Bending Field Research Articles

A stress-driven local-nonlocal mixture model for Timoshenko nano-beams

A well-posed stress-driven mixture is proposed for Timoshenko nano-beams. The model is a convex combination of local and nonlocal phases and circumvents some problems of ill-posedness emerged in strain-driven Eringen-like formulations for structures of nanotechnological interest. The nonlocal part of the mixture is the integral convolution between stress field and a bi-exponential averaging kernel function characterized by a scale parameter. The stress-driven mixture is equivalent to a differential problem equipped with constitutive boundary conditions involving bending and shear fields. Closed-form solutions of Timoshenko nano-beams for selected boundary and loading conditions are established by an effective analytical strategy. The numerical results exhibit a stiffening behavior in terms of scale parameter.

Recent progress of the JEDI collaboration

This article describes two different methods to measure the electric dipole moment (EDM) of charged particles in storage rings. The frozen spin concept relies on a large fraction (possibly 100%) of the bending field to be electric while an rf Wien filter method can also be used in purely magnetic rings albeit at lower sensitivity. In recent years the JEDI collaboration has developed several important techniques that are applicable for both methods and major milestones are presented here.

Future colliders for particle physics—“Big and small”

Discoveries at high-energy particle colliders have established the standard model of particle physics. Technological innovation has helped to increase the collider energy at a much faster pace than the corresponding costs. New concepts will allow reaching ever higher luminosities and energies throughout the coming century. Cost-effective strategies for the collider implementation include staging. For example, a future circular collider could first provide electron–positron collisions, then hadron collisions (proton–proton and heavy-ion), and finally the collision of muons. Cooling-free muon colliders, realizable in a number of ways, promise an attractive and energy-efficient path towards lepton collisions at tens of TeV. While plasma accelerators and dielectric accelerators offer unprecedented gradients, the construction of a high-energy collider based on these new technologies still calls for significant improvements in cost and performance. Pushing the accelerating gradients or bending fields ever further, the breakdown of the QED vacuum may set an ultimate limit to electromagnetic acceleration. Finally, some ideas are sketched for reaching, or exceeding, the Planck energy.

Infinitesimal bendings of complete Euclidean hypersurfaces

A local description of the non-flat infinitesimally bendable Euclidean hypersurfaces was recently given by Dajczer and Vlachos (Ann Mat 196:1961–1979, 2017. https://doi.org/10.1007/s10231-017-0641-8). From their classification, it follows that there is an abundance of infinitesimally bendable hypersurfaces that are not isometrically bendable. In this paper we consider the case of complete hypersurfaces \(f:M^n\rightarrow \mathbb {R}^{n+1}\), \(n\ge 4\). If there is no open subset where f is either totally geodesic or a cylinder over an unbounded hypersurface of \(\mathbb {R}^4\), we prove that f is infinitesimally bendable only along ruled strips. In particular, if the hypersurface is simply connected, this implies that any infinitesimal bending of f is the variational field of an isometric bending.

Accelerators in the 21st Century

More than 30,000 accelerators are in operation worldwide. Of these less than 1% are devoted to basic research. Prominent among the latter are high-energy particle colliders - powerful engines of discovery and precision measurement, which have played an essential role in establishing the standard model of particle physics. Technological innovation has allowed building colliders for ever higher energy and better performance, at decreasing specific cost. New concepts will allow reaching even higher luminosities and energies throughout the coming century. One cost-effective strategy for future collider implementation is staging. For example, a future circular collider could first provide electron-positron collisions, then hadron collisions (proton-proton and heavy-ion), and, finally, the collision of muons. Indeed, cooling-free muon colliders, realizable in a number of ways, promise an attractive and energy-efficient path towards lepton collisions at tens of TeV. While plasma accelerators and dielectric accelerators offer unprecedented gradients, the construction of a high-energy collider based on these advanced technologies still faces a number of challenges. Pushing the accelerating gradients or bending fields ever further, the breakdown of the QED vacuum may, or may not, set an ultimate limit to electromagnetic acceleration.

LeRoy Johnson at McGee Bend Reservoir (Lake Sam Rayburn) in 1956

McGee Bend was one of some 40 reservoir and dam projects in Texas where salvage archaeological excavations were carried out as part of the nationwide River Basin Surveys program administered by the Smithsonian Institution and the National Park Service between 1947 and 1968 (see Jelks 1965, 2006, 2014, 2017). In 1956, I rented an old vacant farmhouse for our McGee Bend field headquarters where our crew lived without indoor plumbing, and it was there that the photo of LeRoy Johnson bathing in a washtub was taken by one of our crew, Milburn Lathan (Figure 1).

Free edge stress field in smart piezoelectric composite structures and its control: An accurate multiphysics solution

A three-dimensional piezoelasticity based iterative analytical solution is presented for the multiphysics problem of free edge stresses in hybrid composite panels integrated with piezoelectric transducer layers. The formulation, applicable for general laminate configurations with arbitrary layup and materials properties, considers extension, bending, twisting and electric field actuation loadings with full electromechanical coupling. A mixed formulation approach based on the Reissner-type mixed variational principle is adopted to ensure exact point-wise satisfaction of all boundary and interlaminar continuity conditions, which is key to obtaining accurate results for the localized stresses near free edges. The solution is obtained using the recently developed mixed-field multiterm extended Kantorovich method, whose robustness, convergence and accuracy are illustrated for various laminates and loadings. The solution successfully captures singular nature of free edge interlaminar stresses under different loadings. The results show a significant effect of electromechanical coupling on the free edge stresses in hybrid laminates under mechanical loading. The effect of actuator thickness on the interfacial stress distributions under electric load is studied. Finally, it is shown how the free edge stresses due to extension, bending and twisting loads can be controlled by applying appropriate actuation potential.

Experimental study on alarming of concrete micro-crack initiation based on wavelet packet analysis

Wavelet packet analysis and digital image correlation (DIC) have been applied extensively owing to their respective advantages. In this work, a novel approach is presented to realize micro-damage alarming and identification of concrete by the coupling of wavelet packet transform and DIC technology. DIC measurement system is utilized to measure the damage strain field of static three-point bending of concrete specimen. According to excellent characteristic of time-frequency localization of wavelet packet analysis, the strain nephograms are decomposed by wavelet packet transform. Through wavelet packet analysis, the eigenvectors of the component energy are constructed, based on which the alarming index of concrete micro-crack initiation is obtained. The experimental results show that our novel approach can effectively predict the micro-crack initiation and can automatically achieve alarming. Furthermore, the initiation position of micro-cracks and micro-damage degree can be accurately determined by damage strain field. It is beneficial to damage prediction and safety control of concrete structures.

Probing the surface potential of oxidized silicon by assessing terahertz emission

Using laser terahertz emission microscopy, we measured laser-excited terahertz (THz) emission from silicon wafers with silicon-oxide passivation layers, revealing a strong correlation between the THz waveform and the surface potential. The surface potential was electrically tuned by a semitransparent top electrode disc and evaluated by measuring capacitance–voltage characteristics. The waveform changed with external bias and inverted near the flatband voltage, and changes appeared in the peak amplitude were similar to the capacitance–voltage characteristics. These results indicate that by analyzing the waveform of laser-excited THz emission generated by laser terahertz emission microscopy, we could quantitatively measure and map the internal field of surface band bending in semiconductors.

Nonlocal elasticity in nanobeams: the stress-driven integral model

Nonlocal elastic models have attracted an increasing amount of attention in the past years, due to the promising feature of providing a viable simulation for scale effects in nano-structures and especially in nano-beams designed for use as actuators or sensors. In adapting Eringen’s nonlocal model of elasticity to flexure of nano-beams, the bending field is expressed as convolution between the elastic curvature field and an averaging kernel. Basic difficulties are involved in this approach due to conflicting requirements imposed on the bending field by equilibrium on one hand and by constitutive conditions on the other one. In the newly proposed constitutive theory the bending field is placed in the proper position of input variable, giving to the elastic curvature field the role of output of the constitutive law, evaluated by convolution between the bending field and an averaging kernel. Conflicting restrictions on the bending field are thus eliminated and existence and uniqueness of the solution are assured under any data. Equivalence between integral and differential constitutive relations is proven to hold for nonlocal laws with a special kernel, under constitutive boundary conditions stemming naturally from the integral relation. When compared with the local limit, a stiffer elastic response is evaluated by the stress-driven nonlocal model, due to normalisation of the kernel. The theory provides an effective methodology for investigating small scale effects in nanobeams, by well-posed problems.

A nonlinear strain gradient finite element for microbeams and microframes

The aim of this paper is to develop a total Lagrangian finite element formulation for the geometrically nonlinear deformation analysis of microbeams and microframes. A general form of the first strain gradient elasticity theory of Mindlin is employed to capture size effects at micron scales. Moreover, the von Karman strain tensor and its spatial gradient are used to include the effects of geometric nonlinearity. Due to the higher-order nature of the gradient-based governing equilibrium equations, the interpolation functions of the extension and bending field variables are chosen to be the standard \(C^1\) and \(C^2\) functions, respectively. Accordingly, a novel two-node strain gradient microbeam element is introduced. The formulation is then extended to include the deformation of microbeams with arbitrary orientation in a plane, which enables us to analyze arbitrary planar microframes composed of several microbeams at different orientations. The full Newton–Raphson scheme is used to solve the resulting nonlinear algebraic equations. Three different examples are analyzed to show the accuracy and performance of the proposed element at linear as well as nonlinear regimes of deformation. It is shown that the new element can successfully capture the so-called size effect as well as geometric nonlinearity at large distortion of microbeams and microframes at micron scales.

Development of novel ultra-high performance concrete: From material to structure

This paper investigates effects of nanoscale materials and steel fibres on properties of ultra-high performance concrete (UHPC). Different types of steel fibres including twisted steel fibre (TF), waved steel fibre (WF), and micro steel fibre (MF) together with different kinds of nano materials including Nano-CaCO3, Nano-SiO2, Nano-TiO2 and Nano-Al2O3 are studied in the present research. Material compressive stress–strain relationships, strain energy absorption, the flexural strength and fracture energy absorption of UHPC with different nanoscale materials and steel fibres were compared and discussed. Laboratory static bending tests and field blast tests on structural components made of selected UHPC material composition were carried out, and the results highlighted the superior material ductility and blast resistant capacity of UHPC material developed in the present study.

Recent Developments in Micro-Structured Fiber Optic Sensors

Recent developments in fiber-optic sensing have involved booming research in the design and manufacturing of novel micro-structured optical fiber devices. From the conventional tapered fiber architectures to the novel micro-machined devices by advanced laser systems, thousands of micro-structured fiber-optic sensors have been proposed and fabricated for applications in measuring temperature, strain, refractive index (RI), electric current, displacement, bending, acceleration, force, rotation, acoustic, and magnetic field. The renowned and unparalleled merits of sensors-based micro-machined optical fibers including small footprint, light weight, immunity to electromagnetic interferences, durability to harsh environment, capability of remote control, and flexibility of directly embedding into the structured system have placed them in highly demand for practical use in diverse industries. With the rapid advancement in micro-technology, micro-structured fiber sensors have benefitted from the trends of possessing high performance, versatilities and spatial miniaturization. Here, we comprehensively review the recent progress in the micro-structured fiber-optic sensors with a variety of architectures regarding their fabrications, waveguide properties and sensing applications.

Buffer layer influence on light absorption of electron intersubband transition in binary energy level systems of quantum wells

Due to the restriction of the ZnO buffer layer on the left barrier in a wurtzite asymmetric ZnO/MgxZn1-xO single quantum well (QWs) structure with finite barriers, the other factors such as the size of the well and right barrier, and Mg component, etc. will influence the critical value of the left barrier width to form a binary level energy system. By adopting a finite difference method to solve the Schrdinger equation, the eigenstates and eigenenergies of electrons in a two-dimensional electron gas are obtained, and the influences of buffer layer ZnO, size and ternary mixed crystal effects on the formation of binary energy level system in QW are discussed. In our computation, the influences of energy band bending, material doping and built-in electric fields on a realistic heterostructure potential are considered. Furthermore, based on the Fermi's golden rule, the optical absorption coefficient of electronic intersubband transition in QW and the influences of buffer layer thickness, the widths of left barrier, well and right barrier and ternary mixed crystal effects are discussed. Our results indicate that the critical width of left barrier increases with the increases of the right barrier width and buffer layer thickness for a binary energy level system of ZnO/MgxZn1-xO single quantum well with a ZnO buffer layer on the left side. However, the critical width of left barrier decreases with the increase of well width and Mg component. Besides, the buffer layer thickness, the widths of left barrier, well and right barrier and ternary mixed crystal also affect the light absorption induced by the electronic intersubband transitions. The increases of Mg component, the widths of right barrier and left barrier will increase the absorption peak and produce its blue-shift. Whereas, increasing well width will reduce the absorption peak and produce its red-shift. The above conclusions are expected to give theoretical guidance in improving the opto-electronic properties of materials and devices made of these heterostructures.

Constitutive boundary conditions and paradoxes in nonlocal elastic nanobeams

A debated issue, in applications of Eringen's nonlocal model of elasticity to nanobeams, is the paradox concerning the solution of simple beam problems, such as the cantilever under end-point loading. In the adopted nonlocal model, the bending field is expressed as convolution of elastic curvature with a smoothing kernel. The inversion of the nonlocal elastic law leads to solution of a Fredholm integral equation of the first kind. It is here shown that this problem admits a unique solution or no solution at all, depending on whether the bending field fulfils constitutive boundary conditions or not. Paradoxical results found in solving nonlocal elastostatic problems of simple beams are shown to stem from incompatibility between the constitutive boundary conditions and equilibrium conditions imposed on the bending field. The conclusion is that existence of a solution of nonlocal beam elastostatic problems is an exception, the rule being non-existence for problems of applicative interest. Numerical evaluations reported in the literature hide or shadow this conclusion since nodal forces expressing the elastic response are not checked against equilibrium under the prescribed data. The cantilever problem is investigated as case study and analytically solved to exemplify the matter.

Polar Superhelices in Ferroelectric Chiral Nanosprings.

Topological objects of nontrivial spin or dipolar field textures, such as skyrmions, merons, and vortices, interacting with applied external fields in ferroic materials are of great scientific interest as an intriguing playground of unique physical phenomena and novel technological paradigms. The quest for new topological configurations of such swirling field textures has primarily been done for magnets with Dzyaloshinskii-Moriya interactions, while the absence of such intrinsic chiral interactions among electric dipoles left ferroelectrics aside in this quest. Here, we demonstrate that a helical polarization coiled into another helix, namely a polar superhelix, can be extrinsically stabilized in ferroelectric nanosprings. The interplay between dipolar interactions confined in the chiral geometry and the complex strain field of mixed bending and twisting induces the superhelical configuration of electric polarization. The geometrical structure of the polar superhelix gives rise to electric chiralities at two different length scales and the coexistence of three order parameters, i.e., polarization, toroidization, and hypertoroidization, both of which can be manipulated by homogeneous electric and/or mechanical fields. Our work therefore provides a new geometrical configuration of swirling dipolar fields, which offers the possibility of multiple order-parameters, and electromechanically controllable dipolar chiralities and associated electro-optical responses.

Integrated Analysis of the Formation Mechanism of Cracks in a Concrete Dam Using Microseismic Monitoring and Numerical Simulation

The dam of Guanyinyan hydropower station is composed of a concrete gravity dam in the left bank and a rockfill dam in the right bank. During the operation of the hydropower station, several surface cracks occurred in the concrete gravity dam, which threatened the stability of the dam. To evaluate the evolution trend of the cracks and forecast the potential risk of the dam, the microseismic (MS) monitoring technique and finite-element method were used. First, the concrete three-point bending field test was performed to prove the reliability of the MS technique in monitoring the concrete cracks. The MS monitoring results were consistent with the simulation results. Then, the MS monitoring system was installed in the dam body. By analysing the MS activities before and after the impoundment, the evolution trend of the cracks and potential risk of the dam were evaluated and forecasted. The simulation results were also consistent with the monitoring results. These results can provide significant references for the operation safety of the dam and also present a new thought for the risk evaluation of similar dam engineering.

Nonlocal transient thermal analysis of a single-layered graphene sheet embedded in viscoelastic medium

The transient thermal analysis of a single-layered graphene sheet (SLGS) embedded in viscoelastic medium is presented by using the nonlocal elasticity theory. The elastic medium, which characterized by the linear Winkler’s modulus and Pasternak’s (shear) foundation modulus, is changed to a viscoelastic one by including the viscous damping term. The governing dynamical equation is obtained and solved for simply-supported SLGSs. Firstly; the effect of the nonlocal parameter is discussed carefully for the vibration and bending problems. Secondly, the effects of other parameter like aspect ratio, thickness-to-length ratio, Winkler-Pasternak’s foundation, viscous damping coefficient on bending field quantities of the SLGSs are investigated in detail. The present results are compared with the corresponding available in the literature. Additional results for thermal local and nonlocal deflections and stresses are presented to investigate the thermal visco-Pasternak’s parameters for future comparisons.

Simple modification of Compton polarimeter to redirect synchrotron radiation

Synchrotron radiation produced as an electron beam passes through a bending magnet is a significant source of background in many experiments. Using modeling, we show that simple modifications of the magnet geometry can reduce this background by orders of magnitude in some circumstances. Specifically, we examine possible modifications of the four dipole magnets used in Jefferson Lab’s Hall A Compton polarimeter chicane. This Compton polarimeter has been a crucial part of experiments with polarized beams and the next generation of experiments will utilize increased beam energies, up to 11 GeV, requiring a corresponding increase in Compton dipole field to 1.5 T. In consequence, the synchrotron radiation (SR) from the dipole chicane will be greatly increased. Three possible modifications of the chicane dipoles are studied; each design moves about 2% of the integrated bending field to provide a gentle bend in critical regions along the beam trajectory which, in turn, greatly reduces the synchrotron radiation within the acceptance of the Compton polarimeter photon detector. Each of the modifications studied also softens the SR energy spectrum at the detector sufficiently to allow shielding with 5 mm of lead. Simulations show that these designs are each capable of reducing the background signal duemore » to SR by three orders of magnitude. The three designs considered vary in their need for vacuum vessel changes and in their effectiveness.« less

Investigation of trap states in Al2O3 InAlN/GaN metal–oxide–semiconductor high-electron-mobility transistors**Project supported by the Program for National Natural Science Foundation of China (Grant Nos. 61404100 and 61306017).

In this paper the trapping effects in Al2O3/In0.17Al0.83N/GaN MOS-HEMT (here, HEMT stands for high electron mobility transistor) are investigated by frequency-dependent capacitance and conductance analysis. The trap states are found at both the Al2O3/InAlN and InAlN/GaN interface. Trap states in InAlN/GaN heterostructure are determined to have mixed de-trapping mechanisms, emission, and tunneling. Part of the electrons captured in the trap states are likely to tunnel into the two-dimensional electron gas (2DEG) channel under serious band bending and stronger electric field peak caused by high Al content in the InAlN barrier, which explains the opposite voltage dependence of time constant and relation between the time constant and energy of the trap states.