Constrained layer damping (CLD) for three layer beams with viscoelastic cores and aluminum or graphite/epoxy composite laminate outer layers was investigated for model comparison, sensitivity to design parameters, and evaluation of experimental loss factor estimation techniques. Model comparison for damping estimation and resonant frequency prediction was performed between finite element analysis (FEA) Ross, Kerwin, and Ungar theory (RKU), developed moment predicted loss factor, and experimental results. Investigated design parameters include; treatment application length and placement, relative thickness of core and constraining layers to base layer, core loss factor, and boundary conditions (free/free, fixed/fixed, and cantilever). Experimental damping estimation techniques evaluated include; frequency response function (FRF) based methods of component analysis, circle-fit method a curve-fit algorithm developed by Han [30] and the time domain of log decrement. Model comparison of finite elements to experimental results showed good trend prediction correlation. Only fair prediction of absolute loss factors was achieved, possibly due to the difficulty in characterization of viscoelastic properties. Design parameters analysis showed that treatment application length and placement were critical to effective added damping. In one case, for the same amount of damping material, the effective added damping of a well designed application was seven times greater than that of a poorly considered one. The effectiveness of treatment on a region appears to be strongly related to the magnitude on the moment acting on that region. Parameter analysis also showed that although a symmetric beam realizes the highest damping, in most cases near optimal damping can be obtained with constraining layer one half as thick as the base layer. Experimental methods for damping estimation showed the simple FRF component analysis to be consistent with the other methods (experimental and FEA) and the most computationally efficient. / Master of Science

Significant effort has been made to quantify and predict roll damping of vessels in the past. Similarly, efforts have been made to provide effective methods for calculating the roll gyradius of vessels. Both the damping and the gyradius of a vessel are traditionally quantified through the use of a sally test. Experience with the USS Midway showed that shallow water has significant effect on the rolling period and thus the experimentally determined roll gyradius. To date, little effort has been directed to the problem of the effect of shallow water on roll damping and roll period except when trying to match model and full scale experimental data. No clear guidelines exist for the boundary between deep and shallow water or the amount of overprediction of roll period that is likely for a given water depth. In order to provide greater understanding of the effects of shallow water on roll period and roll damping, this thesis performed experiments in varying scale water depths for 5 models: 4 box barges and a model of the USS Essex. The following conclusions were reached: As water depth to draft ratio, d/T, approaches 1 the roll period can increase as much as 14%. The boundary between deep and shallow water is a water depth somewhere between 4 and 7 times the vessel draft depending on the particulars of the vessel's hull form. Vessels with a larger beam to draft ratio will experience shallow water effects in relatively deeper water, that is to say the depth to draft ratio will be greater at the upper limit of deep water. Additionally, vessels with a higher beam to draft ratio will experience larger shallow water effects for a given depth to draft ratio. Finally, for vessels of very fine hull forms, the boundary between deep and shallow water will occur a relatively shallower depths, in other terms, the boundary will occur at a lower depth to draft ratio. / Master of Science

Much interest in mechanically harvesting apples has been generated, but no completely successful system has yet been developed. Bruising of the fruit has been a prime drawback to current mechanical harvesting methods. One practical approach to mechanized apple harvesting is to shake the trunk or limb of the tree to remove the fruit. With this method, apple detachment appears to be more dependent on length of stroke rather than shaking frequency. Therefore, a variable stroke shaker appeared to be a reasonable approach to a controlled rate of detachment from the tree. The objective of this project was to develop a variable stroke tree shaking mechanism. The variable stroke feature was attained with the use of a hydraulic cylinder and metering valve in the boom of the shaker with the cylinder acting as a variable rate dashpot. The design was tested in the laboratory under varying conditions of effective limb mass and shaking frequency. For comparison, a constant stroke shaker was subjected to similar tests. Both shaking mechanisms were subjected to a theoretical analysis. For the constant stroke shaker, the theoretical and test displacements of the limb were in good agreement, but the theoretical and test limb varied widely. In the variable stroke shaker, the coefficient of damping of the cylinder was determined from test work and checked, using the differential equations for the system. The testing performed in this investigation was limited to the laboratory, but for these conditions an effective variable stroke mechanism was developed. / Master of Science

An optimization algorithm has been developed to study the effectiveness of substituting time tested ODEs with FDEs as applied to ship motions, specifically with an eye toward modeling different forms of roll damping. Relations between the order of differentiation a and damping coefficient b in the FDEs have been drawn for changing damping, added moment of inertia, and initial roll angle. A pitch model has also been studied and compared to the roll model. The error at each of these a and b pairs has also been calculated using an L2-norm. An initial effort was made to correlate the FDE coefficients to differing mechanisms of roll damping as characterized by Himeno. / Master of Science

Ship motions in a seaway have long been of great importance, and today with advanced hull forms and higher speeds they are as important as ever. While one can now often adequately predict heave, pitch, sway, yaw and even surge, roll motions are much more difficult. Roll is the one motion that is very dependent upon viscous effects of the fluid. Recently, at David Taylor Model Basin, there have been model experiments where the bilge keels were instrumented in order to directly measure their damping force upon the vessel. To build upon this work and to validate it when applied to full scale vessels, a trial using the Italian naval vessel Nave Bettica was performed. The objective of this thesis is to describe the experiment, present and analyze the results, and offer some conclusions based upon these results. The process of instrumenting the port bilge keel using strain gages and correlating their output to pressures and total forces is described. Selected results for different forward speeds are presented, with full results in the appendices. Particle image velocimetry (PIV) was also performed during the test and was used to measure the flow field in a three foot by three foot area under the aft end of the same bilge keel. Selected image series are presented, as is a methodology for using these images to calculate the center of pressure and the corresponding results. / Master of Science

A non-intrusive direct-measuring skin friction device with a rubber RTV sheet over the surface of the floating head, gap and housing was developed for application in 3D, unsteady, transonic flight conditions. Design conditions required optimum gage performance at altitudes ranging from 15,000 to 45,000 feet, Mach numbers ranging from 0.6 to 0.99 resulting in shear values of 0.3 to 1.5 psf. under vibration conditions up to 8.0 grms over a 15 - 2,000 Hz frequency range. The gage consisted of a rubber RTV sheet-coated floating element attached to an aluminum cantilevered beam. A dual-axis, full bridge strain gage configuration was used with the application of semi-conductor strain gages to increase instrument sensitivity. The gage was studied with and without a viscous liquid (glycerin) fill in the housing. Vibration verification testing was performed at 1.0 grms in the Virginia Tech modal analysis lab to ensure adequate damping performance over a 0−3200 Hz frequency range. Tests revealed that the rubber RTV compound sheet provided adequate viscoelastic damping, with or without viscous liquid fill. Gage performance verification testing was performed on in the Virginia Tech supersonic wind tunnel at shear levels of tw = 3.9 to 5.3 psf in a Mach 2.4 flow. Skin friction values in good agreement with previous testing and analytical predictions were obtained from the tests with adequate damping in the low vibration environment of the Virginia Tech supersonic wind tunnel. The gage proved robust as it survived repeated runs including the violent start and unstart processes typical of a supersonic, blowdown wind tunnel. Flight tests were performed at NASA Dryden Flight Research Center, with the gage mounted in a plate suspended below an F-15 aircraft. This provided a mildly 3D, turbulent boundary layer on a vibrating surface. The gage was tested without liquid fill in the gage cavity, and it performed satisfactorily in this high vibration environment. The gage demonstrated adequate damping and good robustness, surviving the complete flight test intact and remained fully operational. The sensor measured skin friction values 30%-50% higher than those predicted by indirect methods and analogies generally valid for 2D, steady flows. The gage indicated trends in skin friction values for different flight conditions in good agreement with the other methods. Possible reasons for the differences in numerical values are discussed in detail, including potential uncertainties in the gage output and limitations and uncertainties in the methods used for comparison. Finally, suggestions for further development of such gages are provided for flight test applications. / Master of Science

The main purpose of this report was to test several common viscoelastic polymers and identify key attributes of their applicability to a small aircraft landing gear system for improved damping performance. The applied viscoelastic damping treatment to the gear was of a constrained layer type, promoting increased shear deformation over free surface treatments, and therefore enhanced energy dissipation within the viscoelastic layer. A total of eight materials were tested and analyzed using cyclic loading equipment to establish approximate storage modulus and loss factor data at varying loading frequencies. The three viscoelastic polymers having the highest loss factor to shear modulus ratio were chosen and tested using a cantilever beam system. A Ross, Kerwin, and Ungar analysis was used to predict the loss factor of the cantilever beam system with applied treatment and the predictions were compared to experimental data. Customer requirements often govern the scope and intensity of design in many engineering applications. Limitations and constraints, such as cost, weight, serviceability, landing gear geometry, environmental factors, and manufacturability in regards to the addition of a viscoelastic damping treatment to a landing gear system are discussed. Based on results found from theoretical and experimental testing, application of a damping treatment to a small aircraft landing gear system is very promising. Relatively high loss factors were seen in a cantilever beam for simple single layer constrained treatments for very low strain amplitudes relative to strains seen during loading of the landing gear. With future design iterations, damping levels several times those seen in this document will be seen with a constrained treatment applied to a landing gear system. / Master of Science

This thesis investigates the damping effects of a metamaterial, on structural vibration, by inducing periodic voids in the base damping material as opposed to infusing the damping material with other material. Metamaterials have been used previously to improve the damping of vibrational waves and acoustic waves through wave scattering and wave reflection at periodic impedance changes. Impedance changes can occur at both material boundaries and geometric changes of the medium. Impedance changes cause wave scattering, wave reflection, and changing of wave speed. The low frequency region of the vibration spectrum is generally harder to dampen due to the longer wavelengths. By slowing the waves down, the wavelength can be shortened and the viscoelastic material will be more effective at damping the waves. The metamaterial in the thesis has one, two, three, and four periodically located voids in the viscoelastic damping material to determine the effectiveness of the damping compared to the same beam with no damping material applied and the beam covered completely with the standard viscoelastic damping material. This research will include both finite element models of the beam and concept testing to explore the damping effects of the metamaterial. / Master of Science / In the field of mechanical engineering vibrations are one of the main causes of failure of machinery components. Reducing vibrations greatly effects the longevity and effectiveness of a machine. The research in this thesis focuses on how to reduce the vibration in a beam by using a metamaterial. Standard damping materials provide damping, reduction of vibration, at various quantities depending on the frequency and wavelength of the vibrational wave. Metamaterials are particular materials designed to reduce vibration by influencing the physical phenomena of a wave as it travels through the material usually by periodic wave scatters. The metamaterial in this research is designed to slow the flexural waves down, therefore shortening the wavelength, making it easier to dampen the vibration compared to a standard damping material. The damping effectiveness of the metamaterials explored in this research will be quantified via finite element modeling and testing in a laboratory.

This thesis investigates passive damping system performance in heavy vehicles through analytical expressions, simulations with different vehicle models as well as through experimental evaluation in a tractor semi trailer combination. The objective is to study what levels of chassis suspension damping that are desirable for different vehicle modes and how this may be achieved with passive damping systems. To investigate the influence on performance from damper positioning, analytical expressions for a 2D - suspension model are derived. Geometric key parameters controlling roll and bounce damping are found to be damper vertical aligning and perpendicular distance between damper and suspension roll centre respectively. These parameters are often not easily altered within an already existing vehicle. To investigate performance possibilities from damping not restricted by packaging requirements, the concept with distributed damping is furthermore studied. Theoretical expressions for modally distributed damping are first derived from an analytical tractor model with 7 DOF. Considered motions for which damping is prescribed are bounce, pitch and roll of sprung mass, and axle crossing. These equations are evaluated through various simulations with a 4x2-tractor semi trailer model. Results from simulations show that the conflict in damping demands with passive independent dampers for a single lane change and a one-sided pot hole may be significantly reduced with amplitude dependent modal damping. Vehicle damping performance is not only affected by the dampers positioning and their individual setting, but also by the damper attachment structure. The influence from compliance in e.g. brackets and mounting bushings at damper attachment points is therefore studied. Linear analysis with a simple spring mass damper model shows that damper attachment compliance reduces the damper efficiency. Finite element analyses of both the chassis frame and the tractor are furthermore performed to obtain numerical values of front-axle damper-attachment stiffness. The effect from damper-attachment stiffness is quantified though simulations with a tractor semi trailer model. Simulation results show that it is important to consider the attachment stiffness during vehicle manoeuvres containing high frequency inputs such as the passage over a plank. A methodology and equations for prescribing chassis suspension damping as function of general vehicle modes by using electronically controlled variable dampers is presented. A critical input for such implemented modal damping systems are the real time estimation of modal motions necessary for force calculation. From performed simulations it is shown that geometric calculations of modal velocities based solely on relative damper displacements contain significant discrepancies to actual motion for transient road inputs. To overcome this, a time-domain system identification approach is presented, where models that estimate modal coordinate velocities with considerably higher accuracy are identified. The proposed modal damping approach is implemented on a 4x2 tractor and experimentally evaluated through various road tests. It is shown that the system has the desired ability to control sprung mass bounce and pitch modes separately and that it improves vehicle performance on all tested load cases. / QC 20100830

With advances in material technology, allowing, for instance, engines to withstand higher combustion pressure and consequently improving performance, comes challenges to productivity. These materials are, in fact, more difficult to machine with regards to tool wear and especially machine tool stability. Machining vibrations have historically been one of the major limitations to productivity and product quality and the cost of machining vibration for cylinder head manufacturing has been estimated at 0.35 euro per part. The literature review shows that most of the research on cutting stability has been concentrating on the use of the stability limits diagram (SLD), addressing the limitations of this approach. On the other hand, research dedicated to development of machine tool components designed for chatter avoidance has been concentrating solely on one component at the time. This thesis proposes therefore to extend the stability limits of the machining system by enhancing the structure’s damping capability via a unified concept based on the distribution of damping within the machining system exploiting the joints composing the machine tool structure. The design solution proposed is based on the enhancement of damping of joint through the exploitation of viscoelastic polymers’ damping properties consciously designed as High Damping Interfaces (HDI). The tool-turret joint and the turret-lathe joint have been analysed. The computational models for dimensioning the HDI’s within these joints are presented in the thesis and validated by the experiments. The models offer the possibility of consciously design damping in the machining system structure and balance it with regards to the needed stiffness. These models and the experimental results demonstrate that the approach of enhancing joint damping is viable and effective. The unified concept of the full chain of redesigned components enables the generation of the lowest surface roughness over the whole range of tested cutting parameters. The improved machining system is not affected by instability at any of the tested cutting parameters and offers an outstanding surface quality. The major scientific contribution of this thesis is therefore represented by the proposed unified concept for designing damping in a machining system alongside the models for computation and optimisation of the HDIs. From the industrial application point of view, the presented approach allows the end user to select the most suitable parameters in terms of productivity as the enhanced machine tool system becomes less sensitive to stability issues provoked by difficult-to-machine materials or fluctuations of the work material properties that may occur in ordinary production processes. / <p>QC 20120413</p> / DampComat / Production 4 micro / FFI Robust Machining