Showing papers on "Beam (structure) published in 2006"

Bernoulli–Euler beam model based on a modified couple stress theory

Abstract: A new model for the bending of a Bernoulli–Euler beam is developed using a modified couple stress theory. A variational formulation based on the principle of minimum total potential energy is employed. The new model contains an internal material length scale parameter and can capture the size effect, unlike the classical Bernoulli–Euler beam model. The former reduces to the latter in the absence of the material length scale parameter. As a direct application of the new model, a cantilever beam problem is solved. It is found that the bending rigidity of the cantilever beam predicted by the newly developed model is larger than that predicted by the classical beam model. The difference between the deflections predicted by the two models is very significant when the beam thickness is small, but is diminishing with the increase of the beam thickness. A comparison shows that the predicted size effect agrees fairly well with that observed experimentally.

Dynamic properties of flexural beams using a nonlocal elasticity model

Abstract: In this paper, a nonlocal Bernoulli-Euler beam model is established based on the theory of nonlocal elasticity. Frequency equations and modal shape functions of beam structures with some typical boundary conditions are derived based on the model. The corresponding dynamic properties are presented and discussed in detail, which are shown to be very different from those predicted by classic elasticity theory when nonlocal effects are significant. The results can be applied to modeling and characterization of size-dependent mechanical properties of micro- or nanoelectromechanical system (MEMS or NEMS) devices.

Analysis of Thin Beams and Cables Using the Absolute Nodal Co-ordinate Formulation

Abstract: The purpose of this paper is to present formulations for beam elements based on the absolute nodal co-ordinate formulation that can be effectively and efficiently used in the case of thin structural applications. The numerically stiff behaviour resulting from shear terms in existing absolute nodal co-ordinate formulation beam elements that employ the continuum mechanics approach to formulate the elastic forces and the resulting locking phenomenon make these elements less attractive for slender stiff structures. In this investigation, additional shape functions are introduced for an existing spatial absolute nodal co-ordinate formulation beam element in order to obtain higher accuracy when the continuum mechanics approach is used to formulate the elastic forces. For thin structures where bending stiffness can be important in some applications, a lower order cable element is introduced and the performance of this cable element is evaluated by comparing it with existing formulations using several examples. Cables that experience low tension or catenary systems where bending stiffness has an effect on the wave propagation are examples in which the low order cable element can be used. The cable element, which does not have torsional stiffness, can be effectively used in many problems such as in the formulation of the sliding joints in applications such as the spatial pantograph/catenary systems. The numerical study presented in this paper shows that the use of existing implicit time integration methods enables the simulation of multibody systems with a moderate number of thin and stiff finite elements in reasonable CPU time.

Double-slit interference with Laguerre-Gaussian beams

Abstract: The interference of Laguerre-Gaussian beams carrying orbital angular momentum was demonstrated in Young's double-slit geometry. Double-slit interference is shown to be affected by the azimuthal phase dependence of a Laguerre-Gaussian beam. This interference provides new insight into the helical phase structure of the Laguerre-Gaussian beam and has potential applications for measuring the orbital angular momentum of an arbitrary wavefront.

Multiple Crack Detection of Concrete Structures Using Impedance-based Structural Health Monitoring Techniques

Abstract: This paper presents a feasibility study for practical applications of an impedance-based real-time health monitoring technique applying PZT (Lead–Zirconate–Titanate) patches to concrete structures. First, comparison between experimental and analytical studies for damage detection on a plain concrete beam is made. In the experimental study, progressive surface damage inflicted artificially on the plain concrete beam is assessed by using both lateral and thickness modes of the PZT patches. Then, an analytical study based on finite element (FE) models is carried out to verify the validity of the experimental result. Secondly, multiple (shear and flexural) cracks incurred in a reinforced concrete (RC) beam under a third point bending test are monitored continuously by using a sensor array system composed of the PZT patches. In this study, a root mean square deviation (RMSD) in the impedance signatures of the PZT patches is used as a damage indicator.

Measurement of the Bending Strength of Vapor-Liquid-Solid Grown Silicon Nanowires

Abstract: The fracture strength of silicon nanowires grown on a [111] silicon substrate by the vapor-liquid-solid process was measured. The nanowires, with diameters between 100 and 200 nm and a typical length of 2 Im, were subjected to bending tests using an atomic force microscopy setup inside a scanning electron microscope. The average strength calculated from the maximum nanowire deflection before fracture was around 12 GPa, which is 6% of the Young’s modulus of silicon along the nanowire direction. This value is close to the theoretical fracture strength, which indicates that surface or volume defects, if present, play only a minor role in fracture initiation. Nanowires (NWs) are of interdisciplinary interest to applications in the fields of biomedical sensing, nano- and optoelectronics and photovoltaics due to their electrical, optical, mechanical, and geometrical properties that may deviate substantially from bulk. 1 To name some particularly exciting applications, the reader is referred to the following list: (i) high-frequency electromechanical resonators, 2 (ii) high-aspect ratio tips for surface probe microscopy, 3 (iii) sensor array for electrical detection of cancer markers, 4 (iv) Si NW arrays for photovoltaics, 5 and (v) nanoscale light-emitting diodes. 6 For all these applications the mechanical stability of the NWs is essential for their atomic scale manipulation, functionalization, or integration into device schemes. Several methods were used in the past to access the mechanical properties of silicon NWs and nanobeams. An atomic force microscope (AFM) was used for bending tests of single crystal, micromachined silicon beams (from 1 mm down to 200 nm in width, beam axis oriented in [110] direction). No change in Young’s modulus, but an increase in bending strength by a factor of up to 38 was observed from the millimeter down to the nanometer scale. 7 AFM measurements were also done on silicon NWs (from 10 to 100 nm in diameter, grown along the [111] direction) where a bending modulus of 186 GPa (188 GPa in bulk) was measured. 8

Flexural vibration band gaps in Timoshenko beams with locally resonant structures

Abstract: Flexural vibration in Timoshenko beams with periodically attached local resonators is studied theoretically and experimentally. The existence of a low frequency flexural vibration gap is indicated by the complex band structure calculated with transfer matrix theory for an infinite beam, as well as the frequency response function calculated with the finite element method for a finite Timoshenko beam with finite local resonators. This finite Timoshenko beam was manufactured and vibration experiments generated an experimental frequency response function curve showing a vibration gap as expected. The existence of low frequency gaps in Timoshenko beams with local resonators provides a method of flexural vibration control of beams.

Evanescent Bessel beam generation via surface plasmon resonance excitation by a radially polarized beam

Abstract: A simple setup for generating evanescent Bessel beams is proposed. When a radially polarized beam is strongly focused onto a dielectric-metal interface, the entire beam is p-polarized with respect to the dielectric-metal interface, enabling excitation of surface plasmons from all directions. The angular selectivity of surface plasmon excitation mimics the function of an axicon, leading to an evanescent nondiffracting Bessel beam. The created evanescent Bessel beam may be used as a virtual probe for near-field optical imaging and sensing applications.

Flexural vibration band gaps in Euler-Bernoulli beams with locally resonant structures with two degrees of freedom

Abstract: Using the transfer matrix theory, we provided the band structure of flexural waves in an Euler-Bernoulli beam with locally resonant structures, with two degrees of freedom, i.e., a resonator with vertical and rotational vibration. The frequency response function of a finite periodic system was calculated by the finite element method. The material damping of rubber makes the gaps wider in the calculation. These theoretical results show a good agreement with those of the experiment. The measured result provides an attenuation of over $20\phantom{\rule{0.3em}{0ex}}\mathrm{dB}$ in the frequency range of the band gaps. The existence of low-frequency band gaps in such a beam provides a method of flexural vibration control of beams.

Near surface mounted CFRP laminates for shear strengthening of concrete beams

Abstract: A Near Surface Mounted (NSM) strengthening technique was developed to increase the shear resistance of concrete beams. The NSM technique is based on fixing, by epoxy adhesive, Carbon Fiber Reinforced Polymer (CFRP) laminates into pre-cut slits opened in the concrete cover of lateral surfaces of the beams. To assess the efficacy of this technique, an experimental program of four-point bending tests was carried out with reinforced concrete beams failing in shear. Each of the four tested series was composed of five beams: without any shear reinforcement; reinforced with steel stirrups; strengthened with strips of wet lay-up CFRP sheets, applied according to the externally bonded reinforcement (EBR) technique; and two beams strengthened with NSM precured laminates of CFRP, one of them with laminates positioned at 90° and the other with laminates positioned at 45° in relation to the beam axis. Influences of the laminate inclination, beam depth and longitudinal tensile steel reinforcement ratio on the efficacy of the strengthening techniques were analyzed. Amongst the CFRP strengthening techniques, the NSM with laminates at 45° was the most effective, not only in terms of increasing beam shear resistance but also in assuring larger deformation capacity at beam failure. The NSM was also faster and easier to apply than the EBR technique. The performance of the ACI and fib analytical formulations for the EBR shear strengthening was appraised. In general, the contribution of the CFRP systems predicted by the analytical formulations was slightly larger than the values registered experimentally. Performance of the formulation by Nanni et al. for NSM strengthening technique was also appraised. Using bond stress and CFRP effective strain values obtained in pullout bending tests with NSM CFRP laminate system, the formulation by Nanni et al. predicted a contribution of this CFRP system for the beam shear resistance of 72% the experimentally recorded values.

Systems and methods for imaging large field-of-view objects

Abstract: An imaging apparatus and related method comprising a detector located a distance from a source and positioned to receive a beam of radiation in a trajectory; a detector positioner that translates the detector to an alternate position in a direction that is substantially normal to the trajectory; and a beam positioner that alters the trajectory of the radiation beam to direct the beam onto the detector located at the alternate position

An experimental, analytical and numerical study of the static behavior of steel beams reinforced by pultruded CFRP strips

Abstract: The paper presents the results of an experimental and numerical programme to characterize the static behaviour of steel beams reinforcement by pultruded carbon fibre reinforced polymer (CFRP) strips. Traditional H shaped steel beams with different CFRP reinforcement geometries bonded to the tension flanges using different epoxy adhesives were tested under three points bending configuration. Beams were not naturally or artificially corroded or notched but they were in good conditions before testing. The mid-span deflection and the strain along the whole CFRP lamina were recorded as function of the applied loading. The main objective of the experimental programme was the evaluation of the force transfer mechanism, the increment of the beam load carrying capacity and the bending stiffness. It allowed also to validate different analytical and numerical models for the static analysis of reinforced beams. In particular, a finite element model validated against the experimental data is presented.

Laser-wakefield acceleration of monoenergetic electron beams in the first plasma-wave period.

Abstract: Beam profile measurements of laser-wakefield accelerated electron bunches reveal that in the monoenergetic regime the electrons are injected and accelerated at the back of the first period of the plasma wave. With pulse durations ctau >or= lambda(p), we observe an elliptical beam profile with the axis of the ellipse parallel to the axis of the laser polarization. This increase in divergence in the laser polarization direction indicates that the electrons are accelerated within the laser pulse. Reducing the plasma density (decreasing ctau/lambda(p)) leads to a beam profile with less ellipticity, implying that the self-injection occurs at the rear of the first period of the plasma wave. This also demonstrates that the electron bunches are less than a plasma wavelength long, i.e., have a duration <25 fs. This interpretation is supported by 3D particle-in-cell simulations.

Observation of a fast electron beam emitted along the surface of a target irradiated by intense femtosecond laser pulses.

Abstract: A novel fast electron beam emitting along the surface of a target irradiated by intense laser pulses is observed. The beam is found to appear only when the plasma density scale length is small. Numerical simulations reveal that the electron beam is formed due to the confinement of the surface quasistatic electromagnetic fields. The results are of interest for potential applications of fast electron beams and deep understanding of the cone-target physics in the fast ignition related experiments.

Behavior of Reinforced Concrete T-Beams Strengthened in Shear with Carbon Fiber-Reinforced Polymer--An Experimental Study

Abstract: The results of an extensive experimental investigation on reinforced concrete T-beams retrofitted in shear with externally bonded carbon fiber-reinforced polymer (CFRP) are presented. The authors researched the CFRP ratio, the internal shear steel reinforcement ratio and the shear length to the beam's depth ratio. By studying the aforementioned parameters, the behavior of reinforced concrete T-beams strengthened in shear with externally applied CFRP was analyzed. It appears that the contribution of CFRP to shear resistance depends on if a strengthened beam is reinforced in shear with internal transverse steel reinforcement, and not on the CFRP stiffness provided. It seems that the American Concrete Institute and the Canadian Standards Association guidelines fail to incorporate the presence of the transverse resistance and overestimates the shear resistance for high fiber reinforced polymer thickness.

Charged particle radiation therapy

Abstract: Among other things, an accelerator (502) is mounted on a gantry (504) to enable the accelerator to move through a range of positions around a patient (506) on a patient support. The accelerator is configured to produce a proton or ion beam having an energy level sufficient to reach any arbitrary target in the patient from positions within the range. The proton or ion beam passes essentially directly from the accelerator to the patient. In some examples, the synchrocyclotron has a superconducting electromagnetic structure that generates a field strength of at least 6 Tesla, produces a beam of particles having an energy level of at least 150 MeV, has a volume no larger than 4.5 cubic meters, and has a weight less than 30 Tons.

Theory of electronically phased coherent beam combination without a reference beam

Abstract: The first theory for two novel coherent beam combination architectures that are the first electronic beam combination architectures that completely eliminate the need for a separate reference beam are presented. Detailed theoretical models are developed and presented for the first time.

Three-dimensional quasistatic model for high brightness beam dynamics simulation

Abstract: In this paper, we present a three-dimensional quasistatic model for high brightness beam dynamics simulation in rf/dc photoinjectors, rf linacs, and similar devices on parallel computers. In this model, electrostatic space-charge forces within a charged particle beam are calculated self-consistently at each time step by solving the three-dimensional Poisson equation in the beam frame and then transforming back to the laboratory frame. When the beam has a large energy spread, it is divided into a number of energy bins or slices so that the space-charge forces are calculated from the contribution of each bin and summed together. Image-charge effects from conducting photocathode are also included efficiently using a shifted-Green function method. For a beam with large aspect ratio, e.g., during emission, an integrated Green function method is used to solve the three-dimensional Poisson equation. Using this model, we studied beam transport in one Linac Coherent Light Sources photoinjector design through the first traveling wave linac with initial misalignment with respect to the accelerating axis.