Добірка наукової літератури з теми "Flexural Harmonic Probes"

The dynamic mode atomic force microscope (AFM) is a versatile tool that uses a resonantly excited micro-cantilever probe to obtain a sample’s topography and to characterize its material properties. While a number of useful techniques have been developed for interacting with the sample, conventional AFM probes and AFM systems do not facilitate their effective implementation. This thesis investigates the design of specialized AFM probes and the development of a novel probing system that improves the speed of imaging and enhances the sensitivity to material properties in dynamic mode AFM. In order to perform high speed imaging, an integrated high-bandwidth magnetic actuation system, comprising a special probe and an actuator, is designed and developed. Subsequently, the actuation system is fabricated and evaluated using an in-house developed measurement system to possess an eigen-frequency of 104 kHz and a range of 225 nm. In aqueous medium, the probe is shown to suffer 3 times lesser reduction in eigen-frequency compared to a conventional probe of similar eigen-frequency in air. In order to achieve enhanced sensitivity to material properties, a systematic approach is proposed to design and fabricate probes with specified eigen-frequencies. The proposed approach is employed to design and develop a flexural harmonic probe with eigen-frequencies in the ratio 1:2 and a torsional harmonic probe with eigen-frequencies in the ratio 1:2:3. The experimentally evaluated eigen-frequency ratios are shown to match the specifications to within 0.4% and 2% respectively. Further, harmonic probes with exchangeable tips are proposed and a prototype probe is fabricated and evaluated. The developed harmonic probes are shown to be significantly more sensitive to tip-sample forces. To effectively exploit the high speed and sensitivity of the developed probes, a custom AFM system is designed and developed in-house. The AFM includes a novel Z-magnetic actuation system having bandwidth in excess of 3 MHz and an XY nano-positioning system suitable for video-rate imaging. A novel measurement system based on optical beam deflection is developed and evaluated to measure XY-motion of the positioner. High speed control hardware based on Field Programmable Gate Array (FPGA) has been used for data acquisition and real-time control, with update rates of more than 5 MHz. The developed system is demonstrated to enhance the positioning bandwidth of the high-speed AFM probe, and subsequently employed in high-speed dynamic mode AFM imaging at rates upto 1.25 frames/second. Finally, the potential of the developed system for video-rate dynamic mode AFM imaging, and in-situ material characterization is discussed.

Abstract When a laminated sandwich beam consisting of alternating elastic and viscoelastic layers bends in the plane normal to its plane of lamination, the relatively compliant viscoelastic layers are sheared between the adjoining elastic layers and energy is dissipated. This mechanism, known as constrained-layer or shear damping, is a proven method for damping flexural vibration, but only in the plane normal to the plane of lamination. For vibration parallel to the plane of lamination, the elastic layers need not deflect together, and the viscoelastic layers are sheared when adjoining elastic layers undergo different deflections. This mechanism of damping is discussed herein. In this paper, we study the vibration of a symmetrically laminated five-layer sandwich beam in its plane of lamination. Taking the viscoelastic material to have frequency-independent hysteretic damping, we derive a boundary-value problem governing steady harmonic motion of the sandwich beam and then obtain its discretized counterpart. For the particular case of a beam with simply supported elastic layers, we obtain the resonant frequencies and loss factors of the composite beam, and study their dependence on geometric and material properties.

Flexure pivot based oscillators can advantageously replace the hairspring and balance wheel, the time base used in mechanical watches, by drastically reducing friction. However, flexure pivots have drawbacks including gravity sensitivity and restoring torque nonlinearity. In previous work, we introduced a novel gravity insensitive flexure pivot (GIFP) to solve the problem of gravity sensitivty, but no analytical formulation for the restoring torque nonlinearity was found. In this paper, we use numerical simulation to find an empirical expression for restoring torque nonlinearity. We use this expression to find an analytical formula for the rotational stiffness of GIFP. This formula gives an explicit relationship between restoring torque nonlinearity and the isochronism of the corresponding harmonic oscillator. The results also apply to the widely used generalized cross-spring pivot.

The three-dimensional motions of a clamped-free, inextensible beam subject to lateral harmonic excitation are investigated in this paper. Special attention is given to the nonlinear oscillations of beams with low torsional stiffness and its influence on the bifurcations and instabilities of the structure, a problem not tackled in the previous literature on this subject. For this, the nonlinear integro-differential equations describing the flexural-flexural-torsional couplings of the beam are used, together with the Galerkin method, to obtain a set of discretized equations of motion, which are in turn solved by numerical integration using the Runge-Kutta method. Both inertial and geometric nonlinearities are considered in the present analysis. By varying the beam stiffness parameters, and using several tools of nonlinear dynamics, a complex dynamic behavior of the beam is observed near the region where a 1:1:1 internal resonance occurs. In this region several bifurcations leading to multiple coexisting solutions, including planar and nonplanar motions are obtained. Finally, the paper shows how the tools of nonlinear dynamics can help in the understanding of the global integrity of the model, thus leading to a safe design.

High levels of acoustic energy can be produced at the downstream of pressure-reducing valves, pressure safety valves and control valves in piping systems. The presence of acoustic pressure waves and their coupling with the piping wall flexural modes of vibration can result in high levels of dynamic stresses, which cause acoustic fatigue failure at points of discontinuities on the pipe wall. This work presents a procedure for the assessment of the acoustic fatigue of different piping components by the application of finite element analysis. The piping system process data is used to generate the acoustic pressure and acoustic power spectrum at the downstream of the pressure-reducing valve. This acoustic power spectrum is taken to act at a finite element model of the piping system. Dynamic analysis, by use of power spectrum and harmonic analysis, is performed to obtain the response dynamic stresses, which are used for the fatigue evaluation of the piping component. The methodology presented can be applied during the engineering phase in the design and stress analysis of piping components in critical services subjected to acoustic fatigue as well as in the detailed evaluation of the different proposed acoustic fatigue design solutions.

In this paper, we study the fluid-structure interaction problem of the harmonic oscillations of a flanged lamina in a quiescent, Newtonian, viscous fluid. Here, the flanges are introduced to elicit specific vortex-structure interactions, with the ultimate goal of modulating the nonlinear hydrodynamic damping experienced by the oscillating structure. The hydrodynamic forcing, incorporating added mass and hydrodynamic damping effects, is evaluated through boundary element method and computational fluid dynamics simulations. This allows to identify a model for the hydrodynamic forces in the form of a complex-valued function of three nondimensional parameters, describing oscillation frequency and amplitude and flange size. We find that the presence of the flanges results into larger fluid entrainment during the lamina oscillation, thus affecting the added mass. Further, we highlight the existence of a minimum in the hydrodynamic damping which is governed by complex dynamics of vortex-structure interaction. This peculiar phenomenon is discussed from physical grounds by analysis of the pertinent hydrodynamic fields. Finally, we propose a tractable form for the hydrodynamic function, to be used in the study of large amplitude underwater flexural vibrations of flanged structures.

Continuous current action on risers develops vortices. These vortices cause risers to respond in a dynamic manner, leading to relatively large oscillations in the flexural and circumferential stresses. Vortex-induced vibration (VIV) and the resulting changes in the level of stresses in a riser could therefore cause detrimental fatigue induced problems, ranging from leakage to catastrophic failure of the riser. As a result, the VIV induced fatigue has always been a critical parameter in the design of risers, particularly when considering risers in deep waters. An experimental investigation into the fatigue crack propagation was conducted using a series of field data obtained on the high mode vibration of a relatively long flexible model riser. The field test data clearly illustrated the variable amplitude nature of the loading imposed on the riser by the current profile. There are currently several methods available for consideration of the crack growth rate due to overload and/or underloads; however, none has been developed based on data obtained for an actual riser. Indeed, fatigue experimental data on risers subject to VIV are relatively scarce in the literature. In this study the influence of the different harmonics resulting from VIV on the overall fatigue damage of the material is investigated. It is shown that the available basic approaches for assessing the fatigue crack propagation of components under variable amplitude loadings may significantly underestimate the VIV-induced fatigue damage of risers.

Abstract This work analyses the mooring and power cable dynamics in large-scale experimental tests carried out in the wave-current-tsunami flume (COCOTSU) facility at IHCantabria. The analysis is based on scaled elastic string models for a single chain-nylon mooring line and the dynamic cable of a 15MW floating offshore wind turbine (FOWT) supported by a concrete semi-submersible platform (ActiveFloat) in Gran Canaria Island (Spain). Both scaled concepts in the 100 m deep site are developed within the framework of the project COREWIND. All the test campaign is planned to be fully monitored; hence two overlapped video cameras register the line kinematics while the tensions are recorded in its two extreme points. The most difficult characteristic to fix in an elastic material at laboratory scale is the combined reproduction of axial and bending stiffness. On the one hand, to replicate the real axial stiffness in a chain-nylon mooring line, the first problem lies in finding a material capable of doing it with an acceptable hysteresis. The second issue consists in knowing the axial stiffness of the selected elastic material for each imposed oscillation, as it depends on the loading velocity. On the other hand, the limiting mechanical characteristic of the lazy-wave cable is the bending stiffness. To make the bending stiffness tests, we have previously built our own three-point bending flexural tester. 25 materials have been characterised to identify the best one for testing. From this analysis a database of materials has been derived. For each of the configurations analysed, a set of 40 forced oscillation tests are planned to reproduce either mooring fairlead or power cable connector movements in surge. An initial tension-deformation test is followed by 28 combinations of harmonic excitations with two origins, two amplitudes and seven periods, and 11 irregular time series obtained from the resulting surge displacements of the platform when simulating in OpenFAST extreme and severe Design Load Cases 1.3, 1.6 and 6.1. Experimental data obtained are stored in an online repository to make it freely available to the wind energy sector. The ambitious reduced scale tests proposed provide enough cases to deliver a benchmarking database for numerical models calibration including forces at anchor and fairlead, as well as line shape.