situations are deduced and studied in detail. The shapes of the deformed profiles are discussed and confirmation is provided that the previously adopted assumption of parabolic profiles of the deformed lateral curved surface is only valid for blocks of very small aspect ratio. In simple bending a relationship which is more realistic than those hitherto suggested is derived for the couple required to maintain a specified rotation of the loaded end of the block. In apparent shear an exact expression for the ratio of the true to the apparent shear modulus is derived, and compared with the experimental data. An improved approximate relation is deduced.

A deformation dependent stabilization technique, exemplified by EAS elements at large strains

Stabilized finite element methods have been developed mainly in the context of Computational Fluid Dynamics (CFD) and have shown to be able to add stability to previously unstable formulations in a consistent way. In this contribution a deformation dependent stabilization technique, conceptually based on the above mentioned developments in the CFD area, is developed for Solid Mechanics to cure the well-known enhanced assumed strain (EAS) method from artificial instabilities (hourglass modes) that have been observed in the range of large compressive strains. In investigating the defect of the original formulation the dominating role of the kinematic equation as cause for the instabilities is revealed. This observation serves as key ingredient for the design of the stabilizing term, introduced on the level of the variational equation. A proper design for the stabilization parameter is given based on a mechanical interpretation of the underlying defect as well as of the stabilizing action. This stabilizing action can be thought of an additional constraint, introduced into the reparametrized Hu–Washizu functional in a least-square form, together with a deformation dependent stabilization parameter. Numerical examples show the capability of this approach to effectively eliminate spurious hourglass modes, which otherwise may appear in the presence of large compressive strains, while preserving the advantageous features of the EAS method, namely the reduction of the stiffness for an `in-plane bending' mode, i.e. when plane stress elements are used in a bending situation.

A note on enhanced strain methods for large deformations

Enhanced strain elements have been applied to simulate geometrically and materially non-linear problems. Their main advantage is due to the fact that they perform very good in the incompressible limit as well as in bending situations. However, in compressive deformation states these elements depict stability modes which are associated with hour-glass forms and thus denote a rank deficiency for such deformations. In this paper the appearance of these modes will be investigated analytically by means of a simple representative example.