Showing papers in "Shock and Vibration in 2003"

Influence of Material Properties on the Ballistic Performance of Ceramics for Personal Body Armour

Abstract: In support of improved personal armour development, depth of penetration tests have been conducted on four different ceramic materials including alumina, modified alumina, silicon carbide and boron carbide. These experiments consisted of impacting ceramic tiles bonded to aluminum cylinders with 0.50 caliber armour piercing projectiles. The results are presented in terms of ballistic efficiency, and the validity of using ballistic efficiency as a measure of ceramic performance was examined. In addition, the correlation between ballistic performance and ceramic material properties, such as elastic modulus, hardness, spall strength and Hugoniot Elastic Limit, has been considered.

Enhanced Piezoelectric Shunt Design

Abstract: Piezoceramic material connected to an electronic shunt branch circuit has formed a successful vibration reduction device. One drawback of the conventional electronic shunt circuit is the large inductance required when suppressing low frequency vibration modes. Also, the large internal resistance associated with this large inductance exceeds the optimal design resistance needed for low frequency vibration suppression. To solve this problem, a modified and enhanced piezoelectric shunt circuit is designed and analyzed by using mechanical-electrical analogies to present the physical interpretation. The enhanced shunt circuit developed in this paper is proved to significantly reduce the targeted vibration mode of a cantilever beam, theoretically and experimentally.

A Hybrid Control Policy for Semi-Active Vehicle Suspensions

Abstract: Various control policies, such as skyhook and groundhook control, have often been considered for semi-active vehicle suspensions Past studies have shown the performance limitations of these policies, as well as others that have been considered for vehicle applications This study will provide a look into an alternative control technique called "hybrid control", which attempts to merge the performance benefits of skyhook and groundhook control The results of this study are based on an experimental evaluation of hybrid control using a quarter-car rig and a magneto-rheological damper The control policy is employed and evaluated under a steady-state or pure tone input, and a transient or step input Peak-to-peak displacement and peak-to-peak acceleration are used to evaluate performance The results indicate that hybrid control can offer benefits to both the sprung mass and the unsprung mass The steady-state results reveal that hybrid control can be used to reduce the peak-to- peak displacements and accelerations of both bodies The transient evaluation shows that hybrid control can be effective at reducing the peak-to-peak displacement of the sprung mass

Effects of superstructure flexibility on the response of base-isolated structures

Abstract: A parametric study of base-isolated structure with different isolation systems is conducted for investigating the effects of superstructure flexibility. The superstructure is idealized as a one-storey structure isolated by different systems such as elastomeric bearings (with and without lead core) and sliding systems. The governing equations of motion of the isolated structural system are derived and the response of the system is obtained for stochastic model of earthquake ground motion. The earthquake ground motion is modeled as a uniformly modulated non-stationary random process. The stochastic response of isolated structure is obtained using the state variable approach. An equivalent linearization technique is used for the approximate response of isolated structure with non-linear isolation systems. The mean square superstructure acceleration and bearing displacement of the system are plotted under different system parameters and compared with the corresponding response of rigid sup erstructure condition to study the influence of superstructure flexibility. The comparison of response is made under various isolation system parameters (i.e. isolation period, damping, yield strength of elastomeric bearings and friction coefficient of sliding systems). It is observed that the bearing displacement can accurately obtained by modelling the superstructure as a rigid body in a base-isolated structure. However, such approximation may under-estimates the superstructure acceleration under certain conditions.

Experiments on Optimal Vibration Control of a Flexible Beam Containing Piezoelectric Sensors and Actuators

Abstract: In this paper, a digital regulator is designed and experimentally implemented for a flexible beam type structure containing piezoelectric sensors and actuators by using optimal control design techniques. The controller consists of a linear quadratic regulator with a state estimator, namely a Kalman observer. The structure is a cantilever beam containing a set of sensor/actuator PVDF/PZT ceramic piezoelectric patches bonded to the beam surface at the optimal location obtained for the first three vibration modes. The equations of motion of the beam are developed by using the assumed modes technique for flexible structures in infinite-dimensional models. This paper uses a method of minimizing the effect of the removed higher order modes on the low frequency dynamics of the truncated model by adding a zero frequency term to the low order model of the system. A measure of the controllability and observability of the system based on the modal cost function for flexible structures containing piezoelectric elements (intelligent structures) is used. The observability and controllability measures are determined especially to guide the placement of sensors and actuators, respectively. The experimental and numerical transfer functions are adjusted by using an optimization procedure. Experimental results illustrate the optimal control design of a cantilever beam structure.

Dynamic Analysis of Shell Structures with Application to Blast Resistant Doors

Abstract: This paper concerns the dynamic analysis of shell structures, with emphasis on application to steel and steel-concrete composite blast resistant doors. In view of the short duration and impulsive nature of the blast loading, an explicit integration method is adopted. This approach avoids time-consuming computations of structural stiffness matrix and solving of simultaneous nonlinear equations. Single-point quadrature shell elements are used, with numerical control to suppress spurious hourglass modes. Composite shells are handled by an appropriate integration rule across the thickness. Both material and geometric nonlinearities are accounted for in the formulation. Contact and gap problems are considered using bilinear spring elements in the finite element analysis. Numerical examples are presented for some benchmark problems and application study to blast resistant doors. Good correlation is generally obtained between the numerical results based on the software developed and the results obtained by other means including field blast tests.

Ruggedizing Printed Circuit Boards Using a Wideband Dynamic Absorber

Abstract: The existing approaches to ruggedizing inherently fragile and sensitive critical components of electronic equipment such as printed circuit boards (PCB) for use in hostile industrial and military environment are either insufficient or expensive This paper addresses a novel approach towards ruggedizing commercial-off-the-shelf PCBs using a miniature wideband dynamic absorber The optimisation technique used relies on the experimentally measured vibration spectra and complex receptance of the original PCB The analytical study and full-scale experiment show that an optimised wideband dynamic absorber with the mass of only 15% relative to the total mass of the PCB is quite capable of the essential suppression of all relevant resonant responses of the PCB under shock, wideband random and sinusoidal vibration with variable frequency

Control of rotary cranes using fuzzy logic

Abstract: Rotary cranes (tower cranes) are common industrial structures that are used in building construction, factories, and harbors. These cranes are usually operated manually. With the size of these cranes becoming larger and the motion expected to be faster, the process of controlling them has become difficult without using automatic control methods. In general, the movement of cranes has no prescribed path. Cranes have to be run under different operating conditions, which makes closed-loop control attractive.

Evaluating Noise Sensitivity on the Time Series Determination of Lyapunov Exponents Applied to the Nonlinear Pendulum

Abstract: This contribution presents an investigation on noise sensitivity of some of the most disseminated techniques employed to estimate Lyapunov exponents from time series. Since noise contamination is unavoidable in cases of data acquisition, it is important to recognize techniques that could be employed for a correct identification of chaos. State space reconstruction and the determination of Lyapunov exponents are carried out to investigate the response of a nonlinear pendulum. Signals are generated by numerical integration of the mathematical model, selecting a single variable of the system as a time series. In order to simulate experimental data sets, a random noise is introduced in the signal. Basically, the analyses of periodic and chaotic motions are carried out. Results obtained from mathematical model are compared with the one obtained from time series analysis, evaluating noise sensitivity. This procedure allows the identification of the best techniques to be employed in the analysis of experimental data.

Prediction of fatigue life of a continuous bridge girder based on vehicle induced stress history

Abstract: The fatigue damage assessment of bridge components by conducting a full scale fatigue testing is often prohibitive. A need, therefore, exists to estimate the fatigue damage in bridge components by a simulation of bridge-vehicle interaction dynamics due to the action of the actual traffic. In the present paper, a systematic method has been outlined to find the fatigue damage in the continuous bridge girder based on stress range frequency histogram and fatigue strength parameters of the bridge materials. Vehicle induced time history of maximum flexural stresses has been obtained by Monte Carlo simulation process and utilized to develop the stress range frequency histogram taking into consideration of the annual traffic volume. The linear damage accumulation theory is then applied to calculate cumulative damage index and fatigue life of the bridge. Effect of the bridge span, pavement condition, increase of vehicle operating speed, weight and suspension characteristics on fatigue life of the bridge have been examined.

Measurement of Naval Ship Responses to Underwater Explosion Shock Loadings

Abstract: The shock-resistance capability of battle ships against a non-contact underwater explosion (UNDEX) is a very critical factor of survivability. In July 1987 and April 2000, we successfully conducted UNDEX shock tests for a coastal mine hunter (MHC) and a mine sweeper/hunter (MSH) of Republic of Korea Navy (ROKN), at the Chinhae bay, Korea. Test planning for conducting these shock tests included responsibilities, methods, and procedures. Test instruments were developed and tested on a drop shock machine to confirm availability in the actual shock tests with emphasis on shock resistance, remote control and reliability. All vital systems of the ships were confirmed to be capable of normal operational condition without significant damages during the explosion shot. By analyzing the test results, the tactical operational safety zone of the ships in underwater explosion environments was estimated. In this paper, we described the results of measurement of naval ship responses to underwater explosion shock loadings including test planning, sensor locations, data reduction, explosive devices, instrumentation and damage assessments of MSH.

Vibrational energy flow models of finite orthotropic plates

Abstract: In this paper energy flow models for the transverse vibration of finite orthotropic plates are developed. These models are expressed with time- and locally space-averaged far-field energy density, and show more general forms than the conventional EFA models for isotropic plates. To verify the accuracy of the developed models, numerical analyses are performed for finite rectangular plates vibrating at a single frequency, and the calculated results expressed with the energy and intensity levels are compared with those of classical models by changing the frequency and the damping.

Laser-Generated Shocks and Bubbles as Laboratory-Scale Models of Underwater Explosions

Abstract: Underwater shocks and bubbles were generated using a high energy pulsed laser system. The advantages of this experimental approach are: (1) precisely controlled and measured experimental conditions; (2) improved diagnostics, including extensive imaging capabilities; (3) unique experiments, including a simultaneously detonated line charge; and (4) the ability to provide validation quality data for hydrodynamic simulation codes. Bubble sensitivity to variation of several experimental parameters was examined. Numerical simulations were performed corresponding to the experimental shots, showing that empirical bubble theory, experimental bubble data, and simulations were all in good agreement.

Dynamic Viscoelastic Effects on Sound Wave Diffraction by Spherical and Cylindrical Shells Submerged in and Filled with Viscous Compressible Fluids

Abstract: An analysis for sound scattering by fluid-filled spherical and cylindrical viscoelastic shells immersed in viscous fluids is outlined. The dynamic viscoelastic properties of the scatterer and the viscosity of the surrounding and core fluids are rigorously taken into account in the solution of the acoustic scattering problem. The novel features of Havriliak-Negami model for viscoelastic material dynamic behaviour description along with the appropriate wave-harmonic field expansions and the pertinent boundary conditions are employed to develop a closed-form solution in form of infinite series. Subsequently, the associated acoustic field quantities such as the scattered far-field pressure directivity pattern, form function amplitude, transmitted intensity ratio, and acoustic force magnitude are evaluated for given sets of medium physical properties. Numerical results clearly indicate that in addition to the traditional fluid viscosity-related mechanisms, the dynamic viscoelastic properties of the shell material as well as its thickness can be of major significance in sound scattering. Limiting cases are examined and fair agreements with well-known solutions are established.

An improved wavelet correction for zero shifted accelerometer data

Abstract: Accelerometer data from shock measurements often contains a spurious DC drifting phenomenon known as zero shifting. This erroneous signal can be caused by a variety of sources. The most conservative approach when dealing with such data is to discard it and collect a different set with steps taken to prevent the zero shifting. This approach is rarely practical, however. The test article may have been destroyed or it may be impossible or prohibitively costly to recreate the test. A method has been proposed by which wavelets may be used to correct the acceleration data. By comparing the corrected accelerometer data to an independent measurement of the acceleration from a laser vibrometer this paper shows that the corrected data, in the cases presented, accurately represents the shock. A method is presented by which the analyst may accurately choose the wavelet correction parameters. The comparisons are made in the time and frequency domains, as well as with the shock response spectrum.

On antiresonance in the forced response of mistuned bladed disks

Abstract: Mistuning in bladed disks usually increases the forced response of the maximum responding blade leading to shortened component life in turbine engines. This paper investigates mistuning using a transfer function approach where the frequency response functions (FRFs) are described by natural frequencies and antiresonant frequencies. Using this approach, antiresonant frequencies are shown to be a critical factor in determining the maximum blade response. Two insights are gained by formulating antiresonant frequencies as the eigenvalues of reduced system matrices: 1) Mistuning a particular blade has no effect on that blade's antiresonant frequencies. 2) Engine orders N and N/2, where N is the number of blades on the disk, tend to produce the highest maximum local response. Numerical examples are given using a spring-mass-oscillator model of a bladed disk. Pole-zero loci of mistuned bladed disks show that increased maximum blade response is often due to the damping of antiresonant frequencies. An important conclusion is that antiresonant frequencies can be arranged such that a mistuned bladed disk has a lower maximum blade response than a tuned bladed disk.

How Can Spatially Distributed Uncertainties Be Included in FEA and in Parameter Estimation for Model Updating

Abstract: Owing to manufacturing composite materials and others show considerable uncertainties in wall-thickness, fluctuations in material properties and other parameter, which are spatially distributed over the structure. These uncertainties have a random character and can therefore not being reduced by some kind of mesh refinement within the FE model. What we need is a suitable statistical approach to describe the parameter changing that holds for the statistics of the process and the correlation between the parameter spatially distributed over the structure. The paper presents a solution for a spatial correlated simulation of parameter distribution owing to the manufacturing process or other causes that is suitable to be included in the FEA. The parameter estimation methods used in updating algorithms for FE-models, depend on the choice of a priori to be determined weighting matrices. The weighting matrices are in most cases assumed by engineering judgement of the analyst carrying out the updating procedure and his assessment of uncertainty of parameters chosen and measured and calculated results. With the statistical description of the spatial distribution at hand, we can calculate a parameter weighting matrix for a Baysian estimator. Furthermore, it can be shown in principle that with model updating it is possible to improve the probabilistic parameter distribution itself.

Coupled Bending-Bending-Torsion Vibration of a Pre-Twisted Beam with Aerofoil Cross-Section by the Finite Element Method

Abstract: The present study deals with a finite element model for coupled bending-bending-torsion vibration analysis of a pretwisted Timoshenko beam with varying aerofoil cross-section. The element derived in this paper has two nodes, with seven degrees of freedom at each node. The nodal variables are transverse displacements, cross-section rotations and the shear angles in two planes and torsional displacement. The advantage of the present element is the exclusion of unnecessary derivatives of fundamental nodal variables, which were included to obtain invertable square matrix by other researchers, by choosing proper displacement functions and using relationship between cross-sectional rotation and the shear deformation. Element stiffness and mass matrices are developed from strain and kinetic energy expressions by assigning proper order polynomial expressions for cross-section properties and considering higher order coupling coefficients. The correctness of the present model is confirmed by the experimental results available in the literature. Comparison of the proposed model results with those in the literature indicates that a faster convergence is obtained. The results presented also provide some insights in the formulation by clearly indicating that higher order coupling terms have considerable influence on the natural frequencies.

Approximate Potentials with Applications to Strongly Nonlinear Oscillators with Slowly Varying Parameters

Abstract: A method of approximate potential is presented for the study of certain kinds of strongly nonlinear oscillators. This method is to express the potential for an oscillatory system by a polynomial of degree four such that the leading approximation may be derived in terms of elliptic functions. The advantage of present method is that it is valid for relatively large oscillations. As an application, the elapsed time of periodic motion of a strongly nonlinear oscillator with slowly varying parameters is studied in detail. Comparisons are made with other methods to assess the accuracy of the present method.

Axisymmetric Natural Frequencies of Statically Loaded Annular Plates

Abstract: We present a numerical procedure to solve the axisymmetric vibration problem of statically loaded annular plates. We use the von K´´ nonlinear plate model to account for large deformations and study the effect of static deflections on the natural frequencies and mode shapes for six combinations of boundary conditions. The shooting method is used to solve the resulting eigenvalue problem. Our results show that static deformations have a significant effect on the natural frequencies and small effect on the mode shapes of the plate. Further, the results show that the presence of in-plane stresses has a significant effect on the natural frequencies.

High Strain Rate Characterization of Shock Absorbing Materials for Landmine Protection Concepts

Abstract: Numerical modelling of footwear to protect against anti-personnel landmines requires dynamic material properties in the appropriate strain rate regime to accurately simulate material response. Several materials (foamed metals, honeycombs and polymers) are used in existing protective boots, however published data at high strain rates is limited.

Dynamic stability of rotating blades with transverse cracks

Abstract: In this paper, the main objective is to examine the effects of transverse cracks on the dynamic instability regions of an axially loaded rotating blade. The blade is modeled as an Euler-Bernoulli beam. To reduce the governing equations to a set of ordinary differential equations in matrix form, Hamilton's principle is used in conjunction with the assumed-mode method. The crack is accounted for by considering the energy release rate and the parametric instability regions are obtained using Bolotin's first approximation. Benchmark results are presented for cracked rotating blades at different rotating speeds, crack lengths and crack positions.

A New Simulation Method Providing Shock Mount Selection Assurance

Abstract: This paper presents the development and assessment of a practical and efficient process for assessing and/or designing shock isolation systems This process combines practical methods for determining relative displacements and accelerations (eg shock response spectrum analysis) with a new, efficient, easy to use, 6 degree of freedom (6DOF) simulation method known as Shock Isolation Mount Predictions & Loading Estimates (SIMPLE) SIMPLE is also a tool that can easily account for uncertainties in isolated systems and their environments The implementation of 6DOF rigid body theory in SIMPLE is validated by comparing simulation results with other analytical methods This paper also summarizes assessments and designs for 60 different mounting systems using SIMPLE In addition, experimental results from shock tests are compared with pre-test SIMPLE sensitivity simulations and with results of post-test model calibrations These comparisons show the validity of: 1) using 6DOF analysis; 2) using statically derived load-deflection data for simulations; and 3) assessing and designing isolated systems using uncertainties in model parameters

A reduced-domain method of structural damage identification: Application to a spectral element beam model

Abstract: Though there have been many efforts to make the inverse problem of damage identification small by reducing its finite element degrees-of-freedom, there have been few efforts to make it small by reducing its spatial domain of problem. Thus, as the extension of the author's previous work in which the damage identification algorithm was formulated from the dynamic stiffness equation of motion, the present study introduces a spectral element model (SEM)-based reduced-domain method (RDM) of damage identification. In the present RDM, a three-steps process is used to reduce the domain of problem by iteratively searching out and removing damage-free parts of structure in the course of the damage identification analysis. To validate the present RDM, numerically simulated damage identification tests are conducted. The experimental tests for a damaged cantilevered beam specimen show that the present RDM can fairly well locates and quantifies all local damages (i.e., slots) placed along the beam specimen.

Experimental and dynamic study of the piston rod lateral friction for the twin-tube hydraulic shock absorber

Abstract: In this paper, dynamic loads acting on a twin-tube hydraulic shock absorber are derived out both in wheel and axle planes by modeling mechanically car rear suspensions, and internal and external forces that yield lateral surface damage and wear-out of the piston rod for the absorber are analyzed according to bench and real road test measures. From viewpoint of vehicle system dynamics and experiment, such key factors as road unevenness, very high car speed and severe shock induced vibrations are investigated, by which stochastic bending moments and dramatically increasing shock loading are introduced directly to the piston rod. From viewpoint of the whole car assembly, on the other hand, due to hardly perfectly placements of the piston rods in their positions between the car suspension and body, unacceptable manufacturing quality of the body may cause additional dynamic forces on the piston rod. Significant results obtained by theoretical and experimental analysis of lateral frictions of the piston rod are presented systematically for improving design of the shock absorber.

A Combination of Modal Synthesis and Subspace Iteration for an Efficient Algorithm for Modal Analysis within a FE-Code

Abstract: Various well-known modal synthesis methods exist in the literature, which are all based upon certain assumptions for the relation of generalised modal co-ordinates with internal modal co-ordinates. If employed in a dynamical FE substruc- ture/superelement technique the generalised modal co-ordinates are represented by the master degrees of freedom (DOF) of the master nodes of the substructure. To conduct FE modal analysis the modal synthesis method can be integrated to reduce the number of necessary master nodes or to ease the process of defining additional master points within the structure. The paper presents such a combined method, which can be integrated very efficiently and seamless into a special subspace eigenvalue problem solver with no need to alter the FE system matrices within the FE code. Accordingly, the merits of using the new algorithm are the easy implementation into a FE code, the less effort to carry out modal synthesis, and the versatility in dealing with superelements. The paper presents examples to illustrate the proper work of the algorithm proposed.

The variance of energy estimates for the product model

Abstract: A product model, in which {x(t)} , is the product of a slowly varying random window, {w(t)}, and a stationary random process, {g(t)}, is defined. A single realization of the process will be defined as x(t). This is slightly different from the usual definition of the product model where the window is typically defined as deterministic. An estimate of the energy (the zero order temporal moment, only in special cases is this physical energy) of the random process, {x(t)}, is defined as m0=∫∞∞|x(t)|2dt=∫−∞∞|w(t)g(t)|2dt Relationships for the mean and variance of the energy estimates, m0, are then developed. It is shown that for many cases the uncertainty (4π times the product of rms duration, Dt, and rms bandwidth, Df) is approximately the inverse of the normalized variance of the energy. The uncertainty is a quantitative measure of the expected error in the energy estimate. If a transient has a significant random component, a small uncertainty parameter implies large error in the energy estimate. Attempts to resolve a time/frequency spectrum near the uncertainty limits of a transient with a significant random component will result in large errors in the spectral estimates.

Experimental validation of elliptical fin-opening behavior

Abstract: An effort to improve the performance of ordnance has led to the consideration of the use of folding elliptical fins for projectile stabilization. A second order differential equation was used to model elliptical fin deployment history and accounts for: deployment with respect to the geometric properties of the fin, the variation in fin aerodynamics during deployment, the initial yaw effect on fin opening, and the variation in deployment speed based on changes in projectile spin. This model supports tests conducted at the Transonic Experimental Facility, Aberdeen Proving Ground examining the opening behavior of these uniquely shaped fins. The fins use the centrifugal force from the projectile spin to deploy. During the deployment, the fin aerodynamic forces vary with angle-of-attack changes to the free stream. Model results indicate that projectile spin dominates the initial opening rates and aerodynamics dominate near the fully open state. The model results are examined to explain the observed behaviors, and suggest improvements for later designs.