Showing papers on "Deflection (engineering) published in 2005"

Comparison of laser Doppler vibrometer with contact sensors for monitoring bridge deflection and vibration

Abstract: This paper compares results from dynamic live load tests using the non-contact laser Doppler vibrometer (LDV) system with those from two types of contact sensors. Bridge girder deflections and vibrations are simultaneously measured using a linear variable differential transducer (LVDT)-cable system (deflection) and geophone sensors (velocity), both attached to the girders, and the LDV, equipped with displacement and velocity signal decoders. Live load tests are performed on a 3-span continuous unit of the 9-span Doremus Avenue Bridge Replacement Project using 5-axle trucks of known weight and configuration. The Doremus Avenue Bridge is a composite steel slab-on-girder construction. Bridge response is compared for two of the 10 girders. Overall, the LDV measurements of deflection and velocity compare very well with those recorded by the contact sensors and may be used as an alternative to the two systems. Other advantages and disadvantages are also highlighted.

Development of low noise cantilever deflection sensor for multienvironment frequency-modulation atomic force microscopy

Abstract: We have developed a low noise cantilever deflection sensor with a deflection noise density of 17fm∕Hz by optimizing the parameters used in optical beam deflection (OBD) method. Using this sensor, we have developed a multienvironment frequency-modulation atomic force microscope (FM-AFM) that can achieve true molecular resolution in various environments such as in moderate vacuum, air, and liquid. The low noise characteristic of the deflection sensor makes it possible to obtain a maximum frequency sensitivity limited by the thermal Brownian motion of the cantilever in every environment. In this paper, the major noise sources in OBD method are discussed in both theoretical and experimental aspects. The excellent noise performance of the deflection sensor is demonstrated in deflection and frequency measurements. True molecular-resolution FM-AFM images of a polydiacetylene single crystal taken in vacuum, air, and water are presented.

Enhanced stiffness modeling, identification and characterization for robot manipulators

Abstract: This paper presents the enhanced stiffness modeling and analysis of robot manipulators, and a methodology for their stiffness identification and characterization. Assuming that the manipulator links are infinitely stiff, the enhanced stiffness model contains: 1) the passive and active stiffness of the joints and 2) the active stiffness created by the change in the manipulator configuration, and by external force vector acting upon the manipulator end point. The stiffness formulation not accounting for the latter is known as conventional stiffness formulation, which is obviously not complete and is valid only when: 1) the manipulator is in an unloaded quasistatic configuration and 2) the manipulator Jacobian matrix is constant throughout the workspace. The experimental system considered in this study is a Motoman SK 120 robot manipulator with a closed-chain mechanism. While the deflection of the manipulator end point under a range of external forces is provided by a high precision laser measurement system, a wrist force/torque sensor measures the external forces. Based on the experimental data and the enhanced stiffness model, the joint stiffness values are first identified. These stiffness values are then used to prove that conventional stiffness modeling is incomplete. Finally, they are employed to characterize stiffness properties of the robot manipulator. It has been found that although the component of the stiffness matrix differentiating the enhanced stiffness model from the conventional one is not always positive definite, the resulting stiffness matrix can still be positive definite. This follows that stability of the stiffness matrix is not influenced by this stiffness component. This study contributes to the previously reported work from the point of view of using the enhanced stiffness model for stiffness identification, verification and characterization, and of new experimental results proving that the conventional stiffness matrix is not complete and is valid under certain assumptions.

Flexural strengthening of concrete beams with CFRP laminates bonded into slits

Abstract: Near surface mounted (NSM) strengthening technique using carbon fibre reinforced polymer (CFRP) laminate strips was applied for doubling the load carrying capacity of concrete beams failing in bending. This objective was attained and the deformational capacity of the strengthened beams was similar to the corresponding reference beams. The NSM technique has provided a significant increment of the load at serviceability limit state, as well as, the stiffness after concrete cracking. The maximum strain in the CFRP laminates has attained values between 62% and 91% of its ultimate strain. A numerical strategy was developed to simulate the deformational behaviour of RC beams strengthened by NSM technique. Not only the load carrying capacity of the tested beams was well predicted, but also the corresponding deflection.

Reevaluation of Deflection Prediction for Concrete Beams Reinforced with Steel and Fiber Reinforced Polymer Bars

Abstract: This paper provides a critical evaluation of equations commonly used to compute short-term deflection for steel and fiber reinforced polymer (FRP) reinforced concrete beams Numerous proposals have been made for FRP in particular, and the different approaches are linked together by comparing the tension-stiffening component of each method Tension stiffening reflects the participation of concrete between cracks in stiffening the member response The Branson equation used in North America and other parts of the world is based on an empirically derived effective moment of inertia to calculate deflection The tension-stiffening component with this method is highly dependent on the applied level of loading relative to the cracking load as well as the ratio of uncracked-to-cracked transformed moment of inertia ( Ig ∕ Icr ) for the beam section Tension stiffening is overestimated for the high Ig ∕ Icr ratios typical with FRP concrete, leading to a much stiffer response and underprediction of member deflection

The Mechanical Response of Freestanding Circular Elastic Films Under Point and Pressure Loads

Abstract: This paper provides a comprehensive description of the mechanical response of freestanding circular elastic films subjected to point and pressure loads. Regimes of behavior, such as plate, linear membrane, and nonlinear membrane, are identified in terms of two dimensionless variables that allow the creation of a single map that indicates appropriate closed-form solutions. This map provides a theoretical framework to design experiments and interpret film behavior for all orders of magnitude of: film thickness-to-span ratio, deflection, loads, prestretch, and elastic properties. The normalization approach provides the means to quickly identify appropriate simplifications to the nonlinear governing equations, and identify applicable analytical solutions. Numerical results are used to illustrate behavior in transition regions, e.g., the transition for a given plate thickness from small to large deflections under increasing load. Critical loads, thickness and prestretch are identified which indicate when asymptotic plate or membrane solutions are accurate. Asymptotic and numerical results are presented which illustrate finite-sized regions of bending-influenced deformation near point loads and clamped edges. Theoretical predictions for the width of these regions enable us to estimate the validity of analytical strain distributions, and in turn the maximum strains in the film. These results help avoiding brittle fracture or ductile yielding of the film by identifying physical parameters that limit strains to an acceptable level.

Touchdown and Pull-In Voltage Behavior of a MEMS Device with Varying Dielectric Properties

Abstract: The pull-in voltage instability associated with a simple MEMS device, consisting of a thin dielectric elastic membrane supported above a rigid conducting ground plate, is analyzed. The upper surface of the membrane is coated with a thin conducting film. In a certain asymptotic limit representing a thin device, the mathematical model consists of a nonlinear partial differential equation for the deflection of the thin dielectric membrane. When a voltage V is applied to the conducting film, the dielectric membrane deflects towards the bottom plate. For a slab, a circular cylindrical, and a square domain, numerical results are given for the saddle-node bifurcation value $V_{*}$, also referred to as the pull-in voltage, for which there is no steady-state membrane deflection for $V > V_{*}$. For $V > V_{*}$ it is shown numerically that the membrane dynamics are such that the thin dielectric membrane touches the lower plate in finite time. Results are given for both spatially uniform and nonuniform dielectric pe...

Reinforced high-strength concrete beams in flexure

Abstract: An important design issue is the ductility or the ability of a reinforced concrete (RC) member to deform at or near the ultimate load without significant strength loss. This article reports on a study of reinforced high-strength concrete (HSC) beams in flexure. The study included flexural test results generated on 16 reinforced concrete beams. Test parameters considered include concrete compressive strength, ratios of tensile and compressive reinforcements, and spacing of lateral ties. The authors found that the current code provisions for serviceability requirements of maximum crack width and ultimate strength are adequate up to a concrete strength of approximately 130 MPa. The authors express concerns regarding the adequacy of current code provisions, however, for cracking moment and service load deflection. They show that stresses generated by shrinkage of concrete and the creep associated with it can significantly affect the cracking moment and service load deflection of reinforced HSC beams. The authors recommend that the ACI Code specification for maximum spacing of ties in RC flexural members need to be reduced to d/4, particularly at critical sections. This will prevent premature disintegration of the confined concrete core in the compression zone due to buckling of compression reinforcement.

Evaluation of the stiffness chain on the deflection of end-mills under cutting forces

Abstract: This study presents the investigation of the stiffness of the system formed by the machine-tool, shank and toolholder, collet and tool. Cutting forces induce the deflection of the system, and consequently an error appears on the machined surface. Comparing values obtained from cantilever beam models applied to the cutting tool, analytical or FEM, with those experimentally obtained, large differences have been observed, which in some cases are more than 50%. For this reason, we have proceeded to evaluate the stiffness of each of the existing elements between the machine bed and the tool tip. Thus, deflections of the machine-tool, toolholder and toolholder clamping in the spindle, tool clamping in the toolholder, and tool itself, were measured experimentally under the effects of known forces. The final application is the ball-end milling of complex surfaces, an operation commonly performed in the finishing of moulds or forging dies, where errors of more than 70 μm are not unusual. A great part of this error comes from the deflection of the machine-tool assembly, spindle, shank and tool, due to the high cutting forces of the high speed machining of tempered steels. Cutting forces can be estimated using a semi-empirical approach, and from here some values of probable errors may be taken into account to check if the CNC programs are sufficiently adapted. However, a previous study of the deflection chain in the cutting process is needed, as is presented in this work. Results show that stiffness of the slender and flexible tools is 15 times lower than that of the machine and toolholder system. But this correlation is only 5–7 times lower for shorter and thicker tools.

Observed Performance of a Deep Multistrutted Excavation in Shanghai Soft Clays

Abstract: A 15.5 m deep multistrutted soft clay excavation of a metro station in Shanghai has been monitored. The deep excavation was supported by a concrete diaphragm wall. The monitoring included wall deflections, surface and subsurface ground settlements, total pressures, and pore-water pressures. Relatively small wall deflections and ground settlements were measured as compared with similar case histories worldwide. The small deflections and settlements observed probably occurred because of the use of a short excavation section, application of compaction grouting for improving ground conditions, and the use of active prestress steel struts. No significant “creep” deflection of the diaphragm wall could be identified over a 60 day concrete curing period. Continuous ground settlements accompanied by the dissipation of pore-water pressures were observed. By using Terzaghi’s one-dimensional consolidation theory, the observed settlements were found to be attributed to the primary consolidation rather than creep effects.

Heavily Loaded Gas Foil Bearings: A Model Anchored to Test Data

Abstract: Widespread usage of gas foil bearings (FBs) into microturbomachinery to midsize gas turbine engines requires accurate performance predictions anchored to reliable test data. This paper presents a simple yet accurate model predicting the static and dynamic force characteristics of gas FBs. The analysis couples the Reynolds equation for a thin gas film to a simple elastic foundation model for the top foil and bump strip layer. An exact flow advection model is adopted to solve the partial differential equations for the zeroth- and first-order pressure fields that render the FB load capacity and freguency-dependent force coefficients. As the static load imposed on the foil bearing increases, predictions show that the journal center displaces to eccentricities exceeding the bearing nominal clearance. A nearly constant FB static stiffness, independent of journal speed, is estimated for operation with large loads, and approaching closely the bearing structural stiffness derived from contact operation without rotor spinning. Predicted minimum film thickness and journal attitude angle demonstrate good agreement with archival test data for a first-generation gas FB. The bump-foil-strip structural loss factor, exemplifying a dry-friction dissipation mechanism, aids to largely enhance the bearing direct damping force coefficients. At high loads, the bump-foil structure influences most the stiffness and damping coefficients. The predictions demonstrate that FBs have greatly different static and dynamic force characteristics when operating at journal eccentricities in excess of the bearing clearance from those obtained for operation at low loads, i.e., small journal eccentricity.

Buckling of cracked thin-plates under tension or compression

Abstract: Plates are easily susceptible to buckling under compression, in particular when plate's thickness becomes sufficiently small with respect to others plate's sizes; such a mode of failure is often prevalent with respect to strength failure The buckling phenomena under tension loading can also occur, especially in plates containing defects such as cracks or holes; when the buckling load is reached, complex wrinkling deflection patterns in compressed regions develops around such imperfections In the present paper, the buckling analysis of variously cracked rectangular elastic thin-plates under tension and compression is considered A short explanation of the buckling phenomena in plates is recalled and several numerical analyses, carried out by using the Finite Element Method (FEM), are performed in order to determine the critical load multiplier, both in compression and in tension, by varying some plates' parameters In particular, the critical load multiplier is determined for different relative crack length, crack orientation and Poisson's coefficient of the plate's material which is made to range between 01 and 049 Moreover a simple approximate theoretical model to explain and predict the buckling phenomena in cracked plates under tension is proposed and some comparisons are made with FE numerical results in order to assess its reliability in predicting buckling load multipliers Finally, the obtained results are graphically summarised (in dimensionless form) in several graphs and some interesting conclusions are drawn

Analytical Model to Predict Nonlinear Flexural Behavior of Corroded Reinforced Concrete Beams

Abstract: No suitable analysis is presently available to predict the flexural behavior of corroded reinforced concrete (RC) beams. This article presents a new analytical model that predicts the nonlinear flexural behavior of both corroded and newly constructed reinforced concrete beams. The authors first review previous studies in this field, noting deficiencies that required a new model. They then explain how in the proposed model, the deflection of a reinforced concrete beam is calculated from the elongation of the steel reinforcement between flexural cracks rather than from the curvatures of beam sections. This model accounts for the reduction in the steel area and the change in bond strength at the steel-to-concrete interface caused by corrosion. The effect of a loading-unloading cycle on the flexural behavior is also taken into account. The authors also propose a new bond stress-slip model that accounts for the change in bond strength due to corrosion. A comparison of the model’s predictions with experimental results showed that the model accurately predicts the load-deflection curves of both corroded and newly constructed beams.

Three-Dimensional Dynamic Analysis of Flexible Conventional Pavement Foundation

Abstract: The paper examines the dynamic response of flexible conventional pavement systems to single wheel traffic loads in terms of the pavement design criteria, namely the fatigue strain at the bottom of the asphalt concrete layer and rutting strain at the top of the subgrade material. Model setup including geometry, boundary conditions, and load wave characterization are presented. The effect of elastoplasticity of the base material and elastoplasticity with strain hardening of the subgrade material on the dynamic response of the pavement system are first investigated. A detailed model parametric study then follows to show the effect of the base strength and thickness and the subgrade quality on the fatigue and rutting strains and the vertical surface deflection. The study, conducted with program ADINA, employs a three-dimensional, implicit dynamic, finite element method.

Using Inclinometers to Measure Bridge Deflection

Abstract: Deflection of a bridge span under designed loads is an important parameter for bridge safety evaluation. However, it is inconvenient to obtain the bridge deflections directly. For bridges over rivers, railways, or highways, a direct measurement method is impractical. A promising bridge deflection measurement method (inclinometer method) is presented in this paper. It offers a simple, practical and inexpensive method of measuring static and dynamic deflections of bridge spans under loads, even for bridge spans that traverse great heights. Hundreds of experiments and practical tests on simple and continuous bridges, utilizing dynamic and static loads, under various vehicle speeds, show that the method has very high precision, which provides an authentic basis for new-built bridge acceptance and old bridge safety evaluation. The method does not need fixed observation positions as other deflection measurement methods because the inclinometers are installed on the bridge directly, which increases measurement efficiency greatly. These features indicate that as a potential method of measuring bridge deflection, inclinometers have significant engineering application value and a promising future.

Characterizing effective built-in curling from concrete pavement field measurements

Abstract: Instrumented concrete slabs were constructed in Palmdale, Calif., in order to fail the slab sections under accelerated pavement testing. Prior to fatigue failure testing, these slabs were monitored over 24 h cycles without load, and under a slow-moving 40 kN rolling wheel load. Slab temperature profiles, edge and corner deflections, and interior vertical deflections were collected at 2 h intervals. A finite element program was used to analyze the deflection data and calculate an effective built-in temperature difference (EBITD) through the slab, which represented the combined effects of nonlinear “built-in” temperature gradients, irreversible shrinkage, and creep. Differences in restraints (from adjacent slabs, shoulder, and base friction) and variability in material and structural properties resulted in wide variation in the measured EBITD. High EBITD values (−20 to −35°C) were observed for sections with low restraint, and low to moderate EBITD values (0 to −20°C) observed for sections with higher restraint.

Flexural Reinforcement of Glulam Timber Beams and Joints with Carbon Fiber-Reinforced Polymer Rods

Abstract: Fiber-reinforced polymer (FRP) composites have largely been used in combination with masonry and concrete structural elements in the last decade. Recent applications showed that new advantages may also be achieved in the field of timber structures, even if currently steel fasteners are used mainly in connecting systems. This study investigated the possibility of using carbon FRP (CFRP) rods as glued-in reinforcement of glulam beams and as glued-in connectors for glulam timber head joints that should transfer flexural moment between two adjacent beams. Half-scale beams were tested both with and without the presence of FRP reinforcement. Flexural behavior of CFRP-reinforced beams was compared with unreinforced beams that were used as control specimens. Two different amounts of CFRP reinforcement were used in the beam section. Experimental results showed a significant influence of the CFRP rods, because the reinforced beams demonstrated an increase in ultimate capacity and stiffness. Experimental results were also compared with numerical analysis, which showed good accordance with regard to the load and deflection values. Full-size head joints were prepared and tested. Flexural behavior of the joints was compared with the mechanical properties of monopiece beams that were used as reference specimens. Three different force transfer lengths were used for the construction of CFRP-timber joints. Experimental results showed that the use of CFRP rods in timber joints was successful, because the capacity of the CFRP-jointed beams was almost the same as that of the monolithic beams for the longest bond length that was adopted. This result is important in order to find an adequate alternative to traditional joints made with steel bolts and plates, which are unable to create rigid connections, increase dramatically the weight of timber structures, and may be subjected to corrosion in an aggressive environment. A numerical modeling based on the virtual work principle was also conducted and theoretical results were found in good accordance with the experimental results for the tested joint.

High‐Temperature Stability of the Al2O3‐LaPO4 System

Abstract: The compatibility of Al2O3 and LaPO4 at temperatures up to 1600°C is examined. Provided the ratio of La to P was close to 1:1, no reactions were observed after 200 h at 1600°C. Moreover, the Al2O3/LaPO4 interface remained sufficiently weakly bonded to cause deflection of cracks, as reported previously. In the presence of excess P or La, reactions occurred as expected, forming AlPO4 in the case of excess P, and LaAlO3 and LaAl11O18 in the case of excess La.

Thermal post-buckling and flutter characteristics of composite plates embedded with shape memory alloy fibers

Abstract: It is investigated that the composite plate embedded with shape memory alloy (SMA) fibers is subject to the aerodynamic and thermal loading in the supersonic region. The nonlinear finite element equations based on the first-order shear deformation plate theory (FSDT) are formulated for the laminated composite plate embedded with SMA fibers (SMA composite plate). The von Karman strain–displacement relation is used to account for the large deflection. The incremental method considering the influence of the initial deflections and initial stresses is adopted for the temperature-dependent material properties of SMA fibers and composite matrix. The first-order piston theory is used for modeling aerodynamic loads. This study shows the effect of the SMA on the critical temperature, thermal post-buckling deflection, natural frequency and critical dynamic pressure of the SMA composite plate.